O.K. here's the deal. 3-pole 30amp 480v breaker is tripping on overload. A phase-8 amps, B phase-8 amps, C phase-42 amps. I'm thinking that there's a bad connection in the field somewhere causing the imbalance but I'm not sure. This is a brand new install, so the contractor is contractually responsible for any issues that arise within 1 year. Based on the as-builts, each phase should be drawing around 18 amps. What else would cause these type of readings? Bad ballast? Thanks in advance.
Site Lighting Load Readings
- Thread starter A-1Sparky
- Start date
- Status
broadgage
Senior Member
- Location
- London, England
The most likely reason is a simple wiring mistake that has put most of the load on one phase instead of balancing it.
The TOTAL current is similar to that expected, but is badly unbalanced.
A bad ballast can draw excess current, but that does not account for the low current on the other phases.
Also if a bad ballast was using an extra 20 amps at 480 volts, then that is many killowatts and I would expect it to catch fire or blow up quickly, not persist in drawing more current.
Bad connections are unlikely, a partialy open connection would reduce the current in most cases.
A partial short could draw a lot of current, but probably not for long, it would involve many KW being turned into heat at the fault, and almost certainly result in something catching fire, blowing up, developing into a dead short, or burning clear into an open circuit.
The TOTAL current is similar to that expected, but is badly unbalanced.
A bad ballast can draw excess current, but that does not account for the low current on the other phases.
Also if a bad ballast was using an extra 20 amps at 480 volts, then that is many killowatts and I would expect it to catch fire or blow up quickly, not persist in drawing more current.
Bad connections are unlikely, a partialy open connection would reduce the current in most cases.
A partial short could draw a lot of current, but probably not for long, it would involve many KW being turned into heat at the fault, and almost certainly result in something catching fire, blowing up, developing into a dead short, or burning clear into an open circuit.
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
Since none of us are there, we can only go on what is written. And since what is written does not lend to a solution, I would conclude that the information is somehow faulty. Perhaps you are getting your information from what you consider a reliable source, but should verfy things like actual readings, proper wiring, etc. before proceeding. Not trying to be an ass, but I have made similar assumptions to that which I am referring too many times to mention.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
High impedance ground fault
I've taken the readings myself on two occasions. First time: A phase-12 amps, B phase-12 amps, C phase-32 amps. That was about 2 months ago. The second round of readings was taken yesterday. High impedance ground fault, huh? So that would prevent the breaker from opening right away and would explain the imbalance? Thanks.
- Location
- Massachusetts
I will put my money on kingpb.
Most likely you have a phase wire burned open and it is shorting to water in the pipe.
At least that is what I find in my area often.
THHN/THWN with 480 VAC underground is in my opinion a problem waiting to happen.
cadpoint
Senior Member
- Location
- Durham, NC
- Occupation
- Electrician - but not by NC Law.
Well what are the readings now that it's fixed? 
- Status