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Supercharger - Woburn, MA

Johnny Vector

Member
Jun 21, 2020
298
491
Maryland
Where did you get the √3 from? The formula is P(kW) = S(kVA) × PF, so if the power factor is 1 you get 1000kW available for 1000kVA transformer.
Because it's 3-phase coming in and 2-phase going out. I haven't worked out exactly why that factor is correct, or whether it matters if it's a wye or delta configuration, but it's in every calculation I could find. It is the ratio between phase-to-phase and phase-to-ground voltages, so I'm willing to believe it.

I tried to look up what those ratings actually mean in terms of how transformers are sized, but the National Electrical Code doesn't apply on the utility side of things, and apparently the rules that do apply are some combination of opaque, vague, and massively varying depending on utility. It does appear that the limits on these transformers are driven by long term thermal considerations, not hot-spot wire failure, which implies that they can run way over their ratings for as long as it takes for all the oil to rise substantially in temperature, so long as there's a chance to cool back down. Personally, I don't think the single kVA number is sufficient to describe the transformer's capabilities in enough detail, but what do I know?

Any actual utility engineers here who can chime in?
 
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eladts

Member
Jul 31, 2016
850
1,227
Brookline, MA
Because it's 3-phase coming in and 2-phase going out. I haven't worked out exactly why that factor is correct, or whether it matters if it's a wye or delta configuration, but it's in every calculation I could find. It is the ratio between phase-to-phase and phase-to-ground voltages, so I'm willing to believe it.

Large utility transformers are 3-phase to 3-phase, but even with residential split-phase transformers there is no √3 factor. With 3 phase connection the total power available is the same if you connect the loads in a wye or delta configuration. The voltage in a delta configuration is indeed √3 larger, but the current you can draw is √3 smaller so the √3 factors cancel each other.
 

Johnny Vector

Member
Jun 21, 2020
298
491
Maryland
Large utility transformers are 3-phase to 3-phase, but even with residential split-phase transformers there is no √3 factor. With 3 phase connection the total power available is the same if you connect the loads in a wye or delta configuration. The voltage in a delta configuration is indeed √3 larger, but the current you can draw is √3 smaller so the √3 factors cancel each other.
Wonder why that factor was in all the things I found explaining it? A common error that people see and repeat, like "breaks" for brakes, or "loose" for lose? Am I going to have to write down a bunch of voltages and transformer windings to figure out what they mean? (Spoiler: no, not worth the effort).

Or maybe it's just that the ratings are defined in a way that doesn't make sense to me.

Anyway, even at a full 1000 kW, that's 83 kW per station if they're all in use. I guess if you assume everyone's charging to 80% you'll have about half of them below 100 kW. I guess full load is just pretty rare.
 
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Wonder why that factor was in all the things I found explaining it? A common error that people see and repeat, like "breaks" for brakes, or "loose" for lose? Am I going to have to write down a bunch of voltages and transformer windings to figure out what they mean? (Spoiler: no, not worth the effort).

Or maybe it's just that the ratings are defined in a way that doesn't make sense to me.

Anyway, even at a full 1000 kW, that's 83 kW per station if they're all in use. I guess if you assume everyone's charging to 80% you'll have about half of them below 100 kW. I guess full load is just pretty rare.
Transformers are able to be pushed past 100% for a time, oversized transformers are less efficient, and since the loss is on the power company side of the meter they would rather see a smaller tranformer. Thus until the OIL gets too hot they can be pushed, when the oil gets too hot the FD may get involved.

Where I live houses with 200 amp service normally have 10KVA pole mount transformers. You can tell the people that really push them as the paint will be a bit scorched off the transformer.
 

Johnny Vector

Member
Jun 21, 2020
298
491
Maryland
Transformers are able to be pushed past 100% for a time, oversized transformers are less efficient, and since the loss is on the power company side of the meter they would rather see a smaller tranformer. Thus until the OIL gets too hot they can be pushed, when the oil gets too hot the FD may get involved.

Where I live houses with 200 amp service normally have 10KVA pole mount transformers. You can tell the people that really push them as the paint will be a bit scorched off the transformer.

Now I wonder what we have. I suspect from the markings that it's 50kVA (ground-mounted), and it feeds both us and our neighbor. We have 400A service, which would be 96 kW, but we can't draw more than ~50 kW even if charging both cars, running the heat full blast with the oven on, heating the hot tub, and running the hot water tank and the tankless heater for the outdoor shower. And the neighbor doesn't have EVs or a hot tub, so I guess we're good!
 
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adspguy

Member
Dec 1, 2016
233
224
Bedford MA
Passed by this morning and it looks like they have started work with the horizontal trench.
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adspguy

Member
Dec 1, 2016
233
224
Bedford MA
I stopped by there yesterday evening. It looks like the station mounting bases are in their trench. It also looks like the platform where the cabinets go is framed out almost ready for a concrete pour. I'm pretty sure the horizonal tunnel to where the transformer will go in is done. Making good progress for sure.
 
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adspguy

Member
Dec 1, 2016
233
224
Bedford MA
Me three on the visit. They excavated where the power feed vault is, but there's no lateral trench yet.

I think the number of us casually visiting this site shows how accessible and central the location is, being at the intersections of 93 and 95/128.
It looked like near the beginning of the process they set up to do a horizontal drill under the pavement for the power feed, but I could be mistaken. I also thought that the plans that were published showed that being drilled under rather than digging up the parking area.

The site is very close to my office, so I am psyched. Also the new site at the Burlington Mall is close to my house.
 

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