HVAC Load Calculation for Gas Furnace

Status
Not open for further replies.
B

BJHuffin

New member
I'm trying to run through the appropriate load calculations for my home. This is for two reasons: 1.) To get a baseline as I'm remodelling 2.) To run through a real-world scenario as I prepare for my exam.

Since this is my first real-world calculation, it's been a bit slow. The most confusing seems to be the HVAC unit. I have a gas furnace with electric A/C. The nameplate states the following:

Compressors: 12.1 RLA, 57 LRA
Indoor Blower: 1.7 FLA, 1/5 hp
Outdoor Blower: 1.0 FLA, 1/6 hp
Combustion Blower: 0.67 FLA, 1.30 hp
Min Ckt Ampacity: 17.9 A
Max Fuse Size: 25 A

Does this sound right for the HVAC Branch Circuit Load and its contribution to the Feeder/Service Calculation?

12.1 * 1.25 + 1.7 + 1.0 + 0.67 = 18.5 A

And as a newbie to load calculations, is there anything else I should be considering that I would most likely miss?

Thank you in advance for your assistance.
 
The MCA is not always the correct amperage to use for a service load calculation. The MCA = RLA x 125%. This is used to size the branch circuit conductor. See 220.50, 440.6, 440.7

If the compressor is not the largest motor for the service calculation, then only use the RLA for all motors/compressors that will run at one time. Do not add 25% to the RLA for a service calc, unless it is the largest motor(rated load-current) in the entire service calc. If the compressor happens to be the largest, do not add 25% to the other motors. Only the largest motor/compressor!(440.6, 440.7)

assuming the compressor is the largest motor for the service calculation, then 17.9 amps will be your HVAC calculated load for the service calc, as long as the combustion motor will not run at the same time.

I'm a little confused at the info you gave on your post. Why is the outdoor motor listed on the indoor units nameplate?

Anyway, good luck!

Marty
 
Last edited:
Status
Not open for further replies.
Top