Go Back  DoItYourself.com Community Forums > Electrical, AC & DC. Electronic Equipment and Computers > Electrical - AC & DC
Reload this Page >

Dedicated circuit with a shared neutral. Saw sparks with circuit off, surprise!

Dedicated circuit with a shared neutral. Saw sparks with circuit off, surprise!

Reply

  #1  
Old 12-09-13, 08:23 PM
Member
Thread Starter
Join Date: Dec 2013
Posts: 3
Dedicated circuit with a shared neutral. Saw sparks with circuit off, surprise!

I've been going through my home (purchased two year ago) replacing many of the older, corroded electrical receptacles, many of which were installed 35+ years ago using "push-in" connections. I'm replacing them with good quality side-clamp receptacles.

Yesterday I came across an unusual situation and I would like to ask some advice:

I decided to change the refrigerator receptacle. This was on a (supposedly) dedicated line with its own 20-amp breaker. I switched off the breaker, unplugged the fridge and tested the socket to make sure all three wires were "cold". Then I plugged the fridge into a nearby counter plug (on a separate 20-amp circuit) in order to keep it running. (None of the other counter plugs on this circuit were in use). Then I began to disconnect the refrigerator's receptacle.

First surprise: The receptacle had two white neutral wires connected to it, coming from two different bundles of 12-gauge Romex in the back of the box. There was a single black wire connected to the receptacle, plus a spliced ground from both bundles. I found this to be a bit puzzling, as I thought this was supposed to be a dedicated refrigerator circuit that had only a single outlet, but ok, I proceeded forward.

Second surprise: One of the neutral wires was hot! (I saw sparks when I began to disconnect it.) This was very unexpected, because it wasn't hot when I tested the circuit the first time, nor of course was there any known reason for it to be hot. The only differences were the fact that I had plugged the refrigerator into a nearby counter receptacle, and that I was disconnecting the connection between the two white neutral wires.

It turns out that the nearby counter receptacles were all hooked up to the neutral wire that was connected to the refrigerator receptacle. When I plugged the refrigerator into the counter receptacle it apparently put some juice onto the neutral wire. I quickly unplugged everything and tested the counter circuits and found that their only connection to neutral ran through the refrigerator receptacle.

I had to shut off both circuits so I could work safely on my so-called "dedicated" refrigerator circuit.

Q1: Why on earth would an electrician have hooked it up that way? (It was done back in 1975, apparently). Was this done according to code?

Q2: Is it ok to leave things as they are? At this point I've just reconnected everything as it was, using the new receptacle, but I'm wondering whether or not I ought to redo the counter plug circuit so it doesn't share a neutral wire with my dedicated refrigerator circuit.
 
Sponsored Links
  #2  
Old 12-09-13, 09:37 PM
Justin Smith's Avatar
Member
Join Date: Dec 2010
Location: Cressona, Pa, USA
Posts: 2,546
The neutral needs to be pigtailed. Your breakers will need to be handle tied under current code, but not then.
 
  #3  
Old 12-09-13, 10:41 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 46,084
When you have a three wire circuit like that, the neutral is of the most importance. Like Justin mentioned.... the neutral wires should be in a spliced connection with a tail to the receptacle.

If that neutral opens when the circuit is in operation you can get low voltage on one circuit and high on another.

Turn both circuits off. Remove the whites from the receptacle and splice them together with a short piece of the same gauge white. Connect the tail to the receptacle and you'll be good to go.
 
  #4  
Old 12-10-13, 06:40 AM
Member
Join Date: Mar 2010
Location: USA
Posts: 3,091
It may have been done to code back then (in 1975) but the revisions described above are in the current (National Electric) code to prevent your experiencing the sparking you just experienced.

When you are working on a multiwire branch circuit (with shared neutral), both halves need to be shut off while you are working.

When the incoming neutral and continuing neutral are attached to separate screws on a receptacle or other device then the matching hot(s) must be similarly attached to separate screws on the same device which would mean that if you removed the device thus breaking the neutral path, then the continuing hot path(s) will also be broken.
 
  #5  
Old 12-10-13, 02:04 PM
Member
Join Date: Nov 2013
Posts: 281
You guys better put a sticky up about multiwire branch circuits ... the posts have been popping up.
 
  #6  
Old 12-10-13, 02:58 PM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
Welcome to the forums!

The receptacle had two white neutral wires connected to it, coming from two different bundles of 12-gauge Romex in the back of the box. There was a single black wire connected to the receptacle, plus a spliced ground from both bundles.
I assume you're referring to the two cables as "bundles." If so, I see two white wires, two ground wires and one black wire.

I'm wondering what happened to the other black wire? Is it, by any chance, spliced to a red wire in the other cable?

I'm wondering whether or not I ought to redo the counter plug circuit so it doesn't share a neutral wire with my dedicated refrigerator circuit.
There should be no need, and nothing really to be gained, by doing that, provided that you make the other improvements suggested here. That is, the two vertically adjacent single-pole breakers with their handles joined with a handle tie, and the neutrals spliced with a pigtail.

Your panel listing says that your refrigerator is on a dedicated circuit. It does not say that it's on an IG - Isolated or Independent Ground - circuit. So long as no other load shares the hot/neutral pair with the refrigerator, that's true.
 
  #7  
Old 12-10-13, 03:28 PM
Member
Thread Starter
Join Date: Dec 2013
Posts: 3
Thanks! I understand the situation much better now. I will splice/pigtail the neutrals and will handle-tie the two breakers.

As for the second black wire, I will look again, although I don't recall seeing any splices in the back of the box.

I appreciate all the help.
 
  #8  
Old 12-10-13, 04:04 PM
canuk's Avatar
Member
Join Date: Sep 2013
Posts: 293
This is the reason why codes changed to mandate the double breakers are common trip. This is to ensure if one circuit sharing the neutral goes down the other goes down to prevent an unbalanced load and for safety to anyone working on one of the circuits.
 
  #9  
Old 12-10-13, 04:49 PM
Member
Join Date: Nov 2013
Posts: 281
This is the reason why codes changed to mandate the double breakers are common trip. This is to ensure if one circuit sharing the neutral goes down the other goes down to prevent an unbalanced load and for safety to anyone working on one of the circuits.
Its my understanding that turning off one circuit, and then opening the neutral would not put the unbalanced load over the circuits. The other circuit would just not be complete. Now if you open up the neutral with both circuits ON, then you would have the unbalanced 240 load.

I assume the code requirement for handleties or dp breakers is so someone doesnt get electrocuted by the neutral while working with only one of the circuits off.
 
  #10  
Old 12-10-13, 05:06 PM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
I assume the code requirement for handleties or dp breakers is...
Just to be clear, there is no requirement in the code for either handle ties or double pole (in residential) breakers for the protection for a MWBC. The code is seldom written in a way that will limit options.

The requirement is that the protection will provide for common disconnect, or open. While either of the methots you mentioned will provide that, many of us prefer to only use independent breakers with handle ties, so that each circuit can still trip.

... so someone doesnt get electrocuted by the neutral while working with only one of the circuits off.
Yes. Well, so they don't get shocked, anyway.
 
  #11  
Old 12-11-13, 06:36 AM
Member
Join Date: Mar 2010
Location: USA
Posts: 3,091
The second white wire may not come into the box (refrigerator receptacle) all alone. I would guess that you had the multiwire branch circuit feed (red, white, black) enter that box with white and black connected to the receptacle. Then half of the MWBC becomes the dedicated refrigerator circuit. I would guess that another cable (downstream continuation) with just black and white also enters the box with its black connected to the feed red and its white connected to the receptacle.

When you took the receptacle out, it would have been the second (2 wire) cable's neutral that gave you the spark. That would be for the return current coming back from receptacles with switched on loads downstream.
 
  #12  
Old 12-11-13, 08:50 AM
Member
Thread Starter
Join Date: Dec 2013
Posts: 3
Thanks, I'll pull the refrigerator out again soon and will check for that.
 
  #13  
Old 12-11-13, 11:18 AM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
Originally Posted by AllanJ
I would guess that you had the multiwire branch circuit feed (red, white, black) enter that box with white and black connected to the receptacle. Then half of the MWBC becomes the dedicated refrigerator circuit. I would guess that another cable (downstream continuation) with just black and white also enters the box with its black connected to the feed red and its white connected to the receptacle.
Originally Posted by Nashkat1
I'm wondering what happened to the other black wire? Is it, by any chance, spliced to a red wire in the other cable?
.........................................................
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes