How to Size a Breaker for a Motor Load

Sizing a Circuit Breaker
Summary: Circuit breakers are sized according to rules established by codes and standards. For common applications, a 250% x motor FLA maximum size limitation is used.  However, this is different when drives are used, or when special circuit protection types are used.

Introduction

Circuit breakers are commonly used in circuit protection of motor circuits.  Size it wrong and you could end up with nuisance tripping, or even worse insufficient circuit protection. Sizing a circuit breaker for a motor load isn't overly complicated but there are some rules to follow, and a few things to check.  This article simplifies how circuit breakers are sized for motor loads in the US.

What can go wrong?

Do you ever wonder why a breaker isn't sized to match the motor full load amp (FLA) rating?  What would happen?

Likely your circuit breaker would trip when the motor is turned on.  Electric motors have a higher current at start up.  This is sometimes referred to as inrush. 

Circuit breakers have a magnetic element that protects against sharp rises in current.  This varies by breaker design, but it is not unusual to see magnetic tripping sensitivities between 6 to 8 times the nominal amp rating of the circuit breaker.  Motors may have an inrush 10 times the motor FLA or more.  In fact, higher efficiency motors tend to have even higher inrush levels.

Hence circuit breakers need to be sized to account for this anticipated conflict.

How to size a circuit breaker

Both the NEC and UL 508A describe how circuit protection is to be sized, and fortunately they both are in alignment.

Inverse-time circuit breakers are permitted to be sized a maximum of 250% of the motor full load amps.  Important to note that the full load amp current is based on motor HP tables provided in the code/standard, not the actual motor full load amp rating.

Noticed the maximum is 250%.  Should you size to the full allowable limit?  Maybe not.  In some cases sizing to the full 250% may put you in a larger breaker frame size.  This not only means more space is required, but also a higher material cost also.  Most engineers from my experience apply a 2X or 200% factor.  It doesn't hurt to quickly compare the chosen breaker trip curve against the motor inrush info (if the motor model is known).

It is important to note that different rules apply when using specific circuit protection devices or when using drives.  We discuss this in the following sections.

Industrial circuit breakers

What about sizing a Motor Circuit Protector?

Motor Circuit Protectors (MCP) are similar to molded case circuit breakers but usually have an adjustable magnetic inrush setting, and lack a thermal element.  These are required to be sized and applied as per an approved combination rating.  UL has Type D combination motor controller approvals that identifies the MCP, contactor and overload relay by part number, along with any other application conditions.  You really don't size an MCP.  You select a Type D combination (ok, technically, MCP's should also be sized according to their nominal amp rating, but this should have been factored in when the manufacturer selected the specific parts used in the Type D combination motor controller rating they achieved).

What about sizing a Motor Protective Circuit Breaker?

Motor Protective Circuit Breakers (MPCB) are similar to molded case circuit breakers, but they are modified specifically for motor applications.  This means they have higher magnetic protection setting to account for the higher motor inrush.  They also include motor overload protection.  This eliminates the need for an overload relay.  Given these are often rated to UL 489 (circuit breaker standard), they are not HP rated.  Often motor protective circuit breakers are sized according to the motor full load amp rating using the MPCB nominal amp rating.  Be sure the FLA range covers the motor full load amp rating.

One more IMPORTANT consideration

Some devices may have limitations as to the size or type of circuit breaker used.  A common example is the Short-Circuit-Current Rating (SCCR) marking on a device such as a contactor or overload relay.  This can be a problem especially on large HP size contactors that have sufficient motor HP rating, but a circuit breaker amp size that is too low for the desired motor size at the desired SCCR.  It's always important to check the ratings on the devices protected by the circuit breaker.

Another common example occurs when adjustable frequency drives are used.  Drives are unique in that they essentially modify the power into a form that allows for the frequency to be adjusted, thus changing the motor speed.  This means the circuit breaker is not seeing the loading of the motor directly.  Instead it is seeing the current demand characteristics of the drive.  Drive current demand varies by design.  Thus UL 508A instructs that the drive installation manual be followed regarding circuit protection sizing.  This ensures that the circuit protection device used avoids nuisance tripping and complies with the stated SCCR marking.

Key take aways:

  • Breakers are sized at a maximum of 2.5 times motor FLA for most applications.
  • MCP's and MPCB's are sized differently.
  • Consult the circuit breaker sizing limitations of control devices protected by the circuit breaker.

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