derating of conductors

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scotty

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This is a two part question. First derating of conductors, I have one three phase feeder that consist of 4 4?/0 alum.+ ground, and 3 single phase feeders that consist of 3 2/0 alum + ground. I would like to fuse the 3 phase at 125 amps, and the single phase at 100 amps can this be done?? I will be installing this in a 3 1/2 PVC pipe and a trough (over 24 inches long) to a set of two meter banks. The insulation will be all be same. Second when using the temperature rating chart can I use the 90 degree column for this, and if not what some examples for it's use,some fellow workers seem to think the 90 degree column is good for every use I disagree! Thank you for your help. :cool:
 
Re: derating of conductors

As far as the derating goes, I will let the experts handle that, but it sounds like you are fine with the breaker values you mentioned. In fact, your wire size sounds like major overkill. :cool:
[/quote]More than likely, you will have to use the 75 degree column. It depends on the terminal rating of the equipment. Most breakers, panels and metering equipment are rated for 75 degrees. Some breakers are dual rated 60/75.

You are correct if you disagree with your coworkers, because they are wrong. The 90 degree column is used for derating purposes only.

[ December 21, 2004, 09:56 PM: Message edited by: peter d ]
 
Re: derating of conductors

I'm a little confused about the single phase feeders. you said you had 3 + ground. Is this 2 Phase and 1 Neutral?

You need to see Art 310.15 (4) and figure out how many current carrying conductors there are.

As far as the 90 degree column, only use this for derating. Typically, lugs are rated at 75 degrees so Art 110.14(C) needs to be looked at.

It looks like you have a 3 phase load and 2 Single phase loads with a common Neutral so that would be 5 Current carrying conductors. Derate to 80%. If the wire you are using is rated at 90 degrees you can use that value to derate from.

4/0 Aluminum = 205A x 80% = 164A
2/0 Aluminum = 150A x 80% = 120A

If there were up to 9 CCC's in the conduit it would look like this:

4/0 Aluminum = 205A x 70% = 143.5A
2/0 Aluminum = 150A x 70% = 105A

Either way It looks like you're OK
 
Re: derating of conductors

kpepin, for de-rating conductors, I thought that even if the conductors insulation is rated at 75 deg C, you can still start from the 90 deg C column of table 310.16, but if the de-rated ampacity of the conductor is greater than the 75 deg C column rating, you must go with the lower of the two values. I was looking for a code reference, but cannot find one. Anyone out there support or refute me? :confused:
 
Re: derating of conductors

I?ll refute thee. See the last sentence of 110.14(C). The conductor has to have a temperature rating higher than that of the terminations, in order to use the higher temperature as the starting point for calculating ampacity.
 
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