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#95933 - 10/23/05 09:23 AM conductor derating
shkrg24 Offline
Junior Member

Registered: 10/22/05
Posts: 1
I need to run a 1" cond. from a 120/208V 3ph panel to an area of a bldg. to feed 9ea. 20A recep. circuits in clinical exam rooms. If I pull 3, 3ph and neutral sets with 1 ground, and use #10 wire to the first device on each circuit, does that cover the de-rating issues, or are there other factors to consider?

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#95934 - 10/23/05 09:36 AM Re: conductor derating
e57 Offline
Member

Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
Funny I just posted a topic with this same question....
http://electrical-contractor.net/ubb/Forum2/HTML/002340.html


[This message has been edited by e57 (edited 10-23-2005).]
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#95935 - 10/23/05 09:38 AM Re: conductor derating
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
If you run 3 - 3 phase multiwire branch circuits you will have 9 current carrying conductors.

Assuming you use 90 C conductors you can use 12 AWG all the way for these 20 amp circuits.
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#95936 - 10/24/05 05:09 PM Re: conductor derating
boggerbutt2454 Offline
Member

Registered: 02/27/05
Posts: 44
Loc: Concord NC USA
iwire
9 recept. circuits is 12 current carrying conductors as you forgot the 3 neutrals as well as the 9 ungrounded conductors.
10-20 conductors means your derate factor is 50%or 15 amps. You need to add another conduit. 240.4(D) #10 30 amps.

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#95937 - 10/24/05 05:17 PM Re: conductor derating
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
 Quote:
iwire
9 recept. circuits is 12 current carrying conductors as you forgot the 3 neutrals as well as the 9 ungrounded conductors


The neutrals are not current carrying conductors unless the major potion of the load consists of non-linear loads.

IMO the receptacles described are likely to be feeding linear loads.

Bob
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#95938 - 10/24/05 05:37 PM Re: conductor derating
e57 Offline
Member

Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
Bob as you mentioned in the other post of simular issue "Major Portion" is pretty broad.... 310.15(4) doesn't say anything about "Connected load", but dealing with recepticals you'll never know what they'll throw on down the line. Or for that matter what type of load balance you'll end up with.
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#95939 - 10/25/05 02:09 AM Re: conductor derating
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
The 'balance' is entirely irrelevant, the neutral may end up carrying 20 amps but it is still not a '"current carrying conductor" in regards to derating.

As far as the non-linear load issue that is up to the installer and inspector to determine.

As I said IMO the described receptacles "9ea. 20A recep. circuits in clinical exam rooms." Do not sound to me like the major potion of the load will be non-linear.

Your always free to count the neutral as a CCC if you want to, generally I do not.

Bob
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#95940 - 10/27/05 06:44 PM Re: conductor derating
cgw Offline
Member

Registered: 07/29/04
Posts: 133
Loc: Rochester NY
I agree with iwire. On the other hand if you are feeding non-linear loads I do not believe you should be using a common neutral anyway. In that case (separate neutral) the neutral would be current carrying.

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