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Old 05-18-2010, 10:11 AM   #1
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Hey all,

My buddy tore up the feed to his garage with a bobcat, now he wants to run a new feed. The existing was two 12-2 UF in a heater hose.

He's thinking about pulling a 20-amp MWBC with #12 THWN in PVC. He would also like to run a switch loop back to the house for an outside light, which is what the second UF was for in the old arrangement.

This would be 6 #12s in a conduit. 2 hots, neutral, ground, and switch loop. Does anyone know if this would need to be derated? I think that would be considered 5 current-carrying conductors, and I can't find a clear answer on when derating comes into play.

Thanks for any help,

-McSteve

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Old 05-18-2010, 10:25 AM   #2
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In that setup you would only have 3 current carrying conductors.
2 hots-1 each
Neutral-not counted in a mwbc
ground-never a current carrier
switch leg-1 current carrying

So a total a 3 so no reduction needed, at least in the Canadian Electrical Code.

Maybe someone else will respond about the rules of running power to an out building, there are certain rules but i don't know them off the top of my head.

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Old 05-18-2010, 10:27 AM   #3
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More than 3 conductors in a raceway need to be derated according to code. Conductors are derated using the 90 deg C column of 310.16. #12's are derated from 30 amps by this column. 4-6 conductors are derated to 80%. Therefore your #12 are derated to 24 amps, but can still only be on a 20 amp breaker.

I also agree with Darren, so you're ok either way.

Last edited by brric; 05-18-2010 at 10:29 AM.
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Old 05-18-2010, 10:28 AM   #4
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Where are you located ?
Might want to run a 1" conduit in case he (or future owner) wants to put a sub-panel out there
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Old 05-18-2010, 10:31 AM   #5
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Hm, seems to me that a switch loop should be counted as 2 CCC, since both wires of the switch leg are actually carrying current, but I know the code can be a little weird in places. I'll have to do some more searching I suppose.

Codewise, here under the NEC a detached structure can be fed with an MWBC, with a double-pole light switch as a main disconnect at the structure. I'm still not 100% clear on running a switch loop back to another structure, but so far I haven't found any rule against it.

Thanks for the response!
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Old 05-18-2010, 10:37 AM   #6
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Oops, I replied before I saw all the new posts. Sounds like he's good to go with this plan then. I already talked him into running at least a 1" conduit so he can upgrade to a 60-amp sub-panel in the future. No one likes digging trenches
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Old 05-18-2010, 10:55 AM   #7
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Sorry i read it wrong, the switch loop would count as 2 CCC so you would have 4 and then you would have derate by 80%. So for a 20A circuit you would need to use #10.
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Old 05-18-2010, 11:06 AM   #8
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This is getting a little confusing. Maybe some NEC/CEC differences are in play here?

Figuring that there's 4 CCC in the conduit, derating would be needed. So, per NEC, do we de-rate from the 90C ampacity of 30A, or from the maximum allowed OCPD size of 20A?
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Old 05-18-2010, 11:11 AM   #9
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Quote:
Originally Posted by McSteve View Post
This is getting a little confusing. Maybe some NEC/CEC differences are in play here?

Figuring that there's 4 CCC in the conduit, derating would be needed. So, per NEC, do we de-rate from the 90C ampacity of 30A, or from the maximum allowed OCPD size of 20A?
Are you in US or Canada?
There are differences, I'm thinking US since you mention NEC for derating

My understanding is you derate from the ampacity - 30a

And there are new rules regarding the switch loop
As I understand: the light needs to be fed from the garage, but can be switched from the house
Is that how it is already setup (since you mention switch loop only) ?
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Old 05-18-2010, 11:11 AM   #10
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Run a 1" conduit and forget about derating. It makes for an easier pull.

Dave has made some excellent remarks right above. In particular the switch loop and where it originates. (It must come from the garage or unattached structure).
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Old 05-18-2010, 11:18 AM   #11
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Scuba Dave, my bad, I thought for sure I had my location set. We're in the US, Minnesota.

As far as the switch loop, my friend isn't sure how it was originally set up, and I haven't been over to look yet. Either way it'll be re-wired as a switch loop fed from the garage, which I believe is how it has to be done these days.

J.V., this is going to be a 1" or even 1-1/4" conduit, but my understanding is that conduit size has no effect on de-rating. Something to do with all the conductors still being bunched together, even if they're laying on the bottom of a 4" conduit...
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Old 05-18-2010, 11:28 AM   #12
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OK, had to look up the derating

With 6 current carrying you derate by 80% x 30a = 24a
So Ok to use #12

With 7-9 you derate by 70% x 30a = 21a , still good to go
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Old 05-18-2010, 12:34 PM   #13
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I agree with Dave now, see my post further down.

Last edited by darren; 05-18-2010 at 12:50 PM.
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Old 05-18-2010, 12:45 PM   #14
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We are not talking about continuous loads. Derating is done from the 90 deg column of table 310.16 of the NEC. #12 THHN is derated from 30 amps not 20 amps. #10 is derated from 40 amps not 30 amps.

Last edited by brric; 05-18-2010 at 12:47 PM.
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Old 05-18-2010, 12:49 PM   #15
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That explains it, I didn't realize you have #12 wire that is good for 30A. We don't get ampacity like that until we get to 110 degree rated wire(which is rare).

So I take back my comments and agree with Dave.

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