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#181158 09/24/08 01:18 AM
Joined: Aug 2005
Posts: 265
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Sixer Offline OP
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We're installing receptacles for 120 volt roof heating (snow melting) cables. These must be GFCI protected. We're having some discussion on how to wire these up.

What I want to do is install split duplex receptacles on the eave, and have 2 heating cable on each receptacle. Each of these receptacles will be protected by a double pole GFCI breaker. This way we will only need to run a 3-wire to each receptacle and split the hot tie-bar.

The argument is, if the wattage of 2 heating cables in one receptacle are different, will the GFCI trip? One of my employees thinks the load needs to be balanced and the heating cables each need to be on their own single pole GFCI breaker.

I disagree - I say regardless of what either load is, the neutral will carry the unbalance, and the GFCI won't "see" the difference. The GFCI breaker should act the same as if it were protecting a hot-tub, where the heating load is on 240 volt and the pump, lights, etc are on 120 volts.

Am I correct? I want to ensure this will work before we install the wiring next week.


Sixer

"Will it be cheaper if I drill the holes for you?"
Sixer #181161 09/24/08 04:44 AM
Joined: May 2005
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The GFCI breaker will see the imbalance, and trip. Remember that the GFCI is looking at the difference between the current on the hot and the neutral on the load side of the GFCI.

I've seen a similar installation with a split outlet below the kitchen sink. Split wired, with one side switched for a disposal (fed from a GFCI breaker), and the other side unswitched for a dishwasher (fed from a separate circuit),
using a shared neutral.

It went unnoticed for almost 20 years, until somebody actually tried to use the dishwasher circuit. The GFCI would trip every time, with a very small load on the non-GFCI side. The problem was compounded by the fact that the bathroom on the other side of the wall was also fed from the GFCI circuit, while the second bathroom shared a non-GFCI circuit further down the wall. The solution was to install GFCI outlets where needed, and remove the GFCI breaker.

You will want to run 2 wires for each circuit.

techie #181163 09/24/08 07:12 AM
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If you use a double-pole GFCI breaker designed for this purpose, you'll be fine. The CT will simply enclose both hots and the neutral at the same time, and will still balance the net current, which will be 0 regardless of unbalanced current on the neutral unless there is actual leakage to ground.

You could potentially also install two GFCI receptacles and feed them both with a single neutral, but you would have to split the neutrals on the load side of the GFCI receptacles and keep them segregated from that point on.

What you can't do (and hopefully wouldn't try!) is to share the protected neutral of the GFCI circuit with another circuit like in techie's kitchen.

SteveFehr #181174 09/24/08 01:26 PM
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JBD Offline
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Techie's situation is an good example of how a GFCI compares the current going out to the current coming back in. If there is more than 5ma difference, either too little or too much, the GFCI will trip. Even though a GFCI has the word "ground" in its name, ground/dirt has nothing to do with its operation (unless part of a fault current path happens to include it).

techie #181175 09/24/08 01:45 PM
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Sixer Offline OP
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Originally Posted by techie
The GFCI breaker will see the imbalance, and trip. Remember that the GFCI is looking at the difference between the current on the hot and the neutral on the load side of the GFCI.

I don't see an imbalance here. Example, heating cable #1 draws 7 amps, heating cable #2 draws 10 amps. Neutral would carry the difference of the two, which is 3 amps. As Steve says, the CT will enclose all 3 wires and read "0" unless there's leakage to ground. The example you gave has the dishwasher on a separate circuit (not on GFCI). Because the neutral was shared between the GFCI protected circuit and the other circuit, there would be an imbalance - the GFCI breaker wouldn't "see" the current in the hot wire for the dishwasher, but would "see" the neutral current.

No offense Techie, but I'm siding with Steve on this one - at least until someone can convince me otherwise. I'm quite sure that this will work using 3-wire and a double pole GFCI breaker and splitting the hot on each receptacle, but I want to be 100% sure. It makes for a good argument.


Sixer

"Will it be cheaper if I drill the holes for you?"
Sixer #181181 09/24/08 04:00 PM
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JBD Offline
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Originally Posted by Sixer
I'm quite sure that this will work using 3-wire and a double pole GFCI breaker and splitting the hot on each receptacle, but I want to be 100% sure. It makes for a good argument.


Absolutely, it will work with (1) double pole breaker and 3 wires.

Absolutely, it will not work if (2) single pole breakers are used unless 4 wires run.

Last edited by JBD; 09/24/08 04:03 PM.
JBD #181182 09/24/08 04:21 PM
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A 3-wire circuit with a two-pole GFCI should work the same as with any other 240V GFCI CB installation.

Interesting and opposite for comparison would be when a dead front GFCI is used to feed two single-pole switches with one 3-wire for a bath fan/light combo over a tub. It would seem reasonable that there could be a problem with nuisance tripping, but since they are fed from the same 120V circuit, there is normally no issue. Although… I have heard that excessively long runs of the 3-wire in this instance may cause problems.



KJay #181203 09/25/08 06:56 AM
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In this case, I'd suspect excessive leakage through the fan would be the real culprit for tripping the GFCI.

SteveFehr #181217 09/25/08 08:01 PM
Joined: Nov 2000
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You may have an issue using a GFCI to protect these cables. The code rule requires GFP protection which trips at 30 mA, not the 5 mA that a GFCI trips at.


Don(resqcapt19)
Joined: Nov 2007
Posts: 763
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Yes, but he probably wouldn’t even need GFCI protection for those 120V outlets if they are mounted over 6’6” above grade. Most of the low-density 120V and 240V heat tapes I deal with usually come with their own factory installed GFCI and/or GFP protection anyway.

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