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My 3 prong 240v outlet shows 240v when measuring across red and black wires. However, when testing either hot to the third wire I get 0 volts. My understanding is it should be 120v.

What does this signify?

At the breaker box each wire going into the double breaker show 120v when testing from it to the neutral bar.

  • Is this a dryer outlet (30A) or a range outlet (50A)? Also, are there any junctions between the old breaker box and the outlet? – ThreePhaseEel Jan 7 at 5:07
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The third wire is not connected, either broken somewhere along its path or there's a missing connection in one of the junction box leading to the outlet.

If there's multiple connection boxes from the panel to the outlet, you can test at each to figure where the problem is. Either a nut came loose somewhere or a wire was cut/torn.

The question doesn't make it clear whether you are in a location with 240 and ground, or 240 with neutral and ground, but the answer remains the same, the third wire is disconnected somewhere.

  • Thanks, that is what I was suspecting. Had solar installed recently and the city made them run this 240v all the way to the new main box. All other breakers are still in original location. They just tapped it and ran new wire thru my attic. – jdrich79 Jan 7 at 3:39
  • For testing purposes, could I jumper the neutral at the first service panel to the neutral bar to see if that temporary fixes the issue? – jdrich79 Jan 7 at 3:41
  • Jumper the neutral and what? Can you elaborate on the whole setup? Question mentions only one panel and one outlet. – Jeffrey supports Monica Jan 7 at 3:47
  • Sorry. In my follow-up I started to explain, but the house main service got moved when I had solar installed. All the 120v breakers stayed in the original breaker box, the 240v had to have the breaker at the main (new) panel, but did not require the old wiring to be changed. Therefore, they spliced into wire at the old main and ran new wires the the new main. In the old main, I can see the bare wire neutral, I'm just wondering if I could jumper it directly to the neutral bar to see if the issue is in the new wiring or the the old. (Not a permanent solution, just to narrow down) – jdrich79 Jan 7 at 3:58
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Yes, this is a lost neutral or ground.

Keep in mind that if this is a NEMA 10 type receptacle, hot-hot-neutral (no ground), then a lost neutral is a rather dangerous situation. Such connections typically bond neutral to the chassis on the assumption that the neutral wire rarely fails, and bootlegging ground in this way is less unsafe than having no ground. I wouldn't know; I'm not an actuary.

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