derate parallel conductors

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AntiochDan

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I'm working with an electrical engineer who insists I must use the 75 deg column when derating THHN wire. Where in the code book or other source, does it explicitly state I can use the column appropriate to its type of insulation? (specifically, I'm using parallel 400mcm on a 600 amp breaker. The wire is in and he wants me to use a 500 amp trip instead of a 600 amp trip)
 
Re: derate parallel conductors

Do you need to derate, or are you looking for the ampacity of the wire with no applicable derating?
Derating can begin from the 90 deg column, but you choose the smaller wire between the derated 90 deg rating and the non-derated amapcity of the column appropriate for your terminations (usually 75 deg).
335A x 2 = 670A worth of wire without derating.
 
Re: derate parallel conductors

The wires (6 x 400 mcm THHN) are in one conduit so I must derate according to 310.15(B)(2)(a). If I derate on the 90 deg. column, this will be :
380 x .80 = 304 amps times 2 or 608 amps

If I use the 75 deg column, this will be:
335 x .80 = 268 amps times 2 or 356 amps

I'm wondering where in the code book does it specifically state I can use the 90 deg. column for derating THHN wire?

The engineer's position is that since the termination lugs are rated at 75 deg., I must use the 75 deg column for my derating as well.
 
Re: derate parallel conductors

Originally posted by AntiochDan:I'm wondering where in the code book does it specifically state I can use the 90 deg. column for derating THHN wire?
Click to expand...
110.14(C).
 
Re: derate parallel conductors

110.14(C)

"....Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both."
 
Re: derate parallel conductors

So, what's the final answer? Is the EE correct? What is the final rating of the ampacity of the parrallel 400 kcmil?

Thanks,

Brett
 
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