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Paul
Paul, Electrician
Category: Electrical
Satisfied Customers: 805
Experience:  23 yrs as an electrician, self employed contractor, municipal inspector
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I am losing one leg of power in a sub panel in my house at

Customer Question

I am losing one leg of power in a sub panel in my house at random times 3-6 times a day. Sometimes at a high load......Sometimes with hardly any load. Sometimes it comes right back on.....Sometimes it stays off for hours. Sometimes daytime......Sometimes night time. Sometimes when it is hot outside.....Sometimes cold. The other 2 sub panels do not seem to be affected. When it has occurred, I have put a tester on both legs and verified that that incoming feeder drops to a 0 Amps draw on one leg. The other leg works fine. I have checked connections and they seem solid on both ends. No breaker ever trips.

Now the weird part. To restore power, I can turn on the dryer. It is on a 50A breaker connected to both legs. The amp draw increases by +/- 20A on both legs and all other circuits that were out start working again. When I turn the dryer off, everything stays working until the next time. Sometimes 10 minutes later, sometimes a day later. This does not work if I try to duplicate it with any other 208V circuits such as the oven or range.

I had an electrician out that checked stuff for 3-4 hours. He checked the dryer and it seems fine. He checked how well the breakers were seated and they seemed fine. He flipped a couple breakers from one leg to the other to balance it a little better but, it wasn't that bad anyways. He tightened a couple of the neutral connections on the panelboard and he rewired one GFCI that wasn't wired properly. Unfortunately, we didn't have power drop out while he was there. The problem started again 2 hours after he left.

Recommended next steps?
Submitted: 3 years ago.
Category: Electrical
Expert:  Sir Sparks replied 3 years ago.

Sir Sparks :

A Master Electrician here to help. I am a double qualified; USA & UK electrician. Please give me a moment to formulate a reply.

Sir Sparks :

Your first step is to contact your local Utility company and tell them you have a partial power outage

Sir Sparks :

From what you describe the fault is on their end. Even if it is not they will not charge for checking it out

Customer :

Wouldn't that cause the problem to other sub panels and the main panel as well?

Sir Sparks :

Also you say power drops to zero amps. Do you mean zero volts?

Sir Sparks :

Absolutely it would yes, but if the sub-panels are fed through the main panel as they should be then if they are OK then the main panel should be too

Customer :

No, I mean Amps. Their is no load

Customer :

The main panel is OK

Customer :

This occurs on one of three subpanels

Sir Sparks :

You need to use a 2 lead voltmeter and check for incoming volts across each leg at neutral (120 volts) and across both legs (240 volts) on main panel and subpanesl

Sir Sparks :

I must have misread.

Sir Sparks :

OK so it is not a utility problem

Customer :

The electrican did that but, when the system was working properly. He did it at each breaker as well

Sir Sparks :

still do those voltage checks

Sir Sparks :

It needs to be done at the main lugs of the sub panels and the main panel breakers feeding them when the fault is there

Customer :

What will that determine

Sir Sparks :

The dryer bringing the one leg back on identifies this as a lost leg

Sir Sparks :

That you have lost a leg and the location where it is lost

Customer :

??

Customer :

I know we are losing a leg

Sir Sparks :

Do you know where?

Sir Sparks :

If not follow my procedure

Customer :

All I know is that I lose an entire leg of alternating breakers on the one subpanel

Sir Sparks :

OK so do you weant to follow my advice?

Sir Sparks :

want

Customer :

All I know is that when I put a tester on it, the leg was drawing a 6A load and then it drops to 0A. Then I turn the dryer on and it jumps up to 26A. Then when I turn the dryer off, it returns to 6A ( or whatever it was based on what was on at the time).

Sir Sparks :

You need to identify if the fault is at the sub panel, at the main panel or in the wiring between them

Customer :

I am ready to try anything

Customer :

OK

Sir Sparks :

NOT AMPS We need volts. Please re-read all I have written

Customer :

When the system is working propery, both legs are drawing 120V. The electrician checked it accross both legs and siad it was fine as well (240V I assume)

Sir Sparks :

OK good luck I am opting out. I cannot help if you do not listen.

Expert:  Paul replied 3 years ago.

Hi

 

To start with forget about the amps, we are looking for a loss of voltage.

 

The problem will be with either the breaker feeding the sub-panel or the connection at the sub-panel.

 

What I want you to do is to turn on every appliance that is powered off of the sub-panel and let me know if the power starts to drop.

 

I'll wait here.

 

Paul

Customer: replied 3 years ago.
How do I determine if Power drops. Are you asking if I lose the leg when I turn on everything on that subpanel?
Expert:  Paul replied 3 years ago.

You would use a volt meter.

 

I am asking to to put a load on the circuit so we can find the location of the problem.

Customer: replied 3 years ago.
All appliances are on. Power is still good to both legs
Expert:  Paul replied 3 years ago.

Let it run for a little bit. What this is doing is heating up the bad connection. Once it is heated up, the connection will start causing the problem you described.

 

Check the voltage from hot to neutral, do this for both hot lines and tell me the voltage.

 

Are you checking the voltage at the main panel or the sub-panel?

 

Customer: replied 3 years ago.
Embarrased to say that I am not certain how to test the voltage. Do I out the probe from the red lead on the incoming lug from a feeder to the sub panel and the black on the neutral bar?
Expert:  Paul replied 3 years ago.

Yes, make sure the scale is set to 240v if it is a multi meter. Becareful you don't get shocked.

 

 

 

 

I see that the breaker for this panel is 50A.

 

What appliances are being fed from the sub-panel?

Customer: replied 3 years ago.
range, dryer, washer, oven, fan for furnace, insta hot water heater
Expert:  Paul replied 3 years ago.

Well I can tell you it is probably the 50A breaker that has gone bad.. That is way too much load for that size breaker.

 

How big is the main panel in your home?

How far is the sub-panel from the main panel?

Customer: replied 3 years ago.
At subpanel lugs, one was 120V exactly, the other read 121.8V

The breaker in the main panel is 200A. The breaker feeding the subpanel is 125A. The subpanel is just under 100' from the main.
Expert:  Paul replied 3 years ago.

If all of the connections look good, and there are no splices in the cable and this problem only happens in the sub-panel.

I would say it's a bad breaker or the connection where the breaker connects to the panel bussing.

 

 

 

Let me know if you need more help.

 

Paul



Edited by Paul on 1/22/2011 at 7:23 PM EST
Customer: replied 3 years ago.

 

Correction on the dryer breaker size. It is a 30A breaker. It is a quadplex breaker and the inner 50A section is for the double oven. The outter 30A is for the dryer. Sorry about the confusion. I am going to check on splices in the main feeder to the subpanel tomorrow

Expert:  Craig replied 3 years ago.
I'm going to suggest that the subpanel for the dryer that is dropping has a bad neutral wire connection either in the sub panel or going from the sub panel to, at the main panel. Check both ends. When the dryer is turned on the 120 volts circuits that didn't work will now go back to the panel through the dryer element, by passing the bad connection in the neutral wire. This indicates the neutral being the problem for that sub panel. The oven is probably on another sub panel, that's why it doesn't affect the power.

Edited by Craig on 1/23/2011 at 4:08 AM EST
Customer: replied 3 years ago.

Craig,

I had another person mention a bad neutral to me at work the other day. We tightened all the neutral connections at the subpanel but, I haven't checked the neutral connection for the subpanel feeder at the main panel. I will do that today.

 

You confused though me when you said that "The oven is probably on another sub panel and that's why it doesn't affect the power". The oven is definitely not on another sub panel. In fact, it is off the same quadplex breaker that the dryer is served from. If turning the oven on doesn't restore power to the affected outlets the same way that the dryer does, does that rule out the neutral as being the problem? It seems that if the impacted circuits would pass the bad neutral thru the dryer element, it would do the same thing thru the oven.

 

 

 

Unfortunately (or fortunately), I haven't dropped power in the past 24 hours so, I cannot check the voltage across the impacted leg during an outtage.

Expert:  Paul replied 3 years ago.

If you were dropping the neutral, none of the 110v loads in the panel would work and most likely any 110v electronic loads would have burnt up due to the imbalance.

 

Paul

Expert:  Craig replied 3 years ago.
I do not believe that a bad breaker can feed the dryer load at 20 amps and 240 volts. That is 4800 watts! That doesn't happen. A bad connection in the hot feeders will drop the voltage to zero or very low under load. Your situation is just the opposite, the dryer load restores the feeder to full voltage and powers the dryer. Then all the other circuits come back as well. This is why I believe that a neutral coming back to the sub panel from the affected circuits cold be involved. The electronics in the oven control
Your first post describes the fact that the dryer would run both legs of the 240 and read 20 amps, and the other bad circuits began working when the dryer is running. That's what I'm basing my response on.
Make sure all the neutral wires in the sub panel are checked too, each individual white neutral wire. could cause the difference in the circuit response.

Edited by Craig on 1/23/2011 at 5:06 PM EST
Customer: replied 3 years ago.

More data. Just lost power a few moments ago. Entering the subpanel,I get 120V on one leg. I only get 6V on the other leg. Then went to the main panel and checked the conducter coming out of the breaker that feeds the subpanel. I got 121.5V on both legs. I am guessing that this implies that there must be a splice between the connection to the breaker in the main panel and the connection to the lugs in the subpanel that is causing the problem but, who knows.....

 

Up into the very skinny attic space to see what I can find.

 

I also noticed that the conducters were aluminum instead of copper.

Expert:  Sir Sparks replied 3 years ago.
I am writing on behalf of your Expert Paul as he is not on line this evening and you have already been kept waiting long enough on this.

As your test has now confirmed it was obvious from your previous description that the fault lay in one of the hot wires as Paul told you from the beginning. Therefore It would be only fair if you accepted PAUL'S answer to you.

Now that we know you have aluminum conductors these may well be the problem as surface oxidation is a serious problem with aluminum which often causes the kind of problems you have experienced.

You will need to buy some 240 grit sandpaper and a compound called Noalox from the electrical department of Home Depot. Remove all conductors (both ends)on the sub feed and gently rub the exposed aluminum on all of them with the sandpaper then liberally coat them with noalox before re-inserting them and tightening them firmly.

This may well be the cure the problem you are experiencing, in any case it is either this or the wiring, you may now ignore all other theories.

I am sure Paul will agree with this advice. Please accept his answer.

Edited by Sir Sparks on 1/24/2011 at 2:12 AM EST
Expert:  Paul replied 3 years ago.

I would also check the terminations in both panels very carefully.

 

I have come across a few cases where I could read voltage on the wires and the lugs but not have it on the buss bar.

 

Regardless of where it is I would think you will see some sort of an indication of heating. This can be a discoloration or an actual sign of melting.

 

Please keep me informed of what you find and if you need help fixing it.

 

Paul

Customer: replied 3 years ago.

I have an electrician that specializes in service related issues from a larger electrical firm that I work with regularly coming by tomorrow AM. I am going to give him all the correspondence that was conveyed in this forum and see if it helps him determine the problem. I am tempted to just replace the 125A feeders from the main to the subpanel along with the subpanel breaker located in the main panel and move on.....I am hoping he can find the root cause but, I doubt we will lose power while he is here and I fear that wil make it hard for him to zero in on th issue. Hope still prevails though.

 

FYI, I plan on accepting each of the opinions listed on this forum that the electrician feels helped him diagnose the problem. Unfortunately, I still think electricity is magic and can't determine what the proper fix is from the recomendation listed. Thank all of you for your help though and I will post the findings later tomorrow. - Scott

Expert:  Paul replied 3 years ago.

Sounds good Scott.

 

I'm anxious to know the final out come, so please keep me posted.

 

Thanks

Paul

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