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Part 7 – Sizing Motor Branch Circuit Protection and OCPDs

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How to Select OCPDs and Right Size Breaker / Fuse for Motor Overcurrent Protection?

Sizing Branch Circuit Protection for Motors

The sizing of overcurrent protection devices (OCPDs) for motor branch-circuit short-circuit and ground-fault protection is determined based on 150% to 300% of the motor’s full-load current (FLC) rating – NEC Table 430.52(C)(1). This table provides the maximum rating or setting for motor branch-circuit short-circuit and ground-fault protective devices (such as fuses and circuit breakers) for different types of motors, including single-phase, polyphase, DC motors, squirrel-cage, synchronous, and wound-rotor motors.

It is important to note that OCPDs in a motor branch circuit only protect the motor, control circuit, and branch-circuit conductors from short circuits and ground faults (NEC 430.51). However, OCPDs for overcurrent protection do not provide overload protection for the motor itself. Instead, overload protective devices are used for overload conditions of motors.

Motors operating under continuous duty have different protection requirements compared to personnel circuits, service feeders, and general wiring installations. Since motors draw approximately 6-8 times their normal running current during startup, both overcurrent and overload protection must be carefully designed.

For this reason, OCPDs are sized between 150% and 300% of the motor’s FLC. According to NEC Table 430.52(C)(1) (Given below), the protection device ratings for a typical AC motor are as follows:

  • Non-time-delay fuse: 300% of FLC
  • Time-delay fuse: 175% of FLC
  • Instantaneous-trip circuit breaker: 800% of FLC
  • Inverse-time circuit breaker: 250% of FLC

Sizing Motor Branch Circuit Protection

Sizing OCPD and Selection of Breakers for Motors

NEC Article 430, Part IV provides guidelines for sizing overcurrent protection devices (OCPDs) such as fuses and circuit breakers for motor branch circuits. Unlike conductor sizing, which is based on 125% of the motor’s full-load current (FLC), branch-circuit protection for short circuits and ground faults is sized based on NEC Table 430.52(C)(1).

Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Devices

Type of Motor Non–Time-Delay Fuse1 Dual-Element (Time-Delay) Fuse1 Inverse Time Circuit Breaker2 Instantaneous Trip Circuit Breaker
Single-Phase Motors 300% 175% 250% 800%
Three-Phase Motors (non -wound-rotor) 300% 175% 250% 800%
Squirrel Cage (Except Design B energy-efficient/premium) 300% 175% 250% 800%
Design B energy-efficient and Design B premium efficiency 300% 175% 250% 1100%
Wound-Rotor Motors 300% 175% 250% 800%
Synchronous Motors3 300% 175% 250% 800%
DC Motors 150% 150% 150% 250%

NOTES:

  1. The values in the Non–Time-Delay Fuse column apply to time-delay Class CC fuses.
  2. The values in the Inverse Time Breaker and Instantaneous Trip Breaker columns also apply to nonadjustable inverse-time circuit breakers that may be modified per 430.52(C)(1)(a) and (C)(1)(b).
  3. Synchronous motors (low-torque, low-speed types, typically ≤ 450 RPM) used to drive compressors, pumps, etc., may require special consideration.
  4. See 430.54 for exceptions to the values specified.

Step-by-Step Guide for Sizing Motor Branch-Circuit Protection

Step 1: Find the Motor’s Full-Load Current (FLC)

The FLC of a motor is found in the following NEC tables:

Warning: Do not use the motor nameplate current for sizing; always refer to associated NEC tables for FLC.

Step 2: Select the OCPD Size

The overcurrent protection device (OCPD) size depends on the motor type and the device used (fuse or circuit breaker). NEC Table 430.52(C)(1) provides the maximum percentage of the FLC for different protection devices:

Table Data:

If the standard OCPD size does not correspond exactly to the calculated value, NEC 240.6(A) allows using the next higher standard rating.

Step 3: Check for NEC Exceptions

  • If the circuit breaker trips during motor starting, NEC 430.52(C)(1) Exception No. 1 allows increasing the protection device size up to 400% of the FLC for inverse time circuit breakers.
  • For high-inrush current motors, further increases may be allowed per NEC 430.52(C)(1) Exceptions.

Example 1: Sizing Branch-Circuit Protection for 1-Phase Motor

How to determine the correct size of overcurrent protection devices of time-delay fuse and Inverse-time circuit breaker for a 5 HP, 230V, single-phase motor?

Solution:

Given Data:

  • Motor Type & HP: 5 HP, Single-phase AC
  • Voltage: 230V
  • Required Protection Devices: Time-Delay Fuse & Inverse Circuit Breaker

1. Find the Motor FLC

Using the Table 430.248, the full load current (FLC) of 5 HP, 230V, single phase motor is:

FLC = 28A

2. Calculate the Fuse Size

From NEC Table 430.52(C)(1), for a single-phase motor with a time-delay fuse, use 175% of FLC:

OCPD = 28A × 1.75 = 49A

The next closest standard fuse size is 50A – (NEC 240.6(A)).

3. Calculate the Circuit Breaker Size

Selection of motor circuit breaker for motor branch circuit protection is determined using Table 430.52(C)(1). For an inverse time circuit breaker for branch circuit, use 250% of FLC from the given table.

OCPD = 28A × 2.5 = 70A

The standard breaker size is available in 70A.

Based on the above calculation and tables data, the following fuse and circuit breaker can be used for short-circuit and ground fault protection of 5-HP, 230V single phase AC motor circuit.

    • Time-delay fuse: 50A
    • Circuit breaker: 70A

Example 2: Sizing Circuit Breaker for 3-Phase Motor Branch-Circuit Protection

How to determine the correct size of overcurrent protection devices of time-delay fuse and Inverse-time circuit breaker for a 5 HP, 230V, single-phase motor?

Solution:

1 – Find the Full Load Current

The full load current for 25 HP, 460V, three-phase motor based on Table 430.250 is:

FLC = 34A

2 – Size the Circuit Breaker

The suitable size of inverse time circuit breaker for motor branch circuit protection against overcurrent (short-circuit and ground faults) is determined using Table 430.52(C)(1).

Cricut Braker Rating = 34A × 250% = 85A

The next available closest standard breaker size is 90A.

Series Overview: Motor Circuit Calculations

Resources & Tutorials:

Wire Sizing Guides

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0 5 minutes read

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