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Breaker off, 15V from hot to neutral - why?


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I'm a non-electrician (computer person) charged with replacing some old, cracked outlets that can no longer reliably make contact with a plug. The building is an old Catholic school built in 1945, but the wiring in the area I am working on was upgraded at some point, probably semi-competently; Catholic schools are prone to cut corners on building maintenance.

I replaced four outlets (3-prong, romex wire, grounded to box/no ground wire, replaced with 3-prong GFCI) and all went smoothly until I climbed to the 2nd floor, went to the 2nd floor panel, turned off the circuit breaker for the room, and found to my surprise:

Voltage from hot to neutral: 15V

Voltage from hot to ground: 15V

Voltage from neutral to ground: 0V

Resistance from neutral to ground: ~300 ohm

The same pattern occurs in several rooms on the same floor, but the hot->neutral voltage w/circuit breaker off varies from as little as 5V to as much as 35V in some rooms. A few rooms do show 0V.

I searched the web extensively, but could not find any mention of a situation in which a circuit breaker is turned off yet current can still flow from the hot wire. Is there any chance that anyone might explain the possible cause(s), and whether it's something I can handle myself, or an electrician will be necessary?

Thanks very much!

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Thanks for your response! I'm going to turn off the power to the whole floor at the main distribution point and unscrew the panel door to see if I notice anything unusual. But I can't imagine what would cause this problem, aside from something completely wacky like multiple uninsulated hot wires touching each other.

I have two further questions:

1) How do I convince the owners that this is a safety problem that should be fixed immediately? They will most likely argue that it isn't a big deal since the power can still be turned off at the main disconnect.

2) What do I say if the electrician - who could well be the same guy who did the installation in the first place - takes a look and goes 'that's just normal?'.

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How do you know that this voltage was on the wires? (What kind of tester were you using?)

If it was an electronic multimeter, you might have been measuring phantom voltage -- voltage that's induced from nearby energized wires. This is a fairly common issue with older ungrounded wiring systems. If you have one, use a Wiggy to test the wires. I wouldn't be surprised if the "voltage" readings dissappeared with the Wiggy.

I agree that you need an electrician.

- Jim Katen, Oregon

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If it was an electronic multimeter, you might have been measuring phantom voltage -- voltage that's induced from nearby energized wires.

Ah...thanks very much - I was using an electronic multimeter; this makes perfect sense! I didn't have a solenoid voltage tester until about five minutes ago, but what man wanted, ebay hath provided.

I agree that you need an electrician.

Understood - I would rather not be touching these wires at all, but they figured I'm here and they're paying me and how hard can it be to screw in some wires to an outlet anyway. I just want to have some solid ideas about the problem, lest an electrician is hired at the boss' expense just to come and announce that I don't know how to use a multimeter. 8)

Don't you think maybe the voltage is on the neutral?

I believe that the voltage is on the hot wire because there is hot->ground voltage but no neutral->ground voltage.

The huge (300 ohm) resistance between neutral and ground convinces me that they can't be the same wire (ie, not a bootleg ground). However, I'm now doubting that measurement since it won't be accurate at all if current is being induced on either wire.

Thanks very much for all your help - being in a position of incompetence is quite frustrating for me. I'll let you know what happens after I get the Wiggy. Assuming it's 'phantom' voltage that I'm seeing, I'll probably install the GFCIs and then recommend that they have an electrician take a look at the grounding.

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