Motor Branch Circuit Calculator

Size conductors, OCPD, thermal overload protection, and disconnect switch for AC electric motors per NEC Article 430 and Tables 430.248 & 430.250.

NEC edition

Inputs

Table 430.248 stops at 10 HP for single-phase; larger sizes are three-phase only

115V is single-phase only; 460V and 575V are three-phase only

Affects overload protection size per §430.32

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⚡ Test your knowledge
What is the standard sizing multiplier for branch circuit conductors supplying a single continuous-duty motor?
  • A. 100% of FLA
  • B. 115% of FLA
  • C. 125% of FLA
  • D. 150% of FLA
💡 Under NEC §430.22, conductors supplying a single continuous-duty motor must have an ampacity of not less than 125% of the motor full-load current (FLA).
What is the maximum standard setting for an inverse-time circuit breaker on an AC squirrel-cage induction motor circuit?
  • A. 125%
  • B. 175%
  • C. 250%
  • D. 300%
💡 Per NEC Table 430.52, an inverse-time circuit breaker is permitted to be sized up to 250% of the motor FLA to accommodate starting inrush currents.

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How it works & the NEC rules behind it

Electric motor circuits have unique operating characteristics: during startup, an induction motor draws an inrush / starting current typically 5 to 6 times its running nameplate rating. Therefore, NEC Article 430 separates short-circuit protection from running overload protection.

Four Pillars of Motor Circuit Sizing (NEC Article 430):

  • 1. Full-Load Current (FLA) Sourcing (§430.6(A)(1)): For sizing branch-circuit conductors, disconnects, and OCPDs, you must use the tables (Table 430.248 for 1-phase, Table 430.250 for 3-phase), NOT the actual motor nameplate rating! (Nameplate is only used for sizing overload heaters).
  • 2. Conductor Ampacity (§430.22): Single motor branch-circuit conductors must be sized for at least 125% of the motor FLA.
  • 3. Short-Circuit & Ground-Fault Protection (§430.52): For standard Inverse Time Breakers, the maximum starting rating is 250% of FLA (Table 430.52). Where this value does not correspond to a standard OCPD size, the next higher standard rating is permitted (§430.52(C)(1) Exception 1).
  • 4. Motor Overload Protection (§430.32): Separate running overload protection (thermal heaters or electronic overloads) is sized at 125% of motor nameplate current for motors with a Service Factor (SF) ≥ 1.15, or 115% for all other motors.
  • 5. Disconnecting Means (§430.110): Must be rated for at least 115% of motor FLA.

Formulas & equations

Branch Conductor Minimum Ampacity = Motor Table FLA × 1.25 (§430.22) Max Inverse Time Breaker = Motor Table FLA × 2.50 (§430.52 Table) Overload Protection (SF ≥ 1.15) = Motor Nameplate FLA × 1.25 (§430.32) Disconnect Minimum Amps = Motor Table FLA × 1.15 (§430.110)

NEC reference table

Motor HP 208V 3-Phase FLA 230V 3-Phase FLA 460V 3-Phase FLA Min Cu Conductor (75°C) Max Inverse Time Breaker
1 HP4.6 A4.2 A2.1 A#14 AWG15 A
3 HP10.6 A9.6 A4.8 A#14 AWG30 A (208V) / 15 A (460V)
5 HP16.7 A15.2 A7.6 A#12 AWG (208V)45 A (208V) / 20 A (460V)
10 HP30.8 A28.0 A14.0 A#8 AWG (208V)80 A (208V) / 35 A (460V)
20 HP59.4 A54.0 A27.0 A#4 AWG (208V)150 A (208V) / 70 A (460V)
50 HP143.0 A130.0 A65.0 A#3/0 AWG (208V)400 A (208V) / 175 A (460V)

*Source: NEC Table 430.250 (Full-Load Current, Three-Phase Alternating-Current Motors).

Worked example, step by step

Example Calculation: 10 HP, 460V, 3-Phase Squirrel-Cage Induction Motor

Given: 10 HP, 460V 3-Phase Motor with a Service Factor of 1.15.

  • Step 1 (Find Table FLA): Per Table 430.250, a 10 HP motor @ 460V has an FLA = 14.0 Amperes.
  • Step 2 (Branch Conductors): 14.0A × 1.25 = 17.5 Amps minimum ampacity. Table 310.16 requires #14 AWG Copper (rated 20A @ 75°C).
  • Step 3 (Short-Circuit Breaker): 14.0A × 2.50 = 35.0 Amps. Install standard 35A Inverse Time Circuit Breaker.
  • Step 4 (Overload Heaters): 14.0A × 1.25 = 17.5 Amps maximum overload trip rating.
  • Step 5 (Disconnect Switch): 14.0A × 1.15 = 16.1 Amps minimum rating (install standard 30A/60A disconnect switch).

Frequently asked questions

Under NEC Article 430, the branch-circuit breaker is sized specifically to allow the high inrush starting current (up to 250% FLA) without tripping nuisance. The conductors are protected against overheating from sustained overloads by the separate motor thermal overload relay (§430.32), rather than the branch breaker.

Under NEC §430.6(A)(1), you MUST use the code tables (Table 430.248 and Table 430.250) for sizing conductors, disconnects, fuses, and circuit breakers. Nameplate full-load current is ONLY permitted when selecting and setting running overload protection devices (such as overload heaters or VFD electronic overloads).

Motor branch-circuit conductors are sized at 125% of the motor's Table 430.250 full-load current, not the nameplate amps — this NEC motor calculation differs from standard load-circuit sizing. Enter the motor horsepower and voltage above to get the required conductor size, breaker rating, and overload setting together.

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