Grouped Motor Circuit Sizing Guide — Multiple Motors, One Circuit

Feeder vs Branch — Two Different Multi-Motor Problems

A feeder supplying several motor branch circuits is sized under NEC 430.24: ampacity not less than the sum of the full-load current rating of all motors plus 25% of the highest-rated motor in the group. You are not inventing a “total HP” motor and reading one row of Table 430.250.

A single branch circuit that actually feeds more than one motor is only legal under NEC 430.53. That section is narrower than people think — most “two motors on one breaker” shop hacks fail 430.53 unless they meet (A), (B), or (C).

Run the numbers in the Grouped Motor Circuit Sizing Calculator. Keep single-motor branch math on the single-motor calculator.

NEC 430.24 — Worked Example (25 + 10 + 5 HP @ 460 V)

Three-phase induction motors, Table 430.250 FLA: 25 HP = 34 A, 10 HP = 14 A, 5 HP = 7.6 A.

• Sum of FLAs = 34 + 14 + 7.6 = 55.6 A
• 25% of largest (34 A) = 8.5 A
• Feeder ampacity minimum = 55.6 + 8.5 = 64.1 A
• THWN-2 copper at 75°C: 6 AWG (65 A) meets 64.1 A before derates

If ambient or bundling derates drop 6 AWG below 64.1 A effective, upsize. Long feeder? Check voltage drop on ~64 A, not on 34 A alone. See the voltage-drop notes on the NEC 430 article.

NEC 430.53 — When Multiple Motors May Share One Branch OCPD

430.53(A) Not over 1 HP: Small motors, limited ampacity, and tight OCPD caps. Rare for industrial packs.

430.53(B) If smallest motor protected: The branch device cannot exceed the Table 430.52 maximum for the smallest motor, and each motor has overload protection. Useful for several tiny motors — not for mixing a 25 HP with fractional HP loads on one breaker.

430.53(C) Other group installations: The common listed-controller / group case. Max OCPD = Table 430.52 percentage of the largest motor FLA plus the FLA of all other motors on that branch. Each motor still has individual overload protection. Device rating must not exceed that calculated ceiling.

Example on the same 25+10+5 HP @ 460 V pack as a group branch with inverse-time breakers: 34 × 2.50 + 14 + 7.6 = 85 + 21.6 = 106.6 A max → largest standard size ≤ 106.6 A is 100 A. That is not the same number as the feeder ampacity (64.1 A). Conductor and OCPD are still different jobs.

Overloads Are Still Per Motor

Grouping the short-circuit device does not group the thermal protection. Each motor gets an overload relay (or equivalent) from nameplate FLA per 430.32 — same rule as the single-motor guide. Burning one motor while the group breaker stays closed is how 430.53 gets a bad reputation.

FLA lookup chart for individual motors: Motor Circuit Sizing Chart. Full single-motor walkthrough: Motor Circuit Sizing Guide.