SizingKitNEC 430.7(B) · ISO 8528-1
Generator Size Calculator
This builds a load schedule in two columns — what each item draws running, and what it draws in the second after its contactor closes — and sizes the set on the second one. Motors resolve through the NEC full-load current tables and their Table 430.7(B) code letter; hermetic compressors are taken from their nameplate, as the code requires. The answer comes back in kVA and amperes, with a line naming whether the running total or one motor’s inrush decided it.
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- NEC 430.7(B) code letters
- Running and starting columns
- Printable schedule
Everything without a rotor in it
Heaters, lighting, electronics, chargers, the control side of an appliance. These draw the same current the instant they switch on as they do an hour later, so they contribute one figure rather than two. Power factor is 1.00 for a resistance load and comes off the label for anything with a supply in it.
| Load | Watts | PF | VA | Remove |
|---|---|---|---|---|
| 300 | ||||
| 1,200 | ||||
| 667 |
The three prefilled rows are placeholders to show the arithmetic, not allowances. A nameplate or a clamp meter on the running circuit beats every one of them.
Everything that has to be spun up
A horsepower and a code letter resolve against NEC Tables 430.248, 430.250 and 430.7(B). A hermetic compressor has no horsepower rating you may use for this — switch that row to the nameplate and type its rated-load and locked-rotor amperes off the plate.
8 A running · 30.8 A on start · 3.85× · 230 V 1-phase — Table 430.248 full-load current, code letter H at the top of its 6.3–7.09 kVA/hp band.
18.7 A RLA · 96 A on start · 240 V 1-phase — Nameplate RLA and LRA. NEC 440.4 requires both on a hermetic refrigerant motor-compressor, and 430.6(A)(2) sends you to the nameplate rather than to Table 430.248 or 430.250 for that equipment.
What happens at the worst moment
The second one is not pessimism. Cut power to a house in July and the compressor, the well pump and the furnace blower all have satisfied thermostats and closed contactors waiting; when the transfer switch closes they answer together unless something is staggering them.
Only used to turn the volt-amperes into amperes for the transfer switch and the feeder.
Default 3.0. There is no standard figure here: a soft starter draws whatever its own current-limit parameter is set to, commonly 200 to 450 percent, and starting torque falls with the square of it — set it too low and the motor never comes up to speed.
Default 1.1. A drive accelerates the motor from zero frequency, so there is no inrush to speak of — its limit is the drive’s own overload rating, which is where this figure comes from.
What the set has to deliver
Peak on start
27 kVA
113 A at 240 V
Running
8.49 kVA
35.4 A at 240 V
Badge at PF 0.8
21.6 kW
6.8 kW running
The start decided this, not the total. Everything running at once comes to 8.49 kVA, and Air-conditioning compressor coming on last lifts it to 27 kVA — 3.18 times the running figure, for the second or two the rotor takes to come up to speed. Sizing on the running total alone buys a set that will hold the load and will not start it.
Well pump started last would need 13.7 kVA, so sequencing the two so that Air-conditioning compressor never starts against a loaded alternator is worth 13.3 kVA.
The schedule, both columns
Running against starting, row by row. The right-hand column is empty for everything that does not have to be spun up, and that emptiness is the whole reason the two are separate.
| Item | Running VA | Starting VA | From |
|---|---|---|---|
| Refrigerator and freezer controls | 300 | — | 300 W entered at PF 1.00 |
| Lighting and receptacle circuits | 1,200 | — | 1200 W entered at PF 1.00 |
| Furnace controls and blower drive | 667 | — | 600 W entered at PF 0.90 |
| Well pump | 1,840 | 7,090 | Table 430.248 full-load current, code letter H at the top of its 6.3–7.09 kVA/hp band. |
| Air-conditioning compressor | 4,488 | 23,040 | Nameplate RLA and LRA. NEC 440.4 requires both on a hermetic refrigerant motor-compressor, and 430.6(A)(2) sends you to the nameplate rather than to Table 430.248 or 430.250 for that equipment. |
| Totalspeak is the running total with the worst motor swapped for its own start | 8,495 | 27,047 | ratio 3.18× |
Real power actually entered on this page is 2,100 W, all of it from the non-motor schedule. The motors are carried in volt-amperes because what the code table gives is a current: turning that current into watts needs the motor’s own power factor, which is printed on its nameplate and appears nowhere in Article 430.
The same machine, four different numbers
A kVA figure means nothing until you know which of these ratings it was quoted under.
| Rating | Hours | Load profile |
|---|---|---|
| ESP — emergency standby power | up to 200 h per year | variable, with the mean over any 24 h not above 70% of the ESP rating |
| PRP — prime power | unlimited hours per year | variable, with the mean over any 24 h not above 70% of the PRP rating |
| LTP — limited-time running power | up to 500 h per year | constant, at the full LTP rating |
| COP — continuous operating power | unlimited hours per year | constant, at the full COP rating |
The hour allowances and the overload provisions attached to each rating differ between editions of ISO 8528-1 and between what a manufacturer will warrant, and the plate on the set is what governs. The practical consequence does not change: a 22 kW machine badged for standby duty is not a 22 kW machine you can run a job site on all summer, and reading a standby number as if it were a prime number is the most expensive mistake on this page.
NEC (NFPA 70) 2023, Table 430.7(B) — Locked-Rotor Indicating Code Letters. The letter marked on a motor nameplate under 430.7(A)(9) states the band of kilovolt-amperes per horsepower the motor draws with its rotor held still, which is the condition it is in for the instant after the contactor closes.
The band is the marking's whole meaning: a letter G motor draws somewhere between 5.6 and 6.29 kVA per horsepower locked, and this page uses the top of the band because a generator that only just covers the bottom of it will not start the motor. Code letter V is open-ended and cannot be sized from the letter alone — read the locked-rotor amperes off the nameplate instead. Design letter is a different marking and answers a different question: a NEMA Design B, C or D letter describes torque and slip, not inrush.
These are the code's table values, not the motor's nameplate current, and the two are different on purpose. NEC 430.6(A)(1) requires the values in Tables 430.247 through 430.250 to be used — instead of the nameplate — whenever sizing conductors, ampacity, switches and branch-circuit short-circuit and ground-fault protection. The nameplate governs exactly one thing: the running overload device, under 430.6(A)(2) and 430.32. The table figures are deliberately conservative and run above what a modern high-efficiency motor actually draws, so substituting a lower nameplate current under-sizes the conductor and the disconnect. The two exceptions are a listed motor-operated appliance and a multispeed motor, which use their nameplate for both.
ISO 8528-1, the rating convention for reciprocating internal combustion engine driven alternating current generating sets: three-phase sets are rated at 0.8 lagging power factor, so a set's kW badge is 0.8 of its alternator's kVA. A rating convention for the set, not a property of what you are plugging into it. Single-phase sets are commonly rated at 1.0, and a set with a specified 0.9 alternator exists. The plate on the machine in front of you is the authority, and it states both figures.
Sizing a set is not the same job as installing one. The transfer switch and its rating, whether the neutral is switched, how the set is bonded and grounded, the fuel line, the exhaust termination and the clearance to an opening are all decided by the code and by the manufacturer’s installation instructions, and by somebody licensed to sign the drawing — none of that falls out of the number above.
How to size a set on the surge instead of the sum
The order below is the order the arithmetic falls apart in if you skip a step: get the motors right, get the starting order right, then read the number.
List the loads that never surge, then the ones that do
Heaters, lighting, electronics and controls go in the first schedule at their running watts with a power factor — 1.00 for anything purely resistive, and off the label for anything with a switching supply in it. Everything with a rotor goes in the second schedule instead, because a motor contributes two numbers and the first schedule only has room for one.
Give each motor its code letter, or its nameplate
A horsepower alone does not predict inrush: the code letters run from under 3.14 kVA per horsepower at A to over 22.4 at V, so two motors of one size can differ several times over at locked rotor. The letter is stamped on the plate under 430.7(A)(9). An air conditioner or heat pump compressor is the exception that matters most on a house: it is hermetic, it has no usable horsepower rating, and its plate gives you RLA and LRA directly.
Decide what starts against what
Sequenced means one motor comes on at a time with the rest already running, which is what a set with load management or staggered thermostats sees. Together means everything answers the transfer switch at the same instant, which is what a house sees when power returns on a July afternoon and three closed contactors have been waiting. The gap between those two scenarios is frequently a whole frame size of machine.
Technical specifications
| What it sizes on | Two totals, not one. The running figure is every load at once; the peak figure swaps one motor's running current for its inrush and leaves everything else running, which is the moment a set has to survive. Both come out in kVA and in amperes at the set's own output voltage. |
|---|---|
| Motor running current | NEC 2023 Table 430.248 for single-phase and Table 430.250 for three-phase — 12 and 27 horsepower ratings respectively, read at the motor's rated voltage. 430.6(A)(1) requires these table values rather than the nameplate for everything except the overload device, and they run deliberately above what a modern high-efficiency motor draws. |
| Motor starting current | NEC Table 430.7(B) locked-rotor code letters, all 19 of them from A at under 3.14 kVA per horsepower to V at 22.4 and above. The top of each band is used, because a set that just covers the bottom of the band will not turn the shaft. Code letter V has no ceiling and is refused rather than assumed. |
| Hermetic compressors | Entered from the nameplate instead. NEC 440.4 requires a hermetic refrigerant motor-compressor to be marked with rated-load and locked-rotor amperes, and 430.6(A)(2) sends you to that marking rather than to the horsepower tables — a compressor has no horsepower rating you are permitted to use here. |
| Reduced-voltage starting | Soft starter and variable frequency drive rows take a current limit as a multiple of full-load amperes, prefilled at 3.0 and 1.1. Neither is a standard figure: a soft starter draws whatever its own current-limit parameter is set to, and starting torque falls with the square of it. |
| Set rating power factor | 0.80, the ISO 8528-1 convention for a three-phase generating set, used once and only to translate the required kVA into the kW figure the machine would carry on its plate. It is not applied to any load on the page. |
| Duty ratings | The ISO 8528-1 rating table — emergency standby at up to 200 hours a year, prime power unlimited at a 24-hour mean below 70 percent, limited-time running power up to 500 hours, and continuous operating power at constant load. Four different numbers for one machine, printable. |
| Where the schedule stays | In this browser tab. A load schedule names what somebody owns and when their house is empty, and none of it is transmitted, stored or logged — which also means the page works on a phone standing next to the transfer switch with no signal. |
Frequently asked questions
Why does a 5 hp motor need so much more generator than 5 hp of heaters?
Because for the first second or two it is a short circuit with a shaft through it. A stationary rotor generates no back-EMF, so the only thing limiting current is the winding impedance, and the motor draws its locked-rotor current until it accelerates enough to push back. Table 430.7(B) puts numbers on that: a code letter G motor draws up to 6.29 kVA per horsepower locked, so 5 hp is a 31 kVA demand against roughly 6 kVA running. Resistance heat has no such transient — a 5 kW element draws 5 kW the instant it closes and 5 kW an hour later, which is why an all-electric-heat load schedule sizes almost exactly on its sum and a shop full of motors never does.
The generator has enough kW. Why does the motor still not start?
Because starting is an alternator problem before it is an engine problem. Locked-rotor current is heavily inductive — the load looks like a reactor rather than a resistor for that instant — so it demands volt-amperes from the windings and the excitation system without demanding much real power from the engine. What fails is voltage: the alternator sags under the current, the motor's own torque falls with the square of the voltage it is getting, and the shaft never accelerates out of the condition. ISO 8528-5 puts limits on how far and how long a set may dip, by performance class; a set can hold its kW rating comfortably and still be unable to start a motor its kVA cannot cover.
Do I add the starting figures of every motor together?
Only when they can genuinely all start at once, and after a utility outage they frequently can. Thermostats and pressure switches that were satisfied when the power went out have their contacts closed, so the moment the transfer switch makes, the compressor, the well pump and the blower all call together. A house with nothing staggering them is the together case. A set with load management, a compressor time-delay relay, or simply enough motors on drives is the sequenced case, and this page reports both so the difference is visible rather than assumed.
Is a soft starter or a drive a cheaper answer than a bigger machine?
Often, and it is worth pricing before the next frame size up. A soft starter cuts inrush to whatever its current limit is set to, and a variable frequency drive removes it entirely because it accelerates the motor from zero frequency. The catch is torque: it falls with the square of the current, so a starter set to 300 percent of full load delivers about a third of the starting torque the motor would have across the line. That is plenty for a centrifugal pump or a fan, whose load torque is small at low speed, and not enough for a loaded compressor or anything with real inertia — which is precisely the equipment whose inrush was the problem.
What is the difference between the standby and prime ratings on the same set?
The number of hours you are allowed to use it and the average load you may hold. ISO 8528-1 defines emergency standby power as available for up to about 200 hours a year with the mean load over any 24 hours below 70 percent of it, and prime power as available for unlimited hours at a variable load under the same 70 percent mean. The same physical machine therefore carries two different kW figures, and the standby one is the larger. Buying a standby-rated set for a job site or a farm that runs it all season is buying a machine at the wrong number, and the warranty is written against the rating.
My portable generator is sold as 7,500 running and 9,375 starting watts. How does that map onto this?
Those are the manufacturer's figures at a power factor they chose and did not print, so compare them against the kVA on this page rather than against its kW. The ratio between the pair is usually 1.25 — the surge column is the running column times a fixed multiplier rather than a measurement of anything — and 1.25 is a smaller surge allowance than a single across-the-line motor of any size will ask for. Read the machine's own kVA and its allowable overload duration out of its manual, and treat the carton as marketing.
Does the set have to cover the whole house, or only what I choose to run?
That is a decision about the transfer equipment rather than about arithmetic, and it changes the schedule completely. A whole-house switch means the schedule is every load in the building, minus whatever load management can shed. An essential-loads panel means you decide what moves onto it at installation, and the schedule is only those circuits — which is how a 12 kW set covers a house whose service calculation came to 200 A. The NEC also permits sizing a standby supply on a calculated load rather than on the service size, and which of those routes the jurisdiction wants is a question for the plan reviewer before the equipment is ordered.
Inrush, code letters, and why the sum is the wrong number
A generating set is asked two separate questions and only one of them is a total. The first is whether it can hold everything that is running, which is arithmetic anybody can do. The second is whether it can survive the instant a motor is thrown across the line while everything else is already loaded, and that instant is where sets are chosen or regretted. An induction motor at standstill produces no counter-voltage, so it draws the current its impedance allows — roughly five to eight times its running current for a general-purpose machine — and it draws it as volt-amperes at a very poor power factor. The alternator feels all of it. The engine barely notices, which is exactly why a set can be comfortably within its kW rating and still stall a compressor: the voltage sags, motor torque falls with the square of voltage, and the rotor never breaks away.
The code already carries the number, and almost nothing on the web uses it. NEC Table 430.7(B) assigns every motor a locked-rotor indicating code letter, and the letter is a band of kilovolt-amperes per horsepower with the rotor held still — letter G is 5.6 to 6.29, letter H is 6.3 to 7.09, and a letter apart is worth a real fraction of a frame size. Multiply the top of the band by the horsepower and the starting kVA falls out with no rule of thumb anywhere in it. The one place this breaks is the load that most often governs a residential standby job: an air-conditioning compressor is hermetic, has no horsepower rating you may use, and is marked instead with rated-load and locked-rotor amperes under Article 440 — so that row is entered from the plate, and 430.6(A)(2) is the article that says so. The amperage calculator handles the reverse trip when all you have is a kW figure and need the current, and the electrical load calculator is the other document a standby installation usually needs, since the service load and the generator load are computed under different rules and come out as different numbers.
What you do with the answer splits three ways, and only one of them is buying a larger machine. Sequencing costs nothing when the loads happen to be staggered already and costs a load-management module when they are not. A soft starter or a drive attacks the inrush directly, at the price of starting torque, which is fine on a fan and fatal on a loaded compressor. And shrinking the schedule — moving to an essential-loads panel, leaving the range and the dryer on the utility — is usually the cheapest of the three. None of it is a stamped design: the transfer switch rating, the neutral switching, the bonding and grounding arrangement, the fuel supply and the exhaust termination are all code and manufacturer decisions signed off by somebody licensed to sign them. If the reason you are here is fuel rather than watts, the propane usage calculator works out how long the tank feeds the set, and a standby installation that is really about resilience often ends up comparing against the solar panel output calculator and a battery instead.
What happens to the load list
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
A generator schedule is a list of what a building contains and which circuits matter when the lights go out, which is not a document to hand to a website. It stays in the tab. The feeder between the set and the transfer switch is sized on the peak current this page reports and on its length, which is the wire size calculator; and if the building is losing heat faster than it should, the insulation R-value calculator works on the half of the problem no generator can fix.