calculating multifamily with central heat and no AC

[malachi constant]

I've done a lot of multifamily. I have always been able to use the optional 220.84 multifamily calcs, but those are only allowed when you have elec heat or AC - here I have none. I think I have the right calculation method figured out but would appreciate some feedback.

PART 1, individual unit panel.
For individual dwelling unit panels I can still use the optional 220.82 calcs. They only require a 100A feeder, which will be provided to each unit panel.
220.82(B)(1) Pick largest unit. 725 SF x 3 VA/SF = 2175VA.
220.82(B)(2) 1500VAx2 = 3000VA for kitchen circuits. Central laundry facilities so no third small appliance circuit required.
220.82(B)(3)(a) Add up all appliances: fridge, exhaust fan, microwave/hood (no DW/disposal/etc). 2925VA.
220.82(B)(3)(b) 8000VA range.
220.82(B) 100% of 1st 10kVA plus 40% of remainder: 16100VA -> 10000VA + 0.40 x 6100VA = 12440VA
220.82(C)(1) central heat, no AC. 0 VA.
So....
12440VA / 208V / sqrt(1) = 59.8A. I'll stick with the 100A panel.

PART 2, service panel.
This is where it gets less clear. I can't use 220.84 because of the lack of elec heat or AC in the dwelling units.
1. I figure I take each dwelling unit above (12440VA) and multiply by number of units (twelve) to get that part of the service: 149kVA
2. Then use standard methods for calculating house panel loads (laundry, stairwell and corridor lighting/HVAC/plug loads, exterior lighting, etc). That I have at 57KVA
3. Add them up. 206kVA / 208V / sqrt(3) = 573A.
4. 600A service. Done. (I have metering and distribution worked out with the Owner and Utility, this is just a matter of sizing the service correctly.)

I -think- this is -one- correct way to do it. (Using the "standard" method for individual units would bump me up to an 800A service. House loads would remain the same but each dwelling unit would calc out around 15kVA, times 12 = 180kVA, plus the 57kVA house loads = 237kVA -> ~660A at 208/3.)

Thoughts?

 

[texie]

Interesting question. The thought never occured to me to calc. the apt feeder via the optional method and the service using the standard method. I'm not sure you can mix and match like this. I'll be eagerly awaiting other thoughts on this.

 

[jumper]

I am confused, you show electric heat and then say it is not there?????

 

[texie]

I am confused, you show electric heat and then say it is not there?????

I took that to mean there is a central building heating plant.

 

[malachi constant]

I took that to mean there is a central building heating plant.

Right. Central gas boilers provide heat to each unit via fin tube or something. Won't go on the unit panels. Sorry for the confusion.

Texie - regarding the mix-and-match approach - I was coming at it thinking, well, if this was just a single family house we could just use the 220.82 optional method, right? And if this was a duplex we could use the optional method for each tenant, then add them together for the transformer load - right? (Maybe not? I don't know.) If one were to agree that this approach would work for a duplex I think it is fair to say there is really no difference from that to a twelve-unit apartment with additional house loads.

I'm in uncharted territory here (for me). Not trying to do anything unseemly. Just trying to stick with what I know best, which is 220.82. If I have to throw that out it's not a big deal, just would want to know that sooner rather than later.

Complicating things is the project is way over budget. So they eliminated the AC from each unit. I had right around a 400A service when it had AC (due to being able to derate dwelling units per Table 220.84). Now, without AC, I could have an 800A service. Hard to explain that one to the Owner and Contractor. Hard to explain that to ANYONE, really. If it wasn't for having to explain that I would probably suck it up and put in the 800A.

 

[david luchini]

I've done a lot of multifamily. I have always been able to use the optional 220.84 multifamily calcs, but those are only allowed when you have elec heat or AC - here I have none. I think I have the right calculation method figured out but would appreciate some feedback.

PART 1, individual unit panel.
For individual dwelling unit panels I can still use the optional 220.82 calcs. They only require a 100A feeder, which will be provided to each unit panel.
220.82(B)(1) Pick largest unit. 725 SF x 3 VA/SF = 2175VA.
220.82(B)(2) 1500VAx2 = 3000VA for kitchen circuits. Central laundry facilities so no third small appliance circuit required.
220.82(B)(3)(a) Add up all appliances: fridge, exhaust fan, microwave/hood (no DW/disposal/etc). 2925VA.
220.82(B)(3)(b) 8000VA range.
220.82(B) 100% of 1st 10kVA plus 40% of remainder: 16100VA -> 10000VA + 0.40 x 6100VA = 12440VA
220.82(C)(1) central heat, no AC. 0 VA.
So....
12440VA / 208V / sqrt(1) = 59.8A. I'll stick with the 100A panel.

It wouldn't change your panel size, but when using the optional method in 220.82, you need to use the nameplate rating of the range, not the max demand from 220.55.

PART 2, service panel.
This is where it gets less clear. I can't use 220.84 because of the lack of elec heat or AC in the dwelling units.
1. I figure I take each dwelling unit above (12440VA) and multiply by number of units (twelve) to get that part of the service: 149kVA
2. Then use standard methods for calculating house panel loads (laundry, stairwell and corridor lighting/HVAC/plug loads, exterior lighting, etc). That I have at 57KVA
3. Add them up. 206kVA / 208V / sqrt(3) = 573A.
4. 600A service. Done. (I have metering and distribution worked out with the Owner and Utility, this is just a matter of sizing the service correctly.)

I -think- this is -one- correct way to do it. (Using the "standard" method for individual units would bump me up to an 800A service. House loads would remain the same but each dwelling unit would calc out around 15kVA, times 12 = 180kVA, plus the 57kVA house loads = 237kVA -> ~660A at 208/3.)

Thoughts?

Using the standard method from 220.42 thru 220.55, I get a load for the twelve apartments of 84,510VA, not 149,000VA. Don't just add up the unit load from each apartment, apply the demand factors from 220.42, 220.53 and 220.55 to the TOTAL dwelling unit load.

That puts you at 393A for your apartment and house loads. Probably wouldn't be a 400A service depending on how much of your house loads are "continuous."

 

[texie]

I did not check the computations, rather just commented on the concept. I see that others may be questioning the computations as well. Let's see what everyone comes up with.