I had a service call where a 70 amp Square D QO breaker and loadcenter that serve as a disconnect for an electric strip heat unit had melted down and fused to each other. After installing a new breaker and loadcenter I observed that the new breaker is getting too hot. Read with a Fluke laser thermometer it reaches 120 degrees on the load side lugs of one phase. When reading the current draw with a Fluke meter, A phase reads 59 amps and B phase reads 54 amps, however the A phase is reaching 120 degrees while the B phase is at 84 degrees. The breaker is rated at 40 degrees Celsius ( 104 Fahrenheit). All connections are tight and the breaker is stabbed in correctly and making good contact. Can anyone provide any ideas as to why this is happening? I fear that the new breaker and loadcenter will melt down also. The HVAC man says there is nothing wrong with his unit.
Melted Breaker
- Thread starter patron
- Start date
- Status
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
It sounds to me like you didn't size your breaker at 125% for continuous loads. 59x1.25%=73.75 amps
- Location
- Illinois
- Occupation
- retired electrician
You have a bad connection at the bus, within the breaker or at the conductor termination. There should not be that much difference between the two temperatures you measured. Did you measure the votlage drop between the bus and the load side of the breaker?
Note, I am not saying that the 120?F at the termination point is too hot, but with the load on each pole being as close as they are, the temperatures should not have that much difference.
Note, I am not saying that the 120?F at the termination point is too hot, but with the load on each pole being as close as they are, the temperatures should not have that much difference.
modulating control?
modulating control?
Check the control, how does the heat load modulate? Is it on / off by pulling in staging relays or is it an electronic SCR type modulating control?
If you have on / off staging relays the load is clean and not causing the heating. If you have SCR's that are firing and chopping part of the sinewave, the load is noisy, nasty non sinusoidal with high frequency components, and is likely causing the heating.
In addition to sizing at 125% for continuous loading there would have to be some derating for the high harmonic loading, if present. I do not know exactly what to prescribe, I would call the equipment manufacturer and get their recommended field wiring requirements. Ask them about the type of modulating control if it's not obvious.
modulating control?
Check the control, how does the heat load modulate? Is it on / off by pulling in staging relays or is it an electronic SCR type modulating control?
If you have on / off staging relays the load is clean and not causing the heating. If you have SCR's that are firing and chopping part of the sinewave, the load is noisy, nasty non sinusoidal with high frequency components, and is likely causing the heating.
In addition to sizing at 125% for continuous loading there would have to be some derating for the high harmonic loading, if present. I do not know exactly what to prescribe, I would call the equipment manufacturer and get their recommended field wiring requirements. Ask them about the type of modulating control if it's not obvious.
I'm pretty sure that the 40 deg is the operating air temp of the breaker (ambient)
The breaker lugs are rated for 60 / 75 deg C????
Thats 140-167F.
Sierrasparky
Senior Member
- Location
- USA
- Occupation
- Electrician ,contractor
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Yes, the 40?C on the side of the breaker is its ambient temperature rating.
The lugs of the breaker should always be significantly cooler (10-15?C) than the corresponding conductor insulation. In practice, the conductor acts a heat sink.
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
I honestly couldn't tell you about the breaker contact size either.
Troubleshooting based on assumptions gets me in trouble though. I'd rather install the correct size breaker from the start and then see if they still have problems.
kwired
Electron manager
- Location
- NE Nebraska
the thermal element in a 70 amp breaker better be different than a 100 amp breaker otherwise they have the same trip setting.
HotConductor
Senior Member
- Location
- Philadelphia
Exactly.
What type of loads/heat are on the adjacent breakers? With higher continuous load 2 pole/3 pole circuits I like to leave an open space on either side of the breaker(if possible) for heat dissipation.
- Status