NEC 430.52 (B)

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Romel

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Should article 430.52(B) applies to all types of motors?
or are there any exceptions that I have been missing.

I've seen design/installation that seems to be not following this, and approved by AHJs?
say for example, a 208V 3HP pool pump or a lift station where the branch breaker is sized around 20A.
Should it be sized based on its LRA? or is it because that these are small motors that LRA can be ignored?

Any thoughts?
 
Jraef

Jraef

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I think you might be over interpreting 430.52(B).
(B) All Motors. The motor branch-circuit short-circuit and
ground-fault protective device shall be capable of carrying
the starting current of the motor.
Click to expand...
"Carry" does not mean sized to carry it continuously continuously in this context, because starting current is by definition transitory. It just means the OCPD will not trip / clear on the normal starting current of the motor. It's a way of the NEC way of saying "no duh..."

208V 3HP 3 ph motor, 11A FLC, probably 66A LRC, about 6.6x the breaker rating. A 20A circuit breaker should trip in around 4 seconds at that level. If your pump is not done accelerating in 4 seconds, you probably have something wrong.
 
tom baker

tom baker

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My handy Sq D Motor Data calculator says the short circuit and ground fault protection (circuit breaker) should be 20 A, with a FLC of 11A.
I am a old time fuse guy and I would likely fuse it for 15 A, but the motors I worked on were water pumps with a pretty constant load.
Overloads are sized to the FLC, and the short circuit and ground fault protection (circuit breaker) is sized per code tables.
"The overload protects the motor and the breaker protects the wire".
 
R

Romel

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Jraef said:
I think you might be over interpreting 430.52(B).

"Carry" does not mean sized to carry it continuously continuously in this context, because starting current is by definition transitory. It just means the OCPD will not trip / clear on the normal starting current of the motor. It's a way of the NEC way of saying "no duh..."

208V 3HP 3 ph motor, 11A FLC, probably 66A LRC, about 6.6x the breaker rating. A 20A circuit breaker should trip in around 4 seconds at that level. If your pump is not done accelerating in 4 seconds, you probably have something wrong.
Click to expand...

Thanks. I totally agree. a standard SE thermal magnetic breaker trip curve will tell us that it can sustain the 6xFLC for around 2-3 secs before tripping. So it should be fine. I remember in European countries, they have different types of breaker (B,C,D) for different application to account for inrush current so you don't have to account the inrush current in sizing the OCPD.

Now I get it. that article was meant for protection like fuses, reaction time for 6xFLC would be less than 1sec. So if you're using a typical TM breakers with 2-3 sec, you'll be fine.
 
don_resqcapt19

don_resqcapt19

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The pool pump assembly may be a motor operated appliance covered by Article 422, and not a motor covered by Article 430.
 
tom baker

tom baker

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david luchini said:
I would think both the overload and the breaker protect both the motor and the wire.
Click to expand...
My quoted text is a what some electricians are taught to help understand why the breaker is larger than we typically see. the overload is for slow response and the breaker is for short circuits and ground faults that the overload won't trip on
 
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