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Old 10-03-2011, 05:33 PM   #1
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under floor heat on a 3 wire


A calorique under floor heating system (110V) that I am instaling has A thermostat with a built in gfci. The nearest Junction box I can tap into is running a 3 wire with a ground 12G. red and black hot and a shared common white for both circuits. I believe if I use one of the hot leads the with the shared common the gfci will trip. Am I correct ? and if so can anyone recommend an alternative.

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Old 10-03-2011, 05:37 PM   #2
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under floor heat on a 3 wire


No it will not trip the GFCI but I do not recommend it. I would run a new circuit sized to the wattage of the heat mat. What does the instruction manual say? Often they specify a dedicated circuit.

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Old 10-03-2011, 05:41 PM   #3
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Doesn't that thing come with an installation manual?

Try this:


http://www.calorique.com/knowledge-c...nfo/index.html


Their website is about as useless as boobs on bacon.
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Old 10-03-2011, 05:47 PM   #4
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under floor heat on a 3 wire


Thanks for responding.

They do recommend a dedicated circuit But I have no other available options without tearing into the walls and ceiling to run a new line.

The box is grounded by the ground on the 3-way and another non dedicated line that comes into it. I have heard that I coud use the ground in the 3 wire as the neutral-- like splitting it into two circuits but I think this is bad practice.
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Old 10-03-2011, 06:01 PM   #5
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under floor heat on a 3 wire


Quote:
Originally Posted by dradam View Post

The box is grounded by the ground on the 3-way and another non dedicated line that comes into it. I have heard that I could use the ground in the 3 wire as the neutral-- like splitting it into two circuits but I think this is bad practice.
You cannot use the ground and BTW it is already two circuits
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Old 10-03-2011, 06:02 PM   #6
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under floor heat on a 3 wire


Quote:
Originally Posted by dradam View Post
Thanks for responding.

They do recommend a dedicated circuit But I have no other available options without tearing into the walls and ceiling to run a new line.

The box is grounded by the ground on the 3-way and another non dedicated line that comes into it. I have heard that I coud use the ground in the 3 wire as the neutral-- like splitting it into two circuits but I think this is bad practice.
You really may need a dedicated circuit. What is the total wattage of your installation? I did it in my master bathroom and it was over 1100W. Definitely not something that could be shared with a receptacle circuit.

You cannot use the ground conductor in a cable as a neutral, it is dangerous and a code violation. Why would you want to? There's no problem installing a GFCI on a multi-wire branch circuit (MWBC, multiple hots sharing one neutral).
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Old 10-03-2011, 06:07 PM   #7
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under floor heat on a 3 wire


Thank you all. Rj niles above agrees that it would not trip the gfci but seems to recommend against usung the 3 wire. Total wattage is 540.

who is correct?
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Old 10-03-2011, 08:54 PM   #8
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under floor heat on a 3 wire


You really need to determine if either of the other two circuits has reserve capacity to add 540 watts of load and what the circuits are serving.
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Old 10-03-2011, 08:55 PM   #9
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under floor heat on a 3 wire


Quote:
Originally Posted by Bud Cline
Doesn't that thing come with an installation manual?

Try this:

http://www.calorique.com/knowledge-c...nfo/index.html

Their website is about as useless as boobs on bacon.
You are supposed to throw those away I thought.
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Old 10-03-2011, 09:44 PM   #10
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under floor heat on a 3 wire


I can isolate one of the 3 wire hot circuits so it has no other load. The other hot of the 3 wire and the other circuit in this box can take up the other loads.

I still just don't know if the common neutral is a problem.

how much load can the shared neutral handle ? with the 2 hot leads to different loads does the neutral see both sets of loads? or is it like two conpletely different circuits.

thanks
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Old 10-03-2011, 10:01 PM   #11
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under floor heat on a 3 wire


If you can separate the circuits so one side of the MWBC is dedicated to the heating mat then you will be fine. When wired correctly, the neutral will carry the difference current of the two circuits but never more than what one side by itself will use. In the junction box were the MWBC terminates you will have three white wires connected, the feed neutral and the neutral from each continuing circuit.
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Old 10-03-2011, 10:09 PM   #12
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under floor heat on a 3 wire


Check if this 3-wire is on a 2-pole breaker in the panel also.
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Old 10-03-2011, 10:27 PM   #13
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under floor heat on a 3 wire


thank you all for your help.

to Corsair-- lost me on the part about the 3 white wires in the breaker box.

if I use the one hot lead (red)for the mat, the other (black) for the other loads and share the white common--won't this common be the only white lead coming back to the breaker box for these two circuits ?

sorry if I am being thick.
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Old 10-03-2011, 11:33 PM   #14
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under floor heat on a 3 wire


He meant at the junction box, where you’ll be tying onto the circuit from, for the cable for the mat, not the breaker panel. Yes the breaker panel will only have the 1 white from this circuit.
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Old 10-04-2011, 07:42 AM   #15
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under floor heat on a 3 wire


ok, got it thanks.

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