ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Top Posters(30 Days)
Potseal 11
Recent Posts
600 KW 120/208 3 Phase Y protection
by Yooperup. 07/21/17 09:27 AM
1913 American Electrician's Handbook
by gfretwell. 07/20/17 01:08 PM
Green House wiring
by ghost307. 07/20/17 09:10 AM
Permit Snafus...AHJs and Contractors Jump in
by HotLine1. 07/18/17 08:06 PM
New in the Gallery:
SE cable question
Popular Topics(Views)
239,190 Are you busy
174,582 Re: Forum
167,018 Need opinion
Who's Online Now
2 registered members (Dapo, Trumpy), 36 guests, and 6 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate This Thread
Page 1 of 3 1 2 3
#32138 - 12/16/03 07:06 PM losing neurtal  
sparkync  Offline
Joined: Oct 2002
Posts: 826
Could someone refresh me on the theory, that when a main neutral is lost on a 120/240 volt circuit, it throws 240 volts on one leg?? Thanks, I've run across this several times in the past.. Steve Has something to do with electricity seeking ground, right??

Work Gear for Electricians and the Trades

#32139 - 12/16/03 08:04 PM Re: losing neurtal  
NJwirenut  Offline
Joined: Sep 2001
Posts: 806
Bergen County, NJ
Opening the neutral connection effectively places the 2 halves of the panel in series with each other, across the 240V line, and the voltages across each half divide according to Ohm's law. If the loads on the 2 halves were perfectly balanced, each side would see 120V, and there would be no detectable problem. If one phase is more heavily loaded, it will drop LESS voltage, with the difference appearing across the opposite side.

Worst case would be a small electronic load (VCR?) on one side of the panel, and a space heater on the other side. The VCR is across 240V, with only the low resistance of the heater in series with it. Unless designed to take that voltage (some are), the VCR is toast VERY quickly.

A sure sign of an open neutral somewhere are lamps that get brighter when larger loads cycle on.

#32140 - 12/16/03 08:07 PM Re: losing neurtal  
Bill Addiss  Offline
Joined: Oct 2000
Posts: 3,878
Hopefully someone can offer a more technical answer here, but basically the voltage on each 'leg' will depend on the resistance/impedance of the connected load on that 'leg' at that moment.

Both Voltages could be equal if both loads were perfectly balanced, but Murphy's Law says that one will be high and one low.


#32141 - 12/16/03 08:09 PM Re: losing neurtal  
Bill Addiss  Offline
Joined: Oct 2000
Posts: 3,878

I've got to learn to type faster.
Your post wasn't there when I started typing.
Good response.

[Linked Image]

[This message has been edited by Bill Addiss (edited 12-16-2003).]

#32142 - 12/16/03 08:13 PM Re: losing neurtal  
Roger  Offline
Joined: May 2002
Posts: 1,716
Steve, first of all it will not put 240v on one leg, it will be E = I x R on each load in the circuit, which is now a 240v series circuit.

Now, electricity doesn't seek ground, (earth) it seeks it's source.

It can however use ground (earth) as a conductor.


#32143 - 12/16/03 09:42 PM Re: losing neurtal  
sparky  Offline
Joined: Oct 2000
Posts: 5,311
sometimes a pix helps....
[Linked Image]

#32144 - 12/16/03 09:49 PM Re: losing neurtal  
electure  Online Sleepy

Joined: Dec 2000
Posts: 4,260
Fullerton, CA USA
Any circuit that shares a "NOODLE" [Linked Image] with the other will be a series circuit.
The circuits would be just fine if they were equally balanced, but they aren't usually, therefore the voltages "float", according to the load.
I'm crappy at explaining this...
Help from Scott T??

#32145 - 12/16/03 10:10 PM Re: losing neurtal  
Attic Rat  Offline
Joined: Dec 2003
Posts: 524
Bergen Co.,N.J. USA
y'know, I had this happen...not a week ago...I just got finished making my splices in a j-box in the attic...(of course,where else would I be...?)and I had brought up a 14-3 nm from the basement, to the attic for some receptacles and lighting...the 2 phases were then energized and on finishing, I realized I didn't like the way the noodles were spliced so I untwisted them,and to my surprise...the temporary pig-tail lampholders I had strung up had SUPERNOVA'd...I put the "tidy whities" back together and the lamps went back to normal again...I really got scared...I thought somethin' was really awry here, and I called a buddy o'mine who told me to "calm down" and that "that phenomenon is a normal occurance"...phew, I thought somethin' was wrong with my wiring and the sheet-rocker just got done w/ his 1st coat of spackle...I had split the neutral between the receptacles and the lighting....I'd never run into this until recently....boy did I feel stooopid!!!...I also came up on the reverse of this....after losing the noodle on a 2-wire circuit,that I was troubleshooting, I put my DMM across the hot and neutral and read about 78 volts...I guess thats because the current was reading through the lamp filaments and the resistance thru it lowered the actual 110-120volts supplied to it....I eventually found the bad neutral splice... [Linked Image] [Linked Image]

[This message has been edited by Attic Rat (edited 12-16-2003).]

[This message has been edited by Attic Rat (edited 12-16-2003).]

.."if it ain't fixed,don't break a Licensed Electrician"

#32146 - 12/16/03 11:11 PM Re: losing neurtal  
Roger  Offline
Joined: May 2002
Posts: 1,716
I know this is primitive and simple but here goes. Don't pay attention to the arrows.

[Linked Image]

The circuit has 5 amps flowing.
If we look at each load and use E = I x R

The 36 ohm load would see 180 volts

5 x 36 = 180 volts

The 12 ohm load would see 60 volts

5 x 12 = 60 volts

180 + 60 = 240 volts.

I will practice before I post any more drawings. [Linked Image] [Linked Image]


#32147 - 12/17/03 11:57 AM Re: losing neurtal  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Just to add that the situation will become a little more complex if a parallel neutral path exists.

If you lost the incoming service neutral, there may still be a circuit back to the transformer neutral via the grounding electrode, water pipes, etc.

The resistance of this path will be higher than that of the service neutral. The greater this resistance, the more the imbalance between the two sides of the circuit will be.

Page 1 of 3 1 2 3

Member Spotlight
Campbell, CA
Posts: 356
Joined: August 2006
Show All Member Profiles 

2017 NEC and Related
2017 NEC
Now Available!

Shout Box
Powered by UBB.threads™ PHP Forum Software 7.6.0
Page Time: 0.016s Queries: 15 (0.003s) Memory: 0.8219 MB (Peak: 0.9997 MB) Zlib enabled. Server Time: 2017-07-22 00:53:13 UTC