Okay, let's break down the interaction between Overcurrent Protective Devices (OCPDs) and conductor sizing when using a 250% sizing allowance, particularly in the context of motor circuits (since that's where such allowances are most common).

General Principle

In general, conductor ampacity must be at least equal to the calculated load current, and the OCPD rating must be no greater than the conductor ampacity. However,

motor circuits are a significant exception to this general rule.Motor Circuits and OCPD Sizing



Why 250%? The 250% OCPD sizing is allowed in many motor circuit applications specifically to handle the high inrush current (also known as locked rotor current) that occurs when a motor starts. This inrush is many times higher than the motor's full-load current (FLA). If you used a standard OCPD sized close to the motor's FLA, it would likely trip every time the motor started. The Key Sections of the NEC (U.S. National Electrical Code) to Refer To



430.52

Deals with motor branch-circuit short-circuit and ground-fault protection. This is where you'll find the specific percentage allowances for OCPD sizing (e.g., 250% for inverse time breakers).

430.22

Deals with the minimum ampacity of motor branch-circuit conductors.

Conductor Sizing vs. OCPD Sizing in Motor Circuits



Conductor Sizing

Your primary conductors in a motor circuit do not need to be sized to 250% of the motor's FLA simply because you're using a 250% OCPD. Instead, the conductor ampacity is determined by

430.22. 430.22(A) states that the branch-circuit conductors supplying a single motor shall have an ampacity not less than 125% of the motor's full-load current (FLA) rating. There are additional considerations based on duty cycle, number of motors, ambient temperature, etc. These could increase the required ampacity. OCPD Sizing

You size the OCPD separately, following the guidelines in

430.52. As you mentioned, this can allow for higher percentages (like 250%) of the motor's FLA, to allow for starting currents.

Example to Illustrate

Let's say you have a motor with an FLA of 20 amps.1.

Conductor Sizing

Minimum conductor ampacity = 20 amps 1.25 = 25 amps (per 430.22). You would select a conductor with an ampacity of at least 25 amps, taking into account any derating factors.
2.

OCPD Sizing

You're using an inverse time breaker. According to 430.52, you can size the breaker up to 250% of the FLA. 20 amps 2.50 = 50 amps. You can use a 50 amp breaker if needed to allow the motor to start without tripping.

HOWEVER, the code states that where the values determined do not correspond to the standard ampere ratings, the next higher standard overcurrent device rating shall be permitted. You must also consider if the OCPD is being used for short circuit or ground fault protection and size accordingly.

Important Considerations



Code is the Law

Always consult the latest edition of the NEC and any applicable local codes. Code requirements can change.

Manufacturer's Instructions

Follow the motor manufacturer's recommendations for OCPD sizing. Sometimes they will specify a particular type or size of OCPD.

Coordination

Make sure your OCPD is properly coordinated with any upstream OCPDs to ensure selective tripping.

Derating

Remember to account for conductor derating due to ambient temperature, the number of conductors in a raceway, etc., when determining the final conductor size.

In Summary:No, if you are using a 250% OCPD sizing allowance for a motor circuit, you do not need to size your primary conductors at 250% of the motor's FLA. You will determine your conductor sizing from NEC 430.22, which primarily requires 125% of the motor's FLA. The 250% OCPD sizing is a separate calculation, allowed under NEC 430.52 to handle the motor's inrush current. Flag for review