My problem is a 24 unit co-op building that was renovated in the late 1970s. It has three phase, 4 wire, electrical service conductors that feed a 400 Ampere 3 pole fused switch which supplies the apartments and the public areas. I am trying to work backwards from the 400 amp electrical switch to figure out approximately what connected load per apartment is allowed in view of the 400 amp service. Here are my ballpark calculations.
Convert the 400A demand load back to kVA by multiplying by 360 (208 x 1.73) = 144kVA. From that I subtract demand from common areas (lighting, boiler burner, pumps and fans) 4kVA, leaving 140kVA as calculated load for the 24 apartment units. Assuming a demand factor around 0.4, then the connected load allowed per apartment works out to (140/0.4/24) 14.6kVA. Eighty percent (safety factor) of that is 11.7kVA. Subtracting a window air conditioner (1000W) that runs continuously during the summer leaves 10.7kVA. Am I correct in concluding that, within the limits of the fuses and wire size in each branch circuit, the 400A service makes 10.7kVA available for connected load at the 120V receptacles in each apartment? (The apartments each have 100A panels fed by 3-wire #8 AWG cable - a neutral and two “hot”).
Appreciate comments pointing out any errors I may be making, or whether this looks about right. Thanks.