General wiring questions

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Old 04-08-15, 12:21 AM
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General wiring questions

Trying to figure out why a wire set was run.

In my panel I have a couple of wire "bundles/cables" consisting of 1 Blk, 1 Red, 1 Wht, 1 Grnd all in grey wrap insulation.

On one cable, the Blk connects to a 20A breaker (goes to kitchen plugs), the Red connects to a 20A breaker (goes to dining plugs). The wht connects to the neutral.

Does this mean somewhere in the wall the the neutral in this bundle connects to both dining plugs and kitchen plugs?

Not sure why this would be okay if you have to run separate circuits to each area, seems like the neutral is doing double duty. Of course there's only 2 plugs on each circuit (small rooms), but still...

I would think each circuit would need it's own neutral back to the panel (like standard romex).

Assuming this is ok, curiosity as to the reasoning behind code.

Also, I have a double breaker that has a blk & wht wire attached. I understand this is okay but question about marking. Does the wire only need to be marked black at the termination or also at the panel (understanding that at the panel you are looking at the wire connecting to the breaker and hopefully it's obvious it's powered). Just wondering if code required both ends.

Thanks

Edit: The two breakers are right next to each other so they are offset a phase. Built in early 1980's. Further research shows it's a multibranch circuit and legal. But back to my original curiosity, still seems like twice as much current could run through the neutral. Why doesn't this happen?
 
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Old 04-08-15, 01:03 AM
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Multi wire branch circuits are fairly common. It's basically two circuits sharing the same neutral. The neutral is extremely important in this type of circuit in that if you lose it.... you could end up with more than 120vac on a receptacle.

The two legs of the circuit are 240vac. Each leg shares the one neutral. Since the neutral is the balance point between the two legs..... it will never carry more current than each leg separately.

If the current draw on each leg is identical..... then there is 0 on the neutral.
 
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Old 04-08-15, 01:39 AM
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Ah...that makes sense, correct me if I'm wrong on the following.

Basically, as the voltage of 1 leg is positive, the opposite leg is negative, the potential never gets past the 120v the circuits/neutral are designed for (neutral splitting the two legs). Take the neutral out, now you have the potential bouncing up to 240v as the current path is now between the two circuits via the connected neutrals .

RE: "If the current draw on each leg is identical..... then there is 0 on the neutral."....understand this applies to the neutral going back to the panel, however in a properly functioning circuit isn't there still current on the neutral that bridges the two circuits???

Lastly, is it good/best practices to install a handle tie between the two breakers assuming they are rated 120/240?
 
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Old 04-08-15, 05:46 AM
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Newer codes would require a handle tie or a two pole breaker and a means to identify the conductors. Older codes only called for this if both hots landed on the same yoke.

The neutral of a MWBC carries the difference, not the sum of the neutral current.
 
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Old 04-08-15, 06:33 AM
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Take the neutral out, now you have the potential bouncing up to 240v
On a MWBC, it is important to pigtail your neutrals to a receptacle. You will connect neutrals together with only one neutral wire going to receptacle.
This reduces the chance of losing your neutral if a device should fail.

Do not use receptacle terminals to continue the neutral downstream.
 
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