Almighty Calculator

Motor Full-Load Amps and Wire Size Calculator

The NEC lists a full-load current for each motor horsepower and voltage, and the branch-circuit conductors are sized at 125% of that value.

Estimate only. Results are estimates. Verify against the applicable code and manufacturer specifications before relying on them for safety-related work.

Motor type and voltage
Single-phase tables here end at 10 hp; three-phase go to 30 hp.
Conductor
Temperature column
Lowest rating among wire and terminals.
Full-load current (NEC table)
12A
Conductor ampacity needed (125%)15 A
Smallest wire size14 AWG

Overload protection uses the motor nameplate current, and short-circuit and ground-fault protection follow NEC 430.52; neither is calculated here.

Show the math

Full-load current, Single-phase, 230 V, 2 hp = 12 A (NEC table)
Conductor ampacity = 12 × 1.25 = 15 A (NEC 430.22)
Wire size = 14 AWG (20 A in the 75°C column)

Rounded the same way as the result above.

How it works

For sizing branch-circuit conductors the NEC does not use the motor’s nameplate current. It uses standard full-load currents from Tables 430.248 for single-phase and 430.250 for three-phase motors, listed by horsepower and voltage. Conductors that supply a single continuous-duty motor must carry at least 125% of that value (430.22).

The calculator reads the table, applies 125%, and finds the smallest conductor in NEC Table 310.15(B)(16) in the temperature column you choose. The result is a starting point: overload devices, short-circuit protection, voltage drop, and special motors follow their own rules.

Full-load current = value from NEC Table 430.248 or 430.250 Conductor ampacity ≥ 125% × full-load current (NEC 430.22)

Worked example

A 2 hp single-phase motor on 230 V, copper wire in the 75°C column:

  1. Full-load current, Single-phase, 230 V, 2 hp = 12 A (NEC table)
  2. Conductor ampacity = 12 × 1.25 = 15 A (NEC 430.22)
  3. Wire size = 14 AWG (20 A in the 75°C column)
InputValue
Motor type and voltageSingle-phase, 230 V
Horsepower2 hp
ConductorCopper
Temperature column75°C column
ResultValue
Full-load current (NEC table)12 A
Conductor ampacity needed (125%)15 A
Smallest wire size14 AWG

Assumptions and limits

  • Applies to single-speed induction motors at usual speeds and normal torque, as the tables state. Other motor types differ.
  • Only 115 and 230 V single-phase and 230 and 460 V three-phase columns are included. 208 V, 575 V, and larger motors are not.
  • Wire is sized from the 310.15(B)(16) table at 30°C with three or fewer conductors; no derating is applied.

Common questions

How many amps does a 2 hp 230 V motor draw?

The NEC single-phase table lists 12 A. A particular motor’s nameplate current may be lower or higher.

Why not use the nameplate amps?

The NEC uses its tables for conductor sizing so results do not depend on the particular motor. The nameplate is used for the overload device.

What is the 125% for?

Motor branch-circuit conductors must carry 125% of the motor’s full-load current for continuous duty (NEC 430.22).

Does this size the breaker?

No. Short-circuit and ground-fault protection are set by NEC 430.52, and overloads by 430.32.

Sources

  • NEC Table 430.248 (single-phase motors) and Table 430.250 (three-phase induction motors); NEC 430.22 (125% conductor rule).
  • NEC Table 310.15(B)(16) for conductor ampacity (2011 printing).

Updated 2026-09-30