Four Duties, One Circuit
A motor branch circuit does four things that ordinary branch circuits do with a single device. It protects against short circuits and ground faults. It provides a means of disconnection. It provides control — starting, stopping and, where the process demands it, speed. And it protects the motor against sustained overcurrent that falls short of a fault. Four duties, sized under four different rules, frequently in four separate pieces of hardware.
The separation exists because a motor is the one load on a building that draws six to eight times its running current every time it starts, for a second or two, entirely legitimately. A device sized tightly enough to catch a stalled rotor would clear on every start. So the fault device is deliberately set well above the conductor's ampacity, and a thermal element inside the starter takes responsibility for everything between full load and a bolted fault.
That inversion is the source of most of the trouble on pump, fan and compressor work. An installer who sizes the breaker to the conductor gets a machine that trips at seven every morning. An installer who fits a fault device at the code maximum and then omits the overload relay has built a circuit that protects the wire perfectly and lets the motor cook.
Everything below follows the four duties in the order they get decided, which is not the order they sit in the enclosure. Current first, then conductors, then the two protective devices, then the mechanical arrangement, then the run itself.
The four duties, as five parts
- Dead front and handle — swings into the working space in front of the panel, and the operating handle has to be reachable without opening anything live Electrical Equipment Working Clearance Space Calculator
- Disconnect and fault device — sized as a percentage of tabulated full-load current, far above the conductor ampacity, and rated to interrupt the fault current available at this point Transformer-Limited Short-Circuit Current Calculator
- Contactor — makes and breaks full-load current thousands of times, so its utilisation category and not its ampere badge decides the contact life Three-Phase Motor Full-Load Amperage Calculator
- Overload relay — set from the nameplate current of the specific motor fitted, with a trip class chosen against how long that machine takes to reach speed
- Load conductors — sized at a percentage above tabulated full-load current and then reduced again by the temperature and grouping the route imposes Feeder Conductor Ampacity Derating Calculator
- Enclosure and backplate — carries the assembly short-circuit current rating, which is a property of the whole combination and not of any device inside it
Two Currents, and They Are Not Interchangeable
Every motor circuit works from two different currents. One is the full-load amperes stamped on the nameplate of the machine actually installed. The other is the full-load current tabulated in the adopted code against horsepower and voltage, which is a deliberately conservative figure covering the whole population of motors of that size. Conductors and the fault device are sized from the table. The overload relay is set from the plate. Swapping them is the classic error, and on a modern high-efficiency motor the two figures can differ by ten percent or more in either direction.
Read the rest of the plate while you are there. The connection diagram matters on a dual-voltage winding, where a nine-lead motor wired for the wrong voltage draws twice what anybody expected and destroys itself inside a minute. Service factor — 1.0 or 1.15 — changes the permitted overload setting. Duty class distinguishes a continuously running pump from a hoist that works for thirty seconds in every five minutes, and the conductor percentage differs between them. Insulation class and temperature rise decide how much margin the winding has when the ambient turns out to be a boiler room.
Frequency and connection catch out imported equipment constantly. A 50 Hz motor run at 60 Hz turns twenty percent faster, and on a centrifugal fan or pump the absorbed power climbs with the cube of speed — a 7.5 kW motor on a 50 Hz-rated impeller is overloaded from the first minute at 60 Hz, and no amount of correct cable sizing helps.
European plates give kilowatts of shaft output and an IEC frame size. North American tables want horsepower. Converting between them is trivial arithmetic that nonetheless goes wrong when someone reads the input power off a variable-speed drive display and treats it as shaft power.
An IEC plate states shaft output in kilowatts and the code table is indexed by horsepower, so the conversion has to happen first — every percentage in the rest of the sizing is applied to the table value it produces.
The power value to convert.
The unit your starting value is in.
Converted power
6.705 HP
They open the calculator with your figures already in it
kW to Horsepower Calculator: 6.71 HP — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 6.705 HP — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The horsepower that comes back is arithmetic, not a size anyone builds. IEC motors are sold in kilowatt steps of 3, 4, 5.5, 7.5 and 11, NEMA motors in horsepower steps of 3, 5, 7.5 and 10, and the two ladders do not line up: a 4 kW machine converts to 5.36 HP, which no manufacturer stocks. Dropping to the 5 HP below gives away margin the original selection had; going up to 7.5 HP changes frame size, shaft diameter and starter rating, and the new frame rarely bolts to the old holes.
- A kilowatt figure on a European nameplate is a shaft rating tied to a duty class and to reference conditions, normally S1 continuous duty at 40 degrees C ambient and no more than 1,000 m altitude. An S2 or S3 plate is a short-time or intermittent rating the motor cannot hold all shift, and a machine working hotter or higher than those reference conditions has to be derated before it delivers its plate. The conversion carries the number and none of the conditions printed beside it.
Full-Load Current Before the Machine Exists
Electrical design routinely starts while the mechanical package is still out to tender. There is a duty — a flow and a head, an air volume and a static pressure — and an absorbed shaft power, and no supplier yet. The circuit still has to be drawn, the containment routed and the board scheduled, so a defensible estimate of full-load current has to come from the shaft power, the voltage and reasonable efficiency and power factor assumptions.
Modern induction motors in the IE3 and IE4 efficiency classes sit in the low nineties for efficiency at rated load, and full-load power factor for a four-pole machine typically runs between 0.82 and 0.88. Both figures fall away sharply below about half load. A motor running at a quarter of its rating can show a power factor under 0.5, which is why an oversized pump motor loads a feeder far harder than its measured kilowatts suggest and why the correction capacitors on a switchboard get sized against measured reactive demand instead of nameplates.
Treat the estimate as a design current for routing and containment, and then reconcile it against the code table once the supplier is fixed. The estimate is not a substitute for the table: the table is what the sizing rules are written against, and an inspector reading a schedule built on a calculated figure will ask which document it came from.
Where the estimate lands close to a table step, size up. A conductor chosen on an assumption that turns out five percent light becomes a repull through containment that is already lidded and firestopped.
With the duty fixed and the supplier not yet chosen, shaft power and voltage are all there is to work from, and a current estimated from them is what carries the containment and board schedule through to selection.
The motor's rated output horsepower.
The three-phase line-to-line supply voltage.
The motor's rated efficiency at full load.
The motor's rated power factor at full load.
Estimated full-load amperage
11.7 A
This is a calculated estimate — for actual motor circuit conductor and overcurrent protection sizing, NEC 430.250 REQUIRES using the tabulated FLA values from NEC Table 430.250, not the motor nameplate current or a calculated value, per code.
They open the calculator with your figures already in it
Three-Phase Motor Full-Load Amperage Calculator: 11.73 A — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 11.7 A — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- This is RUNNING current, not starting current. A motor started across the line draws locked-rotor current of roughly five to eight times FLA for the first seconds, fixed by the NEMA code letter on the nameplate, and that inrush is what sets the starter size, the breaker's instantaneous trip point, and how far the voltage dips on everything else in the panel when the motor kicks in. A generator, a soft start or a trip curve chosen from the number on this page will trip on every start.
- Efficiency and power factor are FULL-LOAD figures, and current does not fall in proportion to load. A motor running at half its rated horsepower still carries most of its magnetizing current, so its power factor collapses - 0.85 at full load can be 0.6 or worse at part load - and the measured amps sit well above half of what is computed here. An oversized motor is the usual reason a clamp reading and a calculated figure refuse to agree.
- Assumes rated voltage actually arrives at the motor terminals. On a long feeder the voltage at the motor is lower than the voltage at the panel, and a motor asked for the same torque at lower voltage answers by drawing MORE current, which heats the windings and shortens insulation life. Long runs have to be checked for voltage drop separately; this formula takes whatever voltage you type as arriving intact.
Conductors at 125 Percent, Then the Room Takes It Back
A single continuous-duty motor gets conductors at a hundred and twenty-five percent of the tabulated full-load current. Where several motors share a feeder, the rule becomes a hundred and twenty-five percent of the largest plus a hundred percent of the rest, which quietly rewards putting the big machine on its own circuit. Short-time, intermittent and periodic duty motors work to their own percentages, and those percentages are lower — a hoist circuit sized as though it ran continuously is copper spent on nothing.
Having sized up, the route takes it back down. Pump rooms run hot, plant rooms sit under uninsulated flow and return pipework, rooftop containment in direct sun exceeds published ambient assumptions by a wide margin, and three motor circuits sharing a ladder tray trigger a grouping factor. The two corrections multiply, and both apply to the table ampacity before the comparison against design current happens.
Segregation adds another effect people miss. Where motor cables share a tray with control and instrumentation, the separation needed for electromagnetic reasons often forces conductors into a tighter bundle than the thermal calculation assumed, and the tray fill that satisfied the containment designer produces a grouping factor the electrical designer never applied.
Check the derated ampacity against the design current, and check it again against the setting the overload relay will end up at. A conductor that satisfies the design current but sits below the overload trip point can carry a genuine overload indefinitely with nothing upstream objecting.
Ambient correction and grouping adjustment compound on one another, and a motor run through a hot plant room in a shared tray is where both bite hardest — apply them to the table figure and compare what is left against the design current.
The conductor's tabulated ampacity before any correction or adjustment factors are applied.
The multiplier for the ambient temperature the conductor will actually operate in, from the applicable NEC temperature correction table.
The multiplier for the number of current-carrying conductors bundled together in the same raceway or cable.
Derated conductor ampacity
91.5 A
Verify both correction factors directly against the current NEC tables for your specific ambient temperature and number of current-carrying conductors — this calculator applies factors you supply, it does not look them up for you.
They open the calculator with your figures already in it
Feeder Conductor Ampacity Derating Calculator: 91.52 A — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 91.5 A — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Derating and terminations are two separate ceilings and the lower one governs. NEC 110.14(C) holds the conductor to the 60°C or 75°C column for whatever the breaker and lugs are listed at, so a run derated down from the 90°C column can still not be used above its 75°C table value — the number this page returns may legitimately be higher than the ampacity you are allowed to claim.
- The adjustment factor you type depends on a conductor count this page cannot verify. Equipment grounding conductors and the neutral of a balanced circuit are not current-carrying, while the neutral of a three-phase, four-wire wye feeding significant nonlinear load is — reclassifying one conductor can drop you a whole band in Table 310.15(C)(1). The adjustment also does not apply at all through a nipple of 24 in (610 mm) or less.
- A derated ampacity proves nothing by itself. It still has to cover the load it serves at 125 percent of the continuous portion, sit correctly with the overcurrent device protecting it, and survive a voltage drop check over the length of the run. This page stops one step before all three.
The Relay That Actually Protects the Motor
Overload protection is set from the nameplate current of the motor in front of you, typically at a hundred and fifteen or a hundred and twenty-five percent of it depending on service factor and marked temperature rise, with a limited permission to increase the setting where the motor genuinely will not start. That permission is not a licence to dial out a nuisance trip; a relay repeatedly wound up because a machine keeps tripping is a relay that has stopped protecting anything.
Trip class is the parameter installers most often leave at the factory default. Class 10 trips within ten seconds at six times setting, class 20 within twenty, class 30 within thirty. A class 10 relay on a high-inertia centrifugal fan that takes eighteen seconds to reach speed will trip during every start. A class 30 relay on a small pump lets a jammed impeller sit at locked rotor long enough to char the winding. Match the class to the measured acceleration time of the driven load, and measure it during commissioning.
Bimetallic relays respond to their own ambient as well as to the current through them. A starter in a plant room trips in August and holds in January with nothing about the machine having changed. Ambient-compensated relays and electronic relays remove that behaviour, and on any starter mounted in a hot enclosure the extra cost is trivial against one summer of unexplained shutdowns.
Phase loss is the failure mode that destroys three-phase motors quietly. Lose one supply leg and a running motor continues to turn while the remaining two legs carry roughly one and three quarters of their previous current; a plain three-element thermal relay eventually notices, and eventually is measured in minutes of overheating. Electronic relays with single-phase protection respond in seconds. On any motor that matters — a fire pump, a process compressor, an extract fan serving a kitchen — that function is the difference between a blown fuse and a rewind.
Choose the reset mode deliberately. Automatic reset on a conveyor, a mixer or anything with a guard opening is how people get hurt: the machine restarts by itself once the relay cools, with somebody's hands in it. Manual reset is the default on anything an operator can reach into.
The Device That Watches Only for Faults
Short-circuit and ground-fault protection on a motor branch circuit has four permitted forms and each has its own ceiling expressed as a percentage of tabulated full-load current: a non-time-delay fuse, a dual-element time-delay fuse, an inverse-time circuit breaker, and an instantaneous-trip breaker. The percentages differ substantially, and the instantaneous-trip breaker is only permitted as part of a listed combination controller — it is not a device you may fit on its own because the catalogue offered it.
Interrupting rating is the property that gets checked last and matters most. The device has to be capable of interrupting the fault current available at its own terminals, and that figure comes from the transformer, the impedance of everything between it and the starter, and the utility contribution behind it. A 10 kA breaker on a bus with 22 kA available is not a slow failure; it is an explosion with a person standing in front of it.
The enclosure carries a rating of its own. An industrial control panel has a short-circuit current rating that belongs to the whole assembly — the weakest component in the fault path sets it, and a starter panel labelled 5 kA fed from a switchboard with 22 kA available is a labelling failure that an inspector will find. UL 508A covers how that rating is established for a listed panel.
Selectivity is the last consideration. A dual-element time-delay fuse rides the starting inrush and clears a bolted fault in a fraction of a cycle, which makes it a strong choice on motors that start hard. An inverse-time breaker is resettable, which matters where the machine is remote and the maintenance team is not. Both are legitimate; the choice belongs to the person who will be answering the phone at three in the morning.
The interrupting rating on the fault device has to exceed what the supply can actually deliver at that starter, and a transformer-limited screening figure is the first cut at what that number looks like.
The transformer's nameplate kVA rating.
The three-phase line-to-line secondary voltage.
The transformer's nameplate percent impedance.
Estimated maximum available fault current
11,000 A
This is a transformer-only screening approximation (the 'infinite source' method), and it is NOT a worst case in either direction. It ignores upstream utility source impedance and downstream conductor impedance, which reduce the current available at a point further from the transformer. It also ignores two things that push the real figure ABOVE it: running motors feed a fault for the first few cycles at roughly four to six times their own full-load current, and nameplate impedance carries an ANSI/IEEE tolerance of plus or minus 7.5%, so a transformer at the low end of that band delivers proportionally more. Do not select an interrupting rating from this number. This is NOT a substitute for a complete short-circuit study (per IEEE 141/242 or equivalent software) required for proper overcurrent protective device rating, selective coordination, and arc-flash hazard analysis, which must be performed by a qualified electrical engineer.
- Transformer full-load amps
- 601.41 A
They open the calculator with your figures already in it
Transformer-Limited Short-Circuit Current Calculator: 10,935 A — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 11,000 A — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The secondary voltage is used once, as the nominal line-to-line figure in (kVA × 1000) ÷ (volts × √3), and no input asks what the bus was actually sitting at in the moment before the fault, so a system running a few percent above nominal drives proportionally more current than this arithmetic returns — and typing that elevated voltage into the box moves the answer the wrong way, because the term sits in the denominator.
- Entries outside 15 to 2,500 kVA, 120 to 600 V or 2 to 8% impedance are pulled back to the nearest bound and answered there, so a 3,000 kVA unit or a 1.8% nameplate produces a figure lower than the transformer in front of you would actually deliver.
- One division by the impedance decimal serves for every kind of fault, and no input describes the winding connection or the grounding arrangement, so a line-to-ground fault — whose return path those two things govern — is not the event this number describes.
- The transformer full-load amps carried in the breakdown is the dividend of that same division, derived from nameplate kVA, so it is the working shown rather than an independent check, and it is rated capacity rather than the load actually connected to the secondary.
- Nothing in the arithmetic has a time dimension: the output is a magnitude at the instant of the fault, with no duration behind it, so it says nothing about how long the transformer or the bus it feeds can hold that current.
A Disconnect You Can See From Where You Are Standing
The disconnecting means has to be within sight of the motor and the driven machinery, or capable of being locked in the open position, and the adopted code defines within sight — visible and within a stated distance, fifteen metres or fifty feet under the North American rules. The purpose is straightforward: nobody working on a machine should have to trust that somebody in another room understands what they have isolated.
Rate the switch correctly. Horsepower-rated switches are tested to make and break locked-rotor current; a general-use ampere-rated switch of the same physical size is not, and using one as a motor disconnect means the first person who opens it under load may be holding a fireball. Where a fused switch is used, the fuse type it accepts has to match the class the design assumed.
A variable-speed drive is not a disconnect. Its output can be off while its terminals sit at full supply potential, and its DC bus holds a lethal charge for minutes after the supply is removed — the discharge time is on the label and it is not thirty seconds. Provide a lockable isolator, mark the discharge time on the enclosure, and give the maintenance team a procedure that includes proving dead at the terminals.
The lockout point is the physical thing an isolation procedure attaches to. NFPA 70E and the equivalent regimes elsewhere expect a padlockable device, a documented procedure and a verification step; a local isolator with no provision for a hasp meets none of that, and it is the single cheapest item on the circuit to specify correctly at order stage.
How It Starts Rewrites Everything Upstream
Starting method is a mechanical decision with electrical consequences in both directions. Across-the-line starting is cheapest, gives full torque from standstill, and hits the supply with six to eight times full-load current at a very poor power factor. Every reduced-voltage method trades starting torque away to reduce that, and torque falls with the square of applied voltage — so a method that halves the current has quartered the torque available to break the load away.
What the driven load will accept decides which trades are available. Centrifugal pumps and fans need very little torque at zero speed because their load torque climbs with the square of speed, so reduced-voltage starting suits them well. Positive-displacement pumps, loaded conveyors, reciprocating compressors and anything with a static breakaway need full torque immediately, and a soft starter fitted to one of them stalls the machine and holds it at locked rotor until something trips.
The choice reaches back through the whole circuit. It sets the contactor arrangement, the number of conductors to the motor, whether the winding is brought out to six leads or three, the supply capacity needed at the moment of start, and — where the site runs on a generating set — the rating of the set itself. Changing starting method after the switchroom is built is one of the more expensive variations available on a project.
Star-delta deserves one specific warning. The open transition between the two connections leaves the motor briefly unpowered and then reconnects it out of phase with its own residual field, producing a current transient that can exceed direct-on-line inrush. Closed-transition equipment avoids it, and on anything larger or more delicate than a simple fan it is worth the extra contactor.
| Method | Starting current | Starting torque | What it costs you |
|---|---|---|---|
| Direct on line | 6 to 8 x FLC | Full | Mechanical shock, supply dip, contactor and coupling wear |
| Star-delta | About one third of DOL | About one third | Six leads, open-transition transient, no torque control |
| Autotransformer | Set by tap: 50, 65 or 80 percent | Falls with the square of voltage | Bulk, cost, and a transition step to manage |
| Soft starter | 2 to 4 x FLC, ramped | Ramped, reduced at breakaway | Heat during long ramps; needs a bypass contactor for running |
| Variable-speed drive | Near full-load current | Full torque from zero speed | Harmonics, cable constraints, bearing currents, cost |
What a Drive Does to the Cable Behind It
A variable-speed drive turns the cable between itself and the motor into a transmission line, and three separate effects follow. The first is reflected wave: the drive switches its output in tens of nanoseconds, the cable impedance does not match the motor impedance, and the pulse reflects at the motor terminals to produce a peak that can approach twice the DC bus voltage. Long cables make it worse. Drive manufacturers publish a maximum cable length for exactly this reason, and inverter-duty windings built to the requirements of NEMA MG 1 Part 31 exist to survive it.
The second is charging current. Cable capacitance draws current at the switching frequency that never reaches the motor but does pass through the drive's output stage, so a drive feeding a two hundred metre run reads output current substantially above what the machine is drawing and trips on overcurrent while doing nothing useful. Output reactors and dV/dt filters address both effects, and they belong in the design instead of in the troubleshooting.
The third is bearing current. Common-mode voltage at the motor frame discharges through the lubricant film and pits the bearing races, producing fluting that shows up as noise and then as a failed bearing eighteen months in. A symmetrical shielded cable bonded through a three-hundred-and-sixty degree gland at both ends gives the high-frequency current a path back to the drive; shaft grounding rings and insulated non-drive-end bearings deal with what remains.
Two absolute rules for drive output circuits: never fit power-factor correction capacitors on the output, which destroys the drive, and never break the output with a contactor while the drive is running unless the drive is specifically arranged for it. Both are things somebody eventually does to a circuit you installed, so label the enclosure.
The Long Run to the Pump House
Voltage drop on a motor circuit is a torque problem before it is an efficiency problem. Torque available at the shaft falls with the square of terminal voltage, so ten percent low reads as about nineteen percent less starting torque — enough that a pump which started on the bench refuses to break away against a full column of water on a cold morning. The motor then draws more current attempting the same mechanical work, runs hotter, and eventually the overload relay does its job long after the insulation has aged.
Measure at the machine and not at the board. A run recorded as sixty metres on a drawing becomes a hundred and ten once it climbs a riser, crosses a plant room at high level, drops down a wall and reaches a starter mounted on the far side of a pump skid. Take the reading at the motor terminals during a start, with a meter capable of catching the dip, and compare that with the steady-state reading.
Voltage unbalance is the more damaging cousin and gets measured far less. A small percentage of voltage unbalance between the three phases produces a current unbalance several times larger, and NEMA MG 1 publishes the derating curve that follows: a motor on a noticeably unbalanced supply has to be run below its rating or it overheats in the phase carrying the excess. Rotor heating rises steeply, and the failure looks like a random winding fault long after the cause has stopped being visible.
Where upsizing for drop is the answer, upsize the phase conductors and check three consequences: whether the equipment grounding conductor has to grow with them, whether the larger cable still lands in the terminal box of the motor, and whether the gland plate accepts the new outside diameter. Motor terminal boxes are consistently the smallest enclosure on the circuit.
Torque falls with the square of terminal voltage, so a drop that looks tolerable on a lighting circuit is the reason a small pump at the end of a long run will not break away — run the actual measured route length, not the plan dimension.
Copper, or aluminum — the metal printed on the jacket (CU or AL).
The size printed on the jacket: an AWG number up to 4/0, then kcmil.
Single phase — including a 240 V circuit and DC — or a balanced three-phase circuit.
The distance from the panel to the load, one direction only.
The expected current draw of the load in amps.
The nominal circuit voltage — line to line for three phase.
Voltage drop
1.936 V
- Voltage drop
- 1.61 %
- Voltage at the load
- 118.06 V
- K constant, Ω·cmil per ft
- 12.9
- Conductor area, circular mils
- 6,530
They open the calculator with your figures already in it
Wire Gauge Voltage Drop Calculator (Copper or Aluminum, AWG to 1,000 kcmil): 1.94 V — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 1.936 V — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- VOLTAGE DROP IS NOT AMPACITY, and the two are different questions with different answers. A conductor can stay inside the 3% suggestion and still be too small to carry the current without overheating, and it can be thermally adequate and still drop too much over a long run. Both checks have to be made, and only one of them is made here.
- The K constants are DC resistance at 75 °C (167 °F) for uncoated copper and for aluminum, the basis of NEC Chapter 9 Table 8. The table's size-by-size resistances differ from the single constant by a percent or so either way, and the table is not reproduced here. A conductor running cooler drops a little less and one at a 90 °C (194 °F) rating a little more; tinned (coated) copper has its own, slightly higher resistance.
- Treats the circuit as resistive, which is close for lighting, heating and most branch circuits. On a large AC feeder, and above all one in steel conduit feeding an inductive load, the conductor's reactance adds to the drop and the power factor matters; Table 9 of the same chapter carries the AC figures and is not reproduced here.
- Three phase assumes a balanced load and gives the drop between lines. A single-phase load taken from one line to neutral of a three-phase supply is a single-phase circuit: choose single phase and the line-to-neutral voltage.
- Aluminum conductors need terminations and devices listed for them; the code does not let dissimilar metals be joined except in a device listed for the purpose. Nothing here checks a termination, a lug or a splice.
- The 3% and 5% figures are suggestions in the code's informational notes rather than requirements, though a local amendment, an equipment maker's instructions or a specification can make a tighter figure binding.
Bonding, and the Path a Fault Has to Take Home
The equipment grounding conductor on a motor circuit is sized against the rating of the fault device ahead of it, and on a motor circuit that device is enormous relative to the load. Most adopted codes cap the result: the grounding conductor never has to exceed the size of the circuit conductors it runs with. Apply that cap, because the alternative reading produces a grounding conductor larger than the phases on almost every motor on the job.
Flexible connections at the machine are where the fault path quietly opens. A short length of flexible metal conduit is an acceptable equipment grounding path only under specific listing conditions and only up to a stated length, and the fitting at each end has to be listed for grounding. On any motor subject to vibration — which is all of them — run a wire-type conductor inside the flex and stop asking the pipe to do the job.
A drive changes the character of the problem entirely. The current that has to get home from a drive-fed motor is high-frequency common-mode current, and impedance at those frequencies has almost nothing to do with the DC resistance of a conductor. A long, thin equipment grounding conductor is a good ground at fifty hertz and a poor one at ten kilohertz. The low-impedance path is the cable screen, bonded circumferentially at both ends; the wire-type conductor is there for the fifty hertz fault and for the inspector.
Bond the driven machinery as well as the motor. A pump skid, a fan casing, a conveyor frame and the motor mounted to it are separate pieces of metal joined by painted bolted joints, and paint is an insulator. Star washers, a bonding jumper across flexible couplings and a check with a low-resistance ohmmeter close that out in ten minutes at first fix.
Rotation, Three Legs, and a Baseline Nobody Records
Commissioning a motor circuit is a fixed sequence, and skipping the early parts of it is how a coupling gets destroyed. Rotation is checked with the load disconnected or with the coupling free, because a centrifugal pump run backwards moves water, makes plausible noise and destroys itself slowly while everyone congratulates each other on a working installation.
The reading that matters most is the one almost nobody writes down: current on all three legs, at real load, at commissioning. Every future diagnosis on that machine is a comparison against a baseline, and where no baseline exists the engineer called out to a warm motor two years later has nothing to compare with and guesses. Record the three currents, the three line voltages, the ambient, the overload setting and the acceleration time, and leave them in the panel.
Label the starter with the equipment it serves and label the equipment with the starter that feeds it. On a plant room wall carrying nine identical enclosures, that pair of labels is worth more to the next person than the whole drawing set.
- Insulation-test the motor and its cable with the drive or starter disconnected, and record the value with the winding temperature.
- Confirm the winding connection matches the supply voltage on any dual-voltage machine, before the supply is applied.
- Set the overload relay to the nameplate current of the motor actually installed, and set the trip class against the acceleration time you expect.
- Bump the motor with the coupling broken or the load disconnected, and confirm rotation against the arrow on the machine.
- Recouple, start under load, and time the acceleration to full speed; adjust the trip class if the measured time contradicts the assumption.
- Read current on all three legs and line voltage across all three pairs at steady load, and calculate the unbalance.
- Record the readings, the overload setting and the measured acceleration time inside the enclosure, and label the starter with the machine it drives.
One motor, one page
Everything a motor circuit needs fits on a single sheet, and the sheet gets filled in from the machine outward. Each entry depends on the one above it and none of them can be assumed.
- Nameplate current, service factor, duty class and connection diagram — The plate current sets the overload relay and nothing else; the connection diagram is what stops a dual-voltage winding being wired for the wrong supply.
- Tabulated full-load current for the horsepower and voltage — Conductors and the fault device work from the table, never from the plate, and the two figures genuinely differ on efficient machines.
- Derated conductor ampacity along the real route — Ambient correction and grouping adjustment multiply; plant rooms and shared trays take back more than most designs allow for.
- Fault device type, percentage and interrupting rating — The permitted percentage depends on which of the four device types you chose, and the interrupting rating depends on the supply, not the motor.
- Assembly short-circuit current rating for the enclosure — A property of the whole combination; the weakest component in the fault path sets the number on the label.
- Measured route length and the terminal box that has to accept the cable — Upsizing for voltage drop routinely outgrows the motor terminal box and the gland plate, which are the smallest enclosures on the circuit.
- Starting method agreed with whoever specified the driven machine — It sets the contactor arrangement, the lead count, the supply capacity at start and, on a generator-fed site, the set rating.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.