Wire ampacity less than breaker

[Fitzdrew516]

I'm drawing a blank. I need to find the section in the NEC that states that you can use wire with an ampacity rating that is less than the main breaker size as long as your load does not exceed the wire's rating. For example - your service main is 2000A - I've been told that you coud use (5) sets of #500 kCMIL on the 2000A main breaker as long as your load doesn't exceed (in this case) 1900A. Thanks.

- Drew

 

[luckylerado]

Sorry. Not true for breakers over 800 amp.

240.4(C)

 

[Fitzdrew516]

Sorry. Not true for breakers over 800 amp.

240.4(C)

Cool. Thanks for the quick reply.

 

[infinity]

As Lucky said 5 sets of 500 kcmil would not work, you would need 6 sets of 500's or 5 sets of 600's.

 

[mbrooke]

Only example I can think of is the rule-of-six disconnecting means where the incoming conductors can be smaller then the combined main breaker ratings as along a load calcs are done correctly.

 

[JoeStillman]

It's right in the beginning of article 240.

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[kwired]

Six disconnect rule and the over 800 amp or less "next size up" rule that have already been mentioned plus there is also specific application permissions in some articles like 430, 440, 450, 460...

 

[01jacobite]

Now are we talking about service entry conductors or load side the service switch, sizing of conductors for each is different. Explanations above all seems like load side service switch. How do we calculate conductors line side service switch

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[ActionDave]

Now are we talking about service entry conductors or load side the service switch, sizing of conductors for each is different. Explanations above all seems like load side service switch. How do we calculate conductors line side service switch

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Well, if we are talking about conductors that electricians install then the same NEC rules apply right up to the point where the utility makes their connection. Power company conductors get to be sized by their own rules.

Welcome to the forum.

 

[Strathead]

Well, if we are talking about conductors that electricians install then the same NEC rules apply right up to the point where the utility makes their connection. Power company conductors get to be sized by their own rules.

Welcome to the forum.

This is true in Florida where I reside, however, I disagree with what I interpret you are saying. Service entrance conductors are only covered by the NEC rules if they are overhead conductors at the point of connection to the utility wires. Underground conductors are not covered by the NEC no matter who installs them. Since the NEC rules are stricter than any Utility rules I have seen, I suspect there is never a conflict if you size the to the NEC, that is overkill. Again, that is exactly what they do in much (but not all) of Florida. The Electrical Contractor installs the secondary conductors and they have always ween sized to the NEC as far as I can tell. They just don't have to be. (edit) unless of course, the Utility rules state that they are sized per the NEC, which is perfectly feasible.

 

[kwired]

Now are we talking about service entry conductors or load side the service switch, sizing of conductors for each is different. Explanations above all seems like load side service switch. How do we calculate conductors line side service switch

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Minimum ampacity selection for conductors is described in the applicable articles, 210.19 for branch circuits, 215.2 for feeders and 230.42 for service entrance conductors. All three of those have same basic requirements of 100 percent of non continuous load plus 125 percent of continuous load.

This is true in Florida where I reside, however, I disagree with what I interpret you are saying. Service entrance conductors are only covered by the NEC rules if they are overhead conductors at the point of connection to the utility wires. Underground conductors are not covered by the NEC no matter who installs them. Since the NEC rules are stricter than any Utility rules I have seen, I suspect there is never a conflict if you size the to the NEC, that is overkill. Again, that is exactly what they do in much (but not all) of Florida. The Electrical Contractor installs the secondary conductors and they have always ween sized to the NEC as far as I can tell. They just don't have to be. (edit) unless of course, the Utility rules state that they are sized per the NEC, which is perfectly feasible.

NEC clarified what it applies to in the 2011 edition by adding the defined term "service point".

If the conductor is on the utility side of the service point NEC does not apply to it, if it is on the customer side of the service point it will be sized per NEC requirements. Doesn't matter who installs it. If POCO wants to install something undersized for NEC then they need to keep that portion of the install on their side of the service point.

 

[david luchini]

Underground conductors are not covered by the NEC no matter who installs them.

In light of this, how would you explain 230.31?