heat calc.

[rll77]

ok. got a question , Im wiring a 9000 sq. ft. house. doing load calc. trying to figure the heat load. four central units , all heat pumps with back up. This heat calc. will be the determining factor in whether it will be a 400 or 600 service, so its real important i get this straight. factor 100%, 65%, 40% ????
the code book is a little confusing to me in this area. would this be considered space heating? so i have 4 or more units and can derate it to 40%. or not ?? need help please!

 

[bob]

Look at 220.82.C. Is the house total electric?

 

[rll77]

yes, all electric

 

[suemarkp]

I think you take part (3) of 22.82(C) because this is a heat pump. Another question that can have a large effect is what heating value to start with. Resistance heaters have nameplate values of MCA. This is the full load of the heating element * 1.25. So you may actually end up with .8125 of the full load amps (= 1.25 * .65). If you can use the full load amps instead of nameplate, that will save some too.

Also realize that the compressors in heat pumps run with the strip auxillary heat. So you have to add the compressor MCA's in too -- no leaving it out as if it were only an air conditioner.

Technically, I don't know why you couldn't treat the resistance heat as a part (5) solution. I see 4 separately controlled resistance heat sources (and if any air handler is over 10KW of heat, it will probably be internally staged too).

So this is probably an inspector question if it is going to become a design driver. This part of the code is just too nebulous.