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Old 12-26-2012, 10:04 AM   #1
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allowable ampacities for conducter


on the chart of allowable ampacities of conducters what determines which column you use? i live in south alabama and am getting conflicting reports on whether to use the 75 or 90 degree celcius column.

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Old 12-26-2012, 10:15 AM   #2
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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.

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Old 12-26-2012, 10:30 AM   #3
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allowable ampacities for conducter


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?
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Old 12-26-2012, 10:34 AM   #4
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allowable ampacities for conducter


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.
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Old 12-26-2012, 10:44 AM   #5
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allowable ampacities for conducter


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?
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Old 12-26-2012, 12:03 PM   #6
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allowable ampacities for conducter


Exactly what is it that you are trying to size here?
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Old 12-26-2012, 12:12 PM   #7
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allowable ampacities for conducter


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.
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Old 12-26-2012, 12:40 PM   #8
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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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Old 12-26-2012, 02:03 PM   #9
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Quote:
Originally Posted by buddy builder View Post
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.
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.
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Old 12-26-2012, 02:09 PM   #10
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allowable ampacities for conducter


Why not just set a second 200 amp next to the existing panel?
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Old 12-26-2012, 07:25 PM   #11
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allowable ampacities for conducter


Quote:
Originally Posted by buddy builder View Post
thanks so much. i understand a little better now but am still confused about the temp of the environment part.
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.
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Old 12-27-2012, 07:06 PM   #12
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allowable ampacities for conducter


Quote:
Originally Posted by buddy builder View Post
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.
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:

disagreement on main lug vs main breaker

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