HVAC tripping breaker mystery

[electrofelon]

About a year ago, I hooked up a 20 ton package unit. Its 480, breaker is new Siemens BQD 360 breaker. MOCP listed was 60 amps. Conductors are #6, not particularly long, maybe 90 feet. The breaker has tripped several times recently. The site is rather far from me so I have not been there yet, but owner says the HVAC company has been there and cant find a problem. HVAC tech says he tested all three compressors, and with them all in its drawing 34 amps. I dont know if he tested any inrush currents. So I know its not a lot to go on, but any ideas? Has anyone had units trip when wired per MOCP values? I assume unit is not supposed to start more than one compressor at the same time, could that delay circuit not be working right so two are starting at once?

 

[Chamuit]

Is this an intermittent problem?

If not, check the terminations, check the wire (meg), and check the breaker (amp). Not much else left electrician wise.

 

[electrofelon]

Is this an intermittent problem?

If not, check the terminations, check the wire (meg), and check the breaker (amp). Not much else left electrician wise.

My understanding is it just started happening recently, after running fine for almost a year, and it tripped twice in about two days. The owner says they have had "a bunch of other problems with this unit".

 

[Sahib]

The owner says they have had "a bunch of other problems with this unit".

But per HVAC tech, no problem with the unit. Perhaps replacing the breaker may help.

 

[electrofelon]

But per HVAC tech, no problem with the unit. Perhaps replacing the breaker may help.

I will certainly try that if nothing else comes up.

 

[jrlelectric]

Circuit Breaker tripping

Circuit Breaker tripping

Once a circuit breaker trips the first time, a flash will create carbon deposits that can create resistance. The Siemes BQD Circuit Breakers are very robust but this does not mean the breaker was not defective from the start. Check to make sure the wire terminals are tight and that the breaker is tightly bolted to the panel. A new BQD 3 Pole 60 Amp Circuit Breaker is only $109.00
In order not to violate forum policy, you can contact me and I can direct you where to purchase this at this low price.

 

[ptonsparky]

I certainly wouldn't go without a replacement breaker. Do a thermal scan, VD or FOP across the breaker and maybe the supplier can get you credit for the old. Hopefully you torqued connections.

 

[Jraef]

But yes, if the delayed start system is malfunctioning and allowing two compressors to start at the same time, it absolutely could cause the breaker to trip on inrush current.

 

[ptonsparky]

But yes, if the delayed start system is malfunctioning and allowing two compressors to start at the same time, it absolutely could cause the breaker to trip on inrush current.

You would think that after multiple trips by the HVAC tech, that would have been discovered, but it wouldn't be the first time.

I noticed some PP timing of milk coolers for a good sized dairy and mentioned it to the Manager. "They were just here."

I insisted timing was not correct. It was the single most notable correction made at improving their power factor.

 

[synchro]

A measured 34 amps is somewhat high when the MOCP spec is 60A. Also, was the 34A including all fans? If not then your even closer to running out of breaker, so to speak.
The HVAC guy should provide more detail about how the measured currents compare to the compressor RLA's, and where the running current is falling on the manufacturer's performance curves.

 

[ptonsparky]

The problem could be with one of the condensing fans. That wouldn't be the first time.

 

[Chamuit]

If the OP wired and breakered the unit correctly and there are no issues between the breaker and the unit, there is a problem with the unit. Maybe just maybe, the breaker may have an issue.

 

[StarCat]

HVAC Intermittent Problems

HVAC Intermittent Problems

If the OP wired and breakered the unit correctly and there are no issues between the breaker and the unit, there is a problem with the unit. Maybe just maybe, the breaker may have an issue.

There is a post or response I made somewhere on the forum about something of this nature. My background is I have a Masters License in HVACR and 30 years experience. These kinds of problems can be similar to auotmotive and can be hard even for well seasoned Techs to nail down. Taking a current reading on compressors on a quick service call may not necessarily be enough. A meter that can catch and record peak current is useful such as Fluke 87 series.
The case I am referring to was also 480V and there was an insulation fault on one of the line conductors in the service that was brought up through the unit blower section as they often times were on Lennox GCS series RTUs. I was watching this confounded unit running with the blower panel off, babysitting as we call it and saw an arc flash in the conduit. I had to force the repair electrician to also see it because he was unable to make a leap of perception that I knew what I was speaking about at the time. So afterwards I said again, re-pull the wire to the nearest junction or all the way home if necessary.
With that said there can be all manner of situations that can cause imtermittent faults that can indeed be hard to trace quickly. In some cases they have to be ruled out methodically by " isolation " methods, and inspecting unit wiring runs in detail. Isolation to mean ruling out which loads on board that are not at fault by various methods including shedding the suspected load when possible for a " time."
Starting with the basics, all contactors, relays, and terminations need to be looked at closely. This means all the way to the compressor terminals and so forth.
Cabinet wiring can and does wear out and fail, and can be close to ground but not in direct contact. Compressors that are in a stage of mechanical degradation can run for seemingly long periods of time before they develop secondary electrical failures. It can get time consuming. The trick is to break it down into pieces, working with the HVAC guy if necessary. In my case it was always me who had to find the root of the problem, and I had to " fix " it.

Star

 

[electrofelon]

Ok thanks for the replies everyone. Ill get there when I can, prolly next week, and do my own trouble shooting and see what I find. I have a spare breaker, I'll bring that with me just in case. I'll let y'all know what I find.

 

[Jraef]

Ok thanks for the replies everyone. Ill get there when I can, prolly next week, and do my own trouble shooting and see what I find. I have a spare breaker, I'll bring that with me just in case. I'll let y'all know what I find.

Please do. What you find can benefit a lot of other people.

 

[LarryFine]

A measured 34 amps is somewhat high when the MOCP spec is 60A.

Agree! That doesn't allow for much starting current.

 

[electrofelon]

Agree! That doesn't allow for much starting current.

That is pretty typical, in my experience. I don't remember the exact MCA but it is low 50's, which is right around my rule of thumb that actual current is usually pretty close to 2/3rds of MCA. Seems like these days MCA's are not far below the MOCP.

 

[synchro]

I think compressor motor currents can vary a lot depending on the temperatures, amount of refrigerant, etc. (which I'm sure StarCat can expound about much better than I can). So I think the expectation would be that the worst case running current is going to be higher than that made on one measurement. Hence the concern for adequate margin. But it's good to hear if things are becoming more tightly controlled with newer equipment.

 

[electrofelon]

I dug up the submittal sheet out of the depths of my email. Turns out it's only two compressors - I misheard or someone misspoke. 16.7 each RLA, MCA 49.8, MOCP 60.

 

[Sahib]

A measured 34 amps is somewhat high when the MOCP spec is 60A. Also, was the 34A including all fans? If not then your even closer to running out of breaker, so to speak.

If soft starting feature is present in the unit, it does not matter.