# How to break up a circuit with a shared neutral?

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• 09-19-2008, 08:57 AM
canuk
Re: How to break up a circuit with a shared neutral?
[quote=ZZZ;35755] If you have two 120 volt, 15amp heaters on seperate poles for example, with a shared neutral, the current on the neutral would be zero amps. The same would go for incandescant lamps or any other purely resistive loads. [/quote]
Theoritically ..... only if the loads on the 2 legs are perfectly balanced ... which is unlikely in real world applications.

[quote]
As far as damage from a bad connection on the neutral, it is not as likely as damage from a bad connection on the line where the most amperage is flowing. This is why most burned connections are on the main breaker and not on the service neutral.
[/quote]On branch circuits sharing neutral electronic equipment can be damaged without necessarily seeing burned connections on the main breaker.
• 09-19-2008, 03:03 PM
JLMCDANIEL
Re: How to break up a circuit with a shared neutral?
Although balanced loads or nearly balanced loads work, I always recommend against shared neutral in a residential application. As and example one circuit feeds the bathroom which has a vent fan, a ceiling light, a vanity light, etc. When every thing is on the draw is say 12 amps. Ont the other branch you have an attic vent fan drawing 12 amps. every thing is OK until the neutral break or bad connection, and you have one 60 watt light on in the bath and the vent fan is running. Now you are running 240 volts through the circuit, dropping 220+ volts on the fan and 18+ volts on the light and the circuit is drawing over 22 amps. A 20 amp breaker may or may not trip and you will burn out the light or the attic fan. It's very difficult to maintain balance loads in a residential application.
Jack
• 09-19-2008, 08:14 PM
Ernie_Fergler
Re: How to break up a circuit with a shared neutral?
[QUOTE=canuk;35777]Theoritically ..... only if the loads on the 2 legs are perfectly balanced ... which is unlikely in real world applications.

On branch circuits sharing neutral electronic equipment can be damaged without necessarily seeing burned connections on the main breaker.[/QUOTE]

Unlikely in the real world, but not uncommon.
• 09-19-2008, 08:20 PM
canuk
Re: How to break up a circuit with a shared neutral?
Generally speaking the multiwire branch circuits ( MWBC ) are safe and fine to use providing everything is wired correctly. As I mentioned earlier the service to the home is the same thing ( shared neutral ) without giving it a second thought.

Personally I like to wire most circuits with a seperate neutral but there are times when convienience comes into play and I'lll use the MWBC for lights only.
This way I'd rather have the 240 split on light bulbs than having 180 volts running through my DVD player.
• 09-19-2008, 08:23 PM
JLMCDANIEL
Re: How to break up a circuit with a shared neutral?
[quote=ZZZ;35823]Jack,
You are correct also, it will very likely damage something if you lose the neutral due to the higher than normal voltage that results. I do not like using them in residential either. I was just trying to clear up the originator's fear of high neutral currents, which [B]will never be higher[/B] than the highest line current at any time. Changing one conductor to a ground is a very good idea also.
Nonetheless, is still acceptable and approved to use multiwire branch circuits. On the other hand, it would not bother me a bit if the NEC bans them at some point. It is not necessary to balance the load in these applications either, but it is always good practice. Don't forget, the service to a residence it using a shared neutral for two lines and no one would ever ask for an additional neutral for it.[/quote]
So true, sub panels also, however my concern is that it is many times more likely to have a connection problem with the application of a wire nut than a terminal connection and it's more likely to be done by an armature than the service connections.
Jack
• 09-19-2008, 08:31 PM
canuk
Re: How to break up a circuit with a shared neutral?
[quote=Ernie_Fergler;35825]Unlikely in the real world, but not uncommon.[/quote]Yep ..... pretty smart coal cracker ;):D
• 09-19-2008, 08:34 PM
canuk
Re: How to break up a circuit with a shared neutral?
Jack ... that's no reason to condem MWBC rather consider the consequences when it.s done improperly .... as with anything electrical.
• 09-19-2008, 08:53 PM
JLMCDANIEL
Re: How to break up a circuit with a shared neutral?
[quote=ZZZ;35833]Are they worse than amatures?: )

I dislike them especially because of the double or triple pole breaker requirement, you have to shut down additional circuits to work on them. Worse,if you don't have them (like in his house) you can be electrocuted by the opening the neutral not knowing it has a second feed.[/quote]
It's not nice to make fun of senior citizen's mistypes.:D Besides it's spelled amateurs.:p
Jack
• 09-19-2008, 09:00 PM
JLMCDANIEL
Re: How to break up a circuit with a shared neutral?
But it just doesn't seem to help with "Old-timers" disease.:eek:
Jack
• 09-19-2008, 09:48 PM
canuk
Re: How to break up a circuit with a shared neutral?
[quote=ZZZ;35833]Are they worse than amatures?: )

[COLOR=blue]I dislike them especially because of the double or triple pole breaker requirement,[/COLOR] you have to shut down additional circuits to work on them. Worse,if you don't have them (like in his house) [COLOR=blue]you can be electrocuted by the opening the neutral not knowing it has a second feed[/COLOR].[/quote]
Wouldn't that be the reason for the breakers?;):)
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