allowable ampacities for conducter

The temperature of the environment, the temp rating of the wire, the temp rating of the connector being used. The lowest rating of any of these is the one you use.

environment temp

i don't understand the temp of the environment part. it may be 32 degrees f one day and 72 degrees the next. what determines the temp of the environment? is there a estimated temp chart for this area somewhere that is figured on annual averages?

You must also take into consideration the temp rating for the terminal connections.
A good example would be a panel being fed with THHN/THWN wire.
THHN is listed in the 90 degree column, but the panel lugs are not rated for 90 degrees. So you must use either the 60 or 75 degree column just because the panel lugs are not rated for 90 degree.

Like Joed mentions above. There is more to this article than meets the eye. It is also the most misunderstood and misused articles in the NEC.

thanks so much. i understand a little better now but am still confused about the temp of the environment part. yes i am using thhn and i see it is not listed in the 75 degree column in my ugly's book. that is not a problem is it?

Exactly what is it that you are trying to size here?

Typically, in new installtions:
For THHN/THWN conductors in conduit you would use the 75 deg C column.
For NM cable you are limited to the 60 deg C column.

thanks guys. i will use the 75 degree column. as for what i'm sizing up, it is a 400 amp ext panel with blade disconnect feeding two 200 amp interior panels. i will need to use 3-0 copper.

buddy builder said:

thanks guys. i will use the 75 degree column. as for what i'm sizing up, it is a 400 amp ext panel with blade disconnect feeding two 200 amp interior panels. i will need to use 3-0 copper.

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This throws another wrench into it. Conductor sizing for a residential service is also different. It does not use the same chart as feeders or branch circuits.

This is why when folks ask what size wire for XX amps it is impossible to say without knowing all the details.

Why not just set a second 200 amp next to the existing panel?

buddy builder said:

thanks so much. i understand a little better now but am still confused about the temp of the environment part.

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That refers to the temperature that wire/cable might be exposed to. For example an attic possibly might get very hot in your region. Or in a factory the temperature conditions could be very hot where the wire is being run.

buddy builder said:

thanks guys. i will use the 75 degree column. as for what i'm sizing up, it is a 400 amp ext panel with blade disconnect feeding two 200 amp interior panels. i will need to use 3-0 copper.

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Just did a similar installation back in April, I used two 4/0 conductor per leg, but mine was 3 phase. There are some pictures here:

http://www.diychatroom.com/f18/disagreement-main-lug-vs-main-breaker-133204/