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sluggermike 09-02-2007 12:32 AM

feed back voltage when using double breaker
I'm trying to wire a garbage dispsoal and dishwasher using 12ga, three plus ground romex wire on two 20 amp breakers. The there are two hot wires, a red and black wire and one common white wire plus a copper ground. I've notced that that when I turn them both on I get 120 volts. When I turn one off at the breaker, I get 120 volts on one line and 40 to 50 volts on the breaker wire that has been turned off. I have even disconnected the wire from the breaker that is off and noticed that I still get 40 to 50 volts from that wire when I checked it with my volt meter. There seems to be a feed back from the common white wire that is used by the both circuits. Is this normal? If not do you have any suggestions about what is wrong?

Mike Swearingen 09-02-2007 12:49 AM

I'm not a pro electrician, just a long-time DIYer.
I do not think that you should be doing what you're trying to do with the shared neutral and two single pole breakers. It isn't safe.
As I understand it, most local electrical codes call for dishwashers to be on separate dedicated circuits. A dedicated dw circuit and a separate disposal circuit can be 15 amp circuits wired with 14/2+g, but a 20 amp circuit wired with 12/2+g should work just fine for both a disposal and a dw, IMHO.
You only need to connect one black (or red) hot to a single 20 amp breaker and the same color wire to the dw/disposal end of the circuit, and cut and tape the other hot wire off with good electrical tape on both ends of the circuit. Should solve your problem.
Now, listen to the pros here for the proper way to do it. That's just what I would do with where you're already at.
Good Luck!

sluggermike 09-02-2007 03:46 AM

Feed back voltage
Thanks for your suggestions. It is my understanding that if you use a separate breaker, that each one is a dedicated circuit even if you are using the same common white wire. If what you are saying is true, then there would be no reason to use romex with two hots and a common.

Speedy Petey 09-02-2007 07:37 AM

You can do it the way you intend. It is safer for a DIYer to use a two pole breaker for the exact reason of your problem. Also a two-pole breaker ensures both circuits are off if you need to work on any oart of the multi-wire circuit.

Your problem is that you have the two single pole breakers on the same leg in the panel. With a multi-wire circuit you MUST be sure the two breakers are on different legs, if not the result is your exact problem.

DW's do not typically have to be dedicated by code, although some areas do require it I'm sure. It is VERY common to have the DW and disposal on a shared circuit. I do this all the time.

Stubbie 09-02-2007 12:53 PM

Speedy has explained why your getting those test readings. I'd like to make sure you are getting the wiring right after you make the change with the breakers to get them on opposite legs.

Can you take the time to explain how your wiring this after the breakers?

Are you using a duplex receptacle to plug both the dishwasher and disposal into and switching the disposal from a counter-top switch.....that sorta description.

Or are both hardwired?

Or is the disposal plugged in to a receptacle and switched and the dishwasher hardwired?


sluggermike 09-02-2007 02:57 PM

Feed back voltage
The 12/3 has already been run and it would be very difficult to run new 12/2 romex for each breaker. The two hot wires on are on different legs of the panel. I still get 40 to 50 volts from the off breaker and 120 volts from the on breaker. When I disconnect the wire from the off breaker, I found that I still got the 40 to 50 volts from the wire. The only connection seems to be the white, common wire which leads me to believe that there must be a voltage feed back.
After the the breakers the 12/3 romex goes to an electrcal box where it is then separated into the two different circuits. The dishwasher and garbage disposal have not been hooked up yet, but I plan on using a duplex receptacle with the disposal being switched.
Thank you all for your help

Speedy Petey 09-02-2007 04:29 PM

Could this be a GE or FPE panel?

sluggermike 09-02-2007 04:47 PM

Feed back voltage
I believe it is a Square D panel.

Stubbie 09-02-2007 08:03 PM

I had to read this a couple times but it finally sunk in that your not having any problems your just asking if it is normal to have that 40 to 50 volts on the breaker in the off position or on the wire when you disconnect it from the breaker.

YES that is normal. Digital voltmeters are fine if you learn to ignore these "phantom" readings. I assume you are testing to the neutral bar. Remember the neutral bar has all the other 120 volt circuits neutrals connected to it. If you use a solenoid tester like a wiggy or a lighted two wire tester it will not register any voltage. If you measure 240 volts or there abouts between the two single pole breakers lugs your fine.


Speedy Petey 09-02-2007 08:36 PM

Could I have been misreading what the OP is asking?
I thought he is getting 120v across both hots. This happens when skinny FPE or GE two pole breakers are installed wrong.

Slugger, are you getting the normal "240" volts across the breakers with both breakers on? Or are you getting 120v across the two breakers?

Stubbie 09-02-2007 09:02 PM

I think so Speedy. Big on the 'think'. I had to go over it a few times but I'm getting that he is measuring 120 volts from each breaker in the 'on' position to the neutral bar but has not tested between them for 240 volts. He should do that for sure. He for some reason tested one breaker in the off position and got 40 or 50 volts on his digital. then took the wire loose from that breaker and tested the wire to the neutral bar and still got 40 or 50 volts.


sluggermike 09-02-2007 09:50 PM

Feed back voltage
I did what you said. I measured the voltage from the one wire after I disconnected it with the volt meter to the neutral bar and got around 40 volts. I did not check the voltage on the two lugs for 240 volts, but since they are on different legs of the panel it should be 240 volts. When I tested the circuit that was off with my Greenlee voltage detector, it did not indicate there was any voltage yet the digital volt meter showed about 40 volts. What I don't understand is this only seems to happen when I use 12/3 romex.

Stubbie 09-02-2007 10:34 PM

Your testing a 12/3 not connected to any loads so in essence you are in series with an open circuit in your test method.

If you test otherwise to hot wires that are connected to loads even though they are disconnected from the breaker you may very well get little or no voltage.


sluggermike 09-03-2007 12:17 AM

I will check the voltage with a wiggy and let you know what happens. I am not familiar with the "phatom" voltage reading using a digital volt meter that Stubbie mentioned.
I really appreciate all the help.

sluggermike 09-03-2007 12:30 PM

This problem and the answers I've receive have been rolling around in my head for the last couple days. I think I've finally figured out what I really want to know. Is it normal for there to be a "phatom" voltage reading of around 40 volts from a digital volt meter on the off circuit when one circuit is on and one is off when using 12/3 romex? I noticed that it didn't seem to make a difference whether the two hots were on different legs of the panel. I still got the "phatom" voltage.

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