Thread: Please explain scorched nuetral.

My electrician noticed a scorched nuetral in a panel we're working in for a room add. Can someone please explain why this may be, in "non" electrican jargen. Thank you.

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Most likely it's loose... Contrary to common belief, neutral conductors can carry lots of juice.... plenty of people have had the piss shocked out themselves underestimating them... including me.

He was talking about the circuit being "out of phase" or something. I dont know. I really need to learn more about electrical, not just relying on my subs.

It sounds most likely that both hot legs on a shared neutral circuit got put on the same phase. That essentially overloads the neutral and can fry it. It can (and usually does) lead to fried motors and other stuff on the circuit. Overall, it's a pretty bad thing and a pretty big mistake by an electrician.

I'm no electrical genius, but am I correct to say that there are two seperate circuits on one side of the panel sharing the same busbar and using the same neutral? Can you breifly explain phasing or out of phase?

Originally Posted by Marc M

I'm no electrical genius, but am I correct to say that there are two seperate circuits on one side of the panel sharing the same busbar and using the same neutral? Can you breifly explain phasing or out of phase?

Marc,

Sounds like a "multi-wire" circuit. I have also heard it called a "shared neutral" or a "split neutral" circuit. In essence, it functions the same as a 240 volt circuit, except that it does not go to a single appliance or device, but to two different circuits or devices. My understanding is that the two circuits can share a neutral because only the difference in amperage is running back down the neutral conductor. That is as long as each of the voltage carrying conductors (typically colored red and black) are connected to opposite legs of the 240 volt line (a meter shold read 240 volts between these two legs). However, if the voltage carrying conductors are connected to the same leg, then the neutral can end up carrying twice the rated current (a meter would read 0 volts if connected to the same leg).

There is a wealth of information on the subject here on the board. Probably explained better than I can. Do a search and read a lot.

Gunner,
I do appreciate your time. I have been reading and I think I have figured it out. It appears to me that the time or money saved does not equal the potential danger that a multi circuit represents. But what do I know, im just a GC, not an electrician.

This is a drawing depicting what Gunnar described.

Hey Jerry,
man i hate to sound ignorant, but can you explain exactly how the 20 amps on the neutral got there, or gets there.
1- Dont the two seperate circuits of this split 240 be equal in their maximum ampacity rating? (If this is the incorrect terminology please correct me).
2- I was under the assumption also, that the difference of the two circuits goes back onto the nuetral, or is this the bad diagram?

This is the part of what i read that was a little gray.

For a single phase residential electrical service-- a multi-wire branch circuit must be connected to A phase and B phase. When connected like this the current on the neutral is the difference (subtractive) of the current on A phase and B phase. If you connect the hots to (both) A phase or (both) B phase the current becomes additive. That is what the illustration shows. This can cause overloading of the neutral. Also a loose connection can cause overheating and scorching of the conductor.

Originally Posted by Marc M

Hey Jerry,
man i hate to sound ignorant, but can you explain exactly how the 20 amps on the neutral got there, or gets there.
1- Dont the two seperate circuits of this split 240 be equal in their maximum ampacity rating? (If this is the incorrect terminology please correct me).

When properly wired, sort of what you are saying (but not really).

When properly wired, the multi-wire circuit acts as a 240 volt circuit, with no neutral current (if the loads on both 120 volt legs are equal), the current simply come from the transformer, goes through the loads, then back to the transformer, all on the hot conductors.

Now, however, let us introduce a 10 amp load and a 15 amp load, the first 10 amps goes around the 240 volt circuit, however, the extra 5 amps cannot go through the other load (otherwise you would have two 15 amp loads), so that 5 amps goes back through the neutral.

2- I was under the assumption also, that the difference of the two circuits goes back onto the nuetral, or is this the bad diagram?

Correct, which is what the properly wired drawing is showing.

The improperly wired drawing is showing that both legs of the multi-wire circuit are connected to the same hot leg (thus there is -0- volts between them as shown). With -0- volts between the two legs, all the current for each load must now go through the neutral, making the current additive, which makes the 20 amps shown.

Does that help?

[quote=Jerry Peck;60904]When properly wired, sort of what you are saying (but not really).
Now, however, let us introduce a 10 amp load and a 15 amp load, the first 10 amps goes around the 240 volt circuit, however, the extra 5 amps cannot go through the other load (otherwise you would have two 15 amp loads), so that 5 amps goes back through the neutral.quote]

Yes sir it does. May I ask this question;?
The 5 amps back onto the neutral, can this scortch the conductor at the bus?

No --the 5 amps is well below the current carrying capacity. I would look for a loose neutral termination. It may have been corrected and not loose currently. I would recommend cutting the wire off a couple of inches and re-terminate in a different terminal location.

[quote=Marc M;60912]

Originally Posted by Jerry Peck

The 5 amps back onto the neutral, can this scortch the conductor at the bus?

Originally Posted by Roland Miller

No --the 5 amps is well below the current carrying capacity. I would look for a loose neutral termination. It may have been corrected and not loose currently. I would recommend cutting the wire off a couple of inches and re-terminate in a different terminal location.

Roland answered that pretty well.

Originally Posted by Jerry Peck

This is a drawing depicting what Gunnar described.

Ooooh... You mean, I really got that right? Wow! Cool!

JP

I like your diagram, but it seems to me that it needs another sketch of the "properly wired" with 10 amps on one leg, 15 amps on the other leg and 5 on the neutral to more clearly show the amperage difference that you described. I will see if my computer has some software that I can do that with.

Originally Posted by Gunnar Alquist

I like your diagram, but it seems to me that it needs another sketch of the "properly wired" with 10 amps on one leg, 15 amps on the other leg and 5 on the neutral to more clearly show the amperage difference that you described.

Like this?

Originally Posted by Jerry Peck

Like this?

Oooh! Poifect!

Here is another one, representing yet another operational mode of a multi-wire circuit.

JP.

Pretty soon, I am going to have my own slide show. Thanks for all the diagrams.

Gunnar,

Here's a little something I drew up for another purpose.

And now they are gone!

Originally Posted by Jerry Peck

Gunnar, Here's a little something I drew up for another purpose.

Ok, so the teeth look like they are going to "ratchet" down as pressure is applied. Pressure on the underside of the toothed clasp should unlock the bracket, so it is meant to be reused. I cannot figure scale, but it is clearly a clamp of some kind. Looks like it is designed to be in that particular triple array. Conduit, pipe or large gauge NM cable clamp? Or is it larger? Let's see... Since you live in Florida, could it be some kind of gantry for the next generation of orbital launch vehicle?