Can I downsize existing A/C breaker?

[Zee]

Hi all,

I post a lot on the Photovoltaic Forum. I do not know much about air conditioner circuits.
I do vaguely remember in-rush current as a factor....:blink: I think :?

Can i downsize from the (E) 2p60 A brkr to a (N) 2p50A?

The A/C unit is a BRYANT,
208/230 VAC at a residence.
From the nameplate:
Compressor:
1 PH
60 HZ
26.4 RLA
134.0 LRA

Fan Motor:
1PH
60 HZ
26.4 RLA
1.5 FLA

Minimum Circuit Amps: 34.5
Max Fuse: 60 A
Max Ckt Brkr (*) 60
* HACR Type recommended

Any help is appreciated!

 

[Ponchik]

yes you can.

 

[raider1]

Yes, you can use a 50 amp breaker.

 

[suemarkp]

You could probably drop more if you want. Code minimum would be a 35A breaker. But it may trip one of those. A 40A is legal and may actually work fine.

 

[iwire]

Can i downsize from the (E) 2p60 A brkr to a (N) 2p50A?

I agree with the others that you can but I do wonder why you would want to.

If it has to do with the wire size all you need is conductor with at least a 34.5 amp rating even with the 60 amp breaker.

 

[G._S._Ohm]

I guess you should check whether ~136 LRA for one or two seconds will exceed the lower boundary of the new breaker's trip curve.

 

[Mgraw]

Locked rotor should be 1 or 2 tenths of a second on a compressor.

 

[G._S._Ohm]

Then according to this illegible trip curve

http://www.google.com/imgres?q="trip+curve"+breaker+thermal&um=1&hl=en&safe=off&client=safari&sa=N&tbo=d&rls=en&biw=1039&bih=749&tbm=isch&tbnid=XMsIn1DZ9bnwNM:&imgrefurl=http://electrical-engineering-portal.com/time-current-curves&docid=3lGUk4_RdJSwhM&imgurl=http://electrical-engineering-portal.com/wp-content/uploads/time-current-curves-3.jpg&w=539&h=857&ei=MSfNUJePHKrj0gG89YBw&zoom=1&iact=hc&vpx=4&vpy=73&dur=51&hovh=283&hovw=178&tx=52&ty=132&sig=102793154617015424895&page=1&tbnh=120&tbnw=76&start=0&ndsp=26&ved=1t:429,r:0,s:0,i:90

the breaker can be rated at 136/4.5 = 30A.

 

[templdl]

I would have to varify this but but I'm sure that I down size one for myself because I simply didn't have a 30at breaker and iused a 20at breaker instead.
After energizing the A/C I used a clamp on AM to measure the actual current and it was actualy much less than I had anticipated, less than 80% of the breaker rating. I't been doin fine for 14 years now.
So install the breaker that you would think would work for you, fire up the A/C to see what the laod actually is. Even at 100% of the breaker rating the breaker should never trip.

 

[kwired]

Starting the unit without tripping the breaker is usually the biggest concern.

The breaker is only there for short circuit and ground fault protection, overload protection is provided by a separate assembly within the unit.

 

[iwire]

Here is why I am for using the largest breaker allowed, this is a personal view certainly not a code reason.

From my experience typical small / home condenser units do not have any short cycle controls on them, so if someone is adjusts the T-stat they may stop the compressor and instantly try to restart it.

When this happens the compressor will not turn due to the refrigerant pressure already in the cylinder. Now you have a locked rotor condition for a long period, to protect the motor the internal thermal overloads open. During the period the OLs are cooling the pressure is bleeding off. Once the OLs cool enough they close and the motor tries to start again. At this point it may start or it may cycle the over loads another time.

My point here is that during this time of waiting for the OLs to open you do not want the breaker to trip. The internal OLs will protect the motor and reset automatically vs a nuisance breaker trip.

So to me just the fact the breaker is large enough for start up under normal operating conditions does not mean it is big enough for all expected conditions.

YMMV.

 

[G._S._Ohm]

Here is why I am for using the largest breaker allowed, this is a personal view certainly not a code reason.

From my experience typical small / home condenser units do not have any short cycle controls on them, so if someone is adjusts the T-stat they may stop the compressor and instantly try to restart it.

When this happens the compressor will not turn due to the refrigerant pressure already in the cylinder. Now you have a locked rotor condition for a long period, to protect the motor the internal thermal overloads open. During the period the OLs are cooling the pressure is bleeding off. Once the OLs cool enough they close and the motor tries to start again. At this point it may start or it may cycle the over loads another time.

My point here is that during this time of waiting for the OLs to open you do not want the breaker to trip. The internal OLs will protect the motor and reset automatically vs a nuisance breaker trip.

So to me just the fact the breaker is large enough for start up under normal operating conditions does not mean it is big enough for all expected conditions.

YMMV.

It's not good for the compressor to try to pump liquid and I forget the name for this event, but maybe the breaker should trip so the HO knows he/she is doing a bad thing.

 

[iwire]

It's not good for the compressor to try to pump liquid and I forget the name for this event,

Well when it happens in a supermarket compressor rack it is know as 'Money' to the refrigeration guys.

When it happened to cylinders 6, 7 and 8 of my hot rodded small block because my exwife did not cover the air intake it was known as death to 3 rods and a crank. :rant:

 

[G._S._Ohm]

Well when it happens in a supermarket compressor rack it is know as 'Money' to the refrigeration guys.

When it happened to cylinders 6, 7 and 8 of my hot rodded small block because my exwife did not cover the air intake it was known as death to 3 rods and a crank. :rant:

That was the pressure for me to replace the head gasket in my '94 Civic; I was afraid it would take in so much coolant into cyl #1 that the engine would be wrecked.

I am liking it less and less to work out by the curb with the temps in the 40s.

And I have my GoJo but they don't seem to make Liquid Glove anymore.

 

[Zee]

Thanks everybody!
That helped a lot.

The reason is that i need to install a 2p30A breaker in a full subpanel.
The only breaker i could possibly replace with a physically smaller sized breaker (twin or quad) is the full size 2p60A for the A/C unit. so.....Murray makes the panel and their largest quad breaker is 50A....
So i already got my 30/50 quad for the new circuit and A/C respectively!

Thanks again!
No panel replacement needed!

I appreciate the explanations.

 

[Mgraw]

iwire I agree I always install the Max. breaker. Fortunatly newer T-stats have built in time delays to prevent short cycles.

G_S._Ohm We call it slugging liquid but it is caused by other reasons.

 

[G._S._Ohm]

Thanks, Mgraw. :thumbsup: That sounds right.
This could wreck a positive displacement pump since liquid is pretty much incompressible.

Zee, could you post the trip curve for your chosen breaker?
I can learn more from this thread - I want to model the HVAC current draw between the 0.2 seconds and the long term current draw with an exponential and then compare it to the trip curve, kind of like they do for breaker coordination.

 

[kwired]

It's not good for the compressor to try to pump liquid and I forget the name for this event, but maybe the breaker should trip so the HO knows he/she is doing a bad thing.

Thanks, Mgraw. :thumbsup: That sounds right.
This could wreck a positive displacement pump since liquid is pretty much incompressible.

Zee, could you post the trip curve for your chosen breaker?
I can learn more from this thread - I want to model the HVAC current draw between the 0.2 seconds and the long term current draw with an exponential and then compare it to the trip curve, kind of like they do for breaker coordination.

There is a difference between compressing liquid, and starting against high pressure gas.

If a little liquid passes through the compressor it doesn't hurt much, as long as there is still gas in the high pressure side, then it just passes through and adds to the pressure of the gas. When starting you could have liquid or gas in there but once running the reduced pressure on the suction side should evaporate any liquid, if not there is other problems.

 

[G._S._Ohm]

There is a difference between compressing liquid, and starting against high pressure gas.

If a little liquid passes through the compressor it doesn't hurt much, as long as there is still gas in the high pressure side, then it just passes through and adds to the pressure of the gas. When starting you could have liquid or gas in there but once running the reduced pressure on the suction side should evaporate any liquid, if not there is other problems.

Thanks. . .I guess I didn't hang around long enough on HVAC forums.

 

[kwired]

Thanks. . .I guess I didn't hang around long enough on HVAC forums.

I really don't know if I was correct or not, just seems to make sense when applying some basic physics. Too much liquid I'm sure is a problem, but a little bit probably doesn't hurt much.

A plugged evaporator, or a non working evaporator fan will eventually lead to no evaporation, then you will have a problem when the liquid makes it all the way to the compressor without evaporating. That I do know.

Starting the compressor is hard when there is high pressure whether it is high pressure from gas or liquid.