# BTU Tonnage and to Amperage ratio



## Magnettica

Is there a formula for determining amperage for A/C condensing units if the only info you know is the tonnage? 

like...

1-ton equals 12,000 BTU's = 10 amps
2-ton equals 24,000 BTU's = 20 amps
3-ton equals 36,000 BTU's = 30 amps

This is the way I figure it when the nameplate rating is unknown.


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## 480sparky

Magnettica said:


> Is there a formula for determining amperage for A/C condensing units if the only info you know is the tonnage?
> 
> like...
> 
> 1-ton equals 12,000 BTU's = 10 amps
> 2-ton equals 24,000 BTU's = 20 amps
> 3-ton equals 36,000 BTU's = 30 amps
> 
> This is the way I figure it when the nameplate rating is unknown.


There's no linear relationship that is all that accurate.

Tonnage is a measurement of output. Amperage is a measurement of input. 

FWIW, my conversion calculator has 1 BTU/hour = 0.2928104185 watts. Of course, that means the unit has to run 60 minutes for it to be accurate.

The transfer of one measurement to the other, while going upwards hand-in-hand, isn't that accurate. Efficiency of the unit, age, maintenance, etc. all play a part.

Think of converting automobile horsepower to MPG.


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## MDShunk

1 horsepower per ton has been a pretty decent rule of thumb for decades for air-cooled condensing units (condenser fans included). Smaller units a little more. Larger units, a little less. 

As with everything else, try to get actual dataplate information (or at least a model number, cut sheet, or submittal) so you can get the real information.


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## RePhase277

A ton is not 12,000 BTU. It is 12,000 BTU/h, so it is a unit of power equivalent to about 3517 W. So, at 100% efficiency, a 1-ton, 120 V unit would draw about 29 A.

If you know the efficiency of the unit, you can get a rough estimate of current draw.


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## MDShunk

InPhase277 said:


> A ton is not 12,000 BTU. It is 12,000 BTU/h, so it is a unit of power equivalent to about 3517 W. So, at 100% efficiency, a 1-ton, 120 V unit would draw about 29 A.
> 
> If you know the efficiency of the unit, you can get a rough estimate of current draw.


Uh.... not really. The compressor does not directly change electrical energy to cooling tons. It moves heat from one place to another. The breakdown is more realistically 1200 watts per ton when you consider the effect of the refrigerant, which is where the 1 horsepower per ton rule of thumb comes from. 

Think about it.... if you were right, (and you aren't), consider the size of the circuit that would be required for a 10 or 12 thousand BTU window type air conditioner.


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## Bob Badger

I guess we are assuming 240 volt single phase equipment?

Seems we would need a KW per ton factor for it to be useful for any voltage or phase.


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## MDShunk

Bob Badger said:


> I guess we are assuming 240 volt single phase equipment?
> 
> Seems we would need a KW per ton factor for it to be useful for any voltage or phase.


1.2 kw/ton for air cooled. .75 kw/ton for water cooled (chillers).


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## Magnettica

I'm just having this discussion because often times I've been told "we've got a 4-ton unit needs to be wired up" and that's all information I get until I get there.


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## MDShunk

Magnettica said:


> I'm just having this discussion because often times I've been told "we've got a 4-ton unit needs to be wired up" and that's all information I get until I get there.


You're pretty safe to rough-in wire sized to Table 430.248 for a 4 horsepower motor. You'll have to wait for a dataplate to select a breaker, but 99 times out of 100, the motor amp draw chart will work if you go with one horsepower per ton. I just did a 3-ton last week, and the NEC says 18.7 for a 3 horsepower motor. The actual dataplate was 18.8. Pretty darned close.


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## Magnettica

MDShunk said:


> You're pretty safe to rough-in wire sized to Table 430.248 for a 4 horsepower motor. You'll have to wait for a dataplate to select a breaker, but 99 times out of 100, the motor amp draw chart will work if you go with one horsepower per ton. I just did a 3-ton last week, and the NEC says 18.7 for a 3 horsepower motor. The actual dataplate was 18.8. Pretty darned close.


That's an even better way to know it. Thanks.


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## RePhase277

MDShunk said:


> Uh.... not really. The compressor does not directly change electrical energy to cooling tons. It moves heat from one place to another. The breakdown is more realistically 1200 watts per ton when you consider the effect of the refrigerant, which is where the 1 horsepower per ton rule of thumb comes from.
> 
> Think about it.... if you were right, (and you aren't), consider the size of the circuit that would be required for a 10 or 12 thousand BTU window type air conditioner.


Yeah, absolutely right. I wasn't thinking thermodynamically, or soberly. But can a brutha get a pass? I had four Bacardi and Cokes at the steak house.


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## MDShunk

InPhase277 said:


> Yeah, absolutely right. I wasn't thinking thermodynamically, or soberly. But can a brutha get a pass? I had four Bacardi and Cokes at the steak house.


You bet. Word on the street is that Coke is sometimes unfit to drink without requisite amounts of Bicardi or Captain.


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## Xknob&Tube

Thanks for the chart! An inspector made me replace my 30 amp breaker with a 25 amp once.


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