a/c breaker sizing

[erfitz1208]

im a 3rd yr elect apprentice. my neighbor is having a/c trouble.

problem: in the middle of summer when the ac has been running for about 8 hrs the breaker(main panel) for the condensing unit trips..

specs: 2,000 square ft house with 4 ton rheem split system.

cond. nameplate: minimum circuit ampacity-29amp
minimum fuse or breaker size-35amp
maximum fuse or breaker size-50amp

wire size from the cond. to disconnect-8awg
wire size from disconnect to main panel-10awg
30amp breaker in main panel for cond.

he said it only started tripping within the last yr, unit has been in for 10 yrs.

Im wondering why the electricians installed a 30 amp breaker, when the cond calls for 35amp br minimum.


Would a 40amp breaker be more appropriate????:roll:

 

[Jraef]

Would a 40amp breaker be more appropriate????

Not without changing the conductors. That's probably why they used a 30A breaker. If I had to guess, I'd say there was a smaller A/C unit in there before, someone changed it out with a larger unit but got cheap on the installation and didn't want to pay for having new conductors run to the disconnect. That 30A circuit is too small for that A/C unit, I'd do the entire run in 8AWG and use a 40A CB.

 

[don_resqcapt19]

Jeff,
The code permits the use of #10 with a 50 amp breaker for this application. This is just like a motor circuit where the breaker only provides short circuit and ground fault protection. The overload protection of the supply conductors is provided in the AC unit itself.

 

[iwire]

cond. nameplate: minimum circuit ampacity-29amp
minimum fuse or breaker size-35amp
maximum fuse or breaker size-50amp:

I would first take a couple of amp readings to see that it is not more than what it should be.

If that looked OK I would put a 50 amp breaker in and call it a day.

You may use a 50 amp breaker with 10 AWG for this AC unit.

 

[TerryBarnhill]

Jeff,
T The overload protection of the supply conductors is provided in the AC unit itself.

How would you know this? Is it told on the nameplate or do you just know the specifics of this unit. We always get "It's a 2 ton, 3 ton, etc." I don't speak in tons, I speak in amps, so that tells me nothing. We are always pulling 8/2 and going from there, but would be nice to know if there's some trick to this fortune telling business I don't know about.

 

[don_resqcapt19]

Terry,
That is how the equipment is made and why the code says you can size the conductors to the minimum circuit ampacity as shown on the nameplate and size the OCPD to the maximum as shown on the nameplate. The original poster provided the nameplate information for this unit. It shows a minimum circuit ampacity of 29 amps and a maximum OCPD of 50 amps.

I agree that tons tells me nothing about how I size the circuit. I must use the information from the nameplate to size the circuit.

 

[erickench]

The maximum size of 50 A for the circuit breaker is the short-circuit ground fault protection. The conductor would be sized for the overload which is lower.

 

[Howard Burger]

my 'trick'

my 'trick'

Terry, I wondered the same thing when I had occasion to look at my daughter's a/c unit in Minneapolis this spring. I found a label on the compressor stating that the unit provided overload protection (sorry, I don't recall the wording on the label). The 'trick' I had to use to read the label was to stick my digital camera down into the unit to get a pic of the label - the shrouding of the unit prevented me from seeing it without taking the whole shooting match apart. I regret I didn't save the pic.

 

[Article 90.1]

im a 3rd yr elect apprentice. my neighbor is having a/c trouble.

he said it only started tripping within the last yr, unit has been in for 10 yrs.

erfitz, This is a good lesson in troubleshooting for a third year apprentice. Just like a good journalist, we need to ask ourselves, who, what, when, where, and why, when troubleshooting.

You stated that the HO said the AC unit worked fine for 10 years. So, then, think about what happened, something always happened.

Ask the homeowner what series of events has led to the breaker tripping; has someone been working in the house or on the unit, has the unit been acting differently, try to get to what may have changed.

After playing twenty questions, think to yourself, what may be causing the breaker to trip. Remember that breakers are mechanical devices as well as electrical. Have the moving parts become worn due to excessive heat for great lengths of time. Is a neighboring breaker generating a lot of heat that is adding to the mechanical decay of the AC breaker. Are the motors in the the AC unit having to work harder? Check the unit for blockage or restricted air flow. Often dirty filters, animal and insect infestations, grass clippings, and other things will restrict airflow or fan blade rotation thus causing motors to work harder and activating their overload devices.

But as someone else suggested, get your current probe out and take current and voltage measurements as you go. See if there are any anomalies in your readings. Compare your readings with nameplate readings and see if you are close to the OCPD thresholds.

Best of luck through the remainder of your apprenticeship, keep seeking out greater knowledge of our trade; we never stop learning.

 

[480sparky]

......... Is a neighboring breaker generating a lot of heat that is adding to the mechanical decay of the AC breaker. ..........

I wouldn't say that a neighboring breaker 'adds to the mechanical decay' of another breaker, but the additional heat may push that breaker over it's thermal limit causing it to open.

 

[Article 90.1]

I hear ya 480, just trying to put my thoughts into words, which often gets me in trouble.

My goal was to get the OP to think in terms of cause and effect.

I'm not trying to argue one way or the other, but heat over time is a contributor in mechanical failure in any system, at some level. But I also agree with your point about the thermal trip, which is more likely and more accurate.

My point to the OP is that something happened, either over the last 10 years, or recently that has or is tripping the 30 amp breaker in question.

 

[iwire]

There is a 30 amp breaker supplying a unit with a label requiring at least a 35 amp breaker.

I say go for the obvious first.

 

[jxofaltrds]

Kiss

Kiss

There is a 30 amp breaker supplying a unit with a label requiring at least a 35 amp breaker.

I say go for the obvious first.

We do try to over think things sometimes.

K.I.S.S.

 

[glene77is]

Article,

You gave a Good lesson for the apprentice.

(1) Mechanical wear inside a CB.
I have opened up several CB in order to see the machanical parts and ascertain the cause of failures.
One 20A CB that was tripping at less than 20A showed wear on two mating square corners in the thermal trip mechanism.
The thermal mechanism pushed a square cornered lever across another square cornered 'cliff' until it dropped off the 'cliff'. The HO used the CB as a switch, and over many years had caused the square corners to become rounded. Thus, the CB would trip at 15A. The CB would also trip if you hit the cabinet with your fist. Open a CB and you can find some interesting mechanics.

(2) Temperature ambient effect pm CB.
I did apprenticeship in 1960, in Virginia, and on one big residential the load center was located on the west side, in a utility room. Originally the outside door was painted white, but when the HO painted it dull green he caused a problem, and we were out there every week for CB's doing nuisance tripping. The green door absorbed the western evening sun's heat, and caused the ambient temp of the utility room to go up sufficiently to trip various CB. We ended up putting a heavy tin-foil on the inside of the door to keep the room cooler. Those folks liked their green paint because it matched their garden theme.

 

[infinity]

erfitz, This is a good lesson in troubleshooting for a third year apprentice. Just like a good journalist, we need to ask ourselves, who, what, when, where, and why, when troubleshooting.

You stated that the HO said the AC unit worked fine for 10 years. So, then, think about what happened, something always happened.

Ask the homeowner what series of events has led to the breaker tripping; has someone been working in the house or on the unit, has the unit been acting differently, try to get to what may have changed.

After playing twenty questions, think to yourself, what may be causing the breaker to trip. Remember that breakers are mechanical devices as well as electrical. Have the moving parts become worn due to excessive heat for great lengths of time. Is a neighboring breaker generating a lot of heat that is adding to the mechanical decay of the AC breaker. Are the motors in the the AC unit having to work harder? Check the unit for blockage or restricted air flow. Often dirty filters, animal and insect infestations, grass clippings, and other things will restrict airflow or fan blade rotation thus causing motors to work harder and activating their overload devices.

But as someone else suggested, get your current probe out and take current and voltage measurements as you go. See if there are any anomalies in your readings. Compare your readings with nameplate readings and see if you are close to the OCPD thresholds.

Best of luck through the remainder of your apprenticeship, keep seeking out greater knowledge of our trade; we never stop learning.

Actually a dirty filter would make the unit work less not more which would cause it current draw to decrease not increase.

 

[erfitz1208]

40amp br

40amp br

i put the clamp on ammeter on the circuit conductors: compressor 18amps
fan motor 1.8 amps..i let the unit run for over an hour, it never wnt over 20amps total..

i purchased a 40 amp breaker, and compressor hard-start kit.
installed both and called it a day.

thanks for the All the replys!

 

[iwire]

i put the clamp on ammeter on the circuit conductors: compressor 18amps
fan motor 1.8 amps..i let the unit run for over an hour, it never wnt over 20amps total..

i purchased a 40 amp breaker, and compressor hard-start kit.
installed both and called it a day.

thanks for the All the replys!

Cool, that should take car of it.



Just to be clear, it would not have made it 'less safe' had you installed the 50 amp breaker.


In the interest of preventing call backs, if it is up to me I use the largest breaker allowed.

 

[ptonsparky]

Actually a dirty filter would make the unit work less not more which would cause it current draw to decrease not increase.

It has been a while so bear with me. Dirty air filters may cause a unit to trip on overload. Less air flow across the evaporator will cause the freon to boil later than what is intended. Eventually the evaporation will still be taking place in the return lines to the compressor making it work harder. Compressors are designed to pump gas, not liquid. Checking the air filters was always one of the first things we did on service calls. A dirty evaporator will cause the same problem.

 

[sparky59]

I know it doesn't seem right, but don't be afraid to use a 50 amp breaker.

 

[iwire]

It has been a while so bear with me. Dirty air filters may cause a unit to trip on overload. Less air flow across the evaporator will cause the freon to boil later than what is intended. Eventually the evaporation will still be taking place in the return lines to the compressor making it work harder. Compressors are designed to pump gas, not liquid. Checking the air filters was always one of the first things we did on service calls. A dirty evaporator will cause the same problem.

I see what you where saying, I am sure Infinity was talking about the fan itself not the compressor.

But as far as the compressor it should not trip out the overload it should be shutting down due to low or high pressure not due to current.