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Old 03-17-2011, 12:35 AM   #1
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Wiring Options for 2 Bathroom Heaters


Hello! I have a fairly good background in electrical work, but I have to ask about a situation I am considering. I am installing 2 vent/heater/lights in 2 bathrooms that are right next to each other. They both require at least 20 amp 120 volt dedicated circuits. It is easy enough to run two dedicated 12/2 circuits from the service panel, but I am wondering about another option. I have a sufficient length of 10/3 (with ground) cable that I pulled out of my house that was once supplying a dryer. I also have a 30 amp double pole breaker. I am wondering if code allows me to use this cable to feed these two units. I would run the cable to a (accessible) junction box in the attic between the two units. They would only be about 3-4 feet apart. I would then use 12/2 to break out one of the 120 volt feeds to each of the two units. The neutral would be shared between both. They would both be grounded through the cables ground wire. They would be connected to a single double pole 30 amp breaker. Can anyone see any problems with this setup? The only place I could see any problem is with the shared neutral. Setting it up this way would save me over $135 in wire cost. Thanks in advance for your help!

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Old 03-17-2011, 12:48 AM   #2
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Wiring Options for 2 Bathroom Heaters


Yes I see a problem. You are using a 30 Amp breaker for 12 ga wire.

Use a double pole 20a with the 10/3 wire. You MUST use a double pole breaker, NOT a tandem when sharing a neutral.

Other than that I dont see a problem with it.

Consider getting some higher temp wire nuts or something that can handle a continuous load and the heat buildup.

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Old 03-17-2011, 12:53 AM   #3
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Wiring Options for 2 Bathroom Heaters


Oh, right! I didn't even think that there would be 12 gauge wire in the circuit! What do you mean I can't use a tandem breaker? It looks like I will have to buy a 20A breaker to protect the 12 gauge that would be in the circuit. I was planning on a double pole, the kind with two switch handles that are tied together. Is this what you mean I can't use?
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Old 03-17-2011, 01:27 AM   #4
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Wiring Options for 2 Bathroom Heaters


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Originally Posted by Micah View Post
Oh, right! I didn't even think that there would be 12 gauge wire in the circuit! What do you mean I can't use a tandem breaker? It looks like I will have to buy a 20A breaker to protect the 12 gauge that would be in the circuit. I was planning on a double pole, the kind with two switch handles that are tied together. Is this what you mean I can't use?
A tandem breaker is the same size as a single pole breaker, but allows for two 120v circuits that are on the same leg.

A double pole breaker is the size of two single pole breakers and allow for one 240v circuit, or two 120v circuits when there is a neutral this is also called a multiwire branch circuit.
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Old 03-17-2011, 01:32 AM   #5
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Oh, thanks! I have seen these "tandem" breakers before. Is their only purpose to make more room in a full service panel? Does a tandem breaker open both circuits if EITHER of the 120V feeds exceeds the breaker rating?
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Old 03-17-2011, 01:50 AM   #6
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Wiring Options for 2 Bathroom Heaters


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Oh, thanks! I have seen these "tandem" breakers before. Is their only purpose to make more room in a full service panel? Does a tandem breaker open both circuits if EITHER of the 120V feeds exceeds the breaker rating?
There only purpose is to add circuits to a full panel. They act as separate breakers, only the circuit with the overload will trip. There is no voltage between them, they are on the same phase.

The reason they can not be used for the circuit you want to wire up, is the two 20A loads off the same phase sharing a neutral, would mean there is 40a on the neutral.

With a double pole breaker (multiwire branch circuit) the neutral only carries the difference of the two loads.

http://www.diychatroom.com/attachmen...re-diagram.jpg
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Old 03-17-2011, 01:55 AM   #7
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Thanks for the knowledge! I think I am going to go ahead an use this 10/3 cable that I have for this project unless anyone else has a valid objection. I am not looking forward to trying to fish #10 wire up to the attic though.
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Old 03-17-2011, 03:39 AM   #8
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Wiring Options for 2 Bathroom Heaters


The issue I can see when you bring in the 6mm˛ { #10 AWG } is the junction box size just stay away from handi boxes get the 4X4X2 1/8 box and split it off to the switch box then to the heater this will work the best and oui you will have to change that to 20 amp breaker and make a note of that also make sure you have red , black et white and bare or green ground conductor in there. { If not then you will have new issue with it}

to pull 6mm˛ triplex NM cable { 10-3 NM } is not a super hard item to pull but it will take little more effort than what you used to with 4.0mm˛'s { 12 AWG }

From the switch to the Heater/Fan/Light you will need a 4 conductor cable you may find them in big box store. { either 12-4 or 12-2-2 cables }

Merci.
Marc

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