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First thing, I"m not an electrician but have some electrical knowledge. I'm renting in a house and am trying to help the owner with some issues. This is an older house that has a combination of old knob and tube wiring and new wiring. There is a circuit that contains old wiring with only two lines. Sockets and lights on this circuit don't have power. When one of the breakers is tripped the outlets and lights start working and other lights and outlets on this breaker turn off. What could be causing this?

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    I suspect multi-wire branch circuits with a lost/dropped neutral. Do the circuits turn on all the way? Or are lights dim and voltage low? Does the brightness/voltage change when lights on the other leg are turned on/off? – Harper Jul 3 at 18:42
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    Hello Harper, the lights come all the way on and outlets show 120v with voltage tester. – Eduardo Corrochio Jul 3 at 18:58
  • I have a follow up question. If the circuit is crossed and is getting power from two locations why does it only become hot when the breaker is tripped? – Eduardo Corrochio Jul 5 at 15:44
  • That depends on how you define "hot". It's most likely an erroneous measurement. However if that's really happening, it could be that someone put a breaker on the neutral wire by mistake. In the old days, they separately fused both hot and neutral, so neutral could blow. When that happens, the loads connecting between hot and neutral "pull it up" to hot potential. – Harper Jul 5 at 15:56
  • In the modern age, we know you should not fuse neutral. There's nothing wrong with putting a breaker on neutral as long as it is wired to common-trip with the hot wires, so they're all disconnected at once. – Harper Jul 5 at 15:57
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The circuits are “crossed” at one, or possibly more, points. Be very careful when sorting this out.

This is how people get killed by electricity as they think they have isolated something but it is still live.

The method to find it is to disconnect at each junction until the relevant part goes dead and label wires to work out what goes where.

Had a similar problem where a site had been wired with additions over 30 years and most was not labelled... the law changed and it all had to be identified. Relatively easy as it was between buildings, switch of a breaker and 15 seconds later the phone went : “we have no power.. oh, where are you?” That was that breaker labelled... Good fun, but we did not mention those power cuts were deliberate... :)

  • Thanks Solar Mike, I'm definitely not going to get into something I don't have experience with. Just trying to understand the odd behavior. Wouldn't something like that cause the breaker to trip if it was getting power from another circuit? – Eduardo Corrochio Jul 3 at 19:14
  • Breakers break when an excess current flows above a certain value for a specified amount of time, not because of two supplies. – Solar Mike Jul 3 at 19:16
  • I have a follow up question. If the circuit is crossed and is getting power from two locations why does it only become hot when the breaker is tripped? – Eduardo Corrochio Jul 4 at 18:13

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