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I have a Belkin Wemo switch that requires a neutral wire for installation. There is no neutral in the switch box. However, there is a neutral in the electrical outlet box below it, and a path from the switch to the outlet. Can I run a neutral wire from the switch to the neutral in the outlet box and connect it as a shared neutral? I guess this is another way of asking of all neutral wires are basically interconnected eventually (leading back to the main electrical box) and whether any neutral in the house can be used by anything requiring a neutral.

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Add-on questions that I think would be necessary for a complete answer: What are the any sections of NEC that would apply to this specific situation? How do you actually go about this, eg run a 14/2 wire up to the switch, but leave hot disconnected? Any special markings necessary or expected? –  gregmac Nov 1 '13 at 22:36
    
Is the switched fixture and the nearby outlet box on the same circuit (the same breaker)? –  bib Nov 2 '13 at 0:18
    
To answer the question above, the switch and outlet box are on separate circuits. –  David Toole Nov 12 '13 at 15:04
    
Thanks to everyone for the comments. It sounds like my original plan is not a good idea. How about plan B--which would be to run a wire from the neutral at the light fixture back to the switch box and connect to the neutral on the Wemo switch. Same circuit, and obviously, same switch-light combo. –  David Toole Nov 12 '13 at 15:27

3 Answers 3

According to 2011 National Electrical Code article 300.3(B), No. For verbiage, see this answer. For one explanation why, see this answer.

Another reason not to do it, is because you could end up overloading the grounded (neutral) conductor. Depending on if the two circuits are on the same branch circuit, different branch circuits on the same leg, or different branch circuits on different legs, you'll run into different situations. However, in two out of three of these situations, it is very easy to overload the grounded (neutral) conductor.

Examples:

Let's examine what happens in a 120/240V single split-phase system, in a few different configurations. The circuit breakers in these examples is a combination circuit breaker, with 15 ampere thermal overcurrent protection and magnetic short-circuit protection.

Load on a circuit

Single Load

With a single branch circuit supplying a 15 ampere load, we see that there will be 15 amperes on the grounded (neutral) conductor. This is fine, since the grounded (neutral), and ungrounded (hot) conductors are both sized to carry 15 amperes.

Two loads on the same branch circuit

Two loads

In this situation, we'll see 30 amperes on the grounded (neutral). However, since we're also drawing 30 amperes through a single breaker with 15 ampere overcurrent protection, the breaker will open.

Two loads on the same leg, but different breakers

same leg, different breakers

This is the dangerous situation, since the breakers will not trip. Each circuit will draw 15 amperes on the ungrounded (hot) conductor, but the grounded (neutral) conductor will see 30 amperes. Since the conductor is only rated for 15 amperes, the conductor is in danger of overheating and starting a fire.

Two loads on different legs

Different legs

If the two loads are on different legs. You'll find that while each ungrounded (hot) conductor sees 15 amperes, the grounded (neutral) carries 0 amperes.

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Net necessarily, if the neutral is on a different circuit. First, if either circuit is protected by a GFCI circuit breaker, it will trip. GFCI breakers detect a difference in current flowing on the hot and neutral wires. Normally, this difference would represent current flowing to ground but in this case, it would be current flowing to the other circuit.

The other issue is of load. You are adding load to a neutral wire witch may already be carrying it's full rated load.

I suspect that the device you're adding draws minimal current on its own so issue #2 is probably not a practical issue, though GFCI breakers would still be an issue. Never the less, it may not be to code, though I cannot quote code to prove it.

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Thanks, everyone. To clarify, the switch will draw minimal power. The switch and outlet are on two different circuits, each nowhere close to being overloaded. There are no GFCIs on either circuit. As for code, the residence is in Brasil where code is virtually non existent. However, it is new construction with what appears to be an excellent electrical installation and ample amperage in the main box. I would remove the neutral if I sell the house, although a new owner is unlikely to install anything other than a light switch in a switch box. Any other suggestions would be welcome. –  David Toole Nov 2 '13 at 12:06

(Answer assumes you are in the US with a house relatively up to code)

In this case, yes.

Ultimately, all Neutral wires do come back together in the electrical panel box to answer that question as well.

However, while very close GROUND and NEUTRAL are not the same and should not be swapped so make sure you're tapping a neutral and not ground if neutral is what you need.

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I don't think it's true that this is ok -- neutrals may all end up in the same place, but if you run more than one circuit through a single neutral wire, you can exceed the current rating of that neutral without tripping a breaker. Granted, in this particular case, there should be very little current flowing in the neutral through the switch, but the next homeowner may not realize this and think that the neutral belongs to the circuit and use it for some other purpose, like to power a 1000W aquarium light with a 1500W heater plugged into the outlet. –  Johnny Nov 2 '13 at 4:33
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Can you site the article from NEC that allows this? Article 300.3(B) says you are wrong. –  Tester101 Nov 2 '13 at 12:38
    
I would definitely be tapping a neutral, and not a ground. Both are clearly differentiated by coloured wiring. The switch box has both neutral and ground in a typical (new) 3-prong setup. I have tested to determine the neutral I want to connect to is really neutral. –  David Toole Nov 12 '13 at 15:08

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