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I’ve been planning on rewiring my home for a while – it’s all ungrounded knob and tube. I’m fortunate enough to have a big attic and basement/crawl space (basement under half the house, crawl space under the rest), so there won’t be much wall work. My plan was to put the lighting circuits in the attic with NM cable, and wire all of the outlets from the basement/crawl space.
My question is about wiring from the basement/crawl space.
I was planning on running all of the wires in EMT conduit raceways, which is what was recommended to me by electricians and the inspector while we were discussing the plan during her inspection of my last job. From junction boxes in the raceway, I would run NM cable up into the wall to the outlets.
Anyway, it adds up to a lot of circuits. The max number of circuits running in any one location of conduit will be about 12. I’ve sized the EMT conservatively, so I’ll be using 1.5 inch EMT from the panel to a large distribution box, and 1 inch EMT from there to the outlets – that should be plenty of room. My problem is derating. The inspector never raised that issue on my last job, which used a lot of conduit and more than three current carrying conductors per raceway.
If I read the code right, I’m supposed to derate the wires by 50%!!! That means the 12ga wire I was going to use has to be protected by a 10amp breaker instead of a 20amp? Is that right? Am I missing something?
Please help! Thanks,
Chris Faye
Replies
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You start with the 90 degree rating of the wire, 30amps for 12gage.
20amps is 67% of that.
From the table you can have 9 current carrying wires in conduit.
I would run a 50amp feeder up to a 8 subpanel.
*Hi Chris,Isn't the codebook fun? Yes,you are correct, the more wires you put into conduit the less load you can put on them, and by the time you have 10-20 current carrying conductors in a conduit the allowable ampacity is down to 50%.But,no you won't have to put # 12's on a 10 amp breaker(even if they did make them).Remember you're talking about individual conductors here with different types of insulation available.I've never looked at what Lowe's sells but almost all of the #12 wire we pull is marked THHN/THWN.(T-Thermoplastic insulation,H-Heat rating,2HH's good for 90 degrees C.,W-moisture resistant,N-nylon jacketed)Now,look at the ampacity of # 12 THHN/THWN,it's 30 amps.That is what you derate.Can you put #12's on a 30 amp breaker?No,because of the asterisk that sends you to another section which says overcurrent protection for #12 shall not exceed 20 amps,30 amps for #10,etc.If your inspector says then that, with derating, your #12's are still only good for 15 amps and the Code calls for 20 amp circuits for the laundry,bathroom receptacles,kitchen small appliance circuits,etc.well then F.I. pull #10's (THHN/THWN rating 40 amps).OK?You could make a nice immaculate installation with all oversized conduits,but it's not for everyone.Inch and a half can't be bent by hand and bending box offsets in one inch takes some practice.Have fun in the crawlspace. :)
*Attic eh. Hmm. Elevated temperatures. Run away and hide. Its the dreaded double derate you hear sneaking up behind you.Attics can maintain temp greater than 150 degrees F for hours each day. Last I looked, I don't have a current code nearby, the temperature side of the dreaded double derate for THHN is 58% of rated capacity. Pipe fill of 4-6 conductors is 70%. Combined it works out at 40.6% of rated. #12 THHN is rated, but not fused, at 30 amps. 30 amps at 40.6% = 12.18 amps. To get to 15A capacity you need to go to #10 THHN. 20A will need #8 copper.The good news is that this is seldom enforced or viewed as a case that needs the thermal side of a double derate. Even if it is enforced it need only be applied to the portion of the runs in the hot space and can be avoided by routing the conduits inder the floors or in the walls.The fill problem points out why it is often better to run multiple spaller conduits than a single large one.
*Thanks for all of the help guys,I think I can figure out how to get everything in a couple runs of conduit and avoid the derating issue. Or - maybe run 10 ga to the main junction box and 12 ga from there - everything beyond the main junction box will be less than 9 conductors in conduit. I'd rather not make a subpanel - not for any hard reason I guess, I just prefer to have everything in one place if I can.FYI - in the attic, derating is not an issue. Only four lighting circuits up there, and none in conduit. It's all NM up there. Only the basement and crawl space present the issue.And yes - bending 1" is not fun, but I've had a fair amount of practice already. Most of what I need to do is straight runs and 90 degree elbows - nothing that complicated.Thanks again - you've been a lot of help.