Locked-rotor amps and NEMA code letters
The short answer: locked-rotor amps (LRA) is the current a motor draws at the instant it starts, before it turns. Air conditioners and heat pumps print it on the nameplate. Other motors print a code letter instead, which tells you the same thing per horsepower. Either one gives you the motor's starting load in kVA, the number that decides whether a generator can start it.
What locked-rotor amps means
A motor that isn't turning yet acts almost like a short circuit. It draws a big rush of current, several times its running current, and keeps drawing close to that until it's nearly up to speed. LRA is that current, measured with the motor held still (the rotor "locked") at full rated voltage.
It's like pushing a car from a standstill: the first shove takes far more effort than keeping it rolling. LRA is the size of that first shove. Of all the numbers on a nameplate, it's the one that best predicts whether a generator can start the motor. See running watts vs starting watts.
Where to find it
- Central air conditioners and heat pumps: the outdoor unit's nameplate lists the compressor's RLA (rated load amps, what it draws while running) and LRA. The compressor's LRA is the one to use. The outdoor fan adds its own small FLA (full-load amps).
- Well pumps, sump pumps, blowers and other general-purpose motors: the motor nameplate usually shows horsepower, voltage and a code letter (often labeled "CODE" or "KVA CODE"), instead of LRA. A submersible well pump's motor is down the well, so check the control box label or look up the motor model in the maker's data.
- Don't mix it up with the design letter. Many motor nameplates also show "DESIGN" A, B, C or D. That describes the motor's torque characteristics, not its starting kVA.
Turning a code letter into starting kVA
Each code letter stands for a range of starting kVA per horsepower. Multiply the motor's horsepower by the top of the range for its letter. Using the top of the range stays on the safe side.
| Code letter | Starting kVA per hp |
|---|---|
| A | 0 – 3.14 |
| B | 3.15 – 3.54 |
| C | 3.55 – 3.99 |
| D | 4.0 – 4.49 |
| E | 4.5 – 4.99 |
| F | 5.0 – 5.59 |
| G | 5.6 – 6.29 |
| H | 6.3 – 7.09 |
| J | 7.1 – 7.99 |
| K | 8.0 – 8.99 |
| L | 9.0 – 9.99 |
| M | 10.0 – 11.19 |
| N | 11.2 – 12.49 |
| P | 12.5 – 13.99 |
| R | 14.0 – 15.99 |
| S | 16.0 – 17.99 |
| T | 18.0 – 19.99 |
| U | 20.0 – 22.39 |
| V | 22.4 and up |
There are no letters I, O or Q. Letter V has no upper limit, so for a V motor, find the actual LRA from the maker.
What it means for your generator
When a motor starts, the generator's voltage dips. A bigger starting load means a deeper dip. A small dip just flickers the lights. Too deep, and the motor stalls instead of starting, relays and contactors drop out, electronics reset, or the generator trips.
A motor on a generator rarely draws its full LRA, because as the voltage dips, the starting current drops with it. So the real question isn't "can the generator supply the full LRA?" but "how far will the voltage dip, and is that acceptable?" Some home standby spec sheets list a motor-starting capability. Compare it to your motor's LRA or starting kVA.
The calculator uses typical starting watts for each appliance for now. Inputs for LRA and code letters, and a voltage-dip check, are planned.
If one motor drives your generator size, usually a central air conditioner or a well pump, a soft starter can help. It ramps the motor up instead of hitting it with full voltage, which cuts the starting current. How much it cuts depends on the product; makers publish it. Ask an HVAC technician or electrician before fitting one.
Show the math
From LRA (single phase)
starting kVA = LRA × volts ÷ 1,000
Example: an air conditioner
nameplate says compressor LRA 82 A,
240 V
starting kVA = 82 × 240 ÷ 1,000
= 19.68 → 19.7 kVA
From a code letter
starting kVA = hp × top of range
for the code letter
Example: a 1 hp, 240 V motor,
code letter K (8.0 – 8.99 kVA/hp)
starting kVA = 1 × 8.99
= 8.99 → 9.0 kVA
equivalent LRA = 8,990 ÷ 240
= 37.5 A
Both examples are made-up nameplates for illustration. Results are rounded up, never down.
References
- NFPA 70, National Electrical Code, Article 430 (motors), Table 430.7(B): locked-rotor indicating code letters. Summarized in our own words.
- NEMA MG 1, Motors and Generators (code letters, which are marked on the nameplate, and design letters A to D).
- IEC 60034-12, Rotating electrical machines, Part 12: Starting performance of single-speed three-phase cage induction motors (the international counterpart to NEMA design letters).
- IEEE Std 3002.7-2018, Recommended Practice for Conducting Motor-Starting Studies and Analysis of Industrial and Commercial Power Systems (motor starting current and voltage drop).
- U.S. Department of Energy, Motor Systems Tip Sheet #6 (locked-rotor current and how it lasts until a motor nears full speed).
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