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Old 10-06-2010, 11:18 AM   #1
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How to determine wire size?


A couple NEC questions....

How do you determine what size EGC is needed? I know it can be smaller than the Current Carrying Conductors, but is there a chart somewhere?

I knew this at some point (and it should be added to the "national electric code" sticky). Which columns are used for NM-B and THWN in conduit?

THHN and THWN are in different columns, but most wire is rated for both...

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Old 10-06-2010, 01:49 PM   #2
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How to determine wire size?


yes, there is. It is in the NEC along with all the rest of the numbers.


table 250.122

100 am/ #8
200 amp/ #6
400 amp/ #3

NM is limited to the 60 rated column unless you are calculating derating. Then you can use the 90 column for the calculations BUT the final ampacity still cannot exceed the 60 rating.

Quote:
THHN and THWN are in different columns, but most wire is rated for both...
it depends if you are using it as THHN or THWN but (without a long explanation as to why) just stick with the 75 rating for either.


Quote:
I knew this at some point (and it should be added to the "national electric code" sticky). Which columns are used for NM-B and THWN in conduit?
the right one. For most uses, that would be 310.16.

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Old 10-06-2010, 03:43 PM   #3
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How to determine wire size?


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Originally Posted by secutanudu View Post
THHN and THWN are in different columns, but most wire is rated for both...
If you are using the wire in a wet environment you would use the 60 degree column. Dry locations you can use the 75 degree column.
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Old 10-06-2010, 07:36 PM   #4
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How to determine wire size?


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If you are using the wire in a wet environment you would use the 60 degree column. Dry locations you can use the 75 degree column.
actually, THWN is rated for 75 and THHN is rated for 90. There is no 60 rating to be concerned with at all. (for thwn or thhn)

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Old 10-06-2010, 08:35 PM   #5
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How to determine wire size?


Maybe a mod could add this to the NEC sticky...
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