Three-Phase Motor Full-Load Current

Also known as FLA · motor amps · full load amps

I = \frac{P_{out}}{\sqrt{3} \, V \, \text{PF} \, \eta}

Worked example: 10 hp, 460 V, 0.85 PF, 89% → 12.37 A — press Try an example to run it live, then adjust anything.

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\sqrt{3} = 1.7320508075688772

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Nameplate currents →

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Three-Phase Motor Full-Load Current explained

A motor's rating is what comes out of the shaft; the line must supply that plus the losses, and it must supply it through a phase angle. So the input volt-amperes are P_out/(η · PF), and dividing by √3 V gives the line current. A 10 hp, 460 V motor at 89% efficiency and 0.85 PF draws 7457/(1.732 × 460 × 0.85 × 0.89) ≈ 12.4 A — reassuringly close to the 14 A that NEC Table 430.250 lists for sizing.

Use the code table, not this calculation, for conductor and overload sizing in North America: the NEC deliberately tabulates conservative currents and requires them for branch-circuit design, reserving nameplate amps for overload protection. Use this formula instead when you want to know what a specific machine really draws, to sanity-check a clamp-meter reading, or to see how an unloaded motor's collapsing power factor pushes current up out of proportion to the work being done.

Three-Phase Motor Full-Load Current formula

I = \frac{P_{out}}{\sqrt{3} \, V \, \text{PF} \, \eta}

Where

$I$ = Full-load line current (A)
$P_{out}$ = Shaft output power (W)
$V$ = Line-to-line voltage (V)
$\text{PF}$ = Power factor
$\eta$ = Efficiency (%)

Missing one of these? Work it out first, then come back

Full-load line current — Available Short-Circuit Current from Percent Impedance, Transformer Full-Load Current
Shaft output power — Motor Torque from Power and Speed, Motor Efficiency
Line-to-line voltage — Three-Phase Real Power, Single-Phase Real Power with Power Factor
Power factor — Three-Phase Real Power, Single-Phase Real Power with Power Factor
Efficiency — Motor Efficiency, Transformer Efficiency from Core and Copper Losses

Part of this formula set

Motor calculations

5 formulasFull-load current, synchronous speed, slip, efficiency and input power — reading a motor nameplate and checking what it actually draws.