I have a 3 phase 120/208 service panel. I have circuits A,B & C sharing 1 neutral powered off 3 single pole 20 amp breakers. When they are hooked up correctly to A,B & C phases the circuit on C phase starts to cook the breaker when a load is applied. Replaced breaker 3 times same issue but when I take C circuit and put it on A or B phase no issues with load and breaker doesn't start cooking. I know this is causing a unbalanced load on the neutral but wanted to see if the circuit was trashed. When applying other circuits to the C phase they have no issue. Confused on this one since we moved other breakers around to test and no issues.
3 phase panel with 1 circuit having issues
- Thread starter DRM77
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- Chapel Hill, NC
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- Retired Electrical Contractor
Did you look at the busbar where you have the C phase issue. Could be it is messed up... It doesn't seem to be overloaded as the breaker isn't tripping so my guess is the buss.
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
Welcome to the forum.
First guess is condition of breaker contact with bus.
First guess is condition of breaker contact with bus.
Bus is and panel look great. I moved breaker around to other C phase bus slots and same result but only this circuit. Checked equipment hooked to the circuit and even tried using my heaat gun to draw a load. same issue.
contact is good
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
Well the only thing left is the circuit. I would either open all junction box's and see what's going on. You may have to replace the conductors. There could be a short of some sort but it should trip the breaker eventually. Does it not trip the breaker? Have you put an amp meter on that phase?
ptonsparky
Tom
- Occupation
- EC - retired
I agree, he will need to open to things up.
AC\DC
Senior Member
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- Florence,Oregon,Lane
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- EC
Would not a insulation test verify if there is a short on Phase C.
Amp meter like Dennis said would be my next check
Amp meter like Dennis said would be my next check
Another C10
Electrical Contractor 1987 - present
- Location
- Southern Cal
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- Electrician NEC 2020
what does the amp probe or voltage tester show
hillbilly1
Senior Member
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- North Georgia mountains
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- Owner/electrical contractor
What is the voltage on “C” phase? Since it works fine on A or B, Could you possibly have a Delta connected service, and C phase is the high leg? What is the load connected to that phase? Is that load working fine? Or not? If it is multi volt ballasts or drivers, they may work fine on the 208 instead of the 120, but I wouldn’t think it would cook the breaker, even though it’s not Delta rated. Not really enough info to make an informed swag! LOL!
Another C10
Electrical Contractor 1987 - present
- Location
- Southern Cal
- Occupation
- Electrician NEC 2020
That's what comes to mind for me too ..
Check voltage at the breaker, on the bus, at the load. May be elsewhere. Megger is a waste of the me. You have a high resistance connection somewhere or a high voltage condition on the C phase.
As far as Voltage say you have a transformer built with 3 cans and one can is tapped up or maybe say 240 secondary. Or you assume 208/120 but it’s really high leg delta which is what it sounds like!! This will eat up your “110” equipment/ You can never put single phase loads on a high leg except 240.
The other issue is high resistance connections. While not under load you will see normal voltage. Resistive loads like lights and heaters may not work right but no damage. Motors will be weak so the amps increases dramatically to compensate. These can be very hidden:. Things like a cable was cut orn a diagonal or just not all the way into a terminal, aluminum set screw Halle’s and not fully seated, wrong lug not crimped fully. Even a weak breaker sprint. Start at breaker. If you see low voltage keep going upstream until it is normal…that’s the source. It can be the utility. Remember to them you are the same as three double wides with a bigger bill.
Heat travels too. I have seen both ends of a wire cooked when only the downstream end was loose.
The third possibility related to the last one is a blown primary fuse: These are notoriously tricky to find. If the tranny is delta wye you might see a slightly low voltage unloaded. This train will regenerate the third phase like a static phase converter but it is high impedance. The moment you put load on it, your voltage goes to a low value like you have a weak tranny on one leg.
To T/S the low voltage issues is hard because most of them only happen under load: Often it’s helpful use min/max on your meter or use a big resistive load like a shop heater. Motors and trannys are not good loads for this issue. They just trip breakers and blow fuses.
If you can load it long enough an IR gun helps.
As far as Voltage say you have a transformer built with 3 cans and one can is tapped up or maybe say 240 secondary. Or you assume 208/120 but it’s really high leg delta which is what it sounds like!! This will eat up your “110” equipment/ You can never put single phase loads on a high leg except 240.
The other issue is high resistance connections. While not under load you will see normal voltage. Resistive loads like lights and heaters may not work right but no damage. Motors will be weak so the amps increases dramatically to compensate. These can be very hidden:. Things like a cable was cut orn a diagonal or just not all the way into a terminal, aluminum set screw Halle’s and not fully seated, wrong lug not crimped fully. Even a weak breaker sprint. Start at breaker. If you see low voltage keep going upstream until it is normal…that’s the source. It can be the utility. Remember to them you are the same as three double wides with a bigger bill.
Heat travels too. I have seen both ends of a wire cooked when only the downstream end was loose.
The third possibility related to the last one is a blown primary fuse: These are notoriously tricky to find. If the tranny is delta wye you might see a slightly low voltage unloaded. This train will regenerate the third phase like a static phase converter but it is high impedance. The moment you put load on it, your voltage goes to a low value like you have a weak tranny on one leg.
To T/S the low voltage issues is hard because most of them only happen under load: Often it’s helpful use min/max on your meter or use a big resistive load like a shop heater. Motors and trannys are not good loads for this issue. They just trip breakers and blow fuses.
If you can load it long enough an IR gun helps.
Tulsa Electrician
Senior Member
- Location
- Tulsa
- Occupation
- Electrician
Question 1 : what does this multi wire circuit feed.
Question 2 : why are they Single pole breakers.
Question 3: did you have amp clamp on these wires.
Question 4: all 120 v loads
Comment: if the circuits feed loads that used used C for both A and B it could over load C. Do load checks on all lines with load on them. Also check load on netural as each in energized.
These may not be all 120v loads.
Compare amp load when all hooked up A,B,C and then A,A,B ect. Also have camp on netural while doing this. The math will help you know what's going on. So two amp clamps.
If there lighting and recpt circuits. As others have mentioned start tracing.
If hooked up A,B,C and A and B are off does C still sizzle?
Edit:
Would like to add.
With A and B hooked up and breakers on. Have C wire unhooked from beaker and clear. Turn breaker C on. Use voltage setting. Put meter in series with C wire to netural bar take reading. Then take meter and put in series with C wire and C breaker. This should tell a story.
Question 2 : why are they Single pole breakers.
Question 3: did you have amp clamp on these wires.
Question 4: all 120 v loads
Comment: if the circuits feed loads that used used C for both A and B it could over load C. Do load checks on all lines with load on them. Also check load on netural as each in energized.
These may not be all 120v loads.
Compare amp load when all hooked up A,B,C and then A,A,B ect. Also have camp on netural while doing this. The math will help you know what's going on. So two amp clamps.
If there lighting and recpt circuits. As others have mentioned start tracing.
If hooked up A,B,C and A and B are off does C still sizzle?
Edit:
Would like to add.
With A and B hooked up and breakers on. Have C wire unhooked from beaker and clear. Turn breaker C on. Use voltage setting. Put meter in series with C wire to netural bar take reading. Then take meter and put in series with C wire and C breaker. This should tell a story.
Last edited:
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