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Mike what the heck do you mean?

Multiwire branch circuits (common neutral) are common in all occupancy's.



[This message has been edited by iwire (edited 01-05-2007).]

This is intriguing, but frankly, I'm lost. Can someone explain how a tv can catch fire with loss of neutral? [ We only have 230v 2 wire here.]

Alan

Electure,
Thanks for clearing this up for me. I completely overlooked 300.13(B) I knew it wouldn't work, but thinking now of the wiring scheme there leaves no doubt!

Alan, consider 2 possibilities:

1.) 2, 20A single pole breakers are in adjacent slots 1 & 3 in in a 120/240 single phase panel. Each circuit has its own neutral conductor landed on the neutral bus bar. The circuits supply your living room. (You just moved here.) You happen to have your TV as the only load on one circuit and an 1800W space heater as the only load on the other. If one of those neutral conductors should come loose, you will either be watching your TV in the cold or reading a book in the warmth. You probably won't be letting the smoke out of anything.

2.) Since the two circuits above are on opposite legs, a common neutral is permitted. One white wire is landed on the neutral bus and it is carrying the difference current between your two loads. Today, it happens to be the return current from your space heater. Things get really interesting when that neutral comes loose. Basically, you're left reading a book in the cold wondering what you're doing in USA. Meanwhile, Your TV is climbing through 500 feet thinking it has just been plugged into an outlet in the North of France. The voltage to the TV would have increased as the heating element was cooling, dropping almost all of the available 240 Volts across the TV mains.

We always run the risk of an unequal split from a failed center-tap connection from a transformer. Shared neutrals, while working fine most of the time, do offer the potential for more exciting failures. Even with a proper panel Neutral, loads on breaker pairs with shared neutrals are at a higher risk of over voltage.

Consider a case where the shared neutral is intact but the space heater suffers a direct short. It would not be unreasonable to have half the voltage drop on the hot and half on the neutral until the breaker tripped. This could add 60 volts to the other leg's 120 for a short period of time. (But it doesn't take long to let the smoke out)

Joe

[This message has been edited by JoeTestingEngr (edited 01-06-2007).]

Thank you Joe, for a lucid, calm and impersonal explanation of the issues with MWBC's.

[This message has been edited by mxslick (edited 01-24-2007).]

For my $.02 worth, first I have no problem with MWBCs, I can see the problems with an open neutral or an overloaded neutral, which would usually come from improper installation. I personally like them for a few reasons; first, less wire to pull and second, a less crowded breaker panel (half the neutrals and grounds). I guess I like them since I have yet to see the horrors of a MWBC gone bad.

Second, I have seen something funky like that, but it wasn't because of a bad neutral. It was on a house (my grandmother's...I've been hounding her to get her service upgraded) with a 60A service with a cartridge fuse disconnect (I fused it to 55 A/leg). Anyways, there was alot of cooking, etc, and one of the fuses blew. The strange (at the time, I know what happened) part was the lights on that leg didn't go out, they just dimmed, and the microwave even made an attempt to work. Since the oven was on, the blown side was putting its hot thru the heating element of the stove (hence the dimming) I figured it out quickly after turning the oven off and the lights went completely out. Anyways, it was strange, and strange to see the current cross over a 240V load to get to the other leg.

Open or loose neutrals are the bread and butter of our trade; many are the problems that can be traced to the "less important" neutral wire!

Indeed, I think I became a "real electrician" the day the neutral went from "less" to "most" important wire in my mind.

There was once a splice going bad at the PoCo transformer. This resulted in voltage surges at the house, with much damage to the expensive stuff. Simply put, if two loads are in series, the one with less resistance sees more voltage, and dies.

A shop was experiencing failures of their "surge suppressor" power strips. It was also interesting that some lights got brighter whenever the table saw operated. That problem was traced to an broken nuetral lug at the service.

Seven wires under a wire nut is almost never a good idea. When used to combine three circuits, and run but one wire to the panel, it's a really bad idea. One wire with a poor connection led to $300 of repairs to assorted power supplies. (BTW, in this specific instance, this was NOT a proper 'multi-wire branch circuit. Someone had thought to combine three 20 amp neutrals (single phase) into a single #10 to the panel. The three circuits were so cross-connected that it was not possible to fix this without completely re-wiring the office )

One can almost see a time-line in an electricians' training by watching the attitude toward 'shared neutrals' change over time:
- When first introduced to it, apprentices are mystified;
- The mystery is replaced by enthusiasm as they realize that they don't need to pull as many wires;
- If the guy fails to complete training, he later adds to this 'short cut' by, say, combining neutrals that ought not be joined, putting neutral busses somewhere besides the panel, etc.;
- He begins to see problems that are traced to bad neutral connections, and starts fretting about 'unqualified handymen;' and, finally,
- He gets to buy $300 of smoked power supplies because of an "oops" moment, and he begins to re-think sharing neutrals. Especially when he knows some untrained folks will follow after him, and mess with his fine efforts

I think my attitude towards neutrals changed once I put 2+2 together after seeing sparks from a neutral that was in use. I mean I know that a complete circuit needs a live and a return (neutral) you just don't think too much about it since it usually won't shock you (unless there is something wrong). But what goes in must go back out so the neutral is just as important as the live wire, if not more so since a bad neutral poses even more danger than a bad live wire. Good discussion though.