Motor Circuit Calculator estimates the required MCA (Minimum Circuit Ampacity) and MOCP (Maximum Overcurrent Protection) ratings for electric motors. It is based on NEC 430-style multipliers and helps engineers design motor branch circuits.
About MCA. Minimum Circuit Ampacity (MCA) is the minimum conductor ampacity required for a motor branch circuit. Electrical codes require sizing motor circuit conductors at 125% of the motor’s full-load current (FLA) to handle starting and running currents without overheating. This ensures the conductors do not exceed their thermal limits during prolonged motor operation.
About MOCP. Maximum Overcurrent Protection (MOCP) is the largest breaker or fuse rating allowed to protect a motor branch circuit. MOCP devices must allow the motor to start (handle inrush current) but still protect the circuit from faults. Typical NEC multipliers are 250% of FLA for inverse-time circuit breakers, 175% for time-delay fuses, and 300% for non-time-delay fuses. This calculator applies these multipliers and rounds up to the next standard size.
About FLA and nameplate data. Motor nameplates often provide power (kW or hp), voltage, power factor (PF), efficiency (η), and phase. This calculator allows either direct entry of known FLA or automatic FLA calculation from nameplate data using:
- Single-phase: I = (P × 1000) / (V × PF × η)
- Three-phase: I = (P × 1000) / (√3 × V × PF × η)
About standard series. Breakers and fuses come in standard sizes. IEC/EN examples include 6–630 A, and NEC/UL examples include 15–600 A. This tool rounds up the calculated MOCP to the nearest available standard size in the selected series.
Calculation. This calculator determines the required conductor ampacity (MCA) and protective device size (MOCP) from either direct FLA input or calculated FLA from nameplate data. It applies 125% to get MCA and device multipliers to get MOCP, then rounds up to the nearest standard size.
Determine FLA (either user input or from nameplate)
MCA = 1.25 × FLA
MOCP = next standard size ≥ (device% × FLA × Adjustment Factor)
- Example 1: 28 A FLA, inverse-time CB (250%) → MCA = 35 A, MOCP = 70 A (next IEC size)
- Example 2: 11 kW, 400 V, 3φ, PF 0.9, η 0.92 → FLA ≈ 19.2 A → MCA = 24.0 A → MOCP ≈ 50 A (IEC)
- Example 3: 7.5 hp, 230 V, 1φ, PF 0.85, η 0.9 → FLA ≈ 28.2 A → MCA = 35.3 A → MOCP ≈ 80 A (NEC)
Corresponding tools. Use the Breaker Size Calculator for non-motor loads, and the Voltage Drop Calculator to verify wire sizes and starting voltage performance.
