a/c breaker trip

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jetlag

Senior Member
My son works for a motel chain. He said the outside condenser is supplied by #10 conductors and a 35a breaker. He said the name plate shows 37 a max draw. I guess thats the same as full load amps. He says the breaker trips often. I told him I would check amps while running and if over 30 , I would run #8 conductors first.. If load is in the 20's a 50a breaker or even 2.5 times the load is ok for start load on a regular breaker. He doesnt know if the 35a is a time delay breaker or not . I also told him if the running load is much over the 37a max there is problem with the compressor,, Does this sound like good advise, Hey If I scew up its me but if I cause someone else to do it that bothers me more.
 

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
Tell him to hire an electrician to look at this. First, breaker to bus connections and conductor to breaker connections. Then amprobe while equipment is running, could be an equipment problem.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
Depends by what he means by "max draw". He needs to look at the nameplate and see if it lists Minimum Circuit Ampacity (MCA) and Max Breaker values. If the max breaker is 35A, then the install seems reasonable. If the MCA is 35, then its no wonder it trips often.

The wires need to be sized per the MCA value. The breaker needs to >= MCA is not more than the max breaker.

If you have no MCA value, but have an RLA for the compressor and FLA for the fan, then you'd take RLA*1.25 + FLA and that will be close to the MCA. It is just nicer when the factory calculates the MCA because there could be other loads you don't know about.

The measured running current should be near the sum of the RLA and FLA, or about 80% of the MCA (depending on what numbers he has available).
 

Jim W in Tampa

Senior Member
Location
Tampa Florida
Sounds bit high for normal motel room. How many tons ? He needs to know the running amps total. Is always a chance the unit it over charged. If he is in charge of doing the AC work have him check freon level and put amp probe on it. Anything over 30 after its running few minutes is too high for #10. He might be triping only on start up. Add a hard start cap and it might hold. Without actual numbers its guess work.
 

jetlag

Senior Member
Tell him to hire an electrician to look at this. First, breaker to bus connections and conductor to breaker connections. Then amprobe while equipment is running, could be an equipment problem.

He is an inplant electrician ,originally trained under me and I have a state masters license for 18 years. And the post states that I advised him to check the running amps it could be equiptment problem, so your post is pointless.
 

cadpoint

Senior Member
Location
Durham, NC
Depends by what he means by "max draw". He needs to look at the nameplate and see if it lists Minimum Circuit Ampacity (MCA) and Max Breaker values. If the max breaker is 35A, then the install seems reasonable. If the MCA is 35, then its no wonder it trips often.

The wires need to be sized per the MCA value. The breaker needs to >= MCA is not more than the max breaker.

If you have no MCA value, but have an RLA for the compressor and FLA for the fan, then you'd take RLA*1.25 + FLA and that will be close to the MCA. It is just nicer when the factory calculates the MCA because there could be other loads you don't know about.

The measured running current should be near the sum of the RLA and FLA, or about 80% of the MCA (depending on what numbers he has available).

Thanks for that, that is one great overview. I'll pasted it elsewhere...

For all of you out there that don't deal with motors a lot, well pay attention...
 

jetlag

Senior Member
thanks jim

thanks jim

Sounds bit high for normal motel room. How many tons ? He needs to know the running amps total. Is always a chance the unit it over charged. If he is in charge of doing the AC work have him check freon level and put amp probe on it. Anything over 30 after its running few minutes is too high for #10. He might be triping only on start up. Add a hard start cap and it might hold. Without actual numbers its guess work.

ITs not for a motel room , its for another area I think its 4 tons or more . It was late today so he plans more tests tomorrow. I,,ll remind him to make sure unit is not over charged
 

Mgraw

Senior Member
Location
Opelousas, Louisiana
Occupation
Electrician
If the amps are high have him check the contactor points for the compressor. They can be pitted and will cause a unit to over amp. Also check the wires at the compressor, sometimes they get loose.
 

jetlag

Senior Member
thanks suemarkp

thanks suemarkp

Depends by what he means by "max draw". He needs to look at the nameplate and see if it lists Minimum Circuit Ampacity (MCA) and Max Breaker values. If the max breaker is 35A, then the install seems reasonable. If the MCA is 35, then its no wonder it trips often.

The wires need to be sized per the MCA value. The breaker needs to >= MCA is not more than the max breaker.

If you have no MCA value, but have an RLA for the compressor and FLA for the fan, then you'd take RLA*1.25 + FLA and that will be close to the MCA. It is just nicer when the factory calculates the MCA because there could be other loads you don't know about.

The measured running current should be near the sum of the RLA and FLA, or about 80% of the MCA (depending on what numbers he has available).

Its my fault he calls it max draw, it is the MCA as stated at 37a on the name plate so it seems to me the #10 conductors and 35a breaker was not a proper install. He will look for more spec tomorrow .I dont see how they could expect to start the compressor with a 35a breaker .
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Its my fault he calls it max draw, it is the MCA as stated at 37a on the name plate so it seems to me the #10 conductors and 35a breaker was not a proper install. He will look for more spec tomorrow .I dont see how they could expect to start the compressor with a 35a breaker .
I would agree. If the 37 is actually the MCA, then the #10 would be undersized and 35 would be puhing it as the OCP. Did he mention what the nameplate MOCP is ?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
MCA = minimum circuit ampacity. If it is only 35 amps then the #10 wire is okay at 75C. Use a 50 amp breaker. If it is 37 amps then you need #8
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Its my fault he calls it max draw, it is the MCA as stated at 37a on the name plate so it seems to me the #10 conductors and 35a breaker was not a proper install. He will look for more spec tomorrow .
Look for MO(C)P while you're there.
 

Mgraw

Senior Member
Location
Opelousas, Louisiana
Occupation
Electrician
In my experience it is rare to see a A/C unit run at MCA or more unless it has a problem. Dirty coils, pitted contactor, loose connection, over charged, low voltage, "tight" compressor can all cause a unit to over amp. I have known a few A/C contractors who would fuse a unit at or right above MCA to get more service calls. They would claim it was to protect the compressor. I got alot of good customers because of them.
 

jetlag

Senior Member
Thanks Mgraw

Thanks Mgraw

In my experience it is rare to see a A/C unit run at MCA or more unless it has a problem. Dirty coils, pitted contactor, loose connection, over charged, low voltage, "tight" compressor can all cause a unit to over amp. I have known a few A/C contractors who would fuse a unit at or right above MCA to get more service calls. They would claim it was to protect the compressor. I got alot of good customers because of them.

I think this is the problem and today he will find out if the unit runs below the mca , but the 35a breaker has prob starting the compressor and is also 2 a below the mca of 37a. Before my son was hired they had to call electrician for everything, so like you say , protect the compressor was more like pad the wallet.
 
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