Funky Incoming Voltage

[Tav1980]

I was troubleshooting a detached garage sub panel today. The way the customer explained what was going on, I immediately thought they lost a phase feeding the sub-panel. I checked that first, and sure enough I only had 120v phase to phase. After looking for about an hour for where the phase could have been lost, and not being able to find it, I went back to the subpanel and turned off all the breakers. I immediately got 240v phase to phase. I turned on the breakers one by one, and there were a couple that made phase to phase voltage drop to 120v. When I turned them off right back to 240v. Fast forward eventually the panel was working with 240v phase to phase with all breakers on, but to be honest, I do not know how I fix them problem. I did unplug some things, and maybe one of those loads could have been the issue, but the loads worked fine when I left.

Has anyone run into this issue, and if so, did you figure out what caused the problem? This is the first time I have encountered something like this. Thanks!

 

[HEYDOG]

I was troubleshooting a detached garage sub panel today. The way the customer explained what was going on, I immediately thought they lost a phase feeding the sub-panel. I checked that first, and sure enough I only had 120v phase to phase. After looking for about an hour for where the phase could have been lost, and not being able to find it, I went back to the subpanel and turned off all the breakers. I immediately got 240v phase to phase. I turned on the breakers one by one, and there were a couple that made phase to phase voltage drop to 120v. When I turned them off right back to 240v. Fast forward eventually the panel was working with 240v phase to phase with all breakers on, but to be honest, I do not know how I fix them problem. I did unplug some things, and maybe one of those loads could have been the issue, but the loads worked fine when I left.

Has anyone run into this issue, and if so, did you figure out what caused the problem? This is the first time I have encountered something like this. Thanks!

What are you getting phase to ground?

 

[Joethemechanic]

I think you lost one hot leg, and a 120 volt load was acting like a jumper between neutral and the dead leg. Effectively making the dead leg a neutral. Probably a bad (high impedance) connection on one of the hots somewhere upstream.

 

[Ponchik]

Verify your voltage readings with a load of some sort connected. Or use a wiggy.

 

[gar]

230602-2104 EDT

:Tav1980

What do you have coming from the home to the garage?
Is there a local ground rod at the garage?

With no loads ON in the garage what is the voltage from each hot to neutral, and voltage between neutral and EGC directly on the wires coming into the garage panel?

With bthis information, then take a 1500 W 120 V heater, and connect it to each hot to neutral wire pair and measure those voltages. All of this is done directly on the incoming wires, not on any terminals.

Report back.

.

 

[LarryFine]

Probably a bad (high impedance) connection on one of the hots somewhere upstream.

This.

 

[readydave8]

do not know how I fix

Either bumped something to make connection or intermittent problem

Sometimes turning breaker off and back on

 

[ptonsparky]

Either bumped something to make connection or intermittent problem

Sometimes turning breaker off and back on

Real PIA when you can't duplicate the problem while your there. Get back to the shop, and it will show up again.

 

[retirede]

Since the symptoms changed while you were messing with things in the subpanel, the problem is likely there. Loose connection on an incoming phase or even flaky main breaker.

I would add a fairly heavy load to a circuit on each phase while troubleshooting. Recheck voltages, including phase to phase.

 

[Fred B]

Narrow your search by metering as suggested your L-N. Should see one side a 0V and120V on opposing lines. You will get a feed thru from a 2 pole breaker, so turn off all 240 load and MWBC beakers. 120V L-L likely result of opposing phase feed thru to the neutral on the opposing phase.
I have seen where voltages looked correct until a load is applied then phase would drop out until breaker is opened and then reclosed, then it would return to normal until load applied again. Found broken conductor, in an underground feeder that got damaged from an excavation done several years prior that was "just" making contact until the load was applied.
Un-fortunately no quick check, just process of elimination.