How to use this calculator
- Pull the table FLCs. Adopted-code Table 430.250/248 values for every motor — nameplates are for overloads, not feeders.
- Enter largest and the rest. The highest-rated motor's FLC alone, then the sum of all the others; add any non-motor load split continuous/noncontinuous.
- Take the floor to the ampacity table. The minimum ampacity meets your adopted table with its ambient and bundling corrections — the derating calculator runs that step.
- Cap the feeder device. Largest branch device + other FLCs, rounded DOWN to a standard rating.
How it works
A motor feeder is sized for the day the biggest motor starts while everything else is already running — one inrush on top of steady load. That single idea is the whole rule:
I_min = 1.25·I_largest + ΣI_others + 1.25·I_cont + I_noncont · OCPD_max = OCPD_largest + ΣI_others (round down)
Both rules run on your adopted code's table values — nothing is embedded here. Each motor's own branch circuit comes first (FLA from the motor FLA calculator, overloads from the overload size calculator), the feeder's real-world ampacity passes through the ampacity derating calculator, and the run's voltage drop through the voltage drop calculator.
Worked example
Verified against the live calculator
Three motors at 460 V three-phase — 25 hp (34 A),
10 hp (14 A), 5 hp (7.6 A) — the largest on
a 90 A branch breaker:
I_min = 34 × 1.25 + (14 + 7.6) = 64.1 A · OCPD_max = 90 + 21.6 = 111.6 → 110 A standard
The 64.1 A floor goes to the adopted ampacity table with its correction factors, and the feeder device lands on 110 A — rounded down, because the next-size-up allowance belongs to branch circuits under 430.52, not to feeders. Note the asymmetry doing the work: only the 25 hp motor carries the 125%; the other two count at face value.
Frequently asked questions
How do you size a feeder for multiple motors?
NEC 430.24: feeder ampacity ≥ 125% of the largest motor's table FLC plus the full-load currents of all the other motors — plus 125% of any continuous and 100% of any noncontinuous non-motor load. Three motors of 25, 10 and 5 hp at 460 V (34, 14 and 7.6 A) need 34 × 1.25 + 14 + 7.6 = 64.1 A.
Why only 125% on the largest motor and not on all of them?
The conductor heating case the rule guards is the largest motor starting while the rest run at load — one motor's inrush at a time. Adding 25% to every motor would size for all of them starting simultaneously, which belongs to engineered soft-start/sequencing design, not the code floor.
How big can the feeder breaker be for a motor group?
430.62(A): no larger than the largest branch-circuit device rating plus the other motors' full-load currents — and that result rounds DOWN to a standard 240.6(A) rating. With a 90 A branch breaker on the 25 hp motor, 90 + 21.6 = 111.6 A → a 110 A feeder device. The round-UP allowance people remember is 430.52's, for branch circuits only.
Do I use nameplate amps or table values?
Table values — your adopted code's Table 430.250 (three-phase) or 430.248 (single-phase) — for both 430.24 and 430.62 feeder math. Nameplate FLA is for overload protection (430.32). This calculator embeds no tables: you enter the values from the edition your jurisdiction adopts.
Method & assumptions
- NEC 430.24 / 430.62(A) rule structure (verified 2026-06-11 against agreeing published walkthroughs); the adopted edition and the AHJ govern. No FLC, ampacity or protection-percentage tables are embedded — every current is yours from the adopted code.
- Duty-cycle motors (430.22(E)), interlocked motors that cannot run simultaneously, and adjustable-speed-drive input ratings modify the sums — engineering judgments outside this screen.
- The OCPD ceiling shown is the motors-only 430.62(A) case; feeders that also carry non-motor load follow 430.62(B)/430.63 for protection (flagged in the result when applicable).
- Power-factor correction, transformer in the feeder path and service-level rules bring their own articles.