Motor charts vs NEC

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Alexmc21

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Princeton Kentucky
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Electrician
Hooking up motors using the ugly book motor chart and got into a discussion on if the information was right in the ugly book.. ex. 10 hp motor 240v 3 phase calls for a 60 amp breaker with 10 Awg wire. I’m being told that is against code. Can anyone give me a specific area in the code book to determine weather this is right or wrong.. besides the fact the slide charts have been used for over 30 years.
 
augie47

augie47

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I've not checked other possibilities NEC vs Ugly but for your 10HP 230v that would correspond with NEC requirements.
(assuming 75C° terminations on both ends)
 
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Eddie702

Licensed Electrician
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Western Massachusetts
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Electrician
Code books says 28 amps for the motor x 1.25=35A

#10 is good for 35@75C or 40@90C

That's from 430.250, 310.15(B)16

Unless you have derating to do
60A breaker per 430.52 seems fine
 
tom baker

tom baker

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Ahh, the old "not to code" comment...I appreciate you taking the time to ask, so we can all learn. Typical 10 hp 3 ph 240 motor has a FLC of 28 amps. My square D motor data slide rule calls out 60 TM CB with 10 AWG conductor. (if you can find this calculator its invaluable to have handy, of course today there is probably an app)
For a motor, the branch circuit short circuit protective device (60 CB) is sized larger than normal to allow for starting current.
The breaker protects the wire and the overloads protect the motor.
The allowance for this is in Section 240.4 G, which refers to table 240.4 G and then it sends you to Art 430.
Your coworkers and many electricians get nervous putting a 10 AWG on a 60 CB, its been that way for a very long time for motors.
Here is how the math works out for a 2 hp motor
1636347248532.png
 
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