Acceptable Voltage Drop Across Main Breaker

Status
Not open for further replies.
Little Bill

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
What is a reasonable/acceptable voltage drop across a 200A 120/240V main breaker?
Also known as "fall of potential".
I read .23V on each leg from line to load.

Here is the reason for checking. I had a customer call saying their dryer quit working along with the bath lights. Then it would come back on, on it's own.
So I go to the customer and turned the dryer on and the bath lights. I noticed the light had a slight dimming/flicker. The dryer was still running. I checked the voltage and had around 244V L-L and 122V L-N. So I just left the dryer running and the light on.

After about 5 min. I heard the dryer quit and I looked in the bath and the lights were off. I rushed over to the panel and checked voltage. I read 240+ L-L but the "A" leg only had 70-80V L-N, "B" had 120V. I then went back to "A" leg and it was back up to 120V. The lights were back on but, of course, the dryer had to be restarted, which it did start back.
This all sounds like the classic case of losing a leg and some of the 120V things would come on when a 240V load was on because of back feeding through the bus.
Strange thing is only the bath lights would quit when the dryer stopped. I couldn't find any other load that wasn't working.

I checked all connections on everything in the panel, ie; breaker terminals, main lugs, neutral lug and bus.
I found a few loose but not bad. I stayed about 30 min. more and never could get it to lose voltage again. Long day so I headed home. I talked to the HO on the phone as she wasn't home and told her to call POCO to come and check their connections. I had checked voltage before the main at the lugs so I knew it was either in the meter or one of the POCO connections.

Now after I get home the lady calls me and says POCO came and checked it. The problem showed up while they were there. They told her to tell me to replace the main breaker. I asked if they checked it and she said no they weren't allowed to. I asked if they checked their splices and she said all they did was pull the meter and check the voltage. No load on it or anything.:roll:

Anyway, I don't think it is the main breaker because I saw the low voltage before the main.
Any thoughts?
 
T

templdl

Senior Member
Location
Wisconsin
Exercise the breaker and it most likely willchange as the contacts are rubbed together as they are closed. If you do an ohms law calculation using the voltage drop and the current flowing through the contacrs you can calculate the resistance and that resistance equals heating which is not desirable. Ithout current flowing through the contacts measuring the voltage between the line and load terminal has little meaning. Should you know the voltage drop and know the current you can calculate heating watts using watts law, in this case E x I = P in
watts which is heat. In knowing this yuou want to keep that 'E' minimized.
 
Last edited:
C

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
Is the dryer the only 240v load in the house? Everything else is gas?

I would load that main down with as much equipment as I could turn on. Water heater, oven, furnace/ac, dryer, well pump, you name it. I'd probably even plug a hole hawg and heat gun in and tape the triggers on if I had too, just to see if I could make the main breaker flinch.
 
Little Bill

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
Cow said:
Is the dryer the only 240v load in the house? Everything else is gas?

I would load that main down with as much equipment as I could turn on. Water heater, oven, furnace/ac, dryer, well pump, you name it. I'd probably even plug a hole hawg and heat gun in and tape the triggers on if I had too, just to see if I could make the main breaker flinch.
Click to expand...

I did turn all 4 burners on the stove and the dryer running after I caught it the first time.
I never lost any voltage (other than expected) and the breaker never even felt warm. I didn't turn on the HVAC because I was there by myself and didn't know where it was or if it was even working.
 
J

JFletcher

Senior Member
Location
Williamsburg, VA
Little Bill said:
What is a reasonable/acceptable voltage drop across a 200A 120/240V main breaker?
Also known as "fall of potential".
I read .23V on each leg from line to load.

Here is the reason for checking. I had a customer call saying their dryer quit working along with the bath lights. Then it would come back on, on it's own.
So I go to the customer and turned the dryer on and the bath lights. I noticed the light had a slight dimming/flicker. The dryer was still running. I checked the voltage and had around 244V L-L and 122V L-N. So I just left the dryer running and the light on.

After about 5 min. I heard the dryer quit and I looked in the bath and the lights were off. I rushed over to the panel and checked voltage. I read 240+ L-L but the "A" leg only had 70-80V L-N, "B" had 120V. I then went back to "A" leg and it was back up to 120V. The lights were back on but, of course, the dryer had to be restarted, which it did start back.
This all sounds like the classic case of losing a leg and some of the 120V things would come on when a 240V load was on because of back feeding through the bus.
Strange thing is only the bath lights would quit when the dryer stopped. I couldn't find any other load that wasn't working.

I checked all connections on everything in the panel, ie; breaker terminals, main lugs, neutral lug and bus.
I found a few loose but not bad. I stayed about 30 min. more and never could get it to lose voltage again. Long day so I headed home. I talked to the HO on the phone as she wasn't home and told her to call POCO to come and check their connections. I had checked voltage before the main at the lugs so I knew it was either in the meter or one of the POCO connections.

Now after I get home the lady calls me and says POCO came and checked it. The problem showed up while they were there. They told her to tell me to replace the main breaker. I asked if they checked it and she said no they weren't allowed to. I asked if they checked their splices and she said all they did was pull the meter and check the voltage. No load on it or anything.:roll:

Anyway, I don't think it is the main breaker because I saw the low voltage before the main.
Any thoughts?
Click to expand...

If you measured the A leg 70-80V upstream of the main breaker, then I would think (like you do) it's in the feed from the meter, the meter itself, or an upstream problem.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
160416-0014 EDT

Little Bill:

If the voltage is low at the input to the main breaker, then the main breaker and wiring on the main breaker's load side are not the problem.

The problem is somewhere in the power company supply, meterbase, meter, or the wiring and connections to the main breaker. Where you measured the input voltage to the main breaker, and it was low, will determine how close to the main breaker input terminal you have to start looking for the problem.

.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
160416-0853 EDT

oldsparky52:

In the original post these statements occurred:
I had checked voltage before the main at the lugs so I knew it was either in the meter or one of the POCO connections.
Click to expand...

Anyway, I don't think it is the main breaker because I saw the low voltage before the main.
Click to expand...

.
 
growler

growler

Senior Member
Location
Atlanta,GA
Little Bill said:
I read 240+ L-L but the "A" leg only had 70-80V L-N, "B" had 120V. I then went back to "A" leg and it was back up to 120V.

Anyway, I don't think it is the main breaker because I saw the low voltage before the main.
Any thoughts?
Click to expand...

That's why I like to be there when the power company service guys show up. It's not always possible to do so but if you are there you can get them to do a few more test.


If it's an overhead service I would check the temp. of the connections at the weatherhead.
 
ATSman

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
gar said:
160416-0014 EDT

Little Bill:

If the voltage is low at the input to the main breaker, then the main breaker and wiring on the main breaker's load side are not the problem.

The problem is somewhere in the power company supply, meterbase, meter, or the wiring and connections to the main breaker. Where you measured the input voltage to the main breaker, and it was low, will determine how close to the main breaker input terminal you have to start looking for the problem.

.
Click to expand...

I agree 100%. The problem may be as far up stream as the roof peckerhead connections or the drop connections at the pole transformer.
 
O

oldsparky52

Senior Member
Location
Wilmington, NC USA
ATSman said:
I agree 100%. The problem may be as far up stream as the roof peckerhead connections or the drop connections at the pole transformer.
Click to expand...

Or a deteriorating neutral splice U/G. It happened to my neighbor a few years back. I happened to be home when the POCO truck showed up and I asked my neighbor what the problem was. Short story, the PoCo service man said everything was fine, I pressed him, did a small one leg load test and showed him they had a problem. He didn't believe me, called his boss, boss told him to put the "beast" on the meter.

Digging crew showed up 2 hours later, found the deteriorating u/g splice and fixed it.
 
Little Bill

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
growler said:
That's why I like to be there when the power company service guys show up. It's not always possible to do so but if you are there you can get them to do a few more test.


If it's an overhead service I would check the temp. of the connections at the weatherhead.
Click to expand...

It was late when I was there and even later when I told her to call POCO. I had no idea she was going to call right then. Sure didn't expect POCO to come out that late on a Friday.
I've had to deal with this POCO before.If you don't insist they check their connections they will just pull the meter. They will look up at the connections and say "looks fine" without actually checking them.


pjholguin said:
Little Bill,

What type of POCO meter? Digital or analog?
Click to expand...

Digital
We aren't allowed to touch the meter or connections. Also the digital meter would "tell on us"!
 
M

macmikeman

Senior Member
Location
Planet macmikeman
I need a dollar for every time a poco crew wasn't thorough enough when called out to check low power/ swinging voltage readings ahead of main breakers. They usually get it by about the third call back out. (usually they will drive off after fixing it without telling homeowner that they found and repaired the problem at their end...... )
 
Status
Not open for further replies.
Top