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I'm finally implementing the advice given in this question, I've managed to run the wiring up to the junction box, but I'm a bit confused by the contents of that box.

There are three circuits entering the box, but it appears that two circuits are sharing a single neutral.

Everything appears to work (and has done so for 2+ years), but I'm curious if this is kosher from a code point of view. Should there be an individual neutral for each circuit? If so, is there a brief explanation as to why this is OK?

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This answer and This answer, might be useful. – Tester101 Mar 12 '12 at 11:49
Thanks to all who took the time to answer my question. I am going to research further a way to apply a neutral from the existing circuit...which solves all issues. Ben – Ben May 8 '12 at 16:38
up vote 5 down vote accepted

A multi-wire branch circuit (two hots from different phases sharing 1 neutral) is often found in the kitchen where it powers one receptacle which has the jumper connecting the upper and lower outets removed. The result is that you get two 15amps circuits at one receptacle. At the panel, both breakers should be bonded together so it is not possible to have one on and one off.

Code varies by region, but I do not think it is typically permitted in any other configuration.

There are also restrictions for having multiple circuits in a single junction box. Be careful working on this - even if the breaker is off, check for voltage with a non-contact tester to ensure there are no other live circuits.

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Only if it is a split 220 volt circuit as Steven points out and it It must be on a two pole circuit breaker. This ensures that the circuit is two different legs and not just one, also the entire circuit will be disabled with only one breaker. Each hot leg returns on this shared neutral. The reasoning is that no more than the rated (breaker) current will pass through the neutral in this configuration. The two "Hot" legs will tend to balance the return current. Example: If both legs draw 5A, then no return current passes through the neutral. If leg 1 draws 3A and leg 2 draws 10A then the difference, 7A is returned through the neutral.

It is NOT permitted to share a neutral in any other situation. If you were to share a neutral with two breakers on the same leg of a panel, both circuits could draw the breaker limit (lets say 15A) making the shared neutral as much as 30A return current! That will exceed the limit of the wire size and could cause a fire. Also, GFCI and AFCI breakers will not work on a shared neutral circuit.

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I believe that GFCI's do indeed work in a shared neutral configuration as long as they both have their own pigtail lead to the neutral. – Steven Mar 12 '12 at 17:23
There is only one GFCI breaker in a shared neutral scenario. The sharing of the neutral is elsewhere beyond the circuit breaker panel, and the GFCI will not work as expected in this scheme. The GFCI works by measuring the current in both the hot and neutral wires. If they differ by more than 5 ma the breaker trips. If you have a shared neutral currents are being returned through two neutrals imbalancing the leads. – SteveR Mar 12 '12 at 18:28
@SteveR Remember GFCI receptacles measure the hot and neutral out to a device, so if there is extra load on the neutral outside of the device (returning from down stream device for example) it will not sense an imbalance. This is why (in theory) using a pigtail will allow you to connect GFCI receptacles on multi-wire branch circuit. Take a look at This Answer for more info. – Tester101 Mar 12 '12 at 18:40
I thought you meant GFCI outlets, not GFCI breakers. – Steven Mar 12 '12 at 18:46
@Tester101 - I'm referring to GFCI breakers, not receptacles. – SteveR Mar 12 '12 at 22:38

The "Edison 3 wire" circuit has been used for many years. 240 volts between phases and 120 volts to a neutral that carries the unbalanced load of the two phases. On perfectly balanced loads there will be no amperage on the neutral. It is imperative to have the circuit wires on different phases, i.e. 240 volts across them.

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you definitely do not want to share neutrals from different circuits. This sets up a potential shock hazard if one circuit breaker is open and the other closed. If you assume one circuit is dead by opening the breaker the neutral on that circuit will immediately be energized if any appliance is using the second circuit. If you are doing work on the first circuit and get across neutral and ground you could be electrocuted if an appliance or a light is energized on the second circuit.

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There are code rules in place for multiwire branch circuits (shared "neutral"), to reduce the risk of only deenergizing half the circuit. – Tester101 Jun 30 '15 at 12:55

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