Derating for Conduit Fill vs Voltage Drop

[kpepin]

Here is a situation I have come across that I would like to get a little help with.

When figuring out my conduit runs on different projects, I plan for conduit fill and the number of current carrying conductors and derate accordingly. My question is this: When figuring voltage drop into the mix, are there any other steps involved?

example: I need 8 current carrying conductors, all #12 THHN, in one conduit and derate using 70%. My understanding is I can still use the #12 because the insulation is rated at 90 degrees and at 70% the wire can still handle 21 Amps (If I'm wrong, please let me know). Now because of the distance, I need to make the wire #10. Now, do I have to adjust anything else (except conduit size) or am I ok because I derate for conduit fill first and correcting for voltage drop is makes my wire larger anyway? (derated 8 #10 THHN can handle 28 amps) or am I just not thinking right?

 

[iwire]

Re: Derating for Conduit Fill vs Voltage Drop

I would do it this way.

1)Calculate conductor size needed after derating for conduit fill.

2)Calculate conductor size needed for the voltage drop you are willing to accept.

Now use the larger conductor size from 1 or 2.

Done.

 

[charlie b]

Re: Derating for Conduit Fill vs Voltage Drop

And don't forget to up-size the EGC proportionally to the up-sized phase conductors. That may impact your choice of conduit.

 

[pierre]

Re: Derating for Conduit Fill vs Voltage Drop

Charlie
What if he is using the conduit as the equipment gounding conductor (path)?

Pierre

 

[iwire]

Re: Derating for Conduit Fill vs Voltage Drop

Originally posted by pierre:
What if he is using the conduit as the equipment gounding conductor (path)?

Luckily 250.122(B) does not apply to raceways.