Sizing Circuit Breakers: When to Round Up/Down

[adamscb]

Hi all,

I had a question about sizing circuit breakers, and when to round up/round down. The way I understand the code, you take the FLA of the motor, multiply it by 250%, and that gives you the maximum size of the circuit breaker. If the calculated size is above a standard size, then you round down to the next closest standard size. However, if this causes the breaker to trip during startup, then you're permitted to round up to the next closest standard size (and thus going above the 250% maximum rating). Is this way of thinking correct? If so, where in the code is this found? I was just taught this in school; never learned where in the code this actually is.

How would one go about testing to see if the breaker would trip during startup? I've been told that starting current is ~400% FLA, is this a good practice to use?

Thanks!

 

[ron]

Look at 430.52(C)(1), its exceptions and Table 430.52

 

[Henley]

Motors can be a Tricky issue when trying to apply the NEC to the motor circuits. Remember, the NEC is not a Design Manual, and in such cases
as motors the NEC doesn't really provide a definite answer, just some framework.
Test questions on motors, often can be confusing because they want a specific answer. So Art 430.52 provides some guidelines; for a INV Time
Breaker, the Table gives 250% increase, then there are 2 exceptions. The first allows you to round up after the first (250%) increase to the next standard size OCPD ;then the second allows a further increase (If Starting Current is still not Handled) to 400% of FLC however in this case, you can not exceed 400% so often you must round down. While subjective, most texts refer to the second sizing of the OCPD (Up to but not exceeding 400%) as the Maximum.