Shared Neutrals...

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Danny_K

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I'm working in a house that was built in the 50's. The electrical is, to say the least, a bit of a nightmare in that there are multiple rooms in the house on one circuit, and ones that don't even make sense like the outlets in the basement kitchen and the upstairs bedroom receptacles. I know this was probably the norm for the time, but as the house was updated and devices such as lights and outlets were added, it seems like the electricians just tied into the closest junction box to avoid having to run all the way back to the load center.

Long story short, I'm separating everything slowly but surely, adding a sub-panel for the finished basement, and now adding a transfer switch for a portable generator. I am using a manual transfer switch. I was reading over the instructions and they mentioned circuits that share a neutral must both be tied into the transfer switch. I understand these instructions, sort of understand why, but not entirely. I read everywhere that if a neutral to one circuit is cut (i.e. switching from LINE to GEN on the transfer switch), the other circuit can receive an overload of voltage and can potentially back feed. Am I understanding this correctly?

While separating one specific circuit in the house: Basement Stair lights (this breaker also controlled kitchen lights, parlor lights, breezeway light, back porch light and front porch light HOLY COW!) I noticed that even though this breaker was off when I was working on it, when I was wire nutting the Neutrals, I received a spark when touching two neutral wires together. Come to find out, a basement light (across the basement) is somehow tied into the stair light switch, but no hot lead, just a neutral. I assume somewhere in the basement, these neutrals come together and some 12/3 wire goes up to the switch.

Is the above and example of a shared neutral circuit? Two breakers, one neutral? Can I avoid having to bring both breakers over to the transfer switch simply by making sure they are on separate phases in the main panel?

Lastly, what is the easiest way to make sure there are or aren't anymore multiple circuits in the house that use the same neutral wire? As I continue to separate circuits in the house, most will transfer from the main panel to the sub panel, but if the circuit that ends up in the sub panel shares a neutral with one that remains in the main panel, am I creating an entirely different issue?

Sorry for the long post, this part of electrical is very confusing to me.
 

Danny_K

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I actually poked through that post but it doesn't seem to touch much on circuits in the house that may share a neutral. I'm specifically looking for a way to find these circuits easily and avoid having to carry over circuits to the transfer switch that I may not want to carry over.
 

Jadnashua

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The reason a circuit with a shared neutral can work with it not being a larger gauge wire is that the return current from each leg cancels each other out when equal, and even if one leg is at maximum, it would never exceed the capacity of the neutral, and of both legs had the same current on them, the current in the neutral would be zero because of vector addition.

If you're powering one leg, on a shared neutral you'd potentially have current available on the other leg. As a result, they must be separated if you're only going to power one leg.

Keep in mind what the power coming into the house really is: it's 240vac. You get 120vac available because they put a lead half-way across the transformer secondary, half-way between 240 on one end and the other.
 
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