voltage fluctuations

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tiptop

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A residential service has a 70 amp breaker feeding a sub panel in ser of suficcient size. Voltage at breaker is 123 per leg. 123 out but sub panel shows 120 and 128v. Understandably customer would like an explanation. I told him the ser could be "tracking" or failing and thats all I got. A new two pole 70 is 52$ but does not seem to be the problem. I would appreciate any ideas. A floating neutral could run up one leg but usually to 175v. He called utility before me (because they are free) and the service entrance conductors/voltages are ok. Who,s got the knowledge? Thanks in advance.
 
Sounds like a loose neutral connection. Measure the voltage from the neutral to the ground. (Should be separate if this is a sub panel) unless it is a really long run, should be zero volts between them.
 
230607-1029 EDT

tiptop:

You have said that input voltages at both your main panel and subpanel are 243 V. line to line.

At your main panel what is the voltage from your neutral to a screwdriver in the earth a little ways from the home?
Repeat with a 120 V 1500 W heater from one hot line to neutral. Location of heater is not important. Voltage measurement should be on an input wire directly, not an input panel lug in the main panel, to the screwdriver.

What happens to these two voltages when you load an output circuit on one phase from line to neutral with a 1500 W 120 V space heater, and then separately on the other phase. Record these values.

I will get into more comments on other measurements later. What we are looking for are abnormal results.

..
 
gar said:
230607-1029 EDT

tiptop:

You have said that input voltages at both your main panel and subpanel are 243 V. line to line.

At your main panel what is the voltage from your neutral to a screwdriver in the earth a little ways from the home?
Repeat with a 120 V 1500 W heater from one hot line to neutral. Location of heater is not important. Voltage measurement should be on an input wire directly, not an input panel lug in the main panel, to the screwdriver.

What happens to these two voltages when you load an output circuit on one phase from line to neutral with a 1500 W 120 V space heater, and then separately on the other phase. Record these values.

I will get into more comments on other measurements later. What we are looking for are abnormal results.

..
Click to expand...
Why would you measure the voltages to earth, when you can get the same information much easier by measuring the line to neutral voltages under the same loading conditions that you have specified?
Ideally your utility would have a tester like this, and could do the testing for you at the meter.
 
I have seen Line to neutral voltages change under load due to reduced neutral and loonnng distance. Everything was undersized on the one that stands out in my mind.

A milk house heater or your wifes old hair dryer is all you need for testing. Two meters are nice but not required.
 
230608-1457 EDT

don_resqcapt19:

Many ground rods provide a resistance of 10 to 20 ohms to remote earth. And there are some lower and others higher.

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gar said:
230608-1457 EDT

don_resqcapt19:

Many ground rods provide a resistance of 10 to 20 ohms to remote earth. And there are some lower and others higher.

.
Click to expand...
But testing to the earth does not give me any more helpful information than testing to the neutral when looking at voltage issues that are likely caused by a neutral problem. The connection to earth plays no part in the operation of the electrical system.
 
hillbilly1 said:
Sounds like a loose neutral connection. Measure the voltage from the neutral to the ground. (Should be separate if this is a sub panel) unless it is a really long run, should be zero volts between them.
Click to expand...
Thanks hillbilly1. This morning I revisited the panel and found that the neutral for sub panel was the only neutral landed on the left hand bar. testing neut to line on that bar showed the fluctuation at the service. long story short it was a loose tie bar. I had been testing to the big neutral lug and missed it. Thanks to you and others for the input.
 
All the members who said loose neutral were correct. It was a dirty tie bar. not really loose but salty and loose enough. Thanks to all who responded. I was testing directly to the big lug at incoming neutral and missed it. And I have seen it before, must be getting old.
 
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