NEC 220.82 and 220.84

[abmorse1]

Hi all - PE here, but I mostly do commercial work. In the rare occasion I've done residential, I've been using a spreadsheet that works through 220.82 and 220.84 load calculations that was developed by the Sr. Engineer I worked under when I started. He's since retired, but he's an incredibly smart guy with a wealth of experience. I'm not sure if I've found an error in his formulae or if I'm not reading code properly and thought I'd ask here.

The way I'm reading the multifamily dwelling calculations, and yes they have fixed electric cooking and HVAC
To simplify, I'm ignoring a few items just for this post:
*heat pumps, but the way I'm reading it, I'm not sure how to deal with them in 220.84 either
*house panel loads

Per 220.82 - to size individual unit panels and feeders:
3W/sq ft
2 Appliance circuits (1500VA each)
1 Laundry circuit (1500VA)
Dryer (actual load)
other fixed appliances (dishwasher, disposal, etc)
Subtotal is 100% of the first 10K VA and 40% of the rest.
Added on is largest of:
100% of HVAC Cooling
65% of HVAC Heating (unless more than 4 heating units)

Per 220.84 to size the service to the meter banks:
3W/sq ft
2 Appliance circuits (1500VA each)
1 Laundry circuit (1500VA)
Dryer (actual load)
other fixed appliances (dishwasher, disposal, etc)
Subtotal is 100% of the above.
Added on is largest of:
100% of HVAC Cooling
100% of HVAC Heating (unless more than 4 heating units)
That total is multiplied by unit quantity and the factor in table 220.84

The spreadsheet I got from the retired Sr. Engineer simply takes the total from 220.82 and multiplies it by the factor in table 220.84 to size the service. Am I reading the code wrong, or has there been an error in the spreadsheet all this time?

 

[wwhitney]

The spreadsheet I got from the retired Sr. Engineer simply takes the total from 220.82 and multiplies it by the factor in table 220.84 to size the service.

The "total connected load" tabulation for 220.82 and 220.84 is the same other than HVAC. So for the 220.84 calculation you can take that total connected load, add 100% of the HVAC, and multiply by the table demand factor. While for 220.82 you'd take the total connected load (I'm assuming over 10 kVA), multiply by 40%, add 6 kVA [10 + (x-10)*0.4 = 0.4*x + 6], and add the HVAC times its demand factor (which may or may not be 100%).

If the spreadsheet is saying "220.84 load = 220.82 load * 220.84 table demand factor," that is clearly wrong. But it can use the total connected load and the HVAC load as inputs to the 220.84 calculation.

Cheers, Wayne

 

[abmorse1]

Thanks! There's plenty of good stuff to work from in the spreadsheet, just have to fix a few references and formulae. The longer I do this the more I don't trust spreadsheets that I didn't create myself.
Also, that's a neat trick with the 10K! I don't know why I've never bothered to simplify that calc (well, the similar one for receptacle loads in non-dwelling units.)