Motor Feeder and Branch Circuit Sizing per PEC
Motor circuits use several different multipliers on several different base currents. Mixing them up is the classic motor-circuit error.
Start from table full-load current
The PEC requires conductor and protection sizing to use the full-load current from the code tables (by horsepower and voltage), not the motor nameplate current. The nameplate is used only for overload relay sizing.
Branch-circuit conductors
Select the conductor from the ampacity table at this current, then apply correction factors as normal.
Branch-circuit short-circuit and ground-fault protection
This device protects against faults, not overload, so it is sized well above FLC to ride through starting inrush:
| Protective device | Max % of table FLC |
|---|---|
| Inverse-time circuit breaker | 250% |
| Non-time-delay fuse | 300% |
| Time-delay (dual-element) fuse | 175% |
If the motor will not start at the calculated value, it may be increased to the next standard size (up to higher code ceilings).
Overload protection
Use 125% for service factor ≥ 1.15 or a 40 °C-rise motor; 115% otherwise.
Feeder for several motors
Feeder protection = largest branch-circuit protective device rating + sum of the other motors’ full-load currents.
EngEst Pro’s motor-circuit tool applies each multiplier to the right base current — table FLC for conductors and branch protection, nameplate FLA for overloads — and returns wire size, breaker rating, and overload setting for single motors and multi-motor feeders.
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