Sizing a Standby Generator

The Second That Decides the Machine

Add up every running watt on the panel schedule and you get a figure no generating set ever has to survive. The demand that decides the sizing lasts under a second: the instant a well pump, a hermetic compressor or a lift motor is thrown across the line and pulls locked-rotor current out of an alternator with no utility grid standing behind it. Utility supply absorbs that inrush invisibly. A small standby set feels it as a hammer blow on the crankshaft and a collapse in terminal volts.

Sizing a standby machine therefore means finding the worst starting instant in the building and building everything around it. The fuel line, the transfer switch rating, the pad, the exhaust run and the enclosure all follow from a kW figure that was settled by one motor at zero rpm.

Two numbers describe every motor load. Running kW is what the equipment consumes once it is turning. Starting kVA is what it demands while the rotor is stalled, slip is total, and the winding looks to the alternator like a near short circuit. For across-the-line starting the second commonly runs six to eight times the first in current terms, and because it arrives at a very poor power factor the alternator sees it as reactive burden rather than as engine work.

Practical consequence: the engine can be barely loaded during the surge while the alternator is saturated. Sets are quoted with two separate capacities for that reason, a running kW and a maximum motor-starting kVA, and a machine that clears the first can still fail the second.

Locked Rotor, Read Off the Nameplate

Nameplates carry the starting information, though rarely in the units wanted. Three-phase motors built to NEMA MG 1, Motors and Generators, carry a code letter that expresses locked-rotor kVA per horsepower as a band rather than a single value, and the top of the band is what should be assumed when nothing better exists. Larger motors often print locked-rotor amperes directly, which removes the guesswork entirely.

Hermetic compressors are the awkward case. Air-conditioning and heat-pump condensers publish LRA and RLA on the data plate, and the LRA on a single-phase scroll is brutal relative to its running current. Where the plate is unreadable, painted over or salvaged, the equipment manufacturer published data rather than a generic multiplier is the defensible source.

Deep-well submersibles deserve separate attention, because head, column length and check-valve position all affect how long the motor lingers near locked rotor before it accelerates. A pump that takes noticeably longer to come up to speed holds the alternator down in the dip for longer, and duration is judged against a different limit from peak magnitude.

Resistive loads are honest by comparison. Element heaters, ranges, tank heaters and incandescent lighting draw at starting what they draw running. Collecting them separately from the motor list keeps the surge tally clean, since they raise the base the surge sits on top of without contributing anything to the surge itself.

The Start Method Rewrites the Surge

Before enlarging the set, change how the motor starts. A soft starter ramps applied voltage and typically holds inrush to a small multiple of full-load current instead of the across-the-line figure, and the trade is reduced starting torque, which loaded conveyors and positive-displacement pumps may not tolerate.

Variable-frequency drives go further, starting the motor at low frequency so current barely exceeds the running value. That effectively deletes the motor from the surge tally and replaces it with a rectifier charging characteristic and a harmonic profile the alternator has its own opinion about. Non-linear load on a small machine raises voltage distortion, and the ability of the alternator to hold waveform under it belongs in the specification rather than in an afterthought.

On the residential side the equipment has often solved this already. Inverter-driven ductless heat pumps and ECM furnace blowers start nearer their running draw than the badge suggests, and specifying one during a retrofit can shrink a whole-house set by a rating step. Hard-start kits fitted to older single-phase compressors help less than owners hope, but they are quick to try before a larger machine is ordered.

Reduced-voltage methods carry consequences of their own. Extended acceleration means extended rotor heating and a longer sag on the bus, so a start method chosen purely to shrink the generator can drop out contactors and trip undervoltage protection on equipment that had no complaint before.

Ordering the Starts So Two Never Land Together

Coincidence is a design choice, not fate. If the well pump, the septic pump and the condenser can all call at the same moment, the set must be sized for all three at that moment, or the sequence must be forced so that they cannot.

Load-shed and load-management controls do that forcing. A priority controller sheds the air-conditioning contactor while the pump starts, restores it afterwards, and refuses to close a second compressor until a timer has expired. On larger installations the load-add stages inside the generator controller do the same work, releasing feeders in steps once the set has reached rated speed and voltage.

Restart after an outage deserves separate thought, because everything in the building calls simultaneously the moment power returns. Compressor short-cycle protection, staggered thermostat delays and randomised restart timers spread that pile-up out; without them the largest surge in the life of the installation happens a few seconds after transfer, on a cold engine, unwitnessed.

Where sequencing is used to justify a smaller machine, record it. The control that makes the arithmetic true has become part of the design, and a later trade replacing a failed shed relay with a plain contactor undoes the sizing silently and leaves nobody a way to trace the fault.

Voltage Dip, Not Overload, Is What Fails

Sets rarely trip on overload during a motor start. They dip. Terminal voltage falls, the governor lags, frequency sags with it, and every piece of connected electronics decides for itself whether to ride through. Contactors drop out, drive DC buses undervolt, and the pump that caused the dip is dropped by its own control relay halfway through starting.

How much dip is acceptable is a specification decision, not a habit. ISO 8528, Reciprocating internal combustion engine driven alternating current generating sets, defines performance classes with transient voltage and frequency limits, and the class named in the project documents governs. Sensitive electronics, medical equipment and process control push toward the tighter classes and therefore toward a larger alternator relative to the engine behind it.

Alternator construction matters here as much as kW does. A machine with lower subtransient reactance, and with permanent-magnet or auxiliary-winding excitation that sustains field current into a heavy dip, will start a motor that an identically rated set with shunt excitation stalls on. Two generators with the same badge rating are not interchangeable for motor starting, and the quote sheet rarely says so.

The Badge kW You Will Not Get

Ratings are conditional. ISO 8528 separates standby, prime and continuous duty, and a standby rating assumes limited annual hours at varying load. Running the same machine as a prime mover for a farm, a site cabin or a long seasonal outage is a different rating class and usually a different, larger machine.

Site conditions erode what remains. Altitude and intake air temperature reduce available engine power along curves the manufacturer publishes; naturally aspirated engines lose more than turbocharged ones, and an enclosure that recirculates its own hot discharge air imposes an altitude penalty by another route. Fuel choice does the same, since a set rated on liquid propane delivers less on natural gas, and the two ratings are published separately for exactly that reason.

Gas supply is a recurring field failure rather than a paper one. A meter and service sized for a furnace and a range will not also feed a standby engine at full load, and confirming delivered pressure with the set running under load, not pipe diameter on a drawing, is the check that settles it. The gas utility governs what the meter can be upgraded to, and that lead time frequently exceeds the lead time on the generator.

After the Surge: Load Factor and Fuel

Once the motors are turning the picture inverts. The machine that was strained for a second is now lightly loaded for days, and diesel engines dislike that state: sustained running well below rated load leaves unburned fuel and glazing in the exhaust, the condition known as wet stacking. The cure is a load bank or a design that keeps the engine reasonably loaded, never a larger set.

Runtime carries the expectations the owner actually has. Consumption tracks load rather than rating, so a machine oversized to swallow one compressor start burns through a tank at a rate nobody anticipated when they pictured three days of autonomy on the fuel already in the yard.

Tank sizing also answers to code and to the authority having jurisdiction. NFPA 110, Standard for Emergency and Standby Power Systems, sets minimum on-site fuel provision for classified emergency and standby systems, and NFPA 37, Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines, governs how engine and tank are installed. Optional standby work answers instead to owner tolerance, which tends to be revised sharply upward after the first outage that outlasts the tank.

Transfer, and the Surges Nobody Counted

Motors are not the only inrush on the list. Transformers magnetise, LED drivers and switch-mode supplies charge capacitors, UPS rectifiers either walk in or slam in depending on how they were configured, and a resistance welder or a large table saw presents a transient that looks nothing like its nameplate.

Open-transition transfer adds a hazard of its own. Switching a spinning motor load onto an out-of-phase source can produce a current transient larger than locked rotor together with mechanical shock through the shaft and coupling, and in-phase monitors, timed neutral delay and closed-transition equipment all exist to prevent it. The switch must also suit the total load, the available fault current at its location, and the grounding arrangement the installation actually uses.

Grounding at the transfer point is quietly the most common inspection failure on standby work. Whether the set is a separately derived system, and where the neutral-to-ground bond belongs, depend on whether the switch transfers the neutral, and the governing document differs by jurisdiction: NFPA 70, National Electrical Code, in the United States; CSA C282, Emergency electrical power supply for buildings, in Canada; BS 7671, Requirements for Electrical Installations, in the United Kingdom; AS/NZS 3000, Electrical installations, in Australia and New Zealand. Two bonds and no bond both cause trouble that surfaces long after commissioning.

Proving the First Second on Site

A commissioning run with no load proves nothing about the instant the whole design turns on. Step-load testing does: apply the largest starting block the design was sized for, capture terminal voltage and frequency straight through the transient, and confirm both recover inside the limits the specification named. A recording meter, or a controller with transient capture, turns that into evidence rather than an opinion.

Test in the worst conditions available instead of the mildest. A hot afternoon with a warm enclosure and a fully loaded condenser is the real exam, while a cool morning with the cooling off flatters the machine and proves little. Where the installation falls under a classified emergency system, NFPA 110 sets out acceptance and periodic testing regimes including load requirements, and the authority having jurisdiction signs against those.

Hand the numbers over at the end. The tally of running loads, the coincident starting case that was chosen, the sequencing controls relied on to make it true and the measured dip all belong in the operation and maintenance file, because the next contractor who adds a heat pump will otherwise re-derive them from nothing, or more likely will not derive them at all.