Tripping Sub Feed

[mhanson]

Good Day All,

Looking for thoughts on a tripping 125amp residential sub feed.
Sub panel located in laundry room, fed with Aluminum SER Cable from the 200amp Meter/Main panel
Home Owners claim that the 125amp breaker trips when the dryer reaches the third consecutive load.
the 240v 30amp dryer branch circuit is fed from the 125amp sub panel and does not trip.
additional loads in this sub panel include the typical general purpose lights and plugs, kitchen microwave, dishwasher small appliance branch circuits, etc...
Nothing stands out as unusual. I asked the home owners to run several appliances simultaneously, including the dryer and was only able to read about 45amps on B phase, 25amps on A phase, with no sign of trouble.

Appreciate your comments.

Thank You,
Michael Hanson
California Journeyman Electrician.

 

[CONDUIT]

I would check terminals at 125 amp breaker for possible loose connection.

 

[kwired]

I would check terminals at 125 amp breaker for possible loose connection.

Load things up and check temperature of this breaker/terminations.

 

[templdl]

If the owner present at the time the breaker trips is the breaker warn to the touch? Can the breaker be latched (reset) and closed right away or is there a wait period?
If so it tripped on thermal either from an overload or possible load end terminations that are compromised. Seeing that you are dealing with All cable I would consider that as a cause. The ter,imation gets warn, the heat travels into the breaker and heats the thermal element and the breaker derates and nuisance trips.
Of course a breaker that trips instantaneously can be latched and closed right away which doesn't appear to be the cause of the trip.

 

[mhanson]

Tripping Sub Feed

Thank You all for the suggestions,
I will take your suggestions and give these tests a try,

I did make a visual inspection of the equipment and found no signs of discoloration due to overheating, but did not actually check torque on the 125amp breaker.

One thought I have is damage to the NM sub panel feed.?
would a Megger determine if there is damage to the cable.?
and if there is damage, would this trip the breaker.?

Thank You,
Michael Hanson
California Journeyman Electrician.

 

[templdl]

Thank You all for the suggestions,
I will take your suggestions and give these tests a try,

I did make a visual inspection of the equipment and found no signs of discoloration due to overheating, but did not actually check torque on the 125amp breaker.

One thought I have is damage to the NM sub panel feed.?
would a Megger determine if there is damage to the cable.?
and if there is damage, would this trip the breaker.?

Thank You,
Michael Hanson
California Journeyman Electrician.

Why would you suspect damage to the cable as it would relate to tripping the breaker? The only thing that would be a remote chance is the cable is faulting which be actually a catastrophic event. As afore mentioned as I suggested did the breaker trip thermally or magnetically or are you still guessing? Let's determine that first and go from there.

 

[normbac]

If 125 amp breaker is in solid on a clean buss no arcing and wires are tight then
start with the easiest thing, first change out breaker if you only have a smaller breker in stock use it pig tail if you have to but make sure it is at least 30 amp preferably higher, run dryer only and see if it trips. It is always best to know it is not a breaker issue especially since it is such an easy fix.

 

[mhanson]

my thought is that perhaps the cable was stapled too tightly or even one of the phase conductors may have been nicked or pierced by a staple.
Aluminum conductors have a great deal of thermal expansion during use which may cause a short once the conductors heat up enough to make contact.

I will take your suggestions on my next visit and make some appropriate tests. I just wanted to approach this situation with the knowledge of as many possibilities as I can get my hands on.

Thanks again for your suggestions

 

[mhanson]

If 125 amp breaker is in solid on a clean buss no arcing and wires are tight then
start with the easiest thing, first change out breaker if you only have a smaller breker in stock use it pig tail if you have to but make sure it is at least 30 amp preferably higher, run dryer only and see if it trips. It is always best to know it is not a breaker issue especially since it is such an easy fix.

If the dryer is a problem, don't you think the 30amp breaker at the sub panel would trip.?
I don't believe this is associated with any branch circuit but something with the feed. like the cable or 125amp breaker.

 

[Sierrasparky]

45 amps on a 125 amp feeder should not be causing expansion that would make a staple dig in.
I would however be concerned that there is a nail in the feeder. I would check the feeder for a fault.
Is there moisture where the feeder runs?

 

[mhanson]

45 amps on a 125 amp feeder should not be causing expansion that would make a staple dig in.
I would however be concerned that there is a nail in the feeder. I would check the feeder for a fault.
Is there moisture where the feeder runs?

we are in a fairly dry climate here in the San Francisco Bay Area.
The feed is run is thru ceiling and wall space only.

I agree with the nail theory, do you think using an Megger Insulation tester on the cable will help in determining this condition.?

 

[normbac]

If the dryer is a problem, don't you think the 30amp breaker at the sub panel would trip.?
I don't believe this is associated with any branch circuit but something with the feed. like the cable or 125amp breaker.

it is the 125 amp I was refering to i just said 30 amp or higher in case you only had smaller brekers in stock if you have an alternate 125 then of course use that to check
it is to eliminate if the 125 amp is a faulty breaker not the dryer or dryer cb

 

[edlee]

we are in a fairly dry climate here in the San Francisco Bay Area.
The feed is run is thru ceiling and wall space only.

I agree with the nail theory, do you think using an Megger Insulation tester on the cable will help in determining this condition.?

A megger will tell you if there is damage to the insulation. The nail theory doesn't sound likely to me since it takes more than two full loads of the dryer before it happens, though I suppose it's possible. But if there were sudden shorting due to gradual expansion then wouldn't that burn itself away enough to make it eventually stop blowing the breaker? It's not like there could be a lot of metal-to-metal contact or the breaker would blow immediately.

Slow heating that causes thermal opening of the breaker means there is resistance somewhere. If there were a spot problem that was remote from the breaker, then the heat would also be remote and the resistance would be effectively current-limiting, so the breaker wouldn't trip. Are all the circuit conductors sized properly? An undersized wire would slowly heat up.

I think there's something wrong at the panel: 125 breaker itself, the terminations, etc. The suggestion of temporarily replacing the breaker seems good. Have you clipped on an ammeter just to see what's happening?

 

[mhanson]

A megger will tell you if there is damage to the insulation. The nail theory doesn't sound likely to me since it takes more than two full loads of the dryer before it happens, though I suppose it's possible. But if there were sudden shorting due to gradual expansion then wouldn't that burn itself away enough to make it eventually stop blowing the breaker? It's not like there could be a lot of metal-to-metal contact or the breaker would blow immediately.

Slow heating that causes thermal opening of the breaker means there is resistance somewhere. If there were a spot problem that was remote from the breaker, then the heat would also be remote and the resistance would be effectively current-limiting, so the breaker wouldn't trip. Are all the circuit conductors sized properly? An undersized wire would slowly heat up.

I think there's something wrong at the panel: 125 breaker itself, the terminations, etc. The suggestion of temporarily replacing the breaker seems good. Have you clipped on an ammeter just to see what's happening?

I did have the Home Owners turn on several house loads while I was on site, including the dryer, I was reading only 25amps on phase A, and 40amps on phase B.
All conductors are sized properly

I like the idea of replacing the 125amp breaker as a first step and then performing high load tests. What is the benefit of temporarily installing the smaller breaker size.?

Thank You,
Michael Hanson

 

[templdl]

I did have the Home Owners turn on several house loads while I was on site, including the dryer, I was reading only 25amps on phase A, and 40amps on phase B.
All conductors are sized properly

I like the idea of replacing the 125amp breaker as a first step and then performing high load tests. What is the benefit of temporarily installing the smaller breaker size.?

Thank You,
Michael Hanson

It is all that unusual for those to keep on replacing parts until they finaly end up replacing the defective part. It still hasn't been determined if the breaker was triping on thermal or magnetic or neither has a thermal scan been done on the breaker for the possibility there there may be a termination issue. Who pays for the replacement parts when that doesn't solve the problem?

 

[kwired]

I did have the Home Owners turn on several house loads while I was on site, including the dryer, I was reading only 25amps on phase A, and 40amps on phase B.
All conductors are sized properly

I like the idea of replacing the 125amp breaker as a first step and then performing high load tests. What is the benefit of temporarily installing the smaller breaker size.?

Thank You,
Michael Hanson

The idea of using a smaller breaker is probably suggested because you would be more likely to have a smaller breaker on hand. That said you still should be taking temp readings of the breaker and terminations and not just throwing parts at it. Also look for signs of overheating at the bus connection or the output lugs of this breaker. If the bus connection has been overheated it needs replaced or you will forever have problems even if you replace the breaker.

Another thing not yet mentioned is to check for fall of potential across the breaker while under heavier load. If it is heating up there is resistance there and there will be some voltage drop across the resistance.

 

[guschash]

just a thought. If you are feeding from the meter/disconnect are you using 4 wires. Does your SER have 4 wires.

 

[kwired]

just a thought. If you are feeding from the meter/disconnect are you using 4 wires. Does your SER have 4 wires.

Though that can lead us to other code/safety issues, it really should not have any effect on the problems the OP is having with tripping the 125 amp breaker.

 

[edlee]

It is all that unusual for those to keep on replacing parts until they finaly end up replacing the defective part. It still hasn't been determined if the breaker was triping on thermal or magnetic or neither has a thermal scan been done on the breaker for the possibility there there may be a termination issue. Who pays for the replacement parts when that doesn't solve the problem?

The suggestion to temporarily replace the 125a breaker is strictly diagnostic. The reason for the suggestion of using something smaller is, as someone already said, it is likely to be on hand.

 

[wyreman]

All these posts about nuisance tripping on mains!
Try installing a FP breaker =0 there...
30amps should be plenty!

Sorry... couldn't "resist" :slaphead: