Shared Neutral

[gorehound]

When did the NEC start "re-allowing" shared neutral without a double pole breaker?

Thanks, Shane

[Big_Jake]

When did they stop? My electrician is in his 70's and he constantly wires in what he calls "networks" with 12-3 (or 14-3) romex, running two circuits with a shared neutral. The hots have to be out of phase, of course, but if they were accidentally hooked to the same phase, the neutral could get overloaded until it melts and starts a fire, and the breaker wouldn't necessarily pop, since the neutral is unprotected.

Why would a double pole breaker be required without a 240v circuit?

JK

[gfretwell]

The only time a 2 pole breaker or a breaker tie is required is if both circuits end up on one yoke or if you also serve a line to line load

[Chris Friesen]

As long as you're not split-wiring duplex outlets, I think it's okay. At least, that's how it works here in Canada.

Chris

[lukeborn]

Shane,

It may not be required (and maybe never was) to have a handle tie but it would be a nice idea to keep home-owners from tinkering and moving the wire to the same phase since this would force the red and black to be on separate phases as it is required and shoot why not it doesn't cost any more to get 2 pole breakers over 2 single pole breakers. Also a problem I have been running into recently it that shared neutral circuits will not support an arc fault or a gfci breaker as is now required if your city is up to date on the NEC. Is it worth it to save a small buck?

Thanks and keep the good questions rollin' in, Lucas Lucas Electric, LLC

[Larry W]

In the unlikely event of an open neutral in the circuit, 240V would be still be present across the hot leads. If for example a 15 amp motor was running on one leg and a milliamp device on the other, the lower current device would essentially be wired to 240V.

[gorehound]

The reason I asked when they started allowing it again is because my instructor said that code says you are supposed to use a double pole breaker when sharing a neutral. I work for an electrician and we were wiring up a portable gfci board for the local chili cook off and if we used separate neutrals the pull through the carflex would have been more of a pain than it was. I asked about the double pole and he said they said it was fine then they said it wasn't then they said it was again. What makes me nervous about the practice is when you are working on something and you shut the breaker off you think ok I'm good to go. Now you go and pull down the light fixture and start disconnecting it and get knocked on your ass because the neutral is shared with the refrigerator circuit. If the 2 circuits were on a double pull breaker the neutral wouldn't be carrying the load from the refrigerator because both circuits would be dead. Any thoughts?

Thanks Shane

[Terry]

That is true. In some cases having a double pole breaker is better, like getting zapped by the neutral

Sometimes it is not better, like having the refrigerator go off when you have a problem with the lights :)

[Doug Miller]

Either: a) your instructor is incorrect; b) you misunderstood; or c) your instructor is a Canadian. (AFAIK, this *is* required under the CEC.)

The double-pole breaker is required under the NEC if the circuit supplies multiple devices on a single yoke, or supplies both line-to-neutral and line-to-line loads. It is not required under other circumstances. [2005 NEC, Article 210.4]

Makes me damn nervous too. My current house had three such circuits, all unidentified, all on separate single-pole breakers. All Code-compliant and perfectly legal, too, and I had no idea the circuits were sharing neutrals until I happened to pull off the panel cover and see the red wires.

They're all on double-pole breakers now.

IMO a double-pole breaker should *always* be used on shared-neutral circuits (aka multiwire branch circuits or Edison circuits), even when the NEC does

*not* require it, for exactly the reason you cite. It is IMO a foolish and false economy to not spend the extra ten dollars on a proper double-pole breaker, when failing to do so may cost a human life.

[bud--]

I agree with your code comments above.

Seems this was a discussion about a year ago. It is a common practice for electricians to use multiwire circuits. In 3 phase circuits 3 hots can be run with 1 neutral. Requiring a 3 phase breaker and killing all 3 circuits to work on one would be ridiculous.

Most advice on this newsgroup, probably including yours, is to test all the wires you are working on to verify they are dead. There are some things that people should really know before working on electrical systems.

Use of multiwire circuits in residential is likely to go down drastically with increased requirements for AFCI circuits in the 2008 NEC. (But AFCI circuits will not be required in as many locations as originally approved.)

-- bud--

[Doug Miller]

Agreed, up to a point. Unless all three phases are killed, there *will*be* current in the neutral wire if any load is operating on any phase -- of course, 3-phase power implies a commercial or industrial installation, which in turn implies that maintenance will be performed by qualified electricians, who presumably have the required knowledge and equipment to work on live circuits in relative safety.

I should have specified that my comments apply to *residential* multiwire branch circuits.

Absolutely. I go even farther than that, though: before testing a circuit that you presume to be dead, test one that you *know* is *live*. If the tester doesn't indicate, it's time for a new tester.

[bud--]

The NEC has a cousin - NFPA 70E "Standard for electrical safety in the workplace" - which is somewhat of a defacto standard in most workplaces covered by OSHA. The procedure in 70E is to test the voltage detector before and after testing the circuit. But the circuit voltages and energies can be so much more 'interesting'.

-- bud--