Motor FLA, Branch Circuit, and OCPD Sizing for Electricians

The first rule: FLC vs FLA

Two terms that sound similar but aren’t:

• FLC: Full Load Current. From NEC Tables 430.247-250 (DC, single-phase, three-phase, two-phase). Used for branch-circuit conductor sizing and short-circuit/ground-fault protection.
• FLA: Full Load Amps. Read off the motor nameplate. Used for overload protection.

This split is the single most common motor-question trap. The exam will ask for branch-circuit conductor size (FLC from the table) and the very next question will ask for overload (FLA from the nameplate). If you mix them up, both answers go wrong.

Branch-circuit conductors: 125%

Per 430.22, branch-circuit conductors for a single continuous-duty motor must have an ampacity of at least 125% of the motor’s FLC (from the table, not nameplate).

Example: 25 HP, 460V, three-phase squirrel-cage motor. Table 430.250 → 34 A FLC. Branch-circuit conductor ampacity = 34 × 1.25 = 42.5 A. Pick the next standard wire size with that ampacity.

Overload protection: 115% or 125%

Per 430.32(A)(1), overload protection is sized from the motor nameplate FLA:

• Service factor 1.15 or greater → 125% of nameplate FLA
• Temperature rise 40°C or less → 125% of nameplate FLA
• All other motors → 115% of nameplate FLA

Example: Motor with nameplate FLA of 28 A and SF of 1.15. Overload max setting = 28 × 1.25 = 35 A. Note: this uses 1.25, the SF-1.15 percentage. Common trap: candidates use 1.15 because the SF is 1.15. The percentage and the SF aren’t the same number.

Short-circuit and ground-fault protection: 175% to 800%

Per Table 430.52, the SCGF device (the upstream branch breaker or fuse) is sized as a percentage of the FLC. The percentages vary by motor type and protection device:

• Inverse-time circuit breaker, three-phase squirrel-cage motor → 250% of FLC
• Non-time-delay fuse, same motor → 300% of FLC
• Time-delay fuse, same motor → 175% of FLC
• Instantaneous-trip breaker, same motor → 800% of FLC (with engineering supervision)

The percentages stack: SCGF protection lets a much higher current pass than the conductor ampacity, because the device’s real job is to trip on a short circuit (thousands of amps), not on motor overload (handled separately).

Feeder conductors for multiple motors: 125% of largest, 100% of others

Per 430.24, when one feeder serves multiple motors, the conductor ampacity must be at least 125% of the largest motor’s FLC plus the sum of the other motors’ FLC at 100%.

Common motor traps

• Mixing FLC (table) with FLA (nameplate). The exam writes scenarios where both numbers are given. Read which one the question wants.
• Using 1.15 instead of 1.25 for service factor 1.15 motors. The SF determines which row of 430.32 applies, not the percentage itself.
• Forgetting that 430.52 percentages apply to FLC from the table, not to nameplate FLA.
• Treating the inverse-time breaker the same as the non-time-delay fuse. They have different percentages.