Dealing with high amperages

WA_Sparky

Member
I'm working on an apartment complex that is getting a little uncomfortably-high in the amperage. Anyone want to shed some light on this topic?

This multi-building, 75 unit apartment complex, 208 single phase service is showing roughly 3400A demand after nec factors and all have been applied. Eaton has gear that can handle up to 5000A for group metering but im hesitant to continue with design on one single service feed due to potential SCCR problems down the road.. I'm expecting my MDP and disconnect to be rated for 4000A... Is it pretty common to work with amperages these high in 208-1Ø? Any other possible concerns or problems i should be aware of down the road?
 

MyCleveland

Senior Member
My solution was to split the apartments and house into TWO separate services, both grouped in the same room.
House board has a CT section for single meter.
Apartment service has single main and branches serving the meter stacks on each floor. Designed where NO CBers could be added in this distribution equipment.
120/208V-1P-3W to each apartment panel.
 

WA_Sparky

Member
That's not a bad idea. So technically could i use this method of having a split system but still utilize (1) utility transformer. I've heard of it being done in the past just never personally had experience with it. Essentially show two separate feeders tapped off the one xfmr routed to my (2) 2000A distribution boards?
 

oldsparky52

Senior Member
That's not a bad idea. So technically could i use this method of having a split system but still utilize (1) utility transformer. I've heard of it being done in the past just never personally had experience with it. Essentially show two separate feeders tapped off the one xfmr routed to my (2) 2000A distribution boards?
Would that change any demand factors applied to the loads?

You might wind up with 2 distribution boards each rated more than 2000A.
 

zbang

Senior Member
Essentially show two separate feeders tapped off the one xfmr routed to my (2) 2000A distribution boards?
Two separate services, big difference. And whether you have one or two utility transformers is really up to the PoCo, it might be easier all around for there to be more than one.

How many physical buildings are involved? It could make sense to make each one a separate service; don't know how the PoCo would feel about that, though, you'd have to ask.
 

infinity

Moderator
Staff member
4000 amps 1Ø is not common around here. My current apartment complex project is 4-40000 amp 208Y/120 volt services 3Ø. Why wouldn't you use 3Ø?
 

MyCleveland

Senior Member
4000 amps 1Ø is not common around here. My current apartment complex project is 4-40000 amp 208Y/120 volt services 3Ø. Why wouldn't you use 3Ø?
Your tenants must have like six tesla chargers and a few hot-tubs per apartment....kidding
I thought we were discussing 3P.
The house board ranged from 1200-1600A depending on number apts/bldg. size and number of elevators.
The apt board is where you get all your 220.84 discounts.
 

electrofelon

Senior Member
4000 amps 1Ø is not common around here. My current apartment complex project is 4-40000 amp 208Y/120 volt services 3Ø. Why wouldn't you use 3Ø?

Yeah I was going to say the same thing. Not really sure if the OP meant actual single phase service or three phase service with single phase supply to each unit. I assume the latter but sounded more like the former was described. POCO's usually have a relatively low max demand they will supply to a single phase service.
 

augie47

Moderator
Staff member
My experience mirrors what has been stated. Start with POCO and see what options are available. Keep 230.2(C) in mind. Two service points might prove to be a cost advantage.
 

WA_Sparky

Member
Would that change any demand factors applied to the loads?

You might wind up with 2 distribution boards each rated more than 2000A.
Yeah my demand will change for sure. Demand factor from 0.24 to 0.35 (if i went with 3 services at 25 units each. 1 per building). 220.84

Total of three buildings. They anticipate on adding another (3) buildings later on that are carbon copies of these buildings. Which would mean 150 apts total for the site and 6 service entrances.

I've always designed projects using 3Ø service feeders until this year. Per conversation with utility company 208 is cheaper than 480. And 1Ø is also cheaper than 3Ø. There is an independent Elec Engineering company that designs the 1Ø isolation or converters for projects like these in our area. I would have imagined the 1Ø would cause the gear and feeders to be upsized and ultimately negated any savings from the phase converters or phase isolation.
 

Hv&Lv

Senior Member
What kind of hoops you have to jump through for fault currents? 2000 amps 120/208 three phase is 750kVA XF, over 56k Isc.
Using 10kA breakers there?
 

WA_Sparky

Member
Ok update. Utility only supplies up to 100KVA transformers for 1Ø serving 22-24 units. The theory of one service per building is the ticket. If i had to provide protection against 56K Isc i believe the only option would be to go with series rated breakers. If i remember correctly 20A breakers only go to 22k fully rated but some 20A can be series rated for 65K.
 

electrofelon

Senior Member
Ok update. Utility only supplies up to 100KVA transformers for 1Ø serving 22-24 units. The theory of one service per building is the ticket. If i had to provide protection against 56K Isc i believe the only option would be to go with series rated breakers. If i remember correctly 20A breakers only go to 22k fully rated but some 20A can be series rated for 65K.
I believe they do make miniature circuit breakers up to even 65k, but I am sure they are prohibitively expensive. In practice you will end up using series ratings. You won't be able to get a series rating from a larger than 400 amp frame breaker at 65k down to 10k (IIRC, was looking into this a few months ago). There are some triple series rstings often used with meter centers that can get you where you need to be. Talk to your friendly neighborhood gear guy. Don't make the mistake of combing 2 series ratings into a custom triple rating. Lots of people think it works that way but it doesn't.
 

electrofelon

Senior Member
I've always designed projects using 3Ø service feeders until this year. Per conversation with utility company 208 is cheaper than 480. And 1Ø is also cheaper than 3Ø. There is an independent Elec Engineering company that designs the 1Ø isolation or converters for projects like these in our area. I would have imagined the 1Ø would cause the gear and feeders to be upsized and ultimately negated any savings from the phase converters or phase isolation.
I have never heard of such a thing. I can't imagine how that could be at all practical or cost effective.
 

WA_Sparky

Member
What kind of hoops you have to jump through for fault currents? 2000 amps 120/208 three phase is 750kVA XF, over 56k Isc.
Using 10kA breakers there?
The only way I could see making a system work would be to choose series rated breakers so the 20A/1p are rated for 65K with their upstream device.
 
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