Sizing overcurrent protection for motors

[Johnboy1623]

I recently installed some 10hp motors in a dry kiln. They are 10hp with a FLA of 13.9 and a service factor of 1.15. I used the 125% rule and got around 18amps. I rounded up to the next breaker size and arrived at 20amp. I had an issue with the breaker tripping and noticed my vendor sent me a type c breaker and I ordered a replacement breaker with a type D trip class. There is another electrician telling me that I need a 30amp breaker. I noticed that using a slide chart it will in fact recommend a 30amp circuit for a 10hp motor. I can’t however find anywhere in the NEC requiring one. What’s really going on here ? Who’s right ?

 

[augie47]

Assuming the motor has overload protection and your breaker is for Short-Cicuit-Ground-Faul;t protection you can size the breaker at 250% of the motor FLC in Table 250 (for 3 phase motors). In your case the FLC of a 10HP 480 motor is 14 amps so a 30 amp breaker is well within the limit.
Your NEC reference is 430.6,430.150 and 430.52

 

[Tip DS]

Assuming the motor has overload protection and your breaker is for Short-Cicuit-Ground-Faul;t protection you can size the breaker at 250% of the motor FLC in Table 250 (for 3 phase motors). In your case the FLC of a 10HP 480 motor is 14 amps so a 30 amp breaker is well within the limit.
Your NEC reference is 430.6,430.150 and 430.52

@augie47 - Is it possible your reference to 430.150 is a type-o? I'm not finding that particular section. When you refer to Table 250, just for clarity, I'm going to point out it is table 430.250 in NFPA 70.
When you say the breaker can be sized to 250% of the FLC, you're basing that on the Inverse time breaker from Table 430.52, right?

Sounds like you're proposing OP should be getting a 25A Type D breaker. Thanks for sharing your knowledge, BTW.

 

[augie47]

It was a typo 430.250 is the Table for 3 phase motors.
Yes, Inverse Time.
I would likely use a 30 amp breaker.

 

[Johnboy1623]

So I undesrand that I can use a 30 amp breaker but is using a 20amp type D breaker illegal? Why even have people size overcurrent protection using the 125% rule if it is incorrect ? I may be ignorant and am definitely confused am I missing something obvious?

 

[augie47]

As your installation shows, a 20 amp breaker is prone to trip due to the motor inrush.
That's why Art 430 allows for larger breakers than the conductor ampacity.

 

[Tip DS]

What augie47 said. Also consider you are thinking of the steady state value, that the wire can handle for ever and ever and ever, without over heating. Get a little bit above that current, and it may start warming up a little bit. It would take a LONG time to get to failure at 25% above its "forever current." On top of that, your breakers are generally THERMAL-magnetic, meaning they will trip from over current OR over temperature. So, you have that going for you, too.