SEER and power consumption

[Mr_Bill]

Perhaps somebody can explain for me how to reconcile air conditioner SEER specs with current consumption specs.

I am looking at installing a Trane XL19i split system condenser (compressor). The spec sheet says to use branch circuit protection of 30A minimum and 35A maximum. The spec also says to use wire of at least 22A ampacity. This all seems reasonable so far.

Then the spec sheet says that the "R.L.A." for the compressor is

15.1A, and the RLA for the fan is 2.8A. So the total RLA is 17.9A.

Now to my confusion. This is a 36kBTU unit rated at 19+SEER.

36000/19 = 1895watts or 8.6amps.

So, when the compressor and fan are running (but not including any starting transients) how much current is being drawn?

This is more or less an academic question. I intend to use a 30A breaker and

8 ga. THWN which is 50A ampacity, but I'd like to know in advance when the unit is sitting there running, how many amps will I be drawing?

Grateful for any explanation.

[Joseph Meehan]

I don't think you can do what you want as SEER is only for one set of controls.. Change the test criteria like temperature humidity etc and you will have different results.

[udarrell]

Get an amprobe, it varies due to load conditions and other factors.

There are far too many factors that affect SEER Ratings and energy consumption to even begin to address here. If you want to do some reading on some of the factors affecting SEER & efficiency of operation, here is a link to begin the journey.

udarrell

[Mr_Bill]

Thanks for the link. Very interesting.

In some sense, this all got started several years ago. My wife had a friend, and her husband was a commercial HVAC contractor. He walked me through the correct way to design ductwork. What a revelation. I replaced all the ductwork in my house, and not only is the place a lot more comfortable, but my energy bill went down significantly.

I then started paying attention in other houses, and essentially all the houses I've seen have improperly designed ductwork.

So, long story short, I became interested in the basic physics behind HVAC design. Fast forward to today. I'm putting in an A/C unit, and I just want to connect the dots between the specs on the branch circuit and the SEER specs. Either the branch circuit specs are way too conservative (which is fine, wire is cheap), or the SEER value is meaningless, or somewhere in between. I was just curious. Nothing is going to change. I'm still going to run an 8 ga branch circuit with a 30A breaker. I'm still going to buy the 3 ton condenser unit. I was just curious what is really going on with the energy conversion. Oh, well.

[Dave Martindale]

That's probably worst case conditions.

I expect SEER is based on some kind of expected operating conditions, not worst case. Also, the compressor and fan motor are likely inductive loads, so when the unit is drawing 1895 watts the current is larger than 8.6 A, by a factor of 1 over the power factor.

Under what conditions?

Dave

[Mr_Bill]

Dave -

Thanks for the reply.

I would have thought that the load that the compressor presents to the compressor motor is more or less constant. If so, then the power factor, whatever it might be, is also constant, and is factored into the SEER.

I agree that the SEER rating is based on some set of operating conditions, and that in the real world the efficiencies might go down (for example, if it is really hot then the compressor radiator becomes less efficient). I would think, however, that the BTU would go down, and the electrical power consumption would remain constant. But, obviously this is more complex than it seems at first glance.

Anyway, thanks for the reply.

[mm]

I would think that the outside temperature is to the desired inside temperature, or the closer the inside temperature is to the desired temperature, the less the refrigerant would be warmed in the evaporator. So the less effort it would take to compress it at the next stage.

This is sort of the corollary to, You don't get something for nothing.

You also don't usually have to expend effort for nothing. If you get so much cooling for so much effort, you can usually get half as much cooing for somewhat more than half as much effort.

I dson't think the electric would stay constant. Same reason as above.

[Dave Martindale]

Ok. But my point was that you were dividing watts by volts and expecting to get amps as a result. That only works for resistive loads, where current and voltage are in phase, and VA is equal to watts. But for inductive (or capacitive) loads, there's a phase shift between current and voltage, and watts and VA are not equal.

Yeah, it's got to be more complex than that. For example, suppose your house is quite hot, or the evaporator is small for the house. Then the expansion valve will let all of the liquid refrigerant available into the evaporator, and there will be lots of gas returning to the compressor. This will raise low side pressure, the compressor will take more mass of gas into the cyclinder for each stroke. This requires more shaft horsepower from the motor, which will draw more current.

On the other hand, if your house is already cool, the expansion valve will throttle the refrigerant, the compressor has less gas to pump, and motor current will go down. You're getting fewer BTU/hr of cooling, but using less electricity doing it. Overall efficiency could be higher or lower.

Or suppose outside temperatures are unusually hot. The condensor runs hot, the compressor discharge pressure goes up, and again it takes more shaft power from the motor to run the compressor. More power to provide the same number of BTU/hr cooling when condenser air is hotter than normal.

Dave

[trader4]

"The spec sheet says to use branch circuit protection of 30A minimum and 35A maximum. The spec also says to use wire of at least 22A ampacity. This all seems reasonable so far. "

Am I the only one that sees a problem with the above?

[Mr_Bill]

Actually, this thought occurred to me right off the bat.

I can assure you that the factory spec says 22A ampacity,

30A min breaker, 35A max breaker.

In any case, I am using 50A ampacity (8 ga) wire, and a 30A breaker.

[udarrell]

The conductors you use should always exceed the ampacity of the breakers that are protecting them! The purpose of the breaker or fuse is to prevent exceeding the ampacity rating of the conductors!

If the conductor rating is 22-amps then I would want a 20A or less breaker on it. Run at least 35A rated conductors, then use a 30A breaker. The breaker must be sized no larger in amp size than the ampacity rating of a conductor after de-ration calculations have been performed to find the maximum ampacity of that conductor.

Since when does the Electrical Code allow the ampacity capacity rating of the wire to be lower than the breaker that is supposed to protect it from exceeding its conductor amp ratings!

The ampacity of the conductors should always be greater than the breaker rating that is protecting it from exceeding its amp rating.

-udarrell

[trader4]

If he does what he stated above, the wire will have a greater capacity than the breaker.