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nchvac
07-30-2015, 07:50 PM
I have not had this happen at a customers property before, so this has me scratching my head. I am a licensed HVAC and Electrical Contractor so I think I should know this, but I just don't. Most of my electrical work is on HVAC work so I don't do much troubleshooting panel problems.

I have a 2000 or so model Carrier 3.5 ton Heat Pump. I should change it out because it is on its 3rd compressor, but for some reason things around the house come second and by the time we get done with the day who feels like working at home doing the same thing.

Anyway, about a year ago, maybe 2 years, I heard an arcing noise in the electrical panel. Didn't think too much of it and wasn't even sure I heard anything. The next time I was around that part of the house I heard it and noticed it happened when the Heat pump turned on. I opened the panel and find the problem to be the breaker for the outdoor unit. So I got to looking and whomever put the breaker in used a similar type but it was a forced fit. What it was doing was arcing on one leg. So I changed to the correct 40 amp breaker (max listed on the unit) and moved it to a different area of the panel so it would have 2 fresh legs to make a good connection on. Again, this breaker was brand new.

So 2 weeks ago I am in the same area and I heard a small arcing type noise in the panel again and at the same time the outdoor unit was turning on. I go to the panel, open it and I be darned if the same thing isn't happening. Had a nice pitted busbar leg with plenty of carbon build up. Thinking back I do believe it was on the same leg, which is probably the leg that breaks on the outdoor unit contactor-that would make sense anyway.

Now this was a new breaker and I use a Hard Start Kit on my unit even though it is a scroll compressor and I have a time delay on the thermostat and the outdoor unit circuit board has one as well (no they do not stack as 5 minutes from off time is the same at the unit as it is on the thermostat). The Run capacitor is good and the contactor as well as the Hard Start Kit was replaced during spring PM time. No high Locked Rotor Amp readings have ever been taken on the unit and the RLA is well within spec. All wires are tight and correctly sized.

Any ideas on why this has happened with the correct circuit breaker? And has anyone ever changed a busbar on a 200 AMP panel? I hope they make them to replace because this thing is built into the wall and will not be easy to change, especially not for free to me.

Coretz
08-16-2015, 03:56 AM
I'm too inexperienced to even begin answering how this happened or how to fix it, but... I did come across a reference to the NEC that state the busbar can't be used if it is damaged, so not having replaced it before you installed the new breaker is a violation. I was going to provide a link to the pertinent info, but I can't, so look up NEC 110.12c if you want.

But, that doesn't help you, you want to fix it now. Again, I have no formal experience, but from what I can find, if you have to replace the busbar, you need to replace the panel.

And, my apologies in advance if anything I said is untrue or noobish... I mean this is my first reply on this forum. Best of luck good sir, hope you can figure it out well enough.

billygoat22
08-16-2015, 06:10 PM
If you can by exact box new, could switch out parts into old casing.

not sure why arcing w. second breaker, I've noticed the more common type breakers that sometimes the piece metal inside breaker that grabs onto busbar is sometimes distorted even on new ones. They won't plug in right in those cases, so maybe you got one w/weak connection?

billygoat22
08-16-2015, 06:12 PM
oops, I've ordered the main breaker for a panel before that was going bad and switched out. could take pic of the data plate on casing and door so could potentially order new busbar.

Galt
08-16-2015, 08:25 PM
Get a cutler hammer ch panel or a square d qo panel.

jraef
08-17-2015, 02:04 PM
When you had the initial problem, it heated the bus bars. The bus is not soft copper, it is an annealed alloy with a specific hardness, because the breakers have to stab onto it without causing deformation. When it gets over heated, the alloy changes properties and becomes softer, so from that point on it no longer makes good connections. That's why the mfrs tell you to replace the bus if it is ever damaged. You were thinking, reasonably, that moving to a different location was good enough, but it was not.

You can likely buy just an interior, but often times you will find that the replacement parts cost more than an entire unit as a whole, because of volume buying. So buying the same or similar panels and just removing the guts might cost you less.

nchvac
08-17-2015, 05:40 PM
That makes sense. I was thinking maybe the metal used on the busbar had micro pitts or something that led to the arcing. I used to be a machinist and am familiar with annealing, so yes that makes sense. To be honest I thought that busbars were copper plated steel.

I will change the guts on this panel because it is mounted interior of a wall and I would rather not have to do a lot of sheet rock work that I am pretty sure I would need to do by the time I got all of the wires into the new box. I will look at it again and see. Hopefully they still make this style. If not there will be no choice but to change it. Sounds like a good rainy day job.


When you had the initial problem, it heated the bus bars. The bus is not soft copper, it is an annealed alloy with a specific hardness, because the breakers have to stab onto it without causing deformation. When it gets over heated, the alloy changes properties and becomes softer, so from that point on it no longer makes good connections. That's why the mfrs tell you to replace the bus if it is ever damaged. You were thinking, reasonably, that moving to a different location was good enough, but it was not.

You can likely buy just an interior, but often times you will find that the replacement parts cost more than an entire unit as a whole, because of volume buying. So buying the same or similar panels and just removing the guts might cost you less.

nchvac
08-17-2015, 06:44 PM
This panel is a Siemens M1632MB1200F. I am not finding where to buy bussbars for Siemens on the internet. Any suggestions?

billygoat22
08-18-2015, 10:08 PM
The part I needed was ordered from and elec supply place. Scavanging a new panel for parts, or even higher cost of parts would seem preferable to time to put in new box.

I've put in new boxes w/o having to do any sheetrock work. If the box is longer than old, works out ok since can cut hole bigger, but not smaller.

If same size, can push wires out top of bottom of box, and pull that end out first to avoid wall damage. Bundle the wires you shoved out, and can finesse them back thru hole in box by reversing order of install.