Two Circuits On One Wire - Electrical - Contractor Talk

Two Circuits On One Wire

 
Thread Tools Search this Thread Display Modes
Old 11-14-2018, 12:54 PM   #1
Registered User
 
Occva's Avatar
 
Trade: Home Improvement
Join Date: Oct 2016
Posts: 10
Rewards Points: 24

Two Circuits On One Wire


Had an old customer that wanted a vanity light changed out in a basement bath. I turned the switch off and removed the old light. When putting the new fixture mount on the neutral touched the ground and went zap. Tripped breaker 7. Thought that was strange. Got my volt meter out and checked hot wire with the switch on, read 120. Hot wire is breaker 8. Turned 7 back on and checked neutral, read 120.
This is 40's vintage. Just two wires. The ground is screwed to the back of the metal box.
How is this possible? It works fine.
Brought it to the home owners attention. We agreed to cap both wires and have my electrician look at it. He can't get there until next week.
__________________
Excuse me if I seem winded. I just got back from my French self defence class. Never ran so hard in my life!
Occva is offline  

Warning: The topics covered on this site include activities in which there exists the potential for serious injury or death. ContractorTalk.com DOES NOT guarantee the accuracy or completeness of any information contained on this site. Always use proper safety precaution and reference reliable outside sources before attempting any construction or remodeling task!

   

Advertisement

 

Old 11-14-2018, 01:47 PM   #2
I own stock in FotoMat!
 
480sparky's Avatar
 
Trade: 132 on Bush, I've got him at gunpoint!
Join Date: Jan 2009
Location: Iowa
Posts: 11,830
Rewards Points: 11,412

Re: Two Circuits On One Wire


It's called a multi-wire branch circuit. And if your panel numbering is correct, it's not allowed on circuits 7 and 8. It's basically two circuits with one neutral and it can be overloaded.

Just let your Sparky handle it.

Advertisement

__________________
If there's a highway to hell and only a stairway to heaven, that pretty much tells you what the traffic patterns are expected to be.
480sparky is online now  
The Following 2 Users Say Thank You to 480sparky For This Useful Post:
Occva (11-14-2018), rselectric1 (11-14-2018)
Old 11-14-2018, 09:09 PM   #3
Renaissance Man
 
superseal's Avatar
 
Trade: Masonry, Carpentry and everything else.
Join Date: Feb 2009
Location: Southeastern PA
Posts: 6,788
Rewards Points: 5,774

Re: Two Circuits On One Wire


Shared neutral...I got one at my house on 6/8 with a tie bar which I put on after making sparks when messing with it...vintage 40's home saving some scratch from the war effort.
__________________
"Under promise, over deliver and all that!" JBM
supersealconstruction.com

https://www.facebook.com/Superseal-C...2699413750013/
superseal is offline  
The Following User Says Thank You to superseal For This Useful Post:
Occva (11-14-2018)
Sponsored Links
Advertisement
 
Old 11-14-2018, 09:40 PM   #4
I own stock in FotoMat!
 
480sparky's Avatar
 
Trade: 132 on Bush, I've got him at gunpoint!
Join Date: Jan 2009
Location: Iowa
Posts: 11,830
Rewards Points: 11,412

Re: Two Circuits On One Wire


FYI: Opening a shared neutral can burn things up that are attached to the circuits.
__________________
If there's a highway to hell and only a stairway to heaven, that pretty much tells you what the traffic patterns are expected to be.
480sparky is online now  
The Following User Says Thank You to 480sparky For This Useful Post:
John Valdes (11-23-2018)
Old 11-15-2018, 07:54 PM   #5
back doors to hell put in
 
Irishslave's Avatar
 
Trade: Carpenter
Join Date: Jun 2010
Location: Google knows
Posts: 3,517
Rewards Points: 1,344

Re: Two Circuits On One Wire


Quote:
Originally Posted by 480sparky View Post
FYI: Opening a shared neutral can burn things up that are attached to the circuits.
It's why I don't like them. We've all seen what happens when a service neutral goes wonky. European system has no neutral everything is 240v and if I'm not mistaken 50 cps instead of 60
__________________
Television is fake...The internet is real
Irishslave is online now  
The Following User Says Thank You to Irishslave For This Useful Post:
Tom M (02-11-2019)
Old 11-15-2018, 07:59 PM   #6
I own stock in FotoMat!
 
480sparky's Avatar
 
Trade: 132 on Bush, I've got him at gunpoint!
Join Date: Jan 2009
Location: Iowa
Posts: 11,830
Rewards Points: 11,412

Re: Two Circuits On One Wire


Quote:
Originally Posted by Irishslave View Post
It's why I don't like them. We've all seen what happens when a service neutral goes wonky. European system has no neutral everything is 240v and if I'm not mistaken 50 cps instead of 60
Too bad 'pert near every electrical service in the US is a MW circuit........
__________________
If there's a highway to hell and only a stairway to heaven, that pretty much tells you what the traffic patterns are expected to be.
480sparky is online now  
Old 11-15-2018, 08:13 PM   #7
back doors to hell put in
 
Irishslave's Avatar
 
Trade: Carpenter
Join Date: Jun 2010
Location: Google knows
Posts: 3,517
Rewards Points: 1,344

Re: Two Circuits On One Wire


Quote:
Originally Posted by 480sparky View Post
Too bad 'pert near every electrical service in the US is a MW circuit........
Never argue with an expert..........Thanks
__________________
Television is fake...The internet is real
Irishslave is online now  
Old 11-15-2018, 08:15 PM   #8
Capra aegagrus

 
Tinstaafl's Avatar
 
Trade: Remodeler
Join Date: Jan 2008
Location: Central Pennsylvania
Posts: 22,850
Rewards Points: 703

Re: Two Circuits On One Wire


Ran into that when we bought our house. It was powered by a transformer at the neighbor's, a couple of hundred yards away. A heavy load on one side of the line would pop light bulbs on the other side.

POCO called it a "swinging neutral", and wound up giving us our very own transformer. But I had to fight my way through proving it to 5-6 different linemen and company reps, with my equipment and theirs. I still have a bit of a bad taste from that experience.
Tinstaafl is online now  
The Following User Says Thank You to Tinstaafl For This Useful Post:
Fouthgeneration (02-13-2019)
Old 11-17-2018, 07:43 PM   #9
Pro
 
Fouthgeneration's Avatar
 
Trade: Masonry
Join Date: Jan 2014
Location: Mid west
Posts: 2,148
Rewards Points: 7,689

Re: Two Circuits On One Wire


Another reason to install as many appliances on 240 Volt circuits...
Air compressors, furnace motors, some kitchen appliances.

Suffered the loss of several motors and battery chargers on poorly balance 2-3 phase temp panels in the last ten years...

I am guessing both houses were wired by same sparky, and had systemically uneven loading of the half phase busses. plus under sized service wire

The current generation of lighting has to be changing actual % of electrical loads compared to legacy homes 20-30 years ago, reactive vs resistance, power factor and single pole vs dual pole supply/demand...

As Green energy State & federal mandates further devolve our Nation's Electrical Grid's day in day out performance, The need for constant monitoring of the homes power and other utilities actual values versus nominal numbers will soon be standard, IMHO.
__________________
I might be an Idiot, but I know things that You don't: Please wait till after you get my know-how to insult me....
Fouthgeneration is online now  
Old 11-18-2018, 01:47 PM   #10
Pro
 
rescraft's Avatar
 
Trade: residential remodeling/construction
Join Date: Nov 2007
Posts: 519
Rewards Points: 1,071

Re: Two Circuits On One Wire


Always pigtail the neutrals in a MWBC
rescraft is offline  
Old 12-16-2018, 06:38 AM   #11
Registered User
 
caviepc's Avatar
 
Trade: electric
Join Date: Dec 2018
Location: SW Florida
Posts: 6
Rewards Points: 12

Re: Two Circuits On One Wire


NEC requires multiwire circuits to be on 2 pole breaker so if one has a problem they both trip.
caviepc is offline  
Old 12-16-2018, 10:26 AM   #12
I own stock in FotoMat!
 
480sparky's Avatar
 
Trade: 132 on Bush, I've got him at gunpoint!
Join Date: Jan 2009
Location: Iowa
Posts: 11,830
Rewards Points: 11,412

Re: Two Circuits On One Wire


Quote:
Originally Posted by caviepc View Post
NEC requires multiwire circuits to be on 2 pole breaker so if one has a problem they both trip.
The NEC only requires both circuits get turned off together. A 2-pole breaker (or 3-pole) is merely the simplest and easiest method.
__________________
If there's a highway to hell and only a stairway to heaven, that pretty much tells you what the traffic patterns are expected to be.
480sparky is online now  
The Following User Says Thank You to 480sparky For This Useful Post:
Fouthgeneration (12-17-2018)
Old 02-11-2019, 05:01 PM   #13
Electrical Contractor
 
C'est Moi's Avatar
 
Trade: Electrical
Join Date: Jun 2015
Location: NC
Posts: 243
Rewards Points: 548

Re: Two Circuits On One Wire


Quote:
Originally Posted by caviepc View Post
NEC requires multiwire circuits to be on 2 pole breaker so if one has a problem they both trip.
The nec doesn't really require a dp breaker. You can use 2 sp breakers with a breaker tie. These can actually trip one circuit and not trip the other.

Quote:
210.4(B)Disconnecting Means. Each multiwire branch circuit shall
be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch
circuit originates.
Quote:
240.15(B) Circuit Breaker as Overcurrent Device. Circuit breakers
shall open all ungrounded conductors of the circuit both
manually and automatically unless otherwise permitted in
240.15(B)(1), (B)(2), (B)(3), and (B)(4).

(1) Multiwire Branch Circuits. Individual single-pole circuit
breakers, with identified handle ties,
shall be permitted as the
protection for each ungrounded conductor of multiwire
branch circuits that serve only single-phase line-to-neutral
loads.
C'est Moi is offline  
Old 02-11-2019, 06:11 PM   #14
Registered User
 
caviepc's Avatar
 
Trade: electric
Join Date: Dec 2018
Location: SW Florida
Posts: 6
Rewards Points: 12

Re: Two Circuits On One Wire


Quote:
Originally Posted by C'est Moi View Post
The nec doesn't really require a dp breaker. You can use 2 sp breakers with a breaker tie. These can actually trip one circuit and not trip the other.
For the cost difference I can't be bothered with 2 Single pole breakers and tie handles.
caviepc is offline  
Old 02-12-2019, 06:49 AM   #15
Electrical Contractor
 
C'est Moi's Avatar
 
Trade: Electrical
Join Date: Jun 2015
Location: NC
Posts: 243
Rewards Points: 548

Re: Two Circuits On One Wire


Quote:
Originally Posted by caviepc View Post
For the cost difference I can't be bothered with 2 Single pole breakers and tie handles.
I get that but I wanted to make sure others understood that it did not need to be a dp breaker. No biggie. I don't use multiwire branch circuit because of the afci issue and before that I didn't like them because when you loose a neutral you have problems.

Ken I know the feeder and the service conductors are multiwire branch circuit but I would like to minimize the issues if I can.

Advertisement

C'est Moi is offline  


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Down Idle wire broke on Rigid pressure washer Jas2012 Pressure Washing 8 06-19-2013 05:31 PM
3-way wiring- ceiling fan with remote for two-wire hookup hevnbnd Electrical 29 08-17-2009 10:28 PM

Join Now... It's Fast and FREE!

I am a professional contractor
I am a DIY Homeowner
Drywall Talk is for
PROFESSIONAL CONTRACTORS ONLY!

At DrywallTalk.com we cater exlusivly to professional contractors who make their living as a contractor. Knowing that many homeowners and DIYers are looking for a community to call home, we've created www.DIYChatroom.com DIY Chatroom is full of helpful advices and perfect for DIY homeowners.

Redirecing in 10 seconds
No Thanks
terms of service

Already Have an Account?