ℹ About This Calculator
Every motor circuit design starts with the motor's full-load amperes (FLA) — the current it draws at rated load. This calculator converts motor horsepower to full-load amps for single- and three-phase motors from the voltage, power factor and efficiency, and applies the 125% factor the NEC requires for sizing branch-circuit conductors. It works with US voltages (120, 208, 230, 460, 480 V) and helps size wire, conduit and overload protection. For code conductor and overcurrent sizing, the NEC requires the table values of 430.248/430.250, which the calculator references alongside the computed value.
One horsepower equals 746 watts of mechanical output, so the input power is HP × 746 divided by the motor efficiency, and the current follows from the power equation. Power factor and efficiency both reduce below unity for real motors, raising the current. The computed FLA is the theoretical running current; however, the NEC requires that branch-circuit conductors, overload and short-circuit protection be sized from the standardized table full-load current values in Article 430 (tables 430.248 single-phase and 430.250 three-phase), which are slightly conservative, times 125% for continuous motor duty.
Frequently Asked Questions
How do I convert motor HP to amps?
+
For a three-phase motor, use I = (HP × 746) / (√3 × V × PF × η), and for single-phase drop the √3: I = (HP × 746) / (V × PF × η). HP is horsepower, V the voltage, PF the power factor and η the efficiency. For example, a 10 HP, 460 V three-phase motor at 0.85 PF and 90% efficiency draws about 12.2 A. One horsepower is 746 watts of output, and the current supplies that plus the motor's losses.
What is motor full-load amps (FLA)?
+
Full-load amps is the current a motor draws when operating at its rated horsepower and voltage. It is the basis for sizing the branch-circuit conductors, overload protection and disconnect. The FLA depends on the horsepower, voltage, power factor and efficiency. Note that motors draw far more than FLA at startup — locked-rotor current is typically 6 times FLA — which is handled separately in overcurrent sizing.
Should I use the calculated FLA or the NEC table value?
+
For code sizing of conductors, overloads and short-circuit protection, the NEC requires the table full-load current values from Article 430 (Table 430.248 for single-phase, 430.250 for three-phase), not the motor nameplate or a calculated value. These table values are standardized and slightly conservative. Use the calculated FLA to understand the actual current and for load studies, but size the circuit from the NEC table value times the required factors.
Why is the conductor sized at 125% of FLA?
+
NEC 430.22 requires branch-circuit conductors for a single continuous-duty motor to be sized at no less than 125% of the motor's full-load current. This margin accounts for the continuous nature of motor loads and keeps the conductor and terminals from overheating. So a motor with 14 A table FLC needs conductors rated for at least 17.5 A. The overcurrent (short-circuit) device, by contrast, is sized much higher — up to 250% — to allow for starting inrush.
How does voltage affect motor current?
+
Current is inversely proportional to voltage for the same power, so a motor on 460 V draws roughly half the current of the same motor on 230 V. This is why higher voltages are used for larger motors — they need smaller conductors for the same horsepower. It also means running a 230 V motor on a 208 V supply increases the current and heating, which is why motor voltage must match the supply.
What power factor and efficiency should I assume?
+
For a typical three-phase induction motor at full load, power factor is around 0.85-0.88 and efficiency 88-94%, both improving with motor size and premium-efficiency designs. Small motors have lower values. If the nameplate lists the actual power factor and efficiency, use those; otherwise 0.85 PF and 90% efficiency are reasonable defaults for a mid-size motor. Both drop at part load, raising the current per horsepower delivered.
Is this motor FLA calculator accurate?
+
It computes the theoretical full-load current accurately from the power equation given your power factor and efficiency. Remember that for NEC conductor and overcurrent sizing you must use the Article 430 table values rather than the calculated current, and that motors draw about 6 times FLA at startup. Use this calculator for load estimates and understanding, and the NEC tables for code-compliant circuit design.
🧮 110 Free MEP Calculators
Browse all HVAC, Electrical, Plumbing, Fire, Gas and Mechanical calculators - IS/IBC/ASHRAE compliant, free PDF export.
Browse All Calculators →
