A/C Calculations...

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1793

1793

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Louisville, Kentucky
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Inspector
I have been called a few times to do a panel change where the existing service does not have a main. The wire is has no legible writing so I cannot tell the wire size.

I have been doing a Load Calculation using an Excel sheet I found on this site. My question comes to sizing the A/C. In the Excel program it asks for the tonnage of the unit. When you enter 3 Ton it displays 4,700 VA, if you enter 4 Ton it shows 6,300 VA.

Does anyone know how the VA values were calculated?

If I were to use the nameplate on the unit would I use the Min. Circuit Ampacity or the Max. Circuit Ampacity X 240 for the VA?

I hope this makes sense to someone.

Edited for clarification of Max Circuit Ampacity.
 
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1793

1793

If I were doing that calculation I would use the MCA ( found on nameplate) x 240 for VA.

But that's me,, not really Mr. Vegas..
 
For the A/C you should use FLA (Full load amps) X Voltage = VA. MCA specifies the minimum ampacity of the conductors,not the amperage draw of the unit.
 
edamico11 said:
If I were doing that calculation I would use the MCA ( found on nameplate) x 240 for VA.

But that's me,, not really Mr. Vegas..
Click to expand...

MCA= Max. Circuit Ampacity?:-?
 
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Yes

Yes

That is what I was referring to

MAC= Max. Circuit Ampacity....


Look it's Christmas,, do I really need to spell everything out:grin: :grin:

Ok your right to much egg-nog, to early and to soon.:grin: :grin:
 
Photo...

Photo...

I have a photo of a nameplate I would like to share.

What of the information on this Nameplate would you use?

This is a 240 volt install.

Thanks to all who have replied.

 
Watching for the arrows....

Watching for the arrows....

I have to agree with Paul B

30amp would be the number I would use

30 (amp) x 240V = 7,200VA

now this is a high number, but in the overall load calculation it is negligible.
 
The picture that yu provided gives the running load amps (RLA) of the compressor as 14.8 amps. The full load amps (FLA) of the fan motor is .75 amps. Thus the total amperage of the unit is the sum of the two or 15.55 amps. The minimum circuit amps is calculated by adding 125% of the largest motor 14.8 X 1.25 = 18.5 amps and adding the smaller motor to it for a toatl of 19.25 amps which the manufacturer has rounded down to 19.2 amps. Everything that you need is right there. You would use the MCA to size your circuit conductor an the maximum circuit breaker number to size the breaker.
 
haskindm said:
The picture that yu provided gives the running load amps (RLA) of the compressor as 14.8 amps. The full load amps (FLA) of the fan motor is .75 amps. Thus the total amperage of the unit is the sum of the two or 15.55 amps. The minimum circuit amps is calculated by adding 125% of the largest motor 14.8 X 1.25 = 18.5 amps and adding the smaller motor to it for a toatl of 19.25 amps which the manufacturer has rounded down to 19.2 amps. Everything that you need is right there. You would use the MCA to size your circuit conductor an the maximum circuit breaker number to size the breaker.
Click to expand...

My original post was to gain help in how to calculate the tonnage of an A/C uint using the nameplate info.

1793 said:
I have been doing a Load Calculation using an Excel sheet I found on this site. My question comes to sizing the A/C. In the Excel program it asks for the tonnage of the unit. When you enter 3 Ton it displays 4,700 VA, if you enter 4 Ton it shows 6,300 VA.

Does anyone know how the VA values were calculated?

If I were to use the nameplate on the unit would I use the Min. Circuit Ampacity or the Max. Circuit Ampacity X 240 for the VA?
Click to expand...

I hope this makes sense to someone.

Edited for clarification of Max Circuit Ampacity.
 
haskindm said:
The picture that yu provided gives the running load amps (RLA) of the compressor as 14.8 amps. The full load amps (FLA) of the fan motor is .75 amps. Thus the total amperage of the unit is the sum of the two or 15.55 amps. The minimum circuit amps is calculated by adding 125% of the largest motor 14.8 X 1.25 = 18.5 amps and adding the smaller motor to it for a toatl of 19.25 amps which the manufacturer has rounded down to 19.2 amps. Everything that you need is right there. You would use the MCA to size your circuit conductor an the maximum circuit breaker number to size the breaker.
Click to expand...

So what would he use for the load calculation? I say (15.55amps)(240volts)=3732 VA. I would not use the maximum circuit ampacity.
 
Code says to use the "nameplate amps". On HVAC equipment, that is not a very explicit statement. I think code minimum for the load calculation would be the RLA of the compressor plus the FLA of the motor. If you're calculating a branch circuit, feeder, or service, you need to include 25% of the largest motor or compressor on that circuit/feeder/service in your calculation. So for a whole service, you may have a larger motor somewhere else. The easy and conservative way is to just use the MCA value in your calculations.

There is no reliable way to convert tons to amps. The SEER value is basically an efficiency rating. A more efficient unit typically draws less amps for a given tonnage. If you need to plan for an install but the exact HVAC equipment has not yet been chosen, try to find out what SEER level of equipment the installer will be choosing. Then pick a website that has easy to access ampacity information for their equipment and use it as a planning guide.

You can try this one for ball park numbers: http://www.goodmanmfg.com/consumer/changeTab.act?newTab=Product&requestedSite=Goodman
Click on the Product Specifications and it will show you the compressor RLA, the fan FLA, and the overall MCA. Note that a common numbering scheme for HVAC equipment has 024 for 2 ton units, 036 for 3 ton, 048 for 4 ton, and 060 for 5 ton.
 
suemarkp said:
The easy and conservative way is to just use the MCA value in your calculations.
Click to expand...


Yes this would be the simplest method, but it this what is required by the NEC? If this were a test question would using the MCA in a calculation produce the correct answer?
 
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