50 kva transformer on 24 units.

[stew]

did a troubleshoot today at a duplex unit that had a 100 amp service. found open hot to the meter on one side. half hot as it were. anyway i looked around out side and across the street there is one 50 kva transformer feeding 12 duplexes each duplex has its own 100 amp service. thats 24 100 amp services on one 50 kva transformer. how the heck does this fly? actually there are 12 more duplexes in this development and there are only 2 50 kva transformers in the whole complex.

 

[mdshunk]

What's the calculated load? That's what's unknown. I do a fair amount of apartment building service upgrades, and some only have real loads of like 20-25 amps a unit. That, and consider that the utility transformers can be overloaded by quite a bit and still be withing spec.

 

[iwire]

The utility does not size transformers or protect them as we would under the NEC.

The utility has many years of records of what a dwelling unit will really draw and it is not even close to the NEC load calcs.

The utility will be there forever and if the transformer expires they will replace it.

Lastly if the utility did size the equipment per the NEC our electrical rates would increase.

 

[mikeames]

In addition to that its cheaper for them to run smaller transformer 80 - 90 % than run a larger transformer idle.

 

[stew]

this must be why the birds like to sit on this transformer its nice and toasty warm. what thoughtfull people these poco guys are.

 

[tallguy]

In addition to that its cheaper for them to run smaller transformer 80 - 90 % than run a larger transformer idle.

Makes sense, but...

Anyone know how this is likely to play out as the capacity is exceeded? Do the customers experience imperceptible voltage drop as capacity is approached? Brown outs in July? Do they have to complain for months/years before it's upgraded? Or does the POCO have some proactive means to address functional obsolescence?

I figured it at ~17A per household. Hard to believe that they don't come close to hitting that average on a hot day at peak usage, but it's easy to see how it might work since most of these homes probably have gas appliances, occupants have different work schedules, etc.

 

[Davis9]

I did a small 8 Unit total(4 Duplex's) a couple summers ago and IIRC they dropped a 50KVA 120/208 Y unit.

But right now there is a 500KVA outside a 43 unit building I'm doing as well.

Each unit has 100A but the calcs are much lower. Gas stoves, Central Air 18A, and a Dryer 21A, not much real load. Range from 700 Sq ft to 1450 Sq ft/unit.

Tom:smile:

 

[peter d]

I was just involved with the installation of a 400 amp, 120/208 service with a 7 position modular metering setup for a commercial building.

The poco put in 25 kVA cans on the pole, and the service drop couldn't have been bigger than 1/0 aluminum.

If I do my math correctly, 25 kVA comes out to 69.44 amps. (25000)/(208*sqrt 3.) So that tells me that the realistic load of this building will never be more than 70 or so amps, but if it exceeds that for any amount of time, it's no big deal.