Motor Circuit Sizing Under NEC 430: FLA Tables, Overcurrent Protection, and Conductor Selection

Motor circuit sizing under NEC Article 430 follows a four-part sequence: branch circuit conductors, short-circuit and ground-fault protection, overload protection, and disconnecting means. Each component has its own sizing rule, and each pulls from a different NEC section and table. Mixing them up—using the nameplate full-load amps where the NEC requires Full-Load Current (FLC) table values, or sizing the overload device to the same percentage as the OCPD—produces circuits that either fail inspection or nuisance-trip under normal motor starting conditions.

This guide walks through the four-step NEC 430 motor circuit sizing sequence with a complete worked example.

The Critical Distinction: Nameplate FLA vs. NEC FLC Tables

Before anything else, understand which ampere value applies to each calculation:

• Nameplate FLA (Full-Load Amperes): The actual measured current at rated load, printed on the motor’s nameplate. Used only for overload protection sizing.
• Table FLC (Full-Load Current): The value from NEC Tables 430.247–430.250, based on horsepower and voltage. Used for conductor sizing, OCPD sizing, and feeder calculations. Per NEC 430.6(A)(1), you must use these table values—not the nameplate—for branch circuit and feeder sizing.

Step 1: Identify Full-Load Current from NEC Tables

For three-phase squirrel-cage induction motors, use NEC Table 430.250. For single-phase motors, use Table 430.248. For DC motors, Table 430.247.

Table 430.250 FLC values for common three-phase motor ratings at 460V:

These table values account for the full range of motors at that horsepower rating. An exceptionally efficient motor might draw less than its table FLC at full load—but the NEC requires you to use the table value for circuit sizing to ensure the circuit can handle any motor of that rating.

Step 2: Size the Branch Circuit Conductors (NEC 430.22)

Branch circuit conductors for a single motor must have an ampacity of not less than 125% of the motor’s FLC.

After finding the minimum ampacity, also check voltage drop for long runs—motor circuits to distant equipment often have voltage drop as the controlling factor, not ampacity. Motors are sensitive to low voltage: a motor running at 95% voltage draws more current and runs hotter than at rated voltage.

Step 3: Size the Short-Circuit and Ground-Fault Protection (NEC 430.52)

The branch circuit short-circuit and ground-fault protective device (the circuit breaker or fuse protecting the conductors) is sized using NEC Table 430.52—not the 125% rule used for conductors. This device must handle motor starting current without tripping while still protecting against faults.

Table 430.52 maximum percentages for Design B, E, and squirrel-cage motors (the most common type):

These are maximum ratings. You can use smaller values if the motor starts without nuisance tripping. The percentages are high because motor starting current is typically 6–8 times full-load current—the OCPD must not trip during the 2–5 second starting period.

If the calculated OCPD value does not correspond to a standard size, NEC 430.52(C)(1) Exception No. 1 permits using the next higher standard size.

Step 4: Size the Overload Protection (NEC 430.32)

Overload protection (thermal overload relays or electronic overload relays built into the motor starter) protects the motor winding from sustained overcurrent below the fault level. This is sized to the nameplate FLA—the actual current the motor draws at rated load.

Note the contrast with OCPD sizing: the OCPD uses Table FLC at 250%; the overload uses nameplate FLA at 115%–125%. These are independent devices serving different protective functions. Confusing them is the most common NEC 430 error on licensing exams and field installations.

Worked Example: 10 HP, 460V, Three-Phase Motor

A squirrel-cage induction motor, Design B, 10 HP, 460V, three-phase, with nameplate FLA of 13.6A and Service Factor 1.15. Branch circuit using THWN-2 copper in EMT conduit, single motor on its own circuit.

Multiple Motors on a Common Feeder

When a feeder supplies multiple motors, NEC 430.24 requires the feeder conductor ampacity to be not less than the sum of all motor FLCs plus 25% of the largest motor’s FLC:

For a feeder serving a 10 HP (14A FLC) and two 5 HP (7.6A FLC each) motors at 460V:

From Table 310.16 at 75°C: 8 AWG copper = 50A ≥ 32.7A. The feeder OCPD is sized based on the largest motor’s branch circuit OCPD plus the sum of the remaining motors’ FLCs (NEC 430.62).

For the full load inventory approach used in feeder and service sizing, see the guide on calculating electrical load for new circuits, which covers the continuous vs. non-continuous load methodology that also applies when motors are part of a larger panel schedule.

The sizing rules in NEC Article 430 are consistent across motor sizes and voltage levels. Once you’ve worked through the four steps manually—Table FLC lookup, 125% conductor rule, Table 430.52 OCPD percentage, overload at nameplate FLA—the electrical load calculator handles subsequent calculations faster, particularly for multi-motor feeder sizing where the arithmetic stacks up quickly.