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Need help with 220V circuit

Posted by criticalmass (My Page) on
Sun, May 29, 11 at 23:25

So there's a 220V air conditioner, and it's not running. It ran last year. I tested the voltage from white-ground, and black-ground, and there's about 0.30mv coming through each.

I went into the box and tried to substitute the 30A double-breaker with another 30A double-breaker, but that gave me the same result.

I checked the ground connection, as well as continuity of the white/black, and everything was fine. Stumped, I replaced the 30A double-breaker with two individual 15A single breakers, and it's working fine.

The odds that both of these double-breakers would be defective is too miniscule to calculate, but obviously I did SOMETHING to get it to work. I need some insight into what the problem could be.

Thanks, and have a great holiday!


Follow-Up Postings:

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RE: Need help with 220V circuit

Something is very screwy. A 240V unit should not have had "white" in the circuit at all. I also suspect it shouldn't have had a 30A breaker of any sort on the wiring.

It is unlikely that the dual breakers were the problem. But what you have with the untied singles isn't right either. It's impossible to tell remotely.


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RE: Need help with 220V circuit

"A 240V unit should not have had "white" in the circuit at all."

Very possible, though the white should have been re-marked as a hot.

30 mV readings indicate a broken conductor or connection somewhere.

Try measuring right at the pair pf breakers (on their output side).

If you have 240 V there, start tracing the lines.

You might turn off the power, pull the 240 V receptacles out of the box (but leave all the wires connected), then turn the circuit back on and measure off the receptacle terminals on the side or back.

Be careful.
The screws on the sides of the rece4ptacler are rarely insulated and are HOT if the circuit is turned on.

One problem that occurs with many receptacles and inexperienced folks is how far back in the receptacle the contacts actually are.


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RE: Need help with 220V circuit

Ronnatalie,

I didn't see anything wrong with the way it was. Granted, I'm not an electrician, but what's wrong with a 10-2 wire with white and black each carrying 120v plus a ground?


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RE: Need help with 220V circuit

because normally that white wire is neutral, not 120v. Touch a neutral and it's no big deal. Touch a 120v and things are not so pleasant.


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RE: Need help with 220V circuit

Weed, what do you consider normal?

I consider a XX/2 cable for a straight 240v circuit perfectly normal. Sure, the white is supposed to be re-marked, but that was not always done.


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RE: Need help with 220V circuit

I didn't say there was anything wrong with -2 cable other than the white should have been remarked (if it was installed in the past twenty years). My guess however is that it is NOT 10-2 and hence should not have been protected by a 30A breaker.


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RE: Need help with 220V circuit

Ronnatalie, Like I said, I'm not an electrician, but I know what 10/2 wire is. You can trust me when I say it's 10 AWG wire.

brickeyee, I will be playing with the eletric again today. Checking the actual terminals on the receptacles was the first thing I did. Thanks for the insight.


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RE: Need help with 220V circuit

Might be interesting to see what happens if you put one of the 30A dp breakers back in.


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RE: Need help with 220V circuit

Just remember that once you touch one of the leads to the circuit the other lead is hot (high impedance, but still hot).


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RE: Need help with 220V circuit

Pete: I interpreted his question differently. If I opened a 'normal' wall plate and saw a white and black pair, I would 'normally' expect the black to be 120v and the white to be neutral. That is, it is not 'normal' for the white wire to be carrying 120v even if it is capable of doing so.


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RE: Need help with 220V circuit

"That is, it is not 'normal' for the white wire to be carrying 120v even if it is capable of doing so."

It is supposed to be re-marked, but I have seen so many point to point 240 V cables using 2 conductor cable (black-white) not marked that it barely raises an eyebrow.


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RE: Need help with 220V circuit

I will definitely mark it black for future reference, but to be honest, anybody who doesn't know that it's a 220V air conditioner with a special 220V outlet (i.e. not your standard two prong outlet) shouldn't even be pulling the receptacle out of the wall anyway.

Anyway, upon closer inspection, I notice that the bare end of the black wire appeared "tinned". That soldering term is the closest comparison I could come up with. There was, what appeared to be some silver on the copper. I trimmed an inch off each wire, reattached them to the 30A breaker, and everything seems fine again. Maybe it just wasn't making good contact. Any idea what that could be?

Other than awaiting a response to that question, I guess you could say this topic is closed, and I appreciate all the help. Unless, of course, that is we want to continue debating the gauge or color of the wires :)


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RE: Need help with 220V circuit

How old is the cable?

Does it have actual rubber insulation? Often black with colored cloth braid for identification?

Copper wires needs was lead plated to avoid chemical reaction between rubber insulation and the copper.

The reaction made the copper become hard and brittle (the same as overheating) but at normal room temperatures.

It is one of the reasons you often see K&T feeder lines with all the insulation simply falling off.
The copper wire was not lead plated under the rubber insulation.

The problem went away when various plastics started being used as insulation, though some manufacturers appear to have continued the lead plating for at least a while.


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RE: Need help with 220V circuit

Actually, this cable is probably only about 10-15 years old. Other than the thickness, the insulation is just like normal 12/2 & 14/2 wire I've seen -- with the brown paper around the ground.


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RE: Need help with 220V circuit

Plastic outer jacket?

Probably older NM with PVC insulation (only rated for 60C).

NM-C (the newest stuff) uses 90C insulation,

If the screws are not TIGHT (and they need to be surprisingly tight) the connection may not have been good leading to heating and damage to the insulation.


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