100A feeder

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Toros

Senior Member
Location
Tujunga, CA
Hi
For my friend
on the plan
i have indicated 100a cit breaker through #3 wire thhn cu @75 deg for 130 feet feeder supplying the tenant sub panel

he said code has changed recently and we need #2 tbhn cu
is it true???
 
Toros said:
Hi
For my friend
on the plan
i have indicated 100a cit breaker through #3 wire thhn cu @75 deg for 130 feet feeder supplying the tenant sub panel

he said code has changed recently and we need #2 tbhn cu
is it true???
Click to expand...

In the 2014 NEC, #3cu is still rated 100A @ 75deg.
 
Toros said:
... he said code has changed recently...
Click to expand...
The only code change that I can think of that would be relevant is the provision for reduced ampacity for a service or feeder which supplies the entire load of a dwelling unit. In [2011] Table 310.15(B)(7) shows that for 100A calculated current you can use #4 copper.
The standard wire capacity table (no more than three wires in a conduit), 310.15(B)(16), shows that 100A requires #3 if you are allowed to use the 75C rating.

In [2014] the first table is no longer present, but the text tells you that you can use a conductor with only 83% of the calculated required ampacity after applying adjustments and corrections. The results are exactly the same except that [2011] is not explicitly clear about the need to apply adjustments and corrections to the sizing in the table. The second table, 310.15(B)(16), is as far as I can see identical between {2011] and [2014].

I guess you are assuming that since the breaker is 100A the calculated load must be less than or equal to that?
 
Toros said:
Hi
For my friend on the plan i have indicated 100a cit breaker through #3 wire thhn cu @75 deg for 130 feet feeder supplying the tenant sub panel he said code has changed recently and we need #2 tbhn cu is it true???
Click to expand...

No code change so the answer is no.:)
 
Toros said:
Hi
For my friend
on the plan
i have indicated 100a cit breaker through #3 wire thhn cu @75 deg for 130 feet feeder supplying the tenant sub panel

he said code has changed recently and we need #2 tbhn cu
is it true???
Click to expand...

#3 Cu is correct, as long as:
1. Terminals on both sides are listed and labeled for 75C
2. Wire is listed and labeled for 75C or greater
3. You aren't using NM cable AKA romex
4. No derate factors apply
 
The energy code could limit the voltage drop along this feeder, which could result in a larger required wire size.

But nobody pays any attention to those energy codes :)
 
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