LOCKED-ROTOR AMPS (LRA) CALCULATOR

To correctly size a generator for home use to start a motor load, you obviously need to know its inrush current. This quantity is called Locked-Rotor Amperes (LRA) because at the first moment the rotor is at rest and acts as if it's locked. If a motor nameplate directly states its LRA, you are lucky- that's all you need to know. If not, I'll show you how to estimate it. A nameplate often states "kVA Code Letter" (or simply "Code").

This letter designation represents the ratio between starting kVA and rated horsepower (HP) per the NEC® 2023 Table 430.7(B). To find LRA and starting kVA enter nominal operation voltage (such as 120 or 240), HP value, code letter from the nameplate label, and hit "Calculate" button.

The general equation for single-phase devices is the following:
LRA=1000*(kVA/HP)/Voltage.

Note that the above method defines starting current at full nominal voltage. This value is relevant when you run your appliance from the mains, whose level is pretty much stable if you have proper wiring. When you power a motor-driven device from a generator, its output will sag because of initial load surge. Fortunately, most residential appliances will start at up to 30% voltage drop. For example, if rated input of an air conditioner is 120V, it is supposed to start at 120*0.7=84V. At 30% lower input its actual LRA likewise will be 30% lower. That's why we provided both values in the calculator. Furthermore, the surge volt-amps will be 0.7*0.7=0.49 of that of full voltage. When you size your generator, its surge capability should match these number. Also note that NEC® provides a range of kVA/HP values for each designation letter. Our tool gives the maximum values, i.e. the higher limits.