03:27PM | 01/24/07
Member Since: 01/23/07
2 lifetime posts
I am adding a new addition on to an exixsting building that has 200 amp service. I need to run 100 amp service to the new box. The length of the run is approx 80ft through the attic of the existing structure. I need to know wire size for this application.

Thanks for any info you can offer.....


05:07PM | 01/24/07
Member Since: 04/25/05
1918 lifetime posts
#3 coper or #1 AL.

That assumes that you are using a wire and the the panel terminals are rated for 75C (or better).

And that the attic is not too hot that it requires derate the wiring.


04:25AM | 01/25/07
Member Since: 03/31/05
265 lifetime posts
Since you are routing the feeder through the attic, you will need to use #2 Cu or 1/0 Al according to Table 310-15 (b)(6).


04:43PM | 01/25/07
Member Since: 04/25/05
1918 lifetime posts
Table 310-15(b)(6) is restricted to residential SERVICE Feeders or lateral or feeder conductors that run from the main disconnect to the lighting and appliance branch circuit panelboard(s).

That wording allows the special heavy loads such as an AC to be "tapped off" before the feed to a sub-panel. The the loads on the sub-panel has to be similar to a whole house and the load calculated using the residential demand factors.

It can't be used for a sub-panel that serves an addition, the other end of large house, a workshop, barn, etc.

But evem using 310-154(b)(6) how do you come up with #2CU, 1/0 AL? At 100 amps that would be #4/#2.


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