Derating Wire

[dcsva@aol.com]

I have a customer that wants to supply three 3-phase heater loads, each fused at 30amps.
He wants to install one conduit that is about 100FT to a junction box and then split off to each heat bank.
We must first derate the wire by (0.7).
#8 is good for 50amps x 0.7 = 35amps.
1. Is it okay to carry one #8 EGC (good for 100amps) and jump it to each heat bank?
2. Does the ground get derated as well?
3. Minimum Conduit Size for 10-#8 THHN? I am guessing 2".
My code book is 20 miles away.
I will confirm when I get back to the office. Any help would be appreciated.

[ October 19, 2004, 10:03 AM: Message edited by: dcsva@aol.com ]

 

[bphgravity]

Re: Derating Wire

#8 thhn is actually good for 55-amps. 55 x .7 = 38.5 current-carrying capacity for each.

1. Yes, #8 would be suficient to serve as the EGC for all three circuits. This is permitted per section 250.122(C)

2. no, a ground is not a current-carrying conductor, however it is required to be increased in size when the ungrounded conductors are increased in size. Some would argue that increasing the size of the ungrounded conductors to satisfy derating requirements is essentially increasing in size, however, others will say that you are not increasing the conductors size when the carrying capabilities is reduced because of derating.

3. 2" what? You did not specify raceway type.

 

[dcsva@aol.com]

Re: Derating Wire

Concerning 250-122C

Since all three circuits are each protected by 30AMP fuses. 250-122C says the EGC could be #10.
250-122C does not say anything, that I can see, about increasing ungrounded conductors due to derating having an effect on the EGC size.

I orginally picked a #8 EGC because of it's 100amp rating. (30+30+30=90 therefore 100amp EGC.

However, if you go back to increasing the size of the ungrounded conductors by cmils...you must increase the ground accordingly calculates to say #8EGC is required.
The only place I see this calculation required is in 250-122B...voltage drop

I will install a #8 EGC but it appears that only a #10 is required.

Oh yeah, Table C8...10-#8 in a 1-1/4"RMC conduit.

Thanks for the input bph...

 

[charlie b]

Re: Derating Wire

Originally posted by dcsva@aol.com:250-122C does not say anything, that I can see, about increasing ungrounded conductors due to derating having an effect on the EGC size.

That requirement is in 250.122(B).