Motor branch-circuit calculations (Article 430), step by step
Motor questions use three different percentages on two different currents, and most wrong answers come from mixing them up. The method is always the same: pick the right current, apply the right percentage, then round the way the NEC allows for that item. The examples below are for ordinary three-phase induction motors (Design B). The 2026 NEC adds Design BE and CE energy-efficient motors with their own rules, so read the motor design in the question.
Which current, which percentage
Table 430.250 is for three-phase motors. Use the column for the motor voltage, not the system voltage: a 460 V motor on a 480 V system uses the 460 V column.
| What you size | Current to use | Percentage | Rounding | Where to look (2026 NEC) |
|---|---|---|---|---|
| Branch-circuit conductors, one motor | Table full-load current (FLC) | 125% | Never round an ampacity down | 430.6(A)(1), Table 430.250, 430.22 |
| Short-circuit and ground-fault protection | Table FLC | Inverse time breaker 250%, dual-element fuse 175%, nontime-delay fuse 300% | Next higher standard size allowed | Table 430.52(C)(1), 240.6(A) |
| Overload protection | Nameplate current | 125% (service factor 1.15 or more, or 40 °C rise or less); 115% for other motors | Not more than the result | Motor overload rules in Article 430 |
| Feeder conductors, several motors | Table FLC | 125% of the largest motor + 100% of the others | Never round an ampacity down | 430.24 |
Worked example 1: conductors use the table, not the nameplate
A 20 hp, 460 V, three-phase Design B motor has a nameplate current of 25 A. Table 430.250 gives 27 A.
Branch-circuit conductors: 27 × 1.25 = 33.75 A minimum ampacity.
Common wrong answer: 31.25 A (125% of the nameplate current).
Worked example 2: short-circuit and ground-fault protection
Same motor, table FLC 27 A. Maximum rating for each device type:
Inverse time breaker: 27 × 2.50 = 67.5 A. Not a standard size, so the next higher standard size, 70 A, is allowed.
Dual-element (time-delay) fuse: 27 × 1.75 = 47.25 A, so 50 A.
Nontime-delay fuse: 27 × 3.00 = 81 A. The next standard size above 80 A is 90 A.
Common wrong answers: 60 A (rounded the breaker down) and 33.75 A (used the conductor percentage).
Worked example 3: overload protection uses the nameplate
Same motor: nameplate 25 A, service factor 1.15. Maximum overload setting: 25 × 1.25 = 31.25 A.
If the motor had a service factor of 1.0 and a temperature rise above 40 °C, the limit would be 25 × 1.15 = 28.75 A.
Common wrong answer: 33.75 A (used the 27 A table current).
Worked example 4: feeder for three motors
A feeder supplies three 460 V, three-phase Design B motors: 30 hp (table FLC 40 A), 15 hp (21 A) and 10 hp (14 A). No other loads.
Minimum feeder ampacity: 40 × 1.25 + 21 + 14 = 85 A.
Common wrong answers: 93.75 A (125% applied to every motor) and 75 A (no 125% at all).
Worked example 5: one motor, three answers
A 50 hp, 460 V, three-phase Design B motor. Table FLC: 65 A.
Conductors: 65 × 1.25 = 81.25 A minimum ampacity.
Inverse time breaker: 65 × 2.50 = 162.5 A, so 175 A. Nontime-delay fuse: 65 × 3.00 = 195 A, so 200 A.
The three numbers are not related. A breaker larger than the conductor ampacity is normal for a motor circuit, because the overload device protects the conductors from overload.
Habits that save points
- Circle "nameplate" and "table" in the question. Table FLC for conductors, short-circuit protection and feeders; nameplate only for overloads.
- Check the motor voltage and phase before you read Table 430.250.
- Round short-circuit protection up to the next standard size in 240.6(A). Never round an ampacity or an overload limit up.
- In a feeder, apply 125% to the largest motor only.
- Read the motor design. Design BE and CE motors have their own rules in the 2026 NEC, such as 430.22(H).
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- EC&M Magazine: What's new in Chapter 4 of the 2026 NEC (Design BE and CE motors, 430.22(H), 430.24, Table 430.52; 15 Sep 2026)
- Tradesmance: 2026 NEC changes part 3 (430.22(H), 430.24 Exception No. 4)
- OpenExamPrep: Texas motor overcurrent and short-circuit protection (Table 430.52 percentages, 5 hp 230 V example)
- OpenExamPrep: Motor overload protection and multi-motor feeders (overload percentages, 430.24 example)
- EEPower: Protecting motor branch circuits from short circuits and ground faults (Table 430.52 examples)
- PSI Candidate Information Bulletin, Texas electrician examinations (updated 15 Sep 2026)
Updated 2026-10-22. Found something wrong? Tell us and we will fix it and log the fix on our quality page.