ECN Forum Forums General Topics of the Trade General Discussion Area derating

Sorry to be so dumb here, but I'm trying to figure out the derating in some circuits I will be pulling. Been a long time since I had to derate. I'll be having 9 current carrying conductors in 1 conduit. The total load on 1 circuit will be 49.38 amps. ( 3 phase ). Since I would normally use # 6 wire using the 60 degree table which is good for 55 amps, do I take 70% of the 55 amps, which gives me 38.5 amps? If so, then even if I use #4 wire, which is good for 70 amps, then that still will not be quiet enough to make the 49.38 amps needed, unless I can drop the .38 amps. But then again, if I can verify the terminals good enough for 75 degree wire, then I can go by the 75 degree table. But these machines are so old, I doubt I'll be able to verify that, since there is not a label to go by. I wonder if anyone could give me some input on this? The other two circuits will both be 26.3 amps. apiece. (3 phase also) thanks again

sparks;

Here's some numbers for your derating situation:

If you use THHN insulation, then:

<OL TYPE=A>

[*] 9 # 6 THHN cu derated = 52.5 Amps,

[*] 9 # 4 THHN cu derated = 66.5 Amps.
</OL>

If you use THWN or THW insulation, then:

<OL TYPE=A>

[*] 9 # 6 THHN cu derated = 45.5 Amps,

[*] 9 # 4 THHN cu derated = 59.5 Amps.
</OL>

Hope this is helpful.

Scott

Sparky, the key thing to remember when detrating is to start in the column of the wire insulation and conductor type (cu/al). So for THHN or THWN-2, take the 90 degree column and derate based on the number of conductors. You then use that number as your maximum or the 60 degree column, whichever is less. Clear as mud?

Gee thanks guys, it's coming back now. I was using the 60 degree column for the derating on my thhn wire, makes a lot of difference. It gets "cloudy" sometimes when you don't use it on a regular basis don't mean to sound so dumb sometimes, life just gets to going too fast sometimes and I don't seem to have time to "sort" it out.... Thanks again for the help.. Steve