Shared Neutrals

We were in a commercial panel today where sharing neutrals is pretty common and one neutral must not have been tightened down very well because it was pretty crispy and black. I mentioned to the guy I was with that we should get those circuits off so we could fix that neutral but he didn't want to because he thought it would be a pain to figure out what circuits were sharing that neutral. He just tightened down the screw more. So I ask you, how would you figure out what neutrals go to what circuits?
 

JFletcher

Senior Member
Location
Williamsburg, VA
Visually trace the wire to what conduit it enters from the panel, then trace back what ungrounded conductors come out of that conduit, and kill those circuits.

Tying neutrals together outside a circuit violates at least two rules I can think of: paralleling of smaller conductors and not running all CCC in the same circuit/raceway (too late to look up code articles).
 

James L

Senior Member
If it's a solid conductor, it can be fixed without having to disconnect it, without having to turn anything off.

With it still landed, go back to a spot that's not burnt and skin the insulation off with a knife, about an inch long. Then twist a pigtail into it with your Kleins and land that in the neutral bar. Then cut off the bad one and remove it from the neutral bar. Then wire nut your new splice
 
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Smart $

Esteemed Member
Location
Ohio
If it's a solid conductor, it can be fixed without having to disconnect it, without having to turn anything off.

With it still landed, go back to a spot that's not burnt and skin the insulation off with a knife, about an inch long. Then twist a pigtail into it with your Kleins and land that in the neutral bar. Then cut off the bad one and remove it from the neutral bar. Then wire nut your new splice
What part of that happens with the panel de-energized?
 

JFletcher

Senior Member
Location
Williamsburg, VA
Without having to turn anything off
I think what smart is getting at is that working the panel live is an osha no-no.

Come back after business hours, kill the whole thing, then fix it. even if repairing it hot were ok, you still need to figure out why you fried the noodle in the first place.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Using the amp clamp on the neutral and shutting breakers off until it reads 0 is a good, until that old fridge on circuit Z decides to kick on and you have that bootlegged neutral in hand. Shutting the main off is the safest way. I don't have any customers that say absolutely no shut down

We have a restaurant that has been a customer for years and the CBs for MWBCs are not neccessarily in sequential order in the panel. I am dreading the day we have to start using handle ties.
 

jaggedben

Senior Member
I think it's appropriate to mention that not grouping the neutrals with the hots (e.g. using zipties) is a code violation.

There's a good chance that the charred neutral got that way not merely because of not being tightened, but because it's improperly shared between two circuits on the same phase.
 

kwired

Electron manager
Location
NE Nebraska
I think it's appropriate to mention that not grouping the neutrals with the hots (e.g. using zipties) is a code violation.

There's a good chance that the charred neutral got that way not merely because of not being tightened, but because it's improperly shared between two circuits on the same phase.
Improperly shared or not, if the only overheated part is local to the connection - the connection had resistance. If full length of conductor shows signs of overheating - it obviously was overloaded.
 

jaggedben

Senior Member
Improperly shared or not, if the only overheated part is local to the connection - the connection had resistance. If full length of conductor shows signs of overheating - it obviously was overloaded.
An overloaded conductor will tend to heat up at the terminations first because of the higher resistance there. I've seen it before when there was no evidence of a badly made connection. But I did say "a good chance" and not "for sure."
 

Smart $

Esteemed Member
Location
Ohio
An overloaded conductor will tend to heat up at the terminations first because of the higher resistance there. ...
Actually, a proper termination should have less resistance than the conductor itself. However, a proper termination can degrade when subjected to an overload condition (which is why terminals have temperature limitation ratings).

You are correct. An over-temperature condition escalates faster near the termination as a result of its degradation.
 

The Spunkster

Member
Location
NJ----USA
I've seen burned neutrals at termination points caused by a failed or loose neutral back at the source. The farther from the source the neutral extended,the more severe the the damage. It's one of those things,the simplest approach works best.
 

kwired

Electron manager
Location
NE Nebraska
Actually, a proper termination should have less resistance than the conductor itself. However, a proper termination can degrade when subjected to an overload condition (which is why terminals have temperature limitation ratings).
Your average MWBC will probably be 12 AWG conductors. You will generally have 20 amp overcurrent protection on the ungrounded conductors. The insulation will typically be rated 90C and many neutral bars will also be 90C (as a stand alone value anyway, as part of the panel assembly instructions may still call for 75C) Not trying to suggest pushing things to the limits, just saying if you misconfigured a MWBC and had two 120 volt leads from same leg sharing a common neutral that the heating effects probably are not too dramatic on the neutral and the panelboard neutral termination in most cases as the 12 AWG neutral can probably handle 30 amps before excessive heating starts to become much issue.

Now place several of these overloaded neutrals on same bar and the heat will add up fast if the overloading is pretty frequent, but just a random one once in a while probably lasts a long time on average.

waiting to be shot down:)
 
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