Expert: Master Bob - 3/4/2006

Question
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Followup To
Question -
Dear Bob,

I am wiring my bathroom re-model... I have a six gang switch box... six switches divided among two circuits.  One circuit will be a fan that takes its own 20 amp breaker.  The other circuit is just lights and I don't know the amp of the breaker yet.  Anyway, I'm using all 12 gauge wire.  How many 12 gauge wires can I fish through a 3/4 inch flex conduit?  I'm sure there's a limit, but I don't know what that is.  Also, is there a rule against fishing the feed wires (from the service panel to the switch box) and the hot leads from the switches to the appliances in a single stretch of conduit?  This would all get sorted out in a big junction box in the attic space.  Thanks so much, Bob.  

Best,

Paul Myers
Answer -
Hi Paul,

 Just getting back into my office...been one very LONG week here electrically....straight out.

  Based on your description...can I assume by "flex" you are referring to flexible metal conduit (FMC)..? If so...you can (per Code table C3) put up to 16 - #12 THHN copper wires in a run of 3/4 " flex. (the count will differ depending on wire insulation type...I used THHN in my example because it is the most widely used and readily available). The count is based on "fill" calcs....and I HIGHLY recommend you go lower then the maximum allowed...(such as 10 or 12 total wires max) or it will make for one tough pull....especially if it's a long run.  Please state whether you mean metal flex (as above) or plastic flex ( electrical non-metallic flex or ENT) or liguid tite flexible non-metallic (or LFNM), etc., etc......there are several types of "flex" on the market. Also...what wire "type" you intend to pull into this 3/4"....it all makes a difference as to total wires allowed.

 Not sure why you'd want to run both - your feed wires from the panel and the switched legs from the 6 switches ....all in the same conduit....that is somewhat unusual -  I take it the attic is the best place to install an accessible junction for these feeds and switched legs..? Sounds like you'd only need 2 - 20 amp  circuits...one for the fan...and one for the lighting (and one 20 amp run for the Code required GFCI circuit - if not already run). You might want to consider bringing the 20 amp branch circuit feeders up from the panel to the j-box in a 1/2" flex (it would only need to have the 5 wires - 2 hots, 2 neutrals and 1 shared ground - you can go up to 9 - #12's in 1/2")....hitting the J-box...and run the power down to the switches in 1/2" flex....and the switched legs back up in a 3/4"...if that's possible. Would make for a much easier to follow layout and keep the hot feeds and switched legs seperated...but you may have other ideas..?

 I'm sure you have a plan Paul....hope this helps - if you need more details, please follow-up.

Best Wishes,

Bob Osgood
Master Electrician

RKO ELECTRIC

www.rkoelectric.com

Hi Bob!

Wow!  What a terrific and complete response!  Much, much appreciated!  While I have a well above average set of skills and knowledge about building (for a civilian) I just haven't had enough practice with electrical so I apologize in advance if I seem a bit 'out there'.  Naturally, it's the one trade that presents some real danger.  Anyway, just to pursue this a bit more...

I am using aluminum flex conduit and metal boxes for everything except a short run of Romex for one of two sconces.  There's a problem fitting the conduit in a space shared by a vent pipe... so I need something that takes the least amount of room.  

I don't know the type of insulation on my #12 wire.  It's probably pretty standard stuff since I bought these spools at Home Depot.  It also sounds moot because it appears that I will be well below the number of wires in a single conduit.  

RE: my plan to mix switch legs and power feeds in a single conduit...  Well, this has more to do with sorting out the chaos of this in my brain than anything electrically.  I thought I'd run the power and switch legs of the fan circuit in a single conduit that connects the wall switches and the j-box in the attic.  The same with the lighting circuit.  This would then keep the two circuits, lighting and fan, segregated each in its own conduit.  I don't blame you if you're rolling your eyes, but somehow this seems more organized for me... especially since I don't have that many colors for switch legs.  

Regarding bringing the power from the service panel up to the J-Box... here's where I'll probably show my inexperience.  Each circuit -- GFI for outlet, lighting, and fan -- would have its own hot, neutral, and ground.  That's a total of 9 wires.  I didn't know you could share a ground but even so it seems nice for every circuit to have its own ground wire (eyes rolling again?).  Since I now know you can stuff that many wires in a 3/4 conduit, I'll probably try that.  If that's too tough to pull, I'll use an additional 1/2 conduit for the fan cirucit.  

Thank you again!  I'm sure I will return with more questions!  And if there's anything that I've described that raises a red flag, please don't be bashful about letting me know.  

Best,

Paul Myers  

Answer
Hi again Paul,


Thanks for the kind words here and at the ratings....glad I could help out some.

 Yes...your layout sounds fine...and you can always run the seperate grounds (one for each circuit)...although it's not required....each switch/device can tap to the one main ground wire....as well as the J-box and 6 gang switch box...if metal.

 Sounds like you have a clear and solid plan Paul...if you run into any problems...drop me a line - I'll try to help all I can.

Best Wishes,

Bob