Read the Nameplate Before the Panel Schedule
A branch circuit starts at a piece of equipment, and the only number on the job nobody has argued about is the one stamped on that equipment's nameplate. Find it before you find the panel. Write down full-load amperes, or watts and volts, or — on a listed appliance assembly such as a condensing unit, a tankless water heater or a range top — the minimum circuit ampacity and the maximum overcurrent protective device rating. Record phase, voltage and frequency in the same breath. A large share of sizing disputes trace back to somebody working from a catalogue page for a model number that shipped two revisions ago, and the plate on the side of the actual crate settles it.
Where a nameplate gives a minimum circuit ampacity and a maximum fuse or breaker size, those are instructions rather than suggestions. They were derived by the manufacturer under a listing, and the installation instructions that accompany listed equipment carry code force in most adopting jurisdictions. The maximum device rating is a ceiling, not a target. The minimum circuit ampacity already contains the manufacturer's continuous-duty allowance, so sizing a conductor to that figure and then adding another twenty-five percent on top buys nothing, wastes copper, and can push you into a conductor size the equipment lugs were never designed to accept.
Receptacle and lighting circuits arrive with no nameplate at all, so the load has to be assembled by hand. General-purpose outlets get a volt-ampere allowance per outlet or per unit of floor area depending on occupancy and the code edition in force. Luminaires get their marked input wattage, not the lamp wattage — drivers and ballasts draw more than the lamps hanging under them, and on a long run of linear fixtures that difference is the whole margin.
A receptacle circuit has no plate to photograph, so its count gets built out of the per-outlet allowance instead — which is exactly the arithmetic below.
The circuit's breaker rating.
General-use receptacle circuits are standard 120V.
Estimated maximum outlets
10 outlets (estimated maximum)
This is a general planning guideline based on the NEC's standard per-receptacle load value, not a strict maximum-outlet-count rule — actual safe count depends on what's actually plugged in and used simultaneously, which can be far less than this theoretical maximum.
- Safe continuous circuit capacity
- 1,920 VA
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Outlets Per Circuit Calculator: 10 outlets (estimated maximum) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- IN A DWELLING THIS IS A RULE OF THUMB, NOT A CODE LIMIT. The NEC assigns no load to a general-purpose receptacle outlet in a dwelling unit — lighting and receptacles are covered together by a volt-amps-per-square-foot figure — so there is no code maximum number of outlets on a house circuit. The 180 VA per outlet this uses is the NON-DWELLING rule, and applying it to a house gives a sensible answer for the wrong reason.
- Counts general-purpose outlets only. Kitchen small-appliance circuits, bathroom, laundry and any dedicated appliance circuit are required to be separate and are not part of this count.
- A single high-draw appliance can exceed the whole circuit on its own, so the count is meaningless once anything substantial is plugged in. A space heater is most of a 15 A circuit by itself.
- Says nothing about conductor size, voltage drop over the run, or the arc-fault and ground-fault protection the location requires.
- Local amendments are common on this subject and can impose a hard maximum where the model code has none.
Turning Nameplate Amps into Design Amps
Continuous is a defined word, not a description of how busy the circuit feels. Under NFPA 70 National Electrical Code, a load expected to operate for three hours or more is continuous, and both the branch-circuit conductors and the overcurrent device are sized to 125 percent of it. Other regimes arrange the same physics differently: CSA C22.1 Canadian Electrical Code and BS 7671 Requirements for Electrical Installations each handle sustained loading through their own factors and correction methods. Confirm which document, and which edition, the authority having jurisdiction has actually adopted before committing a number to paper — adoption lags publication by years in plenty of places.
Motors leave that path entirely. Conductor sizing for a motor branch circuit works from the tabulated full-load current for horsepower and voltage rather than the nameplate amps, while the short-circuit and ground-fault device is permitted to sit well above the conductor ampacity because the overload relay in the starter handles sustained overcurrent. Mixing the two methods — table current for the wire, nameplate amps for the breaker — is the classic error on pump, compressor and air-handler work, and it produces a circuit that nuisance-trips on every start while being genuinely unprotected against a stalled rotor.
Ranges, dryers, electric vehicle supply equipment and welders each carry their own demand treatment, and several of them permit a circuit smaller than the connected nameplate total. Apply the demand rule that belongs to that appliance class rather than a general one borrowed from feeder calculations. Note the design current you land on, in ink, at the appliance end of your sketch. Everything that follows only ever makes that number harder to serve.
The three-hour test is what separates connected load from design load, and this is where the breaker size you had in mind either survives that 125 percent or does not.
The amp rating printed on the breaker in your panel.
Most household outlets in North America are 120V; large appliances (dryers, ranges, EV chargers) are often 240V.
The sum of the wattage of everything plugged into this circuit at once.
Current draw
10 A
- Total connected load
- 1,200 W
- Safe continuous limit (80% rule)
- 16 A
- Safe continuous limit
- 1,920 W
- % of safe continuous capacity used
- 62.5 %
They open the calculator with your figures already in it
Electrical Circuit Load Calculator: 10 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 — 10 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
- ONE circuit, not the panel. Whether the service and the panel can carry everything together is a separate calculation under the code's demand-factor rules, and a house full of individually compliant circuits can still overload its supply.
- Says nothing about the WIRE. Conductor ampacity is set by the conductor, its insulation, the ambient temperature and how many current-carrying conductors share a raceway — a breaker rating does not guarantee the cable behind it, and a correctly sized breaker on undersized cable is the dangerous combination.
- Voltage drop over the run is not checked here. A circuit inside its current limit can still deliver too little voltage at the far end, which is a separate calculation.
- Motor and compressor loads draw several times their running current at start-up. Nameplate watts describe the running condition and understate what the breaker sees on a cold morning.
- The 80% figure applies to CONTINUOUS loads — three hours or more at full draw. A load that is genuinely intermittent may use more of the breaker's rating, and which of the two a given appliance is can be a judgement.
Code thresholds this tool can check
Code thresholds this tool can check
Checked for United States. Each check below names the body that published the limit it uses. Switching market re-runs them. This is not a code review and has no official standing.
These checks cover only the specific numeric limits listed below. They are not a complete code review: fire separation, egress, structural capacity and accessibility provisions are outside their scope, and only the handful of local amendments offered in the selector are modelled — your municipality may have others. Passing every check here does not make a design compliant. Final approval rests with your local building authority.
The Termination Picks Your Ampacity Column
Now walk one step back, to the terminations. Conductor insulation may be rated 90 degrees Celsius, but the lug on the breaker and the lug on the appliance are rated too, and the lower of the two governs which ampacity column you are allowed to read. Equipment terminations are commonly listed for 60 degrees Celsius up to a certain circuit size and 75 degrees Celsius above it, with the marking on the device or in its instructions being the authority. UL 486A-486B Standard for Wire Connectors covers the connector side of that relationship. The practical consequence is blunt: a 90 degree conductor almost never gets to use its 90 degree ampacity for the final size, only for the derating arithmetic on the way there.
Aluminium changes the answer again. For the same ampacity an aluminium conductor lands a size or two larger, the lug must be listed for the material, and the antioxidant and torque requirements in the manufacturer's instructions are not optional garnish — they are the difference between a joint that holds and a joint that anneals over a decade of thermal cycling. Torque to the marked value with a calibrated tool. A screwdriver and a feeling in the wrist is what burned lugs look like before they burn.
Feed it the design current you settled on at the appliance and you get the starting conductor size — the one the route is about to attack. It reads Table 310.16's 60 °C column, which is the governing termination rating on most residential work and the conservative choice everywhere else; if your terminations are rated 75 °C throughout, the table permits a size smaller than this.
The amp rating of the breaker this wire will serve.
Minimum wire gauge
12 AWG
This is ampacity-only sizing — long runs may also need a thicker gauge to control voltage drop (see the Wire Gauge Voltage Drop Calculator).
- Ampacity of recommended gauge
- 20 A
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Amperage to Wire Gauge Calculator: 12 AWG — 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 — 12 AWG — 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
- Copper on the 60 °C column, and nothing else. There is no input for conductor material or insulation rating, so aluminium — which carries less than copper at the same gauge — comes out undersized, and 75 °C or 90 °C rated cable comes out conservative even where every termination would allow the higher column.
- No derating is applied. Ambient temperature above the table's basis, several current-carrying conductors bundled in one cable or raceway, and sun-exposed rooftop runs all cut a conductor's usable ampacity below the table figure, and none of those conditions is an input here. The gauge returned is a floor for ideal conditions, not a finished answer.
- Run length and voltage are not part of this calculation. A 3 m run and a 60 m run at the same amps return the same gauge, and on a long run voltage drop, not ampacity, is usually what sets the size.
- This is not a load calculation and not a circuit design. It converts an amp figure you supply into a conductor size; it does not work out what that figure should be, does not apply the continuous-load 1.25 uplift for you, and does not size the neutral, the equipment grounding conductor or the overcurrent device. Motor, air-conditioning and welder circuits follow their own sizing rules and their nameplate ratings rather than a straight table lookup against running current.
- The table ends at 1 AWG (110 A). Anything needing 1/0 or larger — most services and main feeders — plus parallel conductor sets and the reduced-size allowance the code gives dwelling service conductors, is outside what this returns; a figure above 110 A does not get a valid answer here.
What the Route Takes Away
Ampacity tables assume an ambient temperature and an unbundled condition, and the route back to the panel rarely honours either. An attic above an insulated ceiling in summer sun, a boiler room, a rooftop raceway in direct sunlight, a run through a cold-storage envelope — each shifts the correction factor, some of them severely, and rooftop exposure in particular has caught out plenty of otherwise careful installations.
Bundling is the other half. Once more than three current-carrying conductors share a raceway or a cable without maintaining spacing, an adjustment factor applies and it compounds with the ambient correction. Neutrals on a three-phase, four-wire circuit serving nonlinear loads count as current-carrying; a neutral carrying only the unbalance of a multiwire branch circuit generally does not. Count the conductors honestly, apply both factors, and check the derated ampacity against the design current from step one. If it fails, the fix is a larger conductor, a second raceway, or a different route — decided now, on paper, rather than at rough-in with the ceiling closing behind you.
Both corrections multiply, so applying them together — here, while the route is still a pencil line — is what stops a table value being mistaken for what the run will actually deliver.
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.
Length Is the Parameter Nobody Measures
Length is the parameter nobody measures until the trim-out. Voltage drop is not a safety requirement in every code — NFPA 70 treats the familiar three percent on a branch circuit and five percent overall as informative guidance, while BS 7671 Requirements for Electrical Installations imposes limits that must be met — but the equipment at the far end does not care which category the rule falls into. Undervoltage runs motors hot, dims and shortens lamps, and makes electronic power supplies work harder than their designers intended.
Measure the actual conductor path, not the plan-view distance. A run that reads forty feet on a drawing routinely becomes seventy once it climbs a wall, crosses a joist bay perpendicular to the run, drops into a chase and turns three corners at the panel. Where the calculation says upsize, upsize the ungrounded conductors and check whether the equipment grounding conductor has to grow in proportion — and confirm the larger conductor still lands in the device lug and still bends inside the box you specified. Upsizing for voltage drop is the most common reason a circuit outgrows its enclosure.
Raceway and Box Come After the Conductors
Only after the conductors are settled does the raceway get a size. Working the other way — pulling a trade size out of habit and then discovering the derated conductors will not fit — is a rework of the whole run. Fill limits are expressed as a percentage of internal cross-sectional area, with the familiar forty percent applying to three or more conductors under most adopted codes, and the tables in the code annexes carrying the actual dimensions for each insulation type. Insulation matters as much as copper size here; the same conductor in a thicker jacket can cost you a trade size.
Jam ratio deserves a thought where three conductors of the same size go into a conduit whose inside diameter falls in an awkward relationship to their outside diameter. The conductors can wedge at a bend and stop dead partway through a pull that the fill percentage said was comfortable. Add the pull-out and re-pull to your schedule if you ignore it. Box fill is the companion calculation at each end — conductors, devices, clamps and grounds each claim volume, and a stuffed device box is one of the most reliably cited items on any rough-in inspection.
Run it with the conductor count and insulation type you finished with, not the ones you assumed at estimate stage, and order the trade size that result gives you.
The gauge of the conductors being pulled through the conduit.
The trade size of the EMT conduit.
Maximum conductors
16 conductors (maximum)
This uses standard EMT and THHN area tables — different conduit types (PVC, rigid) or conductor insulation types have different area values. Always verify against the current NEC Chapter 9 tables for your specific materials.
- Available fill area (40% of conduit)
- 0.21 in²
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Conduit Fill Calculator: 16 conductors (maximum) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Fill is not ampacity. This says how many conductors physically fit under the 40% rule; it does not derate them, and the NEC requires conductor ampacity to be adjusted downward once more than three current-carrying conductors share a raceway. A bundle that passes fill can still need a larger conductor or a smaller breaker — this is not a load calculation and it does not size the circuit.
- EMT and THHN/THWN only. PVC, IMC, rigid and flexible raceways have different internal areas at the same trade size, and other insulations — XHHW, TW, RHW, anything with a thicker jacket — have different conductor areas. Change either and the count changes.
- Every conductor in the raceway counts toward the 40%, including neutrals and the equipment grounding conductor, and the count assumes they are all the same size. A bundle mixing gauges has to be totalled area by area against 40% of the conduit's internal area rather than read off a single-gauge count.
- Fitting is not pulling. Nothing here accounts for run length, the number and radius of bends, pulling tension, lubricant or conductor jamming — a conduit loaded to the code maximum across a long run with several bends can be impractical to pull.
- These are US NEC figures for a run of three or more conductors. A single-conductor run is permitted 53% fill and a short nipple between enclosures 60%, but a two-conductor run is held to 31% — less than the 40% applied here, not more. Outside North America conduit is sized by different methods entirely, so check the code edition and any local amendments your jurisdiction has adopted.
The Grounding Conductor Looks Forward, Not Back
The equipment grounding conductor is sized from the rating of the overcurrent device ahead of the circuit, not from the load and not from the phase conductor size. That inversion catches people, because it is the one element in the whole sequence that looks forward to the panel rather than backward from the appliance. Where the ungrounded conductors have been upsized for voltage drop, the grounding conductor is increased proportionally in most adopted codes.
Wire-type grounding conductors, metallic raceway used as the equipment grounding path, and cable armour each have their own listing conditions, and a raceway that is a legitimate ground path in one code region is merely a mechanical enclosure in another. Where fittings, couplings or a section of flexible conduit interrupt continuity, run a wire-type conductor and stop relying on the pipe. The cost of that decision is a few dollars of copper; the cost of the alternative is a fault current path that only fails on the day it matters.
Landing at the Panel
The panel end closes the loop. The overcurrent device must protect the conductor at its final derated ampacity, respect any maximum rating printed on the appliance nameplate, and be listed for the panelboard it lands in — series ratings and interrupting ratings are properties of the assembly, not of the breaker alone. Confirm the bus rating, the available fault current at that panel, and whether the branch circuit requires arc-fault protection, ground-fault protection for personnel, or both, according to the occupancy and the code edition adopted locally. UL 1699 Standard for Arc-Fault Circuit-Interrupters governs the device side of that requirement.
Physical space is the last constraint and the one most often discovered too late. Two-pole breakers need adjacent full spaces on opposite phases, tandem breakers are only permitted where the panelboard is listed for them in that position, and a multiwire branch circuit needs a handle tie or a common-trip device. Fill in the panel directory as you land each circuit rather than at the end of the job from memory. A directory written from memory is wrong roughly as often as it is right, and the next person to open that dead-front will trust it.
Rehearsing the Failures
Three failure modes account for most of the callbacks. A circuit sized to the table value with the derating skipped runs warm for years and then fails at a termination, usually in the hottest part of the route rather than at either end. A circuit sized correctly but landed on a 60 degree lug reads fine on a meter and fails inspection on the marking alone. And a circuit sized for the load but not the length delivers full voltage at the panel and something meaningfully less at the appliance, which shows up as a motor that runs hot or a heater that never quite reaches setpoint.
Walk the sequence backwards one more time before you order material: nameplate, design current, termination temperature, ambient and bundling corrections, length, raceway and box, grounding conductor, then device and panel space. Each step can only make the conductor larger, never smaller, which is precisely why starting at the panel and working outward produces a number you have to revise. Get the load right and the panel end takes care of itself.
Backwards takeoff for one branch circuit
Assemble the list in the same direction the sizing ran — appliance first, panel last. Nothing on it can be confirmed until the item above it is settled.
- Nameplate record: FLA or MCA and maximum OCPD, voltage, phase — Photograph the plate on the delivered unit; catalogue data goes stale between revisions.
- Conductor sized at derated ampacity, not the table value — Ambient and bundling factors compound — apply both, then compare against design current.
- Termination temperature marking captured at both ends — The lower of breaker lug and appliance lug decides which ampacity column is legal.
- Measured conductor path length, walls and drops included — Plan-view distance underestimates routinely; upsizing for drop can outgrow the device box.
- Wire-type equipment grounding conductor sized to the breaker — Increase proportionally if the ungrounded conductors were upsized for voltage drop.
- Panel space, protection type, and directory entry — Two-pole and multiwire circuits need adjacent spaces and a common-trip or handle tie.
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.