QuestionsCategory: QuestionsCorrectly Calculating Air Condition Load for Service Calculation NEC 2014
Handyman Staff asked 2 weeks ago

I am looking to install all new kitchen appliances and wanted to see if my current service would be sufficient. I am trying to calculate the service load using Article 220 of the NEC 2014 and I am trying to figure out what my AC/Heating Load would be. I have a Gas Furnace and AC unit which the same blower motor.

My Furnace Unit is the G60UHV−36B−090. The datasheet can be found here: https://tech.lennoxintl.com/C03e7o14l/VIu12Ch2uV/ehb_g60uhv_0812.pdf

My AC Unit is this HS19-411: https://tech.lennoxintl.com/C03e7o14l/VIu12Ch2uV/ehb_hs19_0894.pdf

Blower Motor Calculation: Page 7 of the Furnace datasheet says the blower motor output is 1/2 hp. It also says that the furnace runs of 120V-60Hz-1 phase and no more than 12 amps. Table 430.248 says the full load current for a 1/2 hp motor is 9.8A. Since my motor is running off of 120 V, this means the blower motor load is taken as 9.8A * 120V = 1176W?

AC Unit Calculation My AC unit is running off 240V (30A double pole breaker). Page 6 of the AC Unit datasheet shows the Electrical Data for the unit. Now this is were things get a bit confusing. The table lists three different currents for the AC unit (HS19-411), Rated load amps for the compressor, full load amps for the condenser, and then a minimum circuit ampacity. Which one of these is relevant for the purposes of a load calculation?

Total AC/Heat Load Now since the AC and Heat won't be running at the same time I would need to take the largest of the two loads and use that in the total calculation. Apart from the blower motor running, I don't think the furnace uses much electricity at all since it is a gas furnace. Therefore, I would use the AC Unit Load (240V * whatever current is relevant) plus the blower motor load and then that would be my AC/Heating load correct?

Thanks in advance for any help.