Adding garage space heater to optional method service calcs

[trevorm]

Hi Folks,

I'm doing a load calc to see if I can add an welding outlet and a EV charger to a single family dwelling. I was unsure how to add these in with the optional method but found good info here. Now the part i'm confused with is how to add in the 10kw space heater in the garage to the calculation. There is a central forced air heating system with supplemental electric heating included at 100% of compressor and 65% of supplemental heating.

I've added the 10kw garage heater at 65% on top of this but i don't think that is correct. Although if i follow 220.82(C) I would possibly omit the smaller of these two heat sources? Doesn't seem right since this heater is heating a difference space. Maybe i should try the standard method. Here are my full calculations for reference:

Obviously this is too much load for the service but I'm trying to figure out if there is enough space left to add the welding outlet and then wire the EV charger through a energy management system such as the stepwise tap.

Fixed Appliances and Equipment​	Load (W)​
Square Footage @ 3VA x 2252​	6756​
small appliance circuits(2)​	3000​
laundry circuit​	1500​
range (nameplate)​	12500​
dryer (nameplate)​	6240​
water heater (nameplate)​	4500​
dishwasher (nameplate)​	1071​
microwave (nameplate)​	1200​
Hot tub 40A 240V (nameplate)​	9600​
Welder Receptacle 50A 240V (NEW)​	12000​
EV Charger 60A 240V​	14400​
​	​
Total before demand factor​	72767​
First 10000 @ 100%​	10000​
Remaining @ 40%​	25106.8​
Total with demand factor​	35106.8​
​	​
Heating Loads​	​
Heat pump compressor @ 100% 18.3A x 240V​	4392​
Backup heat @ 65% = 40A+20A x 0.65=39A x 240V​	9360​
Garage heater @ 65% = 10000x0.65​	6500​
Total heating loads​	20252​
​	​
Total demand+total heating (W)​	55358.8​
Total Service Load in Amps​	230.7​

 

[trevorm]

Nobody has done a load calculation with an electric heater in an attached garage?

 

[suemarkp]

I think you treat it like an other electric space heating, so it goes in at nameplate * 65%. like you've done. The only other way to reduce it would be if there is much staging of the heat strips in the heat pump. My heat pump had 3 stages of strip heat. Adding a separate room heater would be a 4th. If you can get to 4 separately controlled heaters or strips, then the 65% multiplier drops to 40%. Things like this in the code book make we want to slap a 500W baseboard heater or two on bathroom walls so I can get more separately controlled heaters and get a nice reduction of the heating load.

Watch your heat pump heat strip values. Looks like you're adding up breakers which is going to give too large of a value. Breakers for heat strips are usually sized at 125% of the heat load. So see if you can find the watts of those strips (code says use KVA, not nameplate amps). Complicating that, the air handler fan motor does need to be counted and is probably on that 40A breaker along with a portion of the strip heat.

Is the 18.3A for the heat pump compressor its MCA or some other amp value? I would take the MCA off the heat pump compressor and use that value as it covers the compressor and outside fan but does have a 25% add on for the compressor. Code says use nameplate value so I would consider MCA for the compressor.

I also suspect the welder calculation. A 50A welder would be huge for a home setup. Welders typically have a circuit breaker much larger than its average current draw. 220.82 says to use nameplate values, but a welder has a lot of different values on its nameplate. To me, the load calc should be based on Ieff and not Imax, but code isn't clear. The breaker size is probably even larger than Imax. But if you think they may repurpose this welder outlet into a 2nd car charger, then I'd leave it at 50A.

 

[trevorm]

Thanks so much for the reply. The values i used for the heat strips are heater amps on the nameplate, breaker sizes are 60A and 25A respectively.

Technically per 220.82(C) i'm supposed to pick the one largest of the combined heat pump & heat strip load or 65% or the electric space heating load but i'm just adding 65% of the garage heater since its in a separate space. I wish the code was more clear on these things but such as it is. I haven't been called by an AHJ on load calcs yet but like to have my i's dotted & t's crossed.

I'll also dig into the welder Ieff since I agree, very unlikely the welder will draw even close to 50A

Thanks again!