# Commercial Service Calculation



## SlapdeeBack (Jun 29, 2012)

Can I calculate the demand on a service for a Roof Top Unit with the breaker size only?


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## Chris Kennedy (Nov 19, 2007)

Welcome to the forum.

You don't have a service for one RTU, need further clarification,

Go Dawgs!


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## SlapdeeBack (Jun 29, 2012)

the service is: three phase 480v
i have a 50 amp breaker too the RTU


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## SlapdeeBack (Jun 29, 2012)

trying to quote a price for generator to hold the RTU's and some 277v lights


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## JohnR (Apr 12, 2010)

Don't forget to double the KW you come up with, for the Genny. No, I wouldn't use the breaker size, but being a generator, it might not make a difference , or could help if you did.


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## SlapdeeBack (Jun 29, 2012)

only info i have sofar is the breaker size to the RTU.....trying to fig the volt amps for the 50 amp breaker that servs it


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## SlapdeeBack (Jun 29, 2012)

same question .....7.5 ton RTU what is the load demand on the service...
My problem is that its a RTU / "all heat and air same unit"....so how do i configure it for volt amps if the Max breaker is 50 amp


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## forrest484 (Jun 10, 2013)

*Calculating VA for HVAC*

Rules of Thumb:
When given the breaker size only, it is safe to assume that the minimum circuit ampacity will not be greater than 80% of the over current protection. In your case, 80% of 50 amps is 40 amps. To determine VA, it is safe to assume 1500-2000 VA per ton. Of course this range is dependent on the SEER rating of the units. In your case, a 7.5 ton RTU will demand 11.25-15kva. Of course this is only the AC portion. 15kva is not even close to 40 amps @ 480 volts. Therefore you multiply the assumed MCA (40 amps) by 830 (480 volts X sq rt 3) which gives us 33.2 kva. This would be your maximum load given only the breaker size. When the specs are finally delivered to you, the MCA will fall between 27 and 34 amps (22.4 - 28.2 kva), but without specs, we play it super safe.


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## skokoskoko (Jun 25, 2013)

forrest484 said:


> Rules of Thumb:
> When given the breaker size only, it is safe to assume that the minimum circuit ampacity will not be greater than 80% of the over current protection. In your case, 80% of 50 amps is 40 amps. To determine VA, it is safe to assume 1500-2000 VA per ton. Of course this range is dependent on the SEER rating of the units. In your case, a 7.5 ton RTU will demand 11.25-15kva. Of course this is only the AC portion. 15kva is not even close to 40 amps @ 480 volts. Therefore you multiply the assumed MCA (40 amps) by 830 (480 volts X sq rt 3) which gives us 33.2 kva. This would be your maximum load given only the breaker size. When the specs are finally delivered to you, the MCA will fall between 27 and 34 amps (22.4 - 28.2 kva), but without specs, we play it super safe.


I calculated about 18.3kVA. Went with fuse ratings for motors from the Canadian Electrical Code. Breaker of 50A roughly 20-22A FLA. 

Check the RTU plate for any ratings. It should be on there.


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## aftershockews (Dec 22, 2012)

Chris Kennedy said:


> Welcome to the forum.
> 
> You don't have a service for one RTU, need further clarification,
> 
> Go Dawgs!


There are still some commercial and residential here that have a separate 3 phase service just for the central air.


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