Feeding (3) 3-wire 120/208V single phase apartments using a 3 phase circuit

[Tainted]

Can I feed three 100 amp, single phase 3-wire 120/208V apartments using a 3 phase 4-wire 208V 100 amp circuit?

Here's my worse case scenario calculation:

100Ax208V x 3 apartments = 62,400VA

Per NEC 220.84 to find 3 phase feeder:

62,400VAx0.45 = 28,080VA

28,080VA/(208V√3) = 78 amps

Calculated 3-phase feeder is 78 amps but I will make the minimum to 100 amps since each apartment is 100amps.

 

[jaggedben]

That seems to check out as long as the load calc for each individual unit was 20,800 VA or less to begin with and all the conditions in 220.84 are met.

 

[Tainted]

That seems to check out as long as the load calc for each individual unit was 20,800 VA or less to begin with and all the conditions in 220.84 are met.

If I'm using 100 amp breaker for each apartment do I still need to include 8kw in my calculation for electric cooking equipment even though everyone has gas range?

Shouldn't each apartment be calculated at 28,800VA instead of 20,800VA because of the 8kw electric cooking as per exception in 220.84?

 

[wwhitney]

Here's my worse case scenario calculation:

100Ax208V x 3 apartments = 62,400VA

That's the worst case if you serve each apartment with a 2-wire 208V feeder. But with a 3-wire 120/208V feeder, the worst case per apartment is 100A L-N on each leg, or 2*120A*100V = 24 kVA. For that loading, you'd have 100A on L1, L2, and N.

I find it weird that 220.84 doesn't include the requirement that there are 3 or more apartments per leg. Anyway, if 220.84 and the 45% diversity factor applies, the VA available from the 100A 4-wire 208Y/120V feeder is 36 kVA. So you'd need the loads of the 3 apartments to add up to no more than 36/0.45 = 80 kVA. But as each apartment would need to be limited to 24 kVA, that would happen automatically.

Cheers, Wayne

 

[Tainted]

That's the worst case if you serve each apartment with a 2-wire 208V feeder. But with a 3-wire 120/208V feeder, the worst case per apartment is 100A L-N on each leg, or 2*120A*100V = 24 kVA. For that loading, you'd have 100A on L1, L2, and N.

I find it weird that 220.84 doesn't include the requirement that there are 3 or more apartments per leg. Anyway, if 220.84 and the 45% diversity factor applies, the VA available from the 100A 4-wire 208Y/120V feeder is 36 kVA. So you'd need the loads of the 3 apartments to add up to no more than 36/0.45 = 80 kVA. But as each apartment would need to be limited to 24 kVA, that would happen automatically.

Cheers, Wayne

should I include 8kw cooking for each dwelling unit as per exception in 220.84? so instead of 36,000VA, should it be 60,000VA/0.45?

 

[kwired]

should I include 8kw cooking for each dwelling unit as per exception in 220.84? so instead of 36,000VA, should it be 60,000VA/0.45?

You must have electric cooking appliances to even be able to use 220.84. The load diversity is significantly different if you don't have that cooking load and the demand factors would be completely different if they had one that accounted for that situation. 8 kW is maybe around half the total load but because of it's operating characteristics has a lot of room to lessen the demand value one can apply which is why you get the 45% factor in the first place even on a small number of units.

If you throw in the 8kW even though you have gas cooking appliances then apply the .45 demand factor- you probably still will come out with somewhat similar results as if you did the standard calculations but with no cooking appliances.

 

[JoeStillman]

Why did you use 100A for the apartment load? When you are using the 220.84 calculation method, you don't add up the individual single-family loads that you calculate using 220.82.