Low Voltage at Subpanel

[calebmund]

Hello all, earlier this year, I ran a 100A subpanel off of a 70A at the main. I just returned home from vacation to find my lights we flickers and some outlets weren’t working.

After looking in the panel, I found I was getting 120v to one leg, but only 80v to the other (120v from both at main panel and at junction box where it is converted to MHF from SER).

Couple things, I understand that there is a possibility something got to the MHF in between house and garage, but I really find that hard to believe considering the cable hasn’t been in the ground 6 months. Everything worked fine a week ago. And could MHF still have corroded underground? Again, I would find that hard to believe.

Currently have all 120v breakers on the good side, but with winter coming, I will need both to run my heater

Any assistance would be appreciated!

 

[sparky 1971]

I know you checked the junction box already, but check it again on both sided of yhe connection. How did you connect the SER to the MHF?

 

[mm08822]

Does the sub have a main cb? If yes, what side did you measure voltage on?

Also when measuring voltages, take these readings: L1 - N, L2 - N, L1 - L2, L1 - G, L2 - G

 

[Szumi]

Voltage imbalance, check neutral, it may be floating.

 

[mike93lx]

Voltage imbalance, check neutral, it may be floating.

Feels that that could be the case if it was 160/80, not 120/80, no?

@calebmund what is voltage between the hots?

 

[wyliesdiesels]

Voltage imbalance, check neutral, it may be floating.

if that were the case then it wouldnt be 120/80

 

[Innovate1]

Much more likely the issue is at a connection point than in the ground IMHO. But if there has been excavation in the area it could always be "backhoe fade". At the connections check both sides of the connection - at the feeding 70A breaker check on the breaker lugs and then on the wires themselves (if there is any bare wire past the lugs. Then at the SER splice check on the MHF wires and then the SER wires. Same at the sub panel. Since you got 120/120 at the splice there is really no need to do further checking at the 70A feeding breaker. Where in the subpanel did you check? You should check on the wires incoming. If that is good check on the incoming lugs. If that's good check on the main output lugs. If you have good voltage at one end of a conductor when touching the probes to the conductors not the lugs and it's bad at the other end of the same wires when probing the conductors then you have a damaged wire.

 

[Szumi]

Feels that that could be the case if it was 160/80, not 120/80, no?

@calebmund what is voltage between the hots?

You are right, it normally sums to 240 or whatever L1 L2 is. Voltages changing as loads are applied and removed.

Feels that that could be the case if it was 160/80, not 120/80, no?

@calebmund what is voltage between the hots?

 

[calebmund]

Much more likely the issue is at a connection point than in the ground IMHO. But if there has been excavation in the area it could always be "backhoe fade". At the connections check both sides of the connection - at the feeding 70A breaker check on the breaker lugs and then on the wires themselves (if there is any bare wire past the lugs. Then at the SER splice check on the MHF wires and then the SER wires. Same at the sub panel. Since you got 120/120 at the splice there is really no need to do further checking at the 70A feeding breaker. Where in the subpanel did you check? You should check on the wires incoming. If that is good check on the incoming lugs. If that's good check on the main output lugs. If you have good voltage at one end of a conductor when touching the probes to the conductors not the lugs and it's bad at the other end of the same wires when probing the conductors then you have a damaged wire.

Just checked this morning and can confirm that voltage drop occurs from junction box (house) to panel (garage 50’). All connections are solid, and voltage is jumping from 80-85v on L2. Since the only the in between is the buried MHF, I am thinking you are right, and the cable is damaged. I was able to identify which cable it is in the junction box, so I will just pull and bury another 2awg to replace and repair. Thanks for your input! Only 50’ so, it could be worse!

 

[Innovate1]

Just checked this morning and can confirm that voltage drop occurs from junction box (house) to panel (garage 50’). All connections are solid, and voltage is jumping from 80-85v on L2. Since the only the in between is the buried MHF, I am thinking you are right, and the cable is damaged. I was able to identify which cable it is in the junction box, so I will just pull and bury another 2awg to replace and repair. Thanks for your input! Only 50’ so, it could be worse!

Might want to think about conduit end to end for the replacement if it is actually damage in the run - less likely to get damaged in the future that way.

 

[mike93lx]

Just checked this morning and can confirm that voltage drop occurs from junction box (house) to panel (garage 50’). All connections are solid, and voltage is jumping from 80-85v on L2. Since the only the in between is the buried MHF, I am thinking you are right, and the cable is damaged. I was able to identify which cable it is in the junction box, so I will just pull and bury another 2awg to replace and repair. Thanks for your input! Only 50’ so, it could be worse!

What #2 are you going to use? Xhhw isn't direct burial rated and I don't think you can separate the conductors for the run.

I think you need to pull it all and redo it. I'd second conduit if you are going through that effort

 

[wssix99]

This exact same issue happened to me. I had a bad bulb and a crappy multi-meter.

I would try another meter before you go tearing into things. There are lots of grounds to compare the voltage to. It either is or is not there on the wire.

 

[nadogail]

I have seen the crazy things that show up when you get an open Neutral.

The term “Back Hoe Fade” is new to me, can someone please explain?

 

[wyliesdiesels]

I have seen the crazy things that show up when you get an open Neutral.

The term “Back Hoe Fade” is new to me, can someone please explain?

the voltage readings he is getting are not indicative of an open neutral

 

[Innovate1]

I have seen the crazy things that show up when you get an open Neutral.

The term “Back Hoe Fade” is new to me, can someone please explain?

It's a joke. When a back hoe hits the line it tends to cause a reduction in voltage/signal. Not sure how common the term is but I didn't come up with it. I think it's more used when working with communication lines.

 

[justsam]

Does it show 80 VAC even with no load on that leg? What happens to the voltage if you do put a load on it? If there is a poor connection, (high resistance), it should pull the voltage lower with a load. Heavy current draw might even seal the connection temporarily or open it. Regardless sounds like you need to get your shovel ready

 

[Innovate1]

I still think a bad wire in the ground is unlikely unless the ground is very rocky with sharp rock - even then it seems remote - or some disturbance/digging around the wires. Are burrowing animals common in the area? Any evidence of that in this case? I would exhaust all other possibilities and double check before digging and replacing the cable. Plumbing not electrical but I just had a sprinkler line leak. Wife noticed water gushing out under a small bush one day when one irrigation zone was on. I thought it was very strange that a huge leak just happened there when it has been fine for 15 years. I dug down to the line and found a huge gash from a shovel that nearly cut the pipe completely through. When I mentioned it to her she said she recently had someone put the bush there. Was a fairly easy fix but the point being it was hit by someone digging. Much more shallow than the electric line should be.

 

[Jim greengo]

Hello all, earlier this year, I ran a 100A subpanel off of a 70A at the main. I just returned home from vacation to find my lights we flickers and some outlets weren’t working.

After looking in the panel, I found I was getting 120v to one leg, but only 80v to the other (120v from both at main panel and at junction box where it is converted to MHF from SER).

Couple things, I understand that there is a possibility something got to the MHF in between house and garage, but I really find that hard to believe considering the cable hasn’t been in the ground 6 months. Everything worked fine a week ago. And could MHF still have corroded underground? Again, I would find that hard to believe.

Currently have all 120v breakers on the good side, but with winter coming, I will need both to run my heater

Any assistance would be appreciated!

Do you have 240 at the breaker feeding panel?
Check your connections at j box ,240v at main breaker on sub panel.

 

[dave*99]

How did you transition in and out of the earth at both ends of your underground cable? That bend and rise could be subject to soil settling and dragging your cable down with it.

You mentioned 6 months since installation...... settling.....

 

[Szumi]

I hate these threads where a question is asked and the OP doesn't come back with a followup.

 

[mm08822]

I hate these threads where a question is asked and the OP doesn't come back with a followup.

Probably too busy digging.