Minimum Service Wire Size

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Strathead

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So, I noticed that while the minimum service size can be 15 amps or 30 amps per 230.79, other than in VERY limited circumstances, the wire must be a minimum of #8 copper per 230.23(B) AND/OR 230.31(B). That kind of defeats the savings of going with a smaller service.
 
There is also a minimum for feeders to an detached structure. I guess the concept is to have enough power available for future use. Why does a home need 100 amp if there is no a/c- gas heat, range and water heater and perhaps 600 sq.ft of space? It all has to do with design for future. I know the nec is not a design manual but that is just not true. They demand many things that may not be needed but are there for future applications.
 
Wire-Smith said:
i would think another reason could be fault current, utility probably isn't going to size protection very tight
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Maybe withstand to at least 10 or 22kaic for a fault after the main. But before the main the POCO is not concerned. There cases where 4/0 will melt its metal during a fault before the main OCPD, an example would be a secondary network.
 
mbrooke said:
Maybe withstand to at least 10 or 22kaic for a fault after the main. But before the main the POCO is not concerned. There cases where 4/0 will melt its metal during a fault before the main OCPD, an example would be a secondary network.
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i'm not talking about for shorts before the main. if there is a fault downstream from the main even with ocpd on the branch circuit, the conductors could be damaged. which isn't as big a deal with branch circuits, they have short circuit and ground fault protection. it is a bigger deal for the service wire because they do not. if the branch circuit wire is damaged in a fault the ocpd should not re-close or when the damage actually becomes a problem it should open again. if service conductors are damaged during the fault even if downstream of branch ocpd, when original fault is corrected and ocpd closed the service conductors could have been damaged and without short circuit and ground fault protection your up a creek without a paddle. its more critical for service conductors to be free of damage and more protected from damage because they dont' have the extra protections that other conductors do, the larger wire does that to a degree for short circuits.

just my thoughts. i think 8's is a decent cutoff. not perfect, won't always save from problems. but i like it. it would be hard to put a code requirement saying they should be sized for short circuit current, resi doesn't require that information right now, poco's change xfmr's,...
 
Wire-Smith said:
i'm not talking about for shorts before the main. if there is a fault downstream from the main even with ocpd on the branch circuit, the conductors could be damaged. which isn't as big a deal with branch circuits, they have short circuit and ground fault protection. it is a bigger deal for the service wire because they do not. if the branch circuit wire is damaged in a fault the ocpd should not re-close or when the damage actually becomes a problem it should open again. if service conductors are damaged during the fault even if downstream of branch ocpd, when original fault is corrected and ocpd closed the service conductors could have been damaged and without short circuit and ground fault protection your up a creek without a paddle. its more critical for service conductors to be free of damage and more protected from damage because they dont' have the extra protections that other conductors do, the larger wire does that to a degree for short circuits.

just my thoughts. i think 8's is a decent cutoff. not perfect, won't always save from problems. but i like it. it would be hard to put a code requirement saying they should be sized for short circuit current, resi doesn't require that information right now, poco's change xfmr's,...
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My thoughts exactly. You want the service conductors to survive a fault after the main. If not, yahhh, it could get ugly.
 
It may be simply for the more rugged conductor you get from using #8.

There is a big difference in the construction of #8 vs. #10.

Solid insulated wire generally stops at #10, and stranded starts at #8.

That may be another reason.

It's like the difference between 3/4" and 1" PVC. Although they are only 1 size apart, the difference is pretty significant in construction and durability.

JAP>
 
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