Breaker question

[pierre]

I was called yesterday about this situation:

Two A/C condensor units outside about 50 feet from the panel at a residence (120/240v).
A 100 amp breaker feeding one set of 4AWG or 2AWG Al conductors.These conductors feed 2-50 amp breaker type disconnects located by the A/C units. This installation is about 12-15 years old.
recently, for the last two weeks, the 100 breaker in the panel is heating up and tripping when they record approximately 38 amps of current on both legs(?).
All connections have been tightened, and a new 100 amp breaker was installed - with the same results of the breaker tripping every 15-30 minutes at 38 amps running current.
After much testing via the phone, I suggested running another set of copper feeders to one of the disconnects and separating the two. Using two 50 amp breakers in the panel.

What do you think is causing this, and what would you do?
BTW, the told me the 100 amp breaker lugs are cherry red when they trip. There is some discoloration in the bus, but as I was not there to see how bad, I did not make any suggestions as to changing the bus - it is a MURRAY panel.

Pierre

 

[electricman2]

Re: Breaker question

Based on what you said, I would take a look at the bus stubs.

 

[friebel]

Re: Breaker question

To Pierre; Let me tell you what I think that your problem is with the circuit breaker tripping out. It was stated that the stubs were cherry red. This means that the connection to the breaker has lost all conductivity. In other words, it is acting just like a heater with high resistance. Now if this is the bars that you plug your breakers into and they are cherry red, then you have lost all conductivity and the bars will need to be replaced. This has happened many times with wiring that becomes cherry red and loses it's conductivity. The only fix is to replace the wire.

 

[hbiss]

Re: Breaker question

By breaker lugs I assume you mean the lugs where the conductors are connected. Obviously there is a high resistance here for them to heat up with 38 amps. It's the conducted heat that is causing the breaker to trip. If the bus is in good condition I would replace the breaker again and this time cut the conductors back beyond any overheating discoloration. You will probably have to splice tails onto them. Hopefully there is enough length in the panel to do this.

-Hal

 

[newt]

Re: Breaker question

Take a voltage reading from line side of main to load side of the breaker in question should be close to 0_.5 anything else would indicate bad connection.Is the brk listed for hvc.

 

[templdl]

Re: Breaker question

Also remember that when there is heating which may be cause by a failing line or load connect it only gets worse. Heating at the load connection commonly heats the thermal elements of the breaker, causing the breaker to derate and trip because of the close proximity of the thermal element to the load side of the breaker which can actually protect the breaker.
However, A line side termination failure more often than not is never sensed by the thermal elements because of the location of the line side terminations. The heating does quite often conduct though the stationary contacts on to the moving contacts to the springs that hold the moving contacts closed tightly against the stationary contacts. This heating can take the tension from the springs and greatly reduce contact pressure which causes the contact heat up and fail.
The heating and cooling cycle of a termination also causes a loose termination to continue to loosen even more which increased the heating even more. I have an example of a 250a frame breaker that I keep for show and tell where a line side terminal failure finally started to arc to itself, the arc extended to the ground and then involved the phase adjacent phase.

 

[pierre]

Re: Breaker question

Just a recap:

The contractor ran two sets of 6AWG copper feeders and used fused disconnects at the two A/C units. The last two days have been quite warm and the units have not tripped once. Go figure.