240 between two legs but can't get voltage out of a single leg to Nuetral

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gibleh

Member
We have a 50amp 3 prong outlet at work. Similar to an Electric Dryer Plug.
When we use a meter and measure across to 2 outside legs we get 240VAC.
Then when we measure from one leg to the center we get 137VAC. And when we measure from the other leg to the center we get 108VAC. We normally use this to test run 230-240VAC motors.

What we tried to do was to use the 108VAC like a regular 120VAC outlet.
For some reason when we loaded it up the voltage would drop down to almost Zero. No fuses blew or anything. Once we remove the load the meter registers 108VAC again.

Any idea of what kind of arrangment this may be and why it won't work????
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
gibleh said:
We have a 50amp 3 prong outlet at work. Similar to an Electric Dryer Plug.
When we use a meter and measure across to 2 outside legs we get 240VAC.
Then when we measure from one leg to the center we get 137VAC. And when we measure from the other leg to the center we get 108VAC. We normally use this to test run 230-240VAC motors.

What we tried to do was to use the 108VAC like a regular 120VAC outlet.
For some reason when we loaded it up the voltage would drop down to almost Zero. No fuses blew or anything. Once we remove the load the meter registers 108VAC again.

Any idea of what kind of arrangment this may be and why it won't work????
You have an almost broken wire or a loose connection somewhere.

Also could be a bad breaker
 
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gibleh said:
We have a 50amp 3 prong outlet at work. Similar to an Electric Dryer Plug.
When we use a meter and measure across to 2 outside legs we get 240VAC.
Then when we measure from one leg to the center we get 137VAC. And when we measure from the other leg to the center we get 108VAC. We normally use this to test run 230-240VAC motors.

What we tried to do was to use the 108VAC like a regular 120VAC outlet.
For some reason when we loaded it up the voltage would drop down to almost Zero. No fuses blew or anything. Once we remove the load the meter registers 108VAC again.

Any idea of what kind of arrangment this may be and why it won't work????
Faulty Neutral. Loose connection in the circuit, at the panel or at the transformer. Check what voltage readings you get "at" the panel from each phase to neutral. If normal, check all Neutral connections in the circuit. If not normal at panel, back track further up line.
 

BAHTAH

Senior Member
Location
United States
Plug configuration

Plug configuration

gibleh said:
We have a 50amp 3 prong outlet at work. Similar to an Electric Dryer Plug.
When we use a meter and measure across to 2 outside legs we get 240VAC.
Then when we measure from one leg to the center we get 137VAC. And when we measure from the other leg to the center we get 108VAC. We normally use this to test run 230-240VAC motors

Any idea of what kind of arrangment this may be and why it won't work????

You mention you use this outlet to run 230-240V motors. Could this be a 50amp 240V 6-50R configuration with no neutral, 2-pole 3-wire grounding?
If so you may not have a neutral or even a ground wire from the panel if the circuit is installed with conduit.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
If it is an open neutral then why is one phase (130 v) working fine. It is the 108V phase that is having the problem. At least that is my understanding.
 

480sparky

Senior Member
Location
Iowegia
Dennis Alwon said:
If it is an open neutral then why is one phase (130 v) working fine. It is the 108V phase that is having the problem. At least that is my understanding.

More than one problem.
 

benaround

Senior Member
Location
Arizona
gibleh,

Does this outlet come from a transformer ? Or , what is the Utility voltage and

phase that feeds the building. I'm thinking that if this is used as a test outlet

for 240vac, that it might originate from an isolation transformer.
 

jghrist

Senior Member
Dennis Alwon said:
If it is an open neutral then why is one phase (130 v) working fine. It is the 108V phase that is having the problem. At least that is my understanding.
In other words, this is what you were told instead of what you actually observed? Make sure you were told right. Most likely the other leg voltage got higher when one went low.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
Dennis Alwon said:
If it is an open neutral then why is one phase (130 v) working fine. It is the 108V phase that is having the problem. At least that is my understanding.

I didn't think the OP ever mentioned that the 130-Volt side was working fine. I bet if they loaded it up it would not work either.
 

Rick Christopherson

Senior Member
Dennis Alwon said:
If it is an open neutral then why is one phase (130 v) working fine. It is the 108V phase that is having the problem. At least that is my understanding.
Because that is exactly what a floating neutral will do. The voltage of the neutral will be dependent on the load between each phase and the neutral. Even though the loads are supposed to be 120 volts, they are acting as series loads between the 240 volt phases, and the neutral floats somewhere in between according to ohms law (or specifically, voltage division).
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Dennis Alwon said:
If it is an open neutral then why is one phase (130 v) working fine. It is the 108V phase that is having the problem. At least that is my understanding.
If one line's voltage-to-ground drops when loaded, the other's is rising. That 240v loads can be carried proves that the third wire is the open one. Try reading the 130v side when the 108v is dropping under load.

The reason you're reading a voltage under no load is because your voltmeter has a high input impedance, and does not present a low-enough impedance to bring down what is an induced (phantom) voltage.
 

mull982

Senior Member
Rick Christopherson said:
Because that is exactly what a floating neutral will do. The voltage of the neutral will be dependent on the load between each phase and the neutral. Even though the loads are supposed to be 120 volts, they are acting as series loads between the 240 volt phases, and the neutral floats somewhere in between according to ohms law (or specifically, voltage division).

Can you give an example of this. I am not seeing it with voltage division.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
hardworkingstiff said:
I didn't think the OP ever mentioned that the 130-Volt side was working fine. I bet if they loaded it up it would not work either.
I agree--- Again my apologies. I was thinking the other leg worked fine. I agree with you all -- now leave me alone while I punish myself.....:grin:
 

bbaumer

Senior Member
Location
Indiana
Lcdrwalker said:
This almost sounds like a corner grounded service.

This could be a number of things, but I don't think corner grounded.

It COULD be an ungrounded 240V delta service or one of the problems described above. In this case your 50A outlet's third prong is just a ground as no neutral is present with this service.

Go look at your incoming service. Does it have 3 dimly lit bulbs on the main distribution panel? If so, you have an ungrounded 240V delta service. Voltage to ground is completely variable. You are only "capacitively coupled" with the earth in that case, assuming you don't have a ground fault which it sounds like you do not.

Also, if the 3 lamps are there I would check them to make sure they are good. If 2 are burning bright and one is out you have a ground fault. If all 3 are burning dimly they are good. They are dim because they are 250V rated and you are applying 137V or 108V or some other completely variable voltage to them less than 240V making them burn dimly.

Just a theory.
 
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bbaumer

Senior Member
Location
Indiana
Oh, I should also mention it is possible that you don't have the ground fault indicator lights (the dim bulbs) but still have an ungrounded service if whoever installed it didn't know better.

In that case I'd check the voltage from phase to ground on all three phases at the service, not just at your receptacle. If you get wacky readings on all three you have an undgrouned service either intentionally or unintentionally.

Unintentional could be one of the neutral or ground problems mentioned above on a high-leg or corner grounded service or you could have an open delta which is another story altogether.

Bottom line is start at the source, confirm what you have then go back to your receptacle.

Please let us know what you find out.
 
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