Alternating voltages

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Edtk999

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Southport, NC, USA
I was called out to a house today that was victim of an electrical fire which happened in a finished room over a detached garage with the intent on checking the remaining system for safety. I found something that alarmed me, yet stumped me at the same time.
Single phase residential dwelling, 200 amp meter main combo servicing a 200 amp main breaker panel in the main house and 100 amp subpanel in the detached garage.
Garage breaker is locked out for the time being, for obvious reasons. The only load being passed thru to indoor panel and 2 breakers for a/c condensers.
Meter socket reads clean, 118v per leg. At the main breaker in the attached outdoor panel, I have the same voltage (with no load to indoor 200 amp panel). When I turn on the indoor panel voltages stay the same, but as I turn on certian breakers the voltages start acting really wierd. Prior to I had an even 118 per, and now it will jump to say 131 and 124, or 129 and 110...and so on and so fourth. There is no consistency to the fluctuation, but it always balances on the opposing circuit. Something keeps screaming neutral problem, but I can't think of what could possibly cause it.
One note I may add is it all has stayed within the + or - 10%, but having just had the fire, which started from "dirty power" (was metered at like roughly 145v over a 110 circuit) which blew up a tv/cable box.
Anyone ever experienced anything like this? Sorry to be so long winded but there was a lot going on in a small cottage on the beach.
 
Open or resistive neutral. Turn enough on to see the imbalance without blowing anything up, and work backwards up the one line until the voltages return to normal.
 
180531-2402 EDT

Edtk999:

George has told you what the problem likely is.

But you post is too confusing to pin point the problem location.

You mentioned 118 balanced which is 118 + 118 = 236 when summed. Possibly this was with no load, not clear.

Then you have 131 +124 = 255, and 129 +110 = 239.

The 120,110 correlates with 118,118. The 131,124 does not correlate.

What does " roughly 145v over a 110 circuit" mean?

At the pole transformer the transformer neutral will connect to a ground rod. If this ground rod is close, say within 100 to 200 ft, then run a test lead from the ground rod, a screwdriver in the soil close to the rod will work, any wire from #24 to #14, to the area where you want to make voltage measurements. This becomes one input to your high impedance meter, like a Fluke 27.

With no load of any kind on the main panel measure the voltages to the main panel neutral bus, the phase A hot, and phase B hot, directly on the incoming wires.

Still with no other loads connect a 1500 W 120 V spacer heater, thus about 10 A, to said three points by some means such as leads to the neutral bus, and to appropriate breakers that have no other load. Voltage measurements are still made to the same points as in the previous paragraph.

Report back.

.


 
Edtk999 said:
I was called out to a house today that was victim of an electrical fire which happened in a finished room over a detached garage with the intent on checking the remaining system for safety. I found something that alarmed me, yet stumped me at the same time.
Single phase residential dwelling, 200 amp meter main combo servicing a 200 amp main breaker panel in the main house and 100 amp subpanel in the detached garage.
Garage breaker is locked out for the time being, for obvious reasons. The only load being passed thru to indoor panel and 2 breakers for a/c condensers.
Meter socket reads clean, 118v per leg. At the main breaker in the attached outdoor panel, I have the same voltage (with no load to indoor 200 amp panel). When I turn on the indoor panel voltages stay the same, but as I turn on certian breakers the voltages start acting really wierd. Prior to I had an even 118 per, and now it will jump to say 131 and 124, or 129 and 110...and so on and so fourth. There is no consistency to the fluctuation, but it always balances on the opposing circuit.Something keeps screaming neutral problem, but I can't think of what could possibly cause it.
One note I may add is it all has stayed within the + or - 10%, but having just had the fire, which started from "dirty power" (was metered at like roughly 145v over a 110 circuit) which blew up a tv/cable box.
Anyone ever experienced anything like this? Sorry to be so long winded but there was a lot going on in a small cottage on the beach.
Click to expand...

The line I quoted in bold says it all. You have a failing or completely open service neutral. This is easy to check by using a 40 watt and a 120 watt light bulb on opposite legs as your sole loads. Under normal conditions both will have 120 volts across them. With an open neutral, they will be in series with each other, and the 40 watt bulb, having three times the resistance of the 120 watt bulb, will have three times the voltage across it, 180 volts to 60 volts.

The reason why electronics tend to fry when a neutral opens up is they are lower wattage/ higher resistance than something like a coffee pot or microwave. If you were to completely lose the service neutral and turn on a coffee pot on one leg and a brand new 65 inch flat screen TV on the other, that flat screen would probably see about 160 volts and go poof... meanwhile the $20 coffee pot would just take about 70% longer to make a pot of coffee.

Edited to add... You could have lost the neutral to the sub panel... If both loads are coming from there, and your voltages on the main panel are stable/ consistent under various loads, you have a break or loose/ corroded connection on your feeder from main panel to subpanel.
 
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[The 120,110 correlates with 118,118. The 131,124 does not correlate.]

OK...maybe I'm slow on the trigger, but I'm a bit confused. The measured voltages with and without load make no sense. You measured 118/118 with no load. Voltage at the main or load side of the meter should be......line to line equals the sum of the line to neutral voltages. A bad neutral can't change that. If you have 110 on one side, you have to have 126 on the other if the total is 236 (118 + 118). Am I missing something? This is a 120/240 single phase, right? I can't see how you could be "making" more than line to line voltage unless your meter is giving you bogus info. If your total line to line voltage drops, you have a phase problem, not a neutral problem. Looks to me like you may have both a line and a neutral problem. I'd be for checking all of your connections and then have the utility check the service using a Beast of Burden.
 
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meternerd said:
[The 120,110 correlates with 118,118. The 131,124 does not correlate.]

OK...maybe I'm slow on the trigger, but I'm a bit confused. The measured voltages with and without load make no sense. You measured 118/118 with no load. Voltage at the main or load side of the meter should be......line to line equals the sum of the line to neutral voltages. A bad neutral can't change that. If you have 110 on one side, you have to have 126 on the other if the total is 236 (118 + 118). Am I missing something? This is a 120/240 single phase, right? I can't see how you could be "making" more than line to line voltage unless your meter is giving you bogus info. If your total line to line voltage drops, you have a phase problem, not a neutral problem. Looks to me like you may have both a line and a neutral problem. I'd be for checking all of your connections and then have the utility check the service using a Beast of Burden.
Click to expand...
Line to line volts may drop but won't increase, this due to voltage drop in the supply conductors or even across the source if it is marginally sized.

Line to neutral volts will go up on one side same amount as it goes down on other side if you have poor path back to source.

Extreme long runs and/or undersized conductor can give you "bad neutral" symptoms even though nothing else is wrong. Voltage drop in that long, undersized neutral is the culprit, as long as it is close to balanced things appear fine.
 
180601-2024 EDT

meternerd:

The 120,110 was a typo and should have been 129,110. 129+110 = 239 which is close enough to 118+118 = 236 and correlates sufficiently well to say the sums were equal. But 131+124 = 255 and that is not an expected correlation, and it is high.

.
 
Thanks guys for your input. I just today got back to site (insurance company and fire marshall had to ok) and I decided to do a swap of both the outdoor panel and the main house sub panel. This is what I found...
6 8-2 romex wires which were "downsized" to fit 30 amp breakers. I corrected this. Also there was tons of overspray paint on the bus bar. Outside the neutral lug had a good amount of corrosion on it, which also could have added to the issue.
Long story short, correcting a few "hacked in" mistakes cleared my issue, so I don't know exactly how it was occurring, nonetheless it is now even 124v on both legs.
Again. Thanks to all who chimed in.
 
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