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Old 04-26-2011, 02:04 PM   #1
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310.16 vs 310.15(B)(6)


Okay, this is the engineer in me trying to understand something that just seems counterintuitive. I think I understand that according to NEC that the allowable ampacity for a wire that is somewhere between what the POCO supplies and what we lay people like to call a main panel, but which is apparently properly referred to as the service panel, can be found in table 310.15(B)(6) but for downstream panels the ampacity must come from table 310.16

What this means effectively is that for a given current you want a wire to handle, you need a bigger wire downstream where you go through your house than you need coming into your house.

The fact that code says this seems pretty well explained here:
http://askcodeman.com/viewtopic.php?f=3&p=1346

So for a 100 amp "main" panel, wiring it from a meter, table 310.15(b)(6) says I can use 2 gage aluminum wire, but if I wire a "sub" panel that same wire could only handle 90 amps and I would need 1 gage or heavier aluminum wire to handle 100 amps.

So my question is why is 2 gage aluminum okay upstream but not okay downstream? I get it for overhead wires where air flow can better cool the wire than in enclosed installations, but your main panel feeder goes through the same walls other wires go through...

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Old 04-26-2011, 03:10 PM   #2
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310.16 vs 310.15(B)(6)


My understanding is because Service Conductors (not Service Entrance Cables - SEC) are, for the most part, outside the building. Notice in 310.16 that #2 AL USE-2 is listed at 100 amp in the 90 column but you don't get to use this.
Something else to remember about Service Conductors is that they are unfused cables.

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Old 04-26-2011, 04:27 PM   #3
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310.16 vs 310.15(B)(6)


310.15(b)(6)if you read it say's single phase dwelling services and feeders.this is to the dwelling(your house)only.
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Old 04-26-2011, 05:32 PM   #4
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310.16 vs 310.15(B)(6)


the reason the feeders can be smaller than usual is that they know that due to demand factors that the cable will not or rarely ever pull its calculated current, therefore you are allowed under table 310.15b6 to use a slightly smaller wire.

It looks like I may be wrong in your other thread that sub panel feeders ALSO fall under that table can someone confirm?
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Old 04-26-2011, 05:40 PM   #5
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310.16 vs 310.15(B)(6)


The handbook makes it clear that the sub panel would not be able to use table 310.15b6, but would have to use table 310.16. Sorry.


throw in a 90a breaker and call it a day
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