Pole transformer sizing (or, can you really feed 7 houses with 25 kva?)

[tga]

The transformer that feeds my house decided to give itself an old fashioned Viking funeral around 9pm Thursday night. Unfortunately I missed most of the excitement, since the power wasn't impacted until the power company cut the power to replace it about 45 minutes later. With the house dark, I could see the police/fire/PoCo vehicles' strobes going. There was still some flames coming out of the transformer, but nothing all the exciting. They got it replaced around midnight.

Friday morning I took a look at the transformer, and it looks like they've installed a 25kva unit (based on the big "25" sticker on it). I'm surprised it isn't bigger or that they didn't hang two. That single transformer feeds 7 houses (was 4 way back when, but 3 decent sized ones have been added in the neighborhood in the last 10 years). I'm basing that assumption off the fact that 7 houses were dark during the event, we're at the end of the street, and this is the only transformer around.

The transformer has probably been flakey for a couple of weeks - I've been noticing lights flickering during the day, but it's stopped now that we have the new one.

So my real question - are there any rules of thumb for sizing these things? I know they're typically overloaded, but really, 7 houses (5 are >3000 ft^2) on 25kva?

 

[Kandiru]

If you have natural gas go for at least 22kW backup automatic switch generator. Dunno about their transformer, ours is also on a pole but holding on so far.

 

[Ampster]

.......
So my real question - are there any rules of thumb for sizing these things? I know they're typically overloaded, but really, 7 houses (5 are >3000 ft^2) on 25kva?

My experience with the utilities is they have their own rules. I also think it matters what voltage is coming into that transformer.
Here is another thread that may answer that:
Do I need 400 amp service? - Page 3

 

[KJD]

In my area it is fairly common to see 4 houses on the same transformer. It is an older neighborhood.

On thing that can lower the load on that transformer is to use scheduled charging to start your EV charge after everyone has gone to bed.

 

[mspohr]

I'm one of seven houses on a 25kw transformer. Voltage drops to 225 when I charge in the evening. Power company is not concerned.

 

[hcsharp]

I have 200A service with 70A and 80A EVSEs. Other than that our electric load is small. No AC, no heat pumps. H2O, Dryer and stove are gas. Power Co switched out our transformer to 25kva and it only serves our house. I asked about what would happen when I'm pulling 150A for charging plus other household loads (minimum 36kva) and he said "It should be fine." He went on to say we worry about the older houses on 5kva transformers.

 

[wk057]

The 25 kVA transformer probably can handle much higher than 25 kVA for short periods (several hours) without issue. 7 houses seems like a stretch, but it'd probably be fine unless they all started charging EVs or something similar.

 

[aesculus]

They look at the historical peak loads and make a determination based on that. However if a bunch of neighbors install more AC units or a bunch of EV's that were never there before, it could change their estimates. If your charging is roughly the same and you are not aware of any big changes in your group of 7 over the last year it's probably OK. The older transformer may have been at end of life or perhaps it had a individual problem. But if you do think there were changes you should contact the PoCo and ask them to run a load calc. My power co thinks it will be OK to stay on my current 200 amp service but I am on the edge. They are willing to add another line from the Tx if I want though that would feed a dedicated 100 amp panel for the EV. My Tx is oversized because I am the only one on it.

 

[cinergi]

When I was having voltage drops while charging at 80, I discovered our 50 kVA was servicing 26 homes! After a battle, I finally found a guy at the powerco who agreed that was ridiculous. They split that up into 13 per 50 kVA xformer -- same ratio as you. The xformer is on the pole right outside my home so I don't lose much when I draw my 80 amps. What amperage do you charge at and what's your voltage before and after the ramp-up period? How far from the xformer are you?
That 25 kVA unit is probably fine. These transformers are built to handle relatively long peaks much higher than their rated load.

 

[tga]

Thanks for the feedback. I was more just curious on what is the norm. Sounds like our setup isn't uncommon.

I don't know what the old transformer was rated at, but I never had any significant voltage drops running oven, dryer, and welder together. Not a big concern; I'm moving before a Tesla arrives. The new house shares the transformer with 2 others. I never looked at it's size, but I wouldn't be surprised if it's smaller.

This is the second transformer fire I've seen. 20+ years ago I saw one go pretty violently - loud bang, lots of sparks, and a shower of burning oil.

 

[AWDtsla]

Anecdotal information: After they extended development of the street I grew up on, we had the pole transformer at the head of the street fail several times. Once, the explosion sent pieces of the transformer probably up to 1500 feet away. I actually remember this happening and pieces of it hitting the wall of our house seconds after the bang and the power going out. If I recall correctly they replaced it 3 times before they decided to upgrade the thing.

They are not the smart. I would think with more modern equipment, like smart meters, they would be able to gather data on the overloads remotely.

 

[wk057]

Anecdotal information: After they extended development of the street I grew up on, we had the pole transformer at the head of the street fail several times. Once, the explosion sent pieces of the transformer probably up to 1500 feet away. I actually remember this happening and pieces of it hitting the wall of our house seconds after the bang and the power going out. If I recall correctly they replaced it 3 times before they decided to upgrade the thing.

They are not the smart. I would think with more modern equipment, like smart meters, they would be able to gather data on the overloads remotely.

It seems like smart meters are still somewhat uncommon in a lot of areas.

For example, my house still has an old fashion mechanical meter, and it was built only ~12 years ago.

 

[AWDtsla]

It seems like smart meters are still somewhat uncommon in a lot of areas.

For example, my house still has an old fashion mechanical meter, and it was built only ~12 years ago.

I appear to be picking up 3k smart meters in my area from a ~3" indoor antenna on an SDR. Maybe the power co has just been replacing them around here. I assume it costs less than paying a guy to walk to every house.

 

[tga]

The new house shares the transformer with 2 others. I never looked at it's size, but I wouldn't be surprised if it's smaller.

Just checked this morning, 15kva for 3 houses.

 

[FlasherZ]

My power company engineer told me that the transformers they use can easily be run at 200% of their load for significant periods of time. Given today's loads, I'm rather surprised @ 7 homes on 25 kVA (my home has a dedicated 37.5 kVA transformer)... but they have more data at their fingertips to help determine it (especially if you have smart meters).

- - - Updated - - -

It seems like smart meters are still somewhat uncommon in a lot of areas.

For example, my house still has an old fashion mechanical meter, and it was built only ~12 years ago.

My electronic meter was replaced with a mechanical meter when I went to net metering.

 

[mknox]

So my real question - are there any rules of thumb for sizing these things? I know they're typically overloaded, but really, 7 houses (5 are >3000 ft^2) on 25kva?

That is not uncommon. The diversification of load is such that utilities can count on concurrent demands of around 3-5 kW per connected house. I used to run an algorithm on the utility's smart meter network to aggregate all the meters connected to each transformer and send an alert when loading got to the point where a transformer upgrade (or sometimes downgrade) was warranted. (I recently retired from an electric utility).

 

[tga]

Both houses have digital/remote reading meters, but I don't think they are smart meters that measure load over short intervals.

The old house (25kva/7 houses) had a standard digital meter. When I added solar, they installed a new meter that recorded inbound and outbound usage. I'm on a municipal utility that doesn't do net metering - they record consumption (billed at retail $0.14/kWh) and excess net production (paying wholesale $0.05/kWh) separately.

The new house (15kva/3 houses) had mechanical meters (separate water heater meter). Both were recently replaced in a state-wide move to remote reading meters. They sent out a letter beforehand emphasizing these were not smart meters - I can only assume to keep the conspiracy nuts at bay.

 

[L.Robinson]

Overloading of the electric delivery infrastructure when EV are in widespread use has been a concern for over a decade in the industry. I seen IEEE articles on the subject occasionally for a number of years. For that reason I charge at 240V 20A starting at 1:00 AM almost always as I know there can be issues in my local lines. There have been more issues with the wires failing than the transformers and there are 3 transformers in parallel feeding the homes on my alley but I've not counted how many homes there are, all mid 1960's construction with most having panel upgrades over the years like mine. The last time a cable failed they experimented with splitting the group but some had too much voltage drop. I just checked my charge voltage graph from last night and didn't see any droop for the 120V per leg so its working. I'll be watching carefully for the first summer with the MS in Dallas to see how it goes.

 

[AWDtsla]

I've seen Model S pull back current even on a chargepoint L2 charger. That should be for 8% voltage drop. I bet there's another current drop after that as well, if not charge abort. I don't think there's much to worry about other than that maybe you didn't get a full charge by morning.

 

[mspohr]

Overloading of the electric delivery infrastructure when EV are in widespread use has been a concern for over a decade in the industry. I seen IEEE articles on the subject occasionally for a number of years. For that reason I charge at 240V 20A starting at 1:00 AM almost always as I know there can be issues in my local lines. There have been more issues with the wires failing than the transformers and there are 3 transformers in parallel feeding the homes on my alley but I've not counted how many homes there are, all mid 1960's construction with most having panel upgrades over the years like mine. The last time a cable failed they experimented with splitting the group but some had too much voltage drop. I just checked my charge voltage graph from last night and didn't see any droop for the 120V per leg so its working. I'll be watching carefully for the first summer with the MS in Dallas to see how it goes.

I think most people charge at night (although most not on TOS rates just plug in rather than set a charge time). Nighttime "base load" is oversupplied now and probably into the forseeable future. I just plug in and charge in the evening since I don't have time of service rates and the one puny 25kW transformer that supplies 7 houses in my neighborhood drops the voltage as low as 229. My neighbor (on the same transformer) just got an X so he will be adding to the load... we'll see if the transformer can handle it.

I do recall reading (unfortunately don't recall where now) that Tesla was floating a scheme for free home charging. Tesla would manage car charging and the utilities would dictate the load they wanted the cars to absorb (and the times) to soak up excess nighttime electricity. In return for managing the load, customers would get free electricity. I can imagine that if Tesla had enough cars available to pull electricity on demand, this could help the utilities manage the grid. Right now, there probably aren't enough cars to make a difference but in a few years there could be enough cars in certain areas to soak up significant overcapacity.