How to use this calculator
- Read the nameplate. Code letter, horsepower, voltage — and the printed LRA if the maker gives one (it governs).
- Compute the band. kVA/hp band × hp ÷ voltage gives the locked-rotor amp range; use the max edge for protection settings.
- Express as a multiple. LRA ÷ FLA tells you what the breaker’s instantaneous trip and the generator’s surge rating must ride through.
- Knock it down if needed. Soft start, wye-delta or a VFD when the supply, the lights or the utility cannot take the hit.
How it works
A motor at standstill is mostly inductance and very little back-EMF — it draws whatever the supply lets it. The nameplate code letter bands that draw in kVA per horsepower, and Ohm's-law arithmetic does the rest:
LRA = kVA/hp × hp × 1000 / (√3 × V) — three-phase · ÷ V — single-phase
Running full-load amps for the same motor come from the motor FLA calculator, the overload protection that rides on them from the overload heater calculator, and the voltage sag the inrush causes along the feeder from the voltage drop calculator.
Worked example
Verified against the live calculator
A 10 hp, 460 V three-phase motor,
nameplate code letter G, FLA 14 A:
LRA = (5.6–6.29) × 10 × 1000 / (1.732 × 460) = 70–79 A ≈ 5.0–5.6 × FLA
The breaker's instantaneous trip and any generator feeding this motor must ride through ~79 A for the second or two of acceleration. Note the voltage lever: the same motor wound for 230 V draws 141–158 A — identical kVA, double the amps — which is why generator sizing questions always start with the code letter and the voltage together.
Frequently asked questions
How do you calculate motor inrush current from the code letter?
LRA = (kVA/hp from the nameplate code letter) × hp × 1,000 ÷ (√3 × V) for three-phase. A 10 hp, 460 V motor with code letter G (5.6–6.29 kVA/hp) draws 70–79 A locked-rotor — about 5 to 5.6 times a typical 14 A full-load current.
What is a motor code letter?
A nameplate letter (A through V, skipping I, O and Q) that bands the motor’s locked-rotor kVA per horsepower per NEC Table 430.7(B). Low letters start soft; high letters hit hard — code letter V is open-ended at 22.4 kVA/hp and up.
How many times the full-load current is motor inrush?
Across-the-line starting commonly lands around 5–8× FLA for ordinary code letters (G through L territory), but it is the code letter and voltage that decide: the same kVA at half the voltage means twice the amps. Use the band max for breaker instantaneous settings and generator sizing.
Does a VFD or soft starter change inrush?
That is their job: reduced-voltage and ramped starting hold current to a programmed limit (often 1.1–1.5× FLA on a VFD). The code-letter math here is the full-voltage worst case — the number the upstream breaker, contactor and generator must survive if the motor ever starts across the line.
Method & assumptions
- Code-letter bands per NEC Table 430.7(B), verified 2026-06-11 against two independent publications of the table (including the Article 430 text) that agree exactly; the scale is nameplate-definitional (NEMA MG-1 carries the same letters) and stable across editions.
- Across-the-line (full-voltage) starting; reduced-voltage methods draw less by design.
- The letter gives a band, not a point — the nameplate LRA, where printed, governs over the band math.
- Protection coordination (instantaneous-trip settings, NEC 430 Part IV sizing) follows the adopted code and the overcurrent device listing — this screen supplies the starting-current number those rules consume.