main breaker sizing

[DrSparks]

So I did the optional method to determine the main service for a 8 unit apartment building. I came up with a demand load of 99kva. So a 600A service will be installed. I'm a little confused on how to determine the panel size for the individual units. Do I use the standard method and treat each unit as a single family?

My other question is this: let's say I have a demand load of 21.5kva. On 120/240 that's 89.5A. Do I need to multiply that by 125% like you normally do for a branch breaker? Would I need more than 100a feeder to each unit is basically what I'm asking, but i've been confused on doing load calls for years. Too much single fam resi.

 

[Smart $]

You can use either standard or optional method for each unit.

You only add 25% for continuous loads and the largest motor. Typically, there is no continuous load in dwelling calc's.

 

[DrSparks]

So you would go with 100A panels and breakers with a demand load of 89A? I neglected to mention that each unit has 6500W of electric heat.

 

[Smart $]

So you would go with 100A panels and breakers with a demand load of 89A? I neglected to mention that each unit has 6500W of electric heat.

And if your 6500W of heat operates continuously at max' current for 3 hours or more, it is seriously undersized (unless we enter into a new ice age )... so still not a continuous load that needs to be factored at 125%.

100A panels looks good to me. :happyyes:

 

[DrSparks]

Yeah and 6500W is the sum of the nameplates of 6 separately controlled units. So I agree. Thanks a lot for your help!

 

[Smart $]

Your Welcome :thumbsup:

 

[curt swartz]

The 99KVA seems high if you only have 8 units @21KVA. Are you sure you used all of the demand factors in the service calculation?

 

[DrSparks]

The 99KVA seems high if you only have 8 units @21KVA. Are you sure you used all of the demand factors in the service calculation?

Well, here's the breakdown:

Gen light: 21,505 s.f. x 3va = 64.5k
Small appl + laundry = 4.5k x 8 = 36k
Water heaters = 4.5k x 8 = 36k
Dryers = 5k x 7 = 35k
Ranges = 10k x 7 = 70k
Heat = 6.5k x 8 = 52k (6 separately controlled units)

Total connected load = 229k
Optional method demand load = 229k x 0.43 = 99k
Amps = 99k / 240 = 412.5A

Sound right?