Motor full-load amps, branch-circuit conductor ampacity, and protective device size.
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Motor Nameplate Data
Motor Electrical Results
Full Load Current (FLC)
—
A — NEC Table 430.250 / 430.248
Min. Conductor Ampacity
—
A (FLC × 1.25, NEC 430.22)
Max. Inverse-Time Breaker
—
A (next std. ≥ FLC × 2.5, NEC 430.52)
Input kW
—
kW (HP × 0.746 ÷ efficiency)
Conductor Size — NEC Table 310.16, Copper, 75°C
AWG / kcmil
Ampacity (75°C)
≥ FLC × 1.25?
NEC 430.22: branch-circuit conductors at least 125% of the table full-load current. NEC 430.52: inverse-time breaker up to 250% of it, next standard size up (NEC 240.6(A): 15, 20, 25 … 500, 600, 700, 800, 1000 … 6000 A). Ampacities are the 75°C column for copper with no more than three current-carrying conductors at 30°C ambient, and assume 75°C-rated terminations (NEC 110.14(C)). Not included: ambient and conduit-fill adjustments (310.15), voltage drop, and the separate running overload (430.32, set from nameplate amps). Verify with the AHJ and local amendments.
Inputs, results, and method — as an Excel workbook or plain CSV.
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This motor full load amp calculator looks up the NEC table full-load current for a single- or three-phase motor from its horsepower and voltage, then derives the minimum branch-circuit conductor ampacity (125%), the smallest 75°C THHN/THWN copper conductor, and the maximum inverse-time breaker. Power factor and efficiency feed a power-equation cross-check and the input kW.
It is a quick design aid for motor branch circuits. For permit and construction documents, conductor and protection sizing must use the table full-load current per the National Electrical Code, and the design must be verified with the authority having jurisdiction and any local amendments.
Formula & Method
FLC = NEC Table 430.250 (3-phase) or 430.248 (1-phase) at the HP and voltage
Conductor ampacity ≥ FLC × 1.25 · Max. breaker = next std. size ≥ FLC × 2.5
Cross-check: I = (HP × 746) ÷ (V × PF × η × √3) (no √3 for 1-phase) · kW = HP × 0.746 ÷ η
NEC 430.6(A)(1) requires the table full-load current, not nameplate amps or a calculated current, for conductor and branch-circuit protection sizing, so every size on this page starts from the table. The 1.25 factor is NEC 430.22; the conductor is the smallest copper size whose 75°C ampacity in Table 310.16 meets it. The 2.5 factor is the NEC 430.52 inverse-time-breaker maximum, taken to the next standard rating. Combinations the tables do not list (for example single-phase above 10 HP, or 460 V single-phase) get no code sizes. The power equation — 746 W per HP, √3 ≈ 1.732 for three-phase line-to-line voltage — is shown only as a cross-check.
Frequently Asked Questions
How is motor full load current (FLA) determined?
For conductor and branch-circuit protection sizing you look it up rather than calculate it: NEC 430.6(A)(1) requires the full-load current from Table 430.250 (three-phase) or Table 430.248 (single-phase) for the motor horsepower and voltage, and this tool uses those table values. The power equation, FLA = HP times 746 divided by (volts times power factor times efficiency times the square root of 3), with the square root of 3 dropped for single phase, estimates the actual running current and is shown only as a cross-check; it usually runs a few percent below the table.
Why does the NEC use table FLC instead of nameplate amps?
NEC 430.6 directs that branch-circuit conductor and protection sizing use the full-load current values from Table 430.247 through 430.250 rather than the motor nameplate amps. The table values are standardized and conservative, so wiring and protection remain adequate across motors of the same horsepower and voltage. Nameplate amps are still used for setting the running overload device.
How are wire size and breaker size determined?
NEC 430.22 requires branch-circuit conductors rated at least 125 percent of the motor full-load current, so this tool multiplies the table current by 1.25 and finds the smallest copper conductor whose 75 C ampacity in Table 310.16 meets it. NEC 430.52 sets the maximum inverse-time breaker at 250 percent of the table current; when that is not a standard size, the next larger standard rating is permitted. That is a maximum, not a required size, and it does not replace the running overload protection.
What power factor and efficiency should I use?
Use the motor nameplate values when you have them. As defaults, a power factor near 0.85 and an efficiency in the low 90 percent range are reasonable for typical NEMA Premium induction motors. In this tool they only drive the power-equation cross-check and the input kW; lower power factor or efficiency raises the calculated current. The NEC table current governs conductor and breaker sizing.
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