How to calculate.

[Chamuit]

I don't hardly do apartments. But I came across this today.

100A 3P 208V breaker feeding three apartments. #2 THHN.
Looks like the panels are fed. A-B, B-C, A-C.

When I do the load calc for these three units, do I treat them as one unit because of the way it is wired? Or, do I treat them as three individual units add the calculated loads then divide by 208V*√3?

 

[Dennis Alwon]

I would use Table 220.84 and the accompanying article

 

[charlie b]

When I do the load calc for these three units, do I treat them as one unit because of the way it is wired? Or, do I treat them as three individual units add the calculated loads then divide by 208V*√3?

Those two are the same thing. When you "treat them as one unit" you calculate the total load current by "adding the calculated loads then divide by 208V*√3." That is how you verify that the 100 amp feeder serving the three units is adequate. Separate from that calculation, you need to know the loads on each unit individually, in order to verify that the panel serving each unit, and the feeder serving each panel, are adequately sized.

Welcome to the forum.

 

[chris kennedy]

Welcome to the forum.

Your about 3? years late with that one.

 

[Chamuit]

Your about 3? years late with that one.

LOL :lol:

 

[Chamuit]

So, each unit has 1000 ft?, 2 appliance circuits, one stove 8kW, no water heater, laundry, heating or cooling.

9000 VA/ft?
9000 VA/3*2 appliance circuits/ea unit
13200 VA/ 3 - 8kW stoves * 55% demand factor

31200 VA
-10000 VA

21200 @ 40% = 8480 VA

18480 VA / 360V = 51A total load

 

[Chamuit]

Okay, so the other thing is that none of the load centers have main breakers. If I feed three dwelling units off of one feeder shouldn't there be a main at each load center?

 

[david luchini]

So, each unit has 1000 ft?, 2 appliance circuits, one stove 8kW, no water heater, laundry, heating or cooling.

9000 VA/ft?
9000 VA/3*2 appliance circuits/ea unit
13200 VA/ 3 - 8kW stoves * 55% demand factor

31200 VA
-10000 VA

21200 @ 40% = 8480 VA

18480 VA / 360V = 51A total load

This looks like you are using 220.82. When using 220.82, you can't take a demand factor for the ranges from 220.55. You would use the nameplate rating of the range and include it in the 100%/40% calculation.

But I don't believe you can use 220.82 for your common feeder. I think it is a calculation for a single dwelling unit. You would've been able to use 220.84 for Multifamily Dwelling, except you indicated no heating or cooling, so 220.84 can't be used. So that leaves you with the method in Part III of Art. 220.

So, you have 9000VA of lighting and 9000VA of small appliance ckts. You have 3 ranges, but you have them on a 3-phase, 4-wire feeder so you base the load on twice the number connected between any two phases. That means 2 ranges = 11kW; 11kW/2 = 5.5kW per phase * 3 phases = 16.5kW.

Your 9kVA lighting and 9kVA appliance can have the demand factor in 220.42 applied, so;

18000VA applied to 220.42:
3000VA @ 100% = 3000VA
15000VA @ 35% = 5250VA
Subtotal = 8250VA
Plus Range dem = 16500VA
Total = 24750VA

24750VA/360= 68.7A demand.

 

[jpage]

Okay, so the other thing is that none of the load centers have main breakers. If I feed three dwelling units off of one feeder shouldn't there be a main at each load center?

As long as the load centers are rated for 100A they are ok. The 100A/3P breaker will trip if it sees 100A on any phase.