NEC 220.84 Transformer Sizing for Multiple Mulifamily Buildings

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wowsk

Member
Location
Iowa
Occupation
Electrical Engineer
Hi,
I've used this forum often, but always someone else had asked my question. I signed up as I've got one that I haven't been able to find an answer for yet.

I have a project with a MV transformer feeding 5 multifamily buildings, each with 4 units. Following 220.84, for one building I get a connected load of 156kVA and a demand load of 70.2kVA.

When sizing the transformer for all 5 dwellings how do I determine the demand load? Is it 156kVA * 5 * 0.38 (demand factor for 20 units from Table 220.84) or is it 70.2kVA * 5?

In my mind the first option makes more sense because I am essentially looking at 20 units from the transformer perspective. It doesn't matter whether they are in different buildings or not.

Thanks for any and all advice.
 
wowsk said:
Hi,
I've used this forum often, but always someone else had asked my question. I signed up as I've got one that I haven't been able to find an answer for yet.

I have a project with a MV transformer feeding 5 multifamily buildings, each with 4 units. Following 220.84, for one building I get a connected load of 156kVA and a demand load of 70.2kVA.

When sizing the transformer for all 5 dwellings how do I determine the demand load? Is it 156kVA * 5 * 0.38 (demand factor for 20 units from Table 220.84) or is it 70.2kVA * 5?

In my mind the first option makes more sense because I am essentially looking at 20 units from the transformer perspective. It doesn't matter whether they are in different buildings or not.

Thanks for any and all advice.
Click to expand...
I believe it would be the first option + any house or site loads.
Are you providing the transformer? Receiving power at MV with a single meter ahead of tranny?
Have you considered voltage regulation...one tranny serving 5 buildings. Are the lengths to each building approximately the same?
 
Ok thanks glad what I was thinking made sense and only site loads are going to be some LED parking lot lighting, nothing major

Yes we are providing the transformer.

Pretty early in the design phase so I haven't modeled load flow yet, that's to come. But longest run to a building is about 275' and shortest is 40'. The transformer is roughly placed in the middle so 2 - 275', 2 - 150', and 40' runs
 
wowsk said:
Ok thanks glad what I was thinking made sense and only site loads are going to be some LED parking lot lighting, nothing major

Yes we are providing the transformer.

Pretty early in the design phase so I haven't modeled load flow yet, that's to come. But longest run to a building is about 275' and shortest is 40'. The transformer is roughly placed in the middle so 2 - 275', 2 - 150', and 40' runs
Click to expand...
Utility & State PUC rules allow for the owner to bill each tenant individually?
Have had numerous developers talk about doing this and zero followed through.
Keep an eye on what your delivery voltage will be at each building...you have to set the taps on ONE value.
 
I would say that the concern that 220.84 refers to "a multifamily dwelling" and therefore the AHJ could interpret that as requiring 70.2 kVA * 5 is a plausible concern. But I see no good reason for that interpretation and would think that 156kVA * 5 * 0.38 makes more sense.

Cheers, Wayne
 
Will do. There is a long ways to go yet on this project, but I wanted to get this to the client first as transformers are big cost and schedule impacts.

I appreciate the help everyone. This is a good forum.
 
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