Unit feeder in multifamily dwelling

[Luke Neilson]

I'm working on a job that is a few hundred apartment unit complex. I'm feeding each unit with 100A into a subpanel and some of the lengths to these units from the distribution panels are going to be 200 feet or more. I know that none of these units will actually draw 100 amps so is there a formula or a rule of thumb for how many amps it will draw or use at most that I can use when I do my load calculations for that unit? It has a 208v Magic pak (self contained heating and cooling unit), electric dryer, and electric range.

 

[electrofelon]

I'm working on a job that is a few hundred apartment unit complex. I'm feeding each unit with 100A into a subpanel and some of the lengths to these units from the distribution panels are going to be 200 feet or more. I know that none of these units will actually draw 100 amps so is there a formula or a rule of thumb for how many amps it will draw or use at most that I can use when I do my load calculations for that unit? It has a 208v Magic pak (self contained heating and cooling unit), electric dryer, and electric range.

It seems a bit odd to me that a project of this size is not already engineered. Not sure how the owner can get comparable numbers for the work when somone bids a hundred or so 2/0 feeders and the other guy does them all in #2. I might bump some of the longer ones up to 1/0, but I would have to think about it more, want more specifics, and to know the cost structure of all this. Was also thinking about terminating larger conductors on the equipment, but I believe most 100A equipment usually takes a 2/0.

 

[Luke Neilson]

Our company constructs all of their apartment buildings in-house, so it's never gone out for bids. The electrical is also never engineered, it's all design-build. This year we'll be building the biggest one yet and so we're into uncharted territory now and could use any expertise or experience you can offer.

 

[electrofelon]

I would just come up with some realistic figure for what the max amps will be that will occur for more than a very brief period of say 20-30 seconds - you know I would want that figure to likely occur for over say a minute or two before I would worry about it. You didnt mention domestic hot water, what is that? What is the MCA of the magic pack? I would guess you would be somewhere around 60-70 amps as a design current for VD and maybe try to keep that below 5% - just real quick and dirty numbers.

Added: and then compare what you come up with to the increased cost of various wire sizes. Will this be SER cable?