Motor Circuit, 3/4HP 120V

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tryinghard

tryinghard

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If I have a ? HP 120V motor with thermal protection built in can I consider this thermal overload as all the OL needed/required?

In this example if I understand 430 correctly my circuit will include:
40A 1P circuit breaker (13.8x250%) ? 430-52
#14cu THHN (13.8x125%) ? 430-22(A)
30A non-fused disconnect (13.8x115%) ? 430-110(A)
Is this example correct?

Does anyone have a good website or resource about motor circuiting?
 
I was using that little slider calculator from Square-D and couldn't figure out why it said a 25A circuit breaker is okay when 430.52 was giving a 35A value. It took awhile but I got it, and felt silly when I realized it.

430.52 is the MOCP. Maximum overcurrent circuit protection.

430.22 is the MCA. Minimum circuit ampacity. Exception (b) for single phase.

13.8A * 250% = 34.5A -> 35A MOCP
13.8A * 150% = 20.7A -> 25A MCA

So a 25A, 30A, or 35A circuit breaker is allowed by Code. Personal preference or field conditions can determine which breaker to select.
 
I suppose the thermal protection is protecting the motor and circuit from overloading, therefore the breaker simply provides the short-circuit & ground-fault protection, it does look strange to see a 35A breaker with #14?s for the circuit.
This source breaker does need to cover the start up which is often 300% or it may trip so I would go with 35A breaker if I can get one.

Thanks for your input
 
I think you have the wrong wire size.

430.248 shows 3/4Hp 120V motor at 13.8amps. 13.8 x 125% =17.25amps.

Even though the CB is not protecting the conductors and the motor OL is, 240.4(D) does limit 14AWG to 15amps.
 
Michael15956 said:
I think you have the wrong wire size...Even though the CB is not protecting the conductors and the motor OL is, 240.4(D) does limit 14AWG to 15amps.
Click to expand...

I want to agree with you ? and normally would ? but to take 430 literally notice 430-51 ??These rules add to or amend the provisions of Article 240.? So in this light I can use 14?s, it doesn?t mean I necessarily will :)
 
What limits a #14 to 15a is 240.4d. Basically, 240.4G exception takes the asteriks off of #10-#14 when sizing a conductor on T310.16 because you have to be able to size the OCPD for high start up currents.
 
Cable can handle a large amount of current 250% for a short period of time. Look at cable damage curves to see what I mean. Therefore, even though the CB protection may seeem to be a lot higher then the conductor continuous rating, and it appears the conductors would not be protected, the actually are.
 
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