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Motor Full-Load Current & Circuit Sizer (NEC Article 430)

Size motor branch circuit conductors (125% FLC), inverse-time breakers (250% Max), fuses, and overload protection (NEC 430.32).

Motor Rating & Code Parameters

NEC Art. 430
NEC 430.32 Overload Sizing

Per NEC 430.6(A)(2), overload heaters/relays are sized from the motor's actual physical nameplate FLA, while conductors & breakers use NEC Table FLC.

NEC Article 430 Sizing Schedule

NEC Table Full-Load Current (FLC)
14 A

From NEC Table 430.250 (Mandated for Conductors & Breakers)

Min Conductor Ampacity (125% FLC)
17.5 A

Minimum Conductor: 14 AWG Copper (20A @ 75°C (THWN/THHN))

Max Inverse-Time Breaker (250% Max)
35 A

Calculated: 35A (NEC 430.52 & 240.6(A) standard size)

Max Time-Delay Fuse (175% Max)
25 A

Calculated: 24.5A (Dual-element fuse)

Motor Overload Relay Setting
17.5 A

Sized at 125% of Nominal FLA (14 A) per NEC 430.32

Minimum Disconnect Rating
16.1 A (10 HP)

At least 115% of Table FLC and rated for motor HP (NEC 430.110)

Step-by-Step Engineering Calculation

  1. 11. NEC Table Full-Load Current (FLC): 14 A (from NEC Table 430.250). Mandated for sizing conductors, breakers, and disconnect per NEC 430.6(A)(1).
  2. 22. Minimum Conductor Ampacity (NEC 430.22): 1.25×14 A=17.5 A1.25 \times 14\text{ A} = 17.5\text{ A}. Minimum Conductor: 14 AWG Copper (20A rated at 75°C (THWN/THHN) per NEC Table 310.16).
  3. 33. Max Inverse-Time Circuit Breaker (NEC 430.52): 2.50×14 A=35 A2.50 \times 14\text{ A} = 35\text{ A}. Maximum standard rating: 35 A (NEC 240.6(A) & 430.52(C)(1) Exception 1).
  4. 44. Max Dual-Element Time-Delay Fuse (NEC 430.52): 1.75×14 A=24.5 A1.75 \times 14\text{ A} = 24.5\text{ A}. Maximum standard rating: 25 A.
  5. 55. Motor Overload Protection (NEC 430.32): Evaluated strictly from Motor Nameplate FLA (14 A) with Service Factor ≥ 1.15 (125% rule) →\rightarrow Max Overload Setting: 1.25×14 A=17.5 A1.25 \times 14\text{ A} = \mathbf{17.5\text{ A}}.
  6. 66. Disconnecting Means (NEC 430.110): Minimum 1.15×14 A=16.1 A1.15 \times 14\text{ A} = \mathbf{16.1\text{ A}} and horsepower rated for at least 10 HP.

Formula & Theory

I_{\text{conductor}} = 1.25 \times \text{FLC} \quad | \quad I_{\text{breaker}} = 2.50 \times \text{FLC} \quad | \quad I_{\text{overload}} = 1.15\text{ or }1.25 \times \text{FLA}_{\text{nameplate}}

Sizes AC motor branch circuits, conductors (NEC 430.22 & Table 310.16), short-circuit ground-fault protection (NEC 430.52), overload relays (NEC 430.32), and disconnecting means (NEC 430.110).

Variable Definitions
FLC:
Table Full-Load Current (Amperes (A)) – NEC Table 430.248/430.250 mandated current for conductors, breakers, and switches
FLA:
Nameplate Full-Load Amps (Amperes (A)) – Actual motor nameplate rating used strictly for Overload Protection (NEC 430.32)
I_cond:
Min Conductor Ampacity (Amperes (A)) – 125% of NEC Table FLC (NEC 430.22)
I_brkr:
Max Inverse-Time Breaker (Amperes (A)) – Maximum 250% of NEC Table FLC (NEC 430.52)
I_fuse:
Max Time-Delay Fuse (Amperes (A)) – Maximum 175% of NEC Table FLC

Worked Engineering Example

10 HP, 460V 3-Phase Induction Motor (SF 1.15)

Standard industrial pump motor feeder sizing.

Inputs:
  • Rating: 10 HP
  • Voltage: 460V 3-Phase
  • Service Factor: ≥ 1.15
  • Terminals: 75°C Listed
Result: NEC Table FLC = 14.0 A | Min Wire: 14 AWG Cu (17.5 A) | Max Breaker: 35 A | Overload: 17.5 A (125% FLA) | Disconnect: 16.1 A

Key Assumptions

  • •Standard squirrel-cage Design B/C/D continuous-duty induction motors.
  • •Conductor sizing uses NEC Table 310.16 terminal limitations (NEC 110.14(C)).
  • •Overload protection calculation is based on motor physical nameplate rating (NEC 430.6(A)(2)).

Limitations & Safety Scope

  • •High-inrush Design E motors or wound-rotor motors may utilize specific NEC 430.52(C)(1) Exception 2 rules allowing up to 300% breaker rating if the motor fails to start.
  • •Multi-motor group installations require feeder calculations under NEC 430.24 (largest motor at 125% plus sum of all other motor FLCs).

Frequently Asked Questions

Why do we use NEC Table values for wire and breakers, but Nameplate FLA for overloads?

NEC 430.6(A)(1) mandates using Table 430.248/430.250 values for sizing conductors, switches, and branch-circuit overcurrent protection to ensure safe capacity if the motor is later replaced with a lower efficiency motor. However, NEC 430.6(A)(2) and 430.32 require using the actual physical nameplate FLA for overload protection to accurately protect that specific motor's thermal windings from burnout.

What is the overload protection multiplier rule?

Under NEC 430.32(A)(1), motors with a marked Service Factor of 1.15 or greater, or a marked temperature rise of 40°C or less, are sized at a maximum of 125% of nameplate FLA. All other motors (e.g. Service Factor 1.0) are sized at a maximum of 115% of nameplate FLA.

Related Calculators & Engineering Tools

Breaker Sizing Calculator
Wire Size Calculator
Power Factor Calculator
Three-Phase Motor Full-Load Current Calculation Guide

Educational & Reference Notice: This calculator provides technical estimates based on standard physical formulas. Actual electrical installation, protective device sizing, and conductor selection must be verified against the applicable electrical code (NEC, IEC, CEC) by a licensed electrician or qualified professional engineer.