Voltage loss on conductors

[102 Inspector]

Situation - Sub-panel located in garage. House breaker provides 240 volts, conductors at garage receive 240 volts when not connect to disconnect at garage.
When conductors connected to breaker in garage, voltage changes to 120 on one and 75 on other with breaker in off position. When breaker is closed, the voltage changes to 120 on one and 5 volts on other side.
My first guess is an underground conductor has failed and finally showed up with heavy rain last week, but why 240 volt with no load and then change. Any thoughts?

 

[Little Bill]

Sounds like you have lost the neutral (grounded conductor) and maybe a loose connection on one of the hot legs as well. If you check the voltage with either a "Wiggy" or "Lo-Z" meter it probably won't read 240V. The way it is now, you are reading 240V but that is probably a "phantom" voltage, but when there is a load it changes.

 

[102 Inspector]

I get 120 on each leg when I check phase to neutral. This is why I am thinking it is not a lost neutral. That was my first thought also.

 

[winnie]

Measure with a different (low impedance) volt meter.

Standard digital volt meters have very high impedance, meaning that they draw very very little current to make the measurement. The tiniest bit of current leaking across a break in a wire will read full voltage, but as soon as you apply any sort of real load this voltage drops.

-Jon

 

[Little Bill]

I get 120 on each leg when I check phase to neutral. This is why I am thinking it is not a lost neutral. That was my first thought also.

Well you left that part out in your OP!
You definitely have problems with one of the hot legs then.
You could use a small load such as a "pigtail" socket and light bulb and put it across from each leg and neutral. You can check voltage at the same time and find out which hot leg is bad. Chances are the bulb won't light or dimly if it does and you can confirm with that along with a meter reading.

 

[102 Inspector]

Yes, I realize I forgot the other information after I posted. Sorry. I will set up for some additional testing as suggested. Thanks for all the replies.

 

[Edtk999]

I had a similar situation a few years back and actually found the wire had been spliced through in an LB . The connection had loosened causing the voltage drop. Try to trace back your feeder if possible to make sure you have no j boxes in line

 

[gar]

171118-1240 EST

102 Inspector:

In my opinion your first post implied that you made measurements to neutral.

You have close to an open circuit in the one hot supply wire that reads a lower voltage to neutral or ground.

My first guess is an underground conductor has failed and finally showed up with heavy rain last week, but why 240 volt with no load and then change. Any thoughts?

Others have confirmed your judgement.

My guess is that the main breaker in your garage sub-panel, when in its off state, has enough shunt capacitance to ground, or capacitance to load (capacitance can be also read as meaning leakage resistance) to load down the high series impedance in your supply wire, and thus produce a lower voltage than expected at the main panel when the supply line is connected to the open breaker.

If you are using a 10 megohm input impedance meter and it reads close to 120 V with no breaker, then my estimate is that the the source impedance in the supply wire is less than about 100k to 500k.

With the breaker open, but connected, and assuming 100k source impedance to get 75 V would require a load of about 160k. The capacitance would have to be large compared with what I might expect for an open breaker to ground. More likely leakage resistance. But leakage R should not be that low.

Connect an ammeter in series with a 100 W incandescent bulb, and place this series combination between the weak leg and neutral, also try to a screwdriver to earth instead of neutral.

A 100 W bulb is about 10 ohms with no current flow at room temperature. This increases to about 150 ohms with 120 V applied.

The 100 W bulb will provide some protection for your meter.

If the leakage is very small the bulb will remain at about 10 ohms. Change your ammeter range until you get a useful reading. Now you can estimate the impedance of the break in the hot supply wire. This can be very variable, and there might times when the ammeter is pinned.

To look at leakage within the breaker looking toward the open breaker input you could again use the 100 W bulb and ammeter, but use the good 120 V as your test source voltage.

.

 

[kwired]

I also think you lost a "hot" conductor. The one that always reads 120 volts is the good one, the other one varies depending on load conditions. If there are 240 volt loads connected they will cause more confusion, turn off all 2 pole breakers in the outbuilding and your problem may become more obvious.

A lost neutral will yield over 120 volts that directly offsets how much the other line drops below 120 volts, since you always have 120 on one line the other line is the one with a problem. Could be underground, could be blown fuse, bad contact in feeder breaker, failing termination, failing breaker to bus connection, ...