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

Seeing different voltages when measuring across different branch circuits

Seeing different voltages when measuring across different branch circuits

Reply

  #1  
Old 07-29-12, 12:36 PM
Member
Thread Starter
Join Date: Jul 2012
Posts: 5
Seeing different voltages when measuring across different branch circuits

Hello,

I know that the resounding answer here is probably going to be "get an electrician"... which I am going to do in a week or so. But in the meantime I am really wanting to know what's going on here.

I have one circuit in my house that is fairly overloaded. Amperage wise maybe not, but it's just going to too many lights. I'm trying to trim back this circuit and use other circuits to get power.

I converted my stove from electric to gas a while back, and turned that 40amp 240v circuit into a subpanel.

When I measure the voltage from either the 'overloaded' circuit or from a circuit on the sub, things look fine. However, when I measure across the hot wire from the sub panel and the neutral from the overloaded circuit, I am getting a lower voltage.... about 70V. Instinct tells me that this is wrong, but I don't have much AC theory under my belt and this is just boggling my mind. I wouldn't expect exact voltages, but a drop of 50 volts seems pretty weird.

Anyone have any clue what may be going on here?

I really appreciate any honest help here... as I said, I am having an electrician look at this soon.

Dave
Cincy, OH
 
Sponsored Links
  #2  
Old 07-29-12, 12:50 PM
Member
Join Date: Jul 2012
Location: USA
Posts: 54
hmmmm, i just joined this forum because i had an electrical problem too. Im no electrition, but do know basic wiring. Is it old wiring? Because i get a voltage drop on some old wiring in my house, but 70v or so is really low, im wondering if bad ground, or a bad Neutral? i would check the wiring from the panel(s).If you check each leg, it should be about 120v to ground. Great question, cant wait for answers Also, what kind of wire is it.. 8/3 ? ......2 hots, a Neutral, and a ground?
 

Last edited by rodog; 07-29-12 at 12:53 PM. Reason: added text
  #3  
Old 07-29-12, 12:58 PM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: USA
Posts: 9,391
when I measure across the hot wire from the sub panel and the neutral from the overloaded circuit, I am getting a lower voltage.... about 70V. Instinct tells me that this is wrong, but I don't have much AC theory under my belt and this is just boggling my mind. I wouldn't expect exact voltages, but a drop of 50 volts seems pretty weird.
Check voltage hot to ground. You probably have a loose neutral connection somewhere.
 
  #4  
Old 07-29-12, 01:37 PM
Member
Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
Assuming the overloaded circuit is " Circuit # 1" , if you measure 120 volts between the terminal of CB#1 and the Neutral terminal bar at the Service Panel , and 75 volts across the terminals of a receptacle connected to Circuit #1 , then there is a voltage loss or "drop" in Circuit #1 caused by a defective connection which has caused resistance to be "inserted" in the conductors of Circuit #1
The defective connection- point could be at a device terminal or at a "Wire-nut" joint and this could be either in a Neutral, White- wire connection or a "Live", Black-wire connection.

The location of the points where you measure the voltage is very important , and I advise that your 1st is to identify and locate all the outlets supplied by Circuit #1 before attempting to locate the problem-point.
 
  #5  
Old 07-29-12, 01:52 PM
Member
Thread Starter
Join Date: Jul 2012
Posts: 5
Thanks for the quick replies!

The only time I am seeing the.lower.voltage is when I measure between the hot on circuit 1.and.neutral on a circuit.that's on a different subpanel. If I measure across the hot and.neutral from either circuit 1 or 2 individually I get 120. Maybe I wasn't clear before.

If it is in fact.a loose wire somewhere this could be tricky...

Sorry for all the periods... on the cell phone.
 
  #6  
Old 07-29-12, 02:11 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,594
Your old circuit for the stove may not have had enough conductors to be converted to a subpanel feed. Did it have 4 wires?
 
  #7  
Old 07-29-12, 03:19 PM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: USA
Posts: 9,391
The only time I am seeing the.lower.voltage is when I measure between the hot on circuit 1.and.neutral on a circuit.that's on a different subpanel. If I measure across the hot and.neutral from either circuit 1 or 2 individually I get 120.
Why would you check voltage across the hot from a circuit from one source and the neutral from an entirely different source?

If I measure across the hot and.neutral from either circuit 1 or 2 individually I get 120
Which would be normal, so where is the problem? If your question and problem is with the subpanel, let's back up and answer the question pcboss has already asked.

pcboss

Your old circuit for the stove may not have had enough conductors to be converted to a subpanel feed. Did it have 4 wires?
 
  #8  
Old 07-29-12, 03:50 PM
Member
Thread Starter
Join Date: Jul 2012
Posts: 5
Why would you check voltage across the hot from a circuit from one source and the neutral from an entirely different source?
For no reason other than curiosity. There are a lot of circuits tied together at this one junction and I accidentally measured across different circuits, saw a weird voltage, and thought WTF?

Which would be normal, so where is the problem? If your question and problem is with the subpanel, let's back up and answer the question pcboss has already asked.
Well, I guess I simply posted the question to see if it was a problem. I'm not implying it is.

The line for the old electric stove had two hot and a neutral. Didn't have it's own ground, but the box is most definitely grounded to the common ground itself. All circuits off the subpanel work fine... dishwasher, disposal, etc etc.

Hell, I just realized that I can stick one probe on a hot wire (either from the main panel or the sub) and the other on an open neutral (I'm positive it's open cause it's coming out of the ceiling disconnected) and I'm still seeing around 80v. Maybe this really isn't a problem at all. I do really appreciate all these replies though!
 
  #9  
Old 07-29-12, 04:00 PM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 33,054
but the box is most definitely grounded to the common ground itself.
What do you mean by: "common ground". If you only have two hots and a neutral and there is no metallic conduit continuous to the supplying panel you don't have a ground. This may not be causing the voltage variations but it could be a safety issue.
 
  #10  
Old 07-29-12, 04:11 PM
Member
Thread Starter
Join Date: Jul 2012
Posts: 5
I have a pretty thorough grounding system, I'm an amateur radio operator and have several ground rods buried and interconnected; both panels are connected to this system. I could definitely run some #2 from panel to panel if that was necessary. The ungrounded 6-3 (I think it's 6g) that fed the stove was probably installed about 50 years ago when the house was built... guess they didn't require a ground back then?

I just checked and this is definitely not isolated to circuits on the subpanel. I checked the hot from the wacky circuit to an open neutral and I still see 80v or so.
 
  #11  
Old 07-29-12, 05:38 PM
Member
Join Date: Mar 2010
Location: USA
Posts: 3,098
You probably have a bad neutral path back to the panel.

If several circuits in the house drop to 70 or so volts, all dropping the same amount at the same time, then the problem is probably in the panel or outside going out to the meter or even on the way to the utility pole.

If some circuits rise in voltage while other drop then the problem is in the neutral. If some circuits drop but all the rest stay correct then the problem is in one of the hot lines.
 
  #12  
Old 07-29-12, 05:49 PM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 33,054
the stove was probably installed about 50 years ago when the house was built... guess they didn't require a ground back then?
That is correct.
I could definitely run some #2 from panel to panel if that was necessary.
#10 would be fine and a smart upgrade. It can be either green or bare. You will need to add a ground bar to the subpanel and remove the bonding screw or strap on the neutral bar.

If you are using a digital multimeter instead of analog your voltage readings below 110 volts may not be accurate.
 
  #13  
Old 07-29-12, 06:23 PM
Member
Thread Starter
Join Date: Jul 2012
Posts: 5
Yeah, I've got to thinking about it and just knowing that the neutral can be open at the other end leads me to believe that it can't be an issue with the circuit. I guess I'd like to get my hands on an analog meter and see what's up. My meter is a calibrated Fluke 75/III... but who knows.

I'll certainly add that ground soon, I have tons of bare #4 (not 2) but if 10 is good, I'll go that route.
 
  #14  
Old 07-29-12, 07:27 PM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 33,054
Fluke is a good meter. The problem is with cheap digitals that don't reject induced voltages because of their high impedance.
 
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