Irregular voltages with 120v loads

Status
Not open for further replies.
T

tzclark

Member
Location
Vancouver Canada
Hey guys, this problem has me scratching my head. I had a guy call me today staying that all his lights are dim. When I get there and start trouble shooting I notice that with any 120v load, even a 40w lamp the voltage on that phase drops to 65v and the other jumps too approximately 160v. My first guess was a burnt up neutral but what didn't make sense was that the grounding should be doing the same thing as the neutral for an incandescent load. Its a 120/240v 100A residential service, I thought possibly a bad ground on the utilities transformer but there is 3 other houses being fed from same point with no issues.

Here what I did

Turned off all circuits
Metered with no loads, all voltages are OK
Connected 40w load to phase A voltage dropped on that phase
Connected 40w load to phase B voltage dropped on that phase
Tested before meter at the service cap and voltage was OK, with 40w load.
 
tzclark said:
Hey guys, this problem has me scratching my head. I had a guy call me today staying that all his lights are dim. When I get there and start trouble shooting I notice that with any 120v load, even a 40w lamp the voltage on that phase drops to 65v and the other jumps too approximately 160v. My first guess was a burnt up neutral but what didn't make sense was that the grounding should be doing the same thing as the neutral for an incandescent load. Its a 120/240v 100A residential service, I thought possibly a bad ground on the utilities transformer but there is 3 other houses being fed from same point with no issues.

Here what I did

Turned off all circuits
Metered with no loads, all voltages are OK
Connected 40w load to phase A voltage dropped on that phase
Connected 40w load to phase B voltage dropped on that phase
Tested before meter at the service cap and voltage was OK, with 40w load.
Click to expand...

You have a weak or intermittant neutral somewhere. Better find it quick, else you will have damage to sensitive loads, if not already.
 
If you draw the equivalent circuit I can't imagine how it can not be the neutral, unless the problem changes with time.

If 0.3A causes a 60v drop the neutral resistance is 200 ohms.
 
tzclark said:
.....what didn't make sense was that the grounding should be doing the same thing as the neutral for an incandescent load.
Click to expand...
Correct. So if the ground is still maintaing typical voltages, then your break in the neutral is downstream from the bonding point. Find your bonding point (if one exists) and start examining the neutral from that point forward. If there is no bonding point, then it could be anywhere from the transformer forward.
 
Status
Not open for further replies.
Top