Please comment on this wiring approach

kudzu9November 7, 2013

I am putting in a small bathroom. Because it will be in a separate, studio building and not used much, I plan to install an on-demand water heater. The heater requires a pair of 8-2 w/g cables and two, 40 amp, double pole breakers in the subpanel. I also know I need a separate 20-amp circuit to service the receptacles in the bathroom. Here is what I am thinking of doing.

Because running and fishing wire will be a challenge, I'd like to minimize the number of cables. I'm thinking of running two 8-3 cables from my subpanel to a junction box close to the heater. In the junction box I would remove the outer jacket from the two 8 gauge cables, and split off one conductor from each to which I would pigtail 12 gauge Romex; this would be my separate 20 amp circuit. I would also connect the ground wire in the 12 gauge Romex to one of the grounds in the 8 gauge cables. The remaining two pairs of the 8 gauge, along with the two 8 gauge grounds, would continue out of the junction box through conduit or armored cable for a foot or two where they would be connected to the heater. Back In the subpanel, the two 8 gauge conductors I split out for my 20 amp circuit would be protected by a 20 amp breaker. Is this ok? And particularly, do I need to run a single ground wire back from this junction box to the breaker box, or is it ok to just have a ground for the 20-amp circuit that starts in the junction box where it would be connected to one of the 8 gauge grounds that started at the subpanel?

Any thoughts are appreciated. Thanks.

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bus_driver

Code violations galore.

    Bookmark   November 7, 2013 at 11:31PM
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kudzu9

busdriver-
Thanks. I posted for input, but it would help if you could be specific. Or better yet, tell me what you would do so that it was code-compliant.

    Bookmark   November 7, 2013 at 11:53PM
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jreagan_gw

Start with running the right number of wires. If you can find space for two 8/3s then you can find space for two 8/2s and a 12/2.

    Bookmark   November 8, 2013 at 7:57AM
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kudzu9

Ok, I'll figure out how to run a separate 12/2 cable. Still, I'm curious to know what was wrong with my original proposal. Is it only that you can't have a shared ground wire between two different sized circuits, or what? Thanks.

This post was edited by kudzu9 on Fri, Nov 8, 13 at 12:49

    Bookmark   November 8, 2013 at 12:48PM
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jreagan_gw

For starters, multi-wire branch circuits are to be single phase only 240.15(B)(1) plus you have no single way to turn it all off 210.4(B).

    Bookmark   November 8, 2013 at 4:08PM
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kudzu9

jreagan-
Thanks...I understand now.

    Bookmark   November 8, 2013 at 4:16PM
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jreagan_gw

And if your hot water heater isn't within sight of you subpanel, then you'll need disconnects for those 2 lines. You don't want to be working on the heater and have somebody turn the breakers back on (or you can put locking tabs on the two breakers)

    Bookmark   November 8, 2013 at 5:15PM
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kudzu9

jreagan-
Thanks, again. It's 15' away in the same room.

    Bookmark   November 8, 2013 at 5:41PM
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dudleydorightdad

1st, all conductors for a given circuit shall be run in the same cable or conduit.
2nd, you may run NM cable in conduit but you may not strip the outer sheathing from it and run it in conduit.
3rd, a number 10 equipment grounding conductor will serve 60 amps.
Best route. Run a piece of 1" emt from your sub to your general bath area. It will have adequate cross sectional area to allow you to run 2#8 x 2 for 4#8, 1 #10 for ground although technically the conduit may serve as the ground, and a pair of 12's for your required 20 amp bathroom circuit. Even 3/4 will allow 6#8's which will handle 4-8's,2-12's and a 10, using the .213 sq. in. allowable 40%fill and the above combination adding up to .1941 square inches with the conductors being type THHN. But 1 inch would be the smarter choice to allow easier wire pull. You would still have to make sure your derating for fill leaves you adequate ampacity.

    Bookmark   November 11, 2013 at 11:10PM
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kudzu9

dudley-
Thanks..that was really helpful.

    Bookmark   November 12, 2013 at 1:25AM
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