Amp Draw On 3 Ton Ac Unit

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Tons to amps

Thread starter garyfitzgerald
Start date Feb 25, 2005

Status: Not open for further replies.

Is there a way to calculate an air conditioning load if all you have is the size of the unit in tons? Applied voltage is also known. 208v 1 phase - 4 ton unit. This is a hermetically sealed compressor.

Re: Tons to amps

I usually use 1.5 KVA per ton, for AC units in the 2-5 ton range, but it can vary a lot.

Steve

Re: Tons to amps

1 Ton of refrigerant =3.5 KVA. So if you 4 tons, you're looking @ 14 KVA or 14,000 watts. I=P/E. 14,000/208=67.3 amps

Re: Tons to amps

I usually use 1.5 KVA per ton, for AC units in the 2-5 ton range, but it can vary a lot.

1 Ton of refrigerant =3.5 KVA

How about a source??

Re: Tons to amps

I have measured my own, very old, 3-ton unit at 19A @ 240V. That works out to 1.5KVA/ton. The breakers are 30A. 3.5KVA/ton sounds much too high for a modern AC unit.

Re: Tons to amps

"How about a source??"

I used OC Conversion Pro: where 1 ton of refrigeration = 3,516.85284 volt ampere or 3.5 KVA.

"I have measured my own, very old, 3-ton unit at 19A @ 240V. That works out to 1.5KVA/ton. The breakers are 30A. 3.5KVA/ton sounds much too high for a modern AC unit."

Now that I think about it, That does sound kind of high, I may have done something wrong in my calculation. :confused:

Re: Tons to amps

So what do you do when you have to design for an essentially unknown load? Go huge?

Do you try to go as small as you can to get the bid and hope you don't shoot yourself in the foot?

The whole idea of doing this backwards seems unprofessional to me. (No offence intended towards you garyfitzgerald). I see this sort of dilema a lot.

Re: Tons to amps

I've never done a lot of research on it. I do not deal much with compressor/condenser units of this size.

But a general rule-of-thumb that I've used for 1-phase, 240V. units in the appx. 2 - 5 ton range is 1440VA per ton.
This is close to steve66 1.5KVA.

[ February 25, 2005, 08:32 PM: Message edited by: kiloamp7 ]

Re: Tons to amps

A little background is in order here. About 1 KW is needed to drive a 1 hp motor,and about 1 hp is needed to drive a 1 ton AC compressor. Throw in some PF, and you have 1.5KVA/ton. This number may be a bit high for modern AC units though.

Re: Tons to amps

Another way

One ton of frig.=12000 BTU
1 watt=3.412 BTU

4 tons x 12,000=48,000 BTU

48,000/3.412=14,068 watts

14,068/208 volts=67.63 amps.

Ronald

Re: Tons to amps

ronaldrc,

It does not work that way!

The air conditioner is a heat pump, it pumps heat out of the house. You cannot treat it as you would a heating element. You numbers are much too high. The AC is some 5 times as effective at pumping heat than a resistive heater is in converting electrical energy to heat.

Your units are messed up too:

1 watt-hour is equivalent to 3.4 BTU.

1 ton of AC is equivalent to 12000 BTU/hour

You must be scrupulous with your units!

[ February 25, 2005, 09:17 PM: Message edited by: rattus ]

Re: Tons to amps

Rattus I did not mention a heater element.

These figures are form a HVAC tech. book
1 watt=3.412 BTUs we are talking about cooling with a air conditioner not heating with a heat pump or reverse air conditioning. Which they claim is more efficient than resistive electric heat.

Ronald

[ February 25, 2005, 09:25 PM: Message edited by: ronaldrc ]

Re: Tons to amps

ronald, trust me! You cannot do it this way.

rattus

Re: Tons to amps

I'm sorry Rattus the books say you can.

Ronald

Re: Tons to amps

Originally posted by ronaldrc:
Rattus I did not mention a heater element.
These figures are form a HVAC tech. book
1 watt=3.412 BTUs we are talking about cooling with a air conditioner not heating with a heat pump or reverse air conditioning. Which they claim is more efficient than resistive electric heat.

Ronald

Ronald, your book is sloppy if it equates power to energy. Unfortunately, some authors are careless in this respect, and this shows a lack of engineering discipline.

1 watt, a unit of power, is equivalent to 3.4 BTU/hour, or

1 watt-hour, a unit of energy, is equivalent to 3.4 BTU.

If you leave the time element out, the comparison is meaningless.

Your book is correct in saying that the heat pump or air conditioner is more effective in moving heat than a resistive heater is in converting energy to heat. That is the reason your KVA computation is much too high.

Re: Tons to amps

Edited for nonsense

[ February 25, 2005, 11:59 PM: Message edited by: ronaldrc ]

Re: Tons to amps

Ronald, although you left out the time element in your units, they would have divided out anyway, so your arithmetic appears to be correct. Now if you divide by the SEER rating, and guess at the PF, then your result would be reasonable. But, if you know all this about the unit, you probably know the current draw anyway.

Re: Tons to amps

Rattus I just looked up a 4 ton seer 10 condenser unit it pulls around 28 amps.

What I was saying in my post if you do use the 3.4 BTU/hour = one watt you could figure it that way instead of the other way.

As efficient as refrigeration is getting now neither will work.

Have a good night:Ronald

Re: Tons to amps

A electric nickelchrome heater element produces 3.4 BTU/Hour

The very effecient mechanical refrigeration compressor produces 8.533 BTU/Hour

One ton of frig.=12000 BTU/hour
1 watt=8.533 BTU/hour

4 tons x 12,000=48,000 BTU/hour

48,000/8.533=5,625 watts

5,625/208 volts=27 amps

Ronald

Re: Tons to amps

Ronald,

I take it back. If you include the SEER, then you can do it your way. SEER is defined as the heat transferred per unit of energy, that is in BTU/watthour.

P = AC load/SEER

For your 4 ton unit and SEER = 10,

P = (48,000 BTU/hour)/(10 BTU/watthour) = 4,800 watts,

then throwing in PF,

I = P/(VxPF) = 4800W/(240Vx0.8) = 25A

This is comparable to Ronald's result for 208V.

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