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Had an old customer that wanted a vanity light changed out in a basement bath. I turned the switch off and removed the old light. When putting the new fixture mount on the neutral touched the ground and went zap. Tripped breaker 7. Thought that was strange. Got my volt meter out and checked hot wire with the switch on, read 120. Hot wire is breaker 8. Turned 7 back on and checked neutral, read 120.
This is 40's vintage. Just two wires. The ground is screwed to the back of the metal box.
How is this possible? It works fine.
Brought it to the home owners attention. We agreed to cap both wires and have my electrician look at it. He can't get there until next week.;)
 

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It's called a multi-wire branch circuit. And if your panel numbering is correct, it's not allowed on circuits 7 and 8. It's basically two circuits with one neutral and it can be overloaded.

Just let your Sparky handle it.
 

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FYI: Opening a shared neutral can burn things up that are attached to the circuits.
 
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FYI: Opening a shared neutral can burn things up that are attached to the circuits.
It's why I don't like them. We've all seen what happens when a service neutral goes wonky. European system has no neutral everything is 240v and if I'm not mistaken 50 cps instead of 60
 
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It's why I don't like them. We've all seen what happens when a service neutral goes wonky. European system has no neutral everything is 240v and if I'm not mistaken 50 cps instead of 60
Too bad 'pert near every electrical service in the US is a MW circuit........ :whistling
 

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Ran into that when we bought our house. It was powered by a transformer at the neighbor's, a couple of hundred yards away. A heavy load on one side of the line would pop light bulbs on the other side.

POCO called it a "swinging neutral", and wound up giving us our very own transformer. But I had to fight my way through proving it to 5-6 different linemen and company reps, with my equipment and theirs. I still have a bit of a bad taste from that experience.
 

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Another reason to install as many appliances on 240 Volt circuits...
Air compressors, furnace motors, some kitchen appliances.

Suffered the loss of several motors and battery chargers on poorly balance 2-3 phase temp panels in the last ten years...

I am guessing both houses were wired by same sparky, and had systemically uneven loading of the half phase busses. plus under sized service wire

The current generation of lighting has to be changing actual % of electrical loads compared to legacy homes 20-30 years ago, reactive vs resistance, power factor and single pole vs dual pole supply/demand...

As Green energy State & federal mandates further devolve our Nation's Electrical Grid's day in day out performance, The need for constant monitoring of the homes power and other utilities actual values versus nominal numbers will soon be standard, IMHO.
 

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NEC requires multiwire circuits to be on 2 pole breaker so if one has a problem they both trip.
The NEC only requires both circuits get turned off together. A 2-pole breaker (or 3-pole) is merely the simplest and easiest method.
 
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NEC requires multiwire circuits to be on 2 pole breaker so if one has a problem they both trip.
The nec doesn't really require a dp breaker. You can use 2 sp breakers with a breaker tie. These can actually trip one circuit and not trip the other.

210.4(B)Disconnecting Means. Each multiwire branch circuit shall
be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch
circuit originates.
240.15(B) Circuit Breaker as Overcurrent Device. Circuit breakers
shall open all ungrounded conductors of the circuit both
manually and automatically unless otherwise permitted in
240.15(B)(1), (B)(2), (B)(3), and (B)(4).

(1) Multiwire Branch Circuits. Individual single-pole circuit
breakers, with identified handle ties,
shall be permitted as the
protection for each ungrounded conductor of multiwire
branch circuits that serve only single-phase line-to-neutral
loads.
 

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For the cost difference I can't be bothered with 2 Single pole breakers and tie handles.
I get that but I wanted to make sure others understood that it did not need to be a dp breaker. No biggie. I don't use multiwire branch circuit because of the afci issue and before that I didn't like them because when you loose a neutral you have problems.

Ken I know the feeder and the service conductors are multiwire branch circuit but I would like to minimize the issues if I can.
 
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