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#201260 - 05/23/11 11:22 AM NEC Art 240.4(B)  
Bravo  Offline
Joined: May 2010
Posts: 30
I have a situation where I client wants to use 500-kcmil THWN copper conductors, 75deg (380A) protected by a 400A OCPD. Please correct me if I am wrong but in accordance to 240.4 this is allowed as long as the calculated load does not exceed the ampacity of the conductors (380A)?

I need help clarifying part (B). The OCPD is 100% rated with adjustable trip unit. Our firms's licensed engineer will not stamp dwgs if the breaker is 100% rated and does not trust that the trip will be adjusted in the field.

again I need understanding what part (B) is stating.

Thanks in advance.

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#201261 - 05/23/11 12:02 PM Re: NEC Art 240.4(B) [Re: Bravo]  
sabrown  Offline
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
According to your reference in 240.4B yes it is allowed. As an Engineer, he has the final say as to what he will stamp and what he won't, whether it meets code or not. Good design may have other reasons may preclude the use of the 400 amp, 100% rated breaker. Anyway, it is his choice to stamp or not, right or perceived wrong.

Look for options acceptable to him, such as a 100% rated braker with (I guess they may be called) replacable current modules. Or a permanent plaque stating maximum rating the breaker may be adjusted to.

#201262 - 05/23/11 12:03 PM Re: NEC Art 240.4(B) [Re: Bravo]  
sabrown  Offline
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
AND or her.

#201266 - 05/23/11 02:45 PM Re: NEC Art 240.4(B) [Re: Bravo]  
HotLine1  Offline

Joined: Apr 2002
Posts: 6,872
Brick, NJ USA
Did you read 240.6 (C)from 2008, that may be an acceptable option to present to the Engineer.

As Sabrown said, the Engineer of record has the ultimate & final say.


#201268 - 05/23/11 05:05 PM Re: NEC Art 240.4(B) [Re: sabrown]  
Bravo  Offline
Joined: May 2010
Posts: 30

Good point, but even a plaque indicating max rated adjustment cant stop someone from setting trip higher than ampacity.


#201269 - 05/23/11 05:08 PM Re: NEC Art 240.4(B) [Re: HotLine1]  
Bravo  Offline
Joined: May 2010
Posts: 30
That is also a valid point. Restricting the access to adjusting the trip unit helps along with a plaque indicating max trip setting allowed. Thank you

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