Motor Circuit Breaker Sizing Calculator

Electric motors are essential devices that convert electrical energy into mechanical energy (in the form of rotational torque of a shaft). The electrical energy could be grid-supplied single-phase or poly-phase alternating current or direct current from an alternative source.

But just like in every device that features an electrical circuit, there is a possibility of wire overheating, fire hazards, and electrocution with electric motors. Engineers mitigate these risks by including circuit breakers in electric motor circuits, which simply makes (or breaks) the electric motor circuit under normal (or fault) conditions.

However, to achieve a desirable performance of motor circuit breakers, engineers must correctly size and specify the ideal circuit breaker for the particular application. This article presents helpful information about motor circuit breakers, including some key design considerations and calculations.

Elecric automatic circuit breaker isolated on white background. 3d illustration

Engineers must correctly size and specify motor circuit breakers for a particular application

How does a motor circuit breaker work?

Circuit breakers are designed to trip (or break the motor circuit from the main supply) when motor circuit current exceeds the circuit breaker rating, preventing motor damage.

Consider a scenario where a 40 amp circuit breaker was specified for an electric motor circuit. It means that the circuit breaker will trip at 40 amp regardless of the type of load the motor is subjected to.

From this example, it would make sense to use a 100 A circuit breaker for a 40 A motor circuit, right? Wrong! An oversized breaker is just as bad (or even worse) than an undersized circuit breaker: it won't protect the motor circuit from fault currents and may burn and damage the motor.

The National Electrical Code (NEC) Article 430 specifies rules and considerations for correctly sizing overload protection devices.

Electric motor circuit breaker size calculator and the National Electrical Code (NEC) article 430

The National Electrical Code (or NFPA 70) is a widely adopted standard for safely installing electrical components and systems, including electric motor circuits. It provides guidelines for electrical installations to prevent fire and other electrical accidents.

The NEC Article 430 states that for continuous duty motors marked with a service factor of 1.15 or greater and a temperature rise of 40¡C, engineers should size the overload protection device at no more than 125% of the motor's full load amperage rating. NEC article 430 also states that all other motors should be sized at 115% of the motor's full load amperage rating.

Consider the example of an electric motor with a service factor of 1.15 and a full load amperage rating of 25 A. The circuit breaker needed for this application should be sized as follows:

Since the overload current rating cannot be greater than 125% of the FLA, engineers can choose the next lower circuit breaker rating, for example, 31 A.

Note: Motor full load amperage ratings can easily be obtained from the motor nameplate or through calculations. Engineers can also obtain these ratings by contacting motor manufacturers.

Circuit Breaker Calculator: Motor Full Load Amperage (FLA)

The formulas for full load amperage rating for single-phase and three-phase circuits are presented below:

FLA for single-phase motors when power rating (Pout) is measured in kilowatts

FLA for single-phase motors when power rating (Pout) is measured in horsepower

FLA for three-phase motors when power rating (Pout) is measured in kilowatts

FLA for three-phase motors when power rating (Pout) is measured in horsepower

Where:

Pout=mechanical power output

PF=Power factor

V=Line voltage

E=The efficiency of the motor

Keep in mind that this circuit breaker sizing only considers motor overload, which happens when the operation of a motor (in excess of its normal, full-load rating) persists for a long enough time that it damages or overheats the motor. When specifying motor circuits, engineers also need to consider overcurrent protection.

Motor Overcurrent Protection

Overcurrent is when the current in the motor wires exceeds the motor's rated current or the ampacity of its conductors. Keep in mind that overcurrent is usually the effect of an overload. However, several other factors also cause motor overcurrent, for example, ground fault and short circuit.

The requirements for motor overcurrent protection are also listed in the NEC Article 430, which includes ground fault and branch circuit short circuit protective devices for the motor. As a rule, these devices must be specified such that they can handle the motor starting current.

The NEC article 430 recommends maximum ratings for ground fault and motor branch circuit short circuit protective devices. Engineers can find more information about different types of motors as well as the corresponding recommended current rating for overcurrent protection devices in Table 430.52 of the NEC article 430.

[Learn more about circuit protection components with Engineering360]

Motor circuit breaker sizing calculator: Engineers should check the NEC 430 article

While this article presents helpful information about motor circuit breaker sizing, there are still several other things to consider when specifying motors for a particular application. For example, there is still the need to specify motor control centers, disconnecting means, adjustable frequency drives, and grounding.

The NEPA 70: National Electrical Code Article 430 covers all of these and more. Check Engineering360 standards library to learn more about the National Electrical Code Article 430.