800 Amp - Parallel 500 MCM ??

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NoPunchList

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I'm looking for the NEC references which allow the secondary service wire to be sized as follows: Parallel 500 MCM to handle 800 Amp Service.
310.16 would indicate this would have to be 600 MCM not 500 to carry the amp load - yet I'm always seeing 500 MCM being used - no one I ask can tell me the exact code reference - they just say it's legal. Help. Thanks.
 
NoPunchList said:
I'm looking for the NEC references which allow the secondary service wire to be sized as follows: Parallel 500 MCM to handle 800 Amp Service.
310.16 would indicate this would have to be 600 MCM not 500 to carry the amp load - yet I'm always seeing 500 MCM being used - no one I ask can tell me the exact code reference - they just say it's legal. Help. Thanks.
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If the actual load to be served is under 760 amps you can use an 800 amp breaker. Based on 240.4 (B)
 
The code gets a bit different if it is a breaker larger than 800A, as 240.4(C) would force you to have wire at least as big as the breaker.
So a 1200A breaker with 3 sets of wire must be at least 600kCMILs
 
Wait a minute. You said "secondary service". Is this a service, or a transformer secondary conductor? If it is a service, the above replies are right. If this is a transformer secondary, it is not permitted, per 240.21(C).
 
ryan_618 said:
Wait a minute. You said "secondary service". Is this a service, or a transformer secondary conductor? If it is a service, the above replies are right. If this is a transformer secondary, it is not permitted, per 240.21(C).
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Thank you. That is a new one for me--
 
Dennis,
That is a new one for me--
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While the wording was changed in the 2005 code, this is not a new rule. The rules in 240.21 talk about the conductor ampacity. New wording was put in the 2005 code because many code users were confusing the ampacity of the conductor with the rule in 240.4 that permits an oversized OCPD.
Don
 
I'm in florida and we refer to the "service" from the Transformer to the bldg as the "Secondary Service" while that from the power source - be it pole or whatever - to the power co's transformer, as the "primary service" - so my question is in relation to the "secondary service" - from the transformer to the bldg ( meter, disconnect, panel, etc ).
 
ryan_618 said:
Whatever you call it, it isn't the right term. Are the conductors on the supply side of the service disconnect? Who owns the transformer?
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Ryan I think he's talking about the feed from the utility transformer to the buildings service...(Main Breaker)
 
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