# Transformer sizing for step up and step down transformers

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#### wwhitney

##### Senior Member
.
Just tell me this-
You have a 50kVA service transformer. That 50kVA available (which I am thinking of as "your service"), at 100% efficiency and rated capacity, is 208.3A at 100% 240V loads and also 416.6A at 100% 120V loads.
Actually, that's a transformer that will provide 50kVA with a certain temperature rise and a certain expected lifetime for that temperature rise. The transformer will transform more than 50kVA with a greater temperature rise and possibly an associated reduction in expected lifetime.

.
So what amperage is that service? As in, when you ask the POCO "what amperage is my service?", what will they say?
Could be 400A, could be 600A, could be more, depends on POCO practice. It will definitely be more than 200A, as the POCO does not provide full sized transformers for the nominal service size.

Cheers, Wayne

#### kwired

##### Electron manager
Originally Posted by PVfarmer
I'm really thinking that two 400A loads is counted as 800A.

I mean two 400A loads at 120V L-N, which is two legs of one main service and switch or breaker.

I know the math here is right, guys.
Just tell me this-
You have a 50kVA service transformer. That 50kVA available (which I am thinking of as "your service"), at 100% efficiency and rated capacity, is 208.3A at 100% 240V loads and also 416.6A at 100% 120V loads.

So what amperage is that service? As in, when you ask the POCO "what amperage is my service?", what will they say?
There is two 120 volt coils but they are connected in series and only rated 25 kVA each, or the 240 volt coil has two halves and the center point is made to be usable - either way you wish to look at it, still gives you same usable results.

#### jim dungar

##### Moderator
Staff member
Originally Posted by PVfarmer
I'm really thinking that two 400A loads is counted as 800A.

I mean two 400A loads at 120V L-N, which is two legs of one main service and switch or breaker.

I know the math here is right, guys.
Just tell me this-
You have a 50kVA service transformer. That 50kVA available (which I am thinking of as "your service"), at 100% efficiency and rated capacity, is 208.3A at 100% 240V loads and also 416.6A at 100% 120V loads.

So what amperage is that service? As in, when you ask the POCO "what amperage is my service?", what will they say?

The utility sizes their equipment any way they want to. They basically have no loading restrictions except for the resultant Voltage Drop. They take a risk that their equipment can handle the way we use electricity, and if it fails they must replace it for free.
Unless my customers are buying voltage above 2300V, in my area many of the utilities will not say what size transformer they have actually installed. I rarely spend any time concerning myself with utility equipment capacity.

#### PVfarmer

##### Senior Member
1 As long as each single conductor current is less than 400A, you are good.
So if L1 = L2, then N = zero, no current on the neutral. From just looking at the currents, you can't tell if that's feeding a single 240V load at 400A, or instead feeding 400A of 120V loads on L1, and 400A of 120V loads on L2.

2 Correct!

3 But the POCO knows you won't be using 400A 24/7, they put in a 50kVA xfmr for your 96kVA service. The transformer is fine to be briefly overloaded if you draw 96kVA, but if you try to do it 24/7, you will burn up their transformer and they will yell at you. From the POCO's point of view, the 400A service is something like 400A peak, maybe 100A on average.

4 It can and it does. Each component of the 400A meter main sees no more than 400A of current when supplying a balanced set 96kVA loads at 120V.
Thanks again Wayne.

1. With 300A of 120V load on the L1 to N leg A, and 150A of 120V on L2-N leg B, you then have 150A on N?
I'm just not seeing how 400A on leg A *and* 400A on leg B is anything but 800A going through the breaker, which is a 400A breaker. Even if neutral is zero, there's still 2x the rated amperage going through the breaker.

2 Thanks!

3 I'd think if you were pulling 96kVA and/or 400A of 240V for motors 24/7, you'd have a 100kVA xfmr. But you could *instead* (not also, or at the same time) pull 800A of 120V.
So a 100kVA xfmr would be the size for an 800A (120V) service and 50VA would be a 400A (120V) service (800A/400A being the POCO name for them), so 100% of that 400A / 50kVA service would be 200A of 240 motors. You can pull 300A of 240V from a 50kVA xfmr, but that's 150% of the 50kVA rating- maybe OK for 3 hours or less but not 24/7.

No yelling by POCO is always good!

4 Not sure what you mean by "components"- if L1 and L@ are 2 components, and if the meter is rated 400A, that means 200A on L1 and 200A on L2, so 400A / 120V rating means 200A of 240. It's rated for the total going through it in any direction.
If you could put 200A of 240V through on L1 and then another 200A /240V through the meter on L2, that would be 400A,and it would have to be rated 800A for it's 120V amp potential.

But... there is no 240V going through the meter at all...is there?

#### jaggedben

##### Senior Member
1. With 300A of 120V load on the L1 to N leg A, and 150A of 120V on L2-N leg B, you then have 150A on N?
Yes, because at any given moment of the AC cycle the currents for those loads are flowing in opposite directions on the neutral, i.e. they cancel out.

I'm just not seeing how 400A on leg A *and* 400A on leg B is anything but 800A going through the breaker, which is a 400A breaker.
L1 and L2 are in series, so it's the same 400A flowing through each leg. You can't have a sum of two currents if you only have one current.

Not sure what you mean by "components"- if L1 and L@ are 2 components, and if the meter is rated 400A, that means 200A on L1 and 200A on L2, so 400A / 120V rating means 200A of 240.
No! Stop repeating what you've been told is wrong!

Try this. 400A*240V = 96000W, which is the max the service can handle.

a) 400A 240V load connected from L1 to L2 = 400A*240V= 96000W

b)400A 120V load connected from L1 to N = 400A*120V = 48000W
c)400A 120V load connected from L2 to N = 400A*120V = 48000W

a = b+c
You can have (a) on the service, or (b+c), but not both.

That is some correct math.

#### PVfarmer

##### Senior Member
You're getting a brand new service.
Two hypothetical scenarios:

A: The loads are- six 5hp 240V fans, so 22,371 watts there, 93.2 amps of 240 load. They only run (all together) intermittently, say 15 minutes out of every 2 hours, 3 hours a day total for 3 months a year. Also 50A of 120V heaters running more continuously for those 3 months (another 6000w) and also 20A of 120V lights all year (another 2400w for lights)

B: Same fans, heaters, and lights, same 30,771w, but running 24/7. Fans running 80% of the time
---

For A, you'd be ok with a 25kVA service xfmr. It would only be overloaded for 15 minutes at a time 3 hours a day total- sounds fine to me. I wouldn't do that, but I see how it can work and be OK with the POCO.

For B, you'd not be OK with a 25kVA xfmr- you'd need a 50kVA so it wouldn't be overloaded 80% of the time.

30,771w (30.8kVA) is 256A of 120V and 128A of 240V.

That's why I'm saying a 25kVA is a 200 amp service and 50kVA xfmr is a 400A service...25kVA is too small for 256A of loads, unless they're sparse.

If the POCO called a 25kVA transformer "400A service" they'd get overloaded everywhere!

#### PVfarmer

##### Senior Member
Try this. 400A*240V = 96000W, which is the max the service can handle.

a) 400A 240V load connected from L1 to L2 = 400A*240V= 96000W

b)400A 120V load connected from L1 to N = 400A*120V = 48000W
c)400A 120V load connected from L2 to N = 400A*120V = 48000W

a = b+c
You can have (a) on the service, or (b+c), but not both.

That is some correct math.
It sure is- it's also an called an 800A single phase service by the POCO here, and involves a 100kVA 1 ph xfmr.

And when-
a= 200A / 240V / L1-L2 / 48kW

b= 200A / 120V / L1-N / 24kW
c= 200A / 120V / L1-N / 24kW

It's called a 400A single/split phase service with a 50kVA xfmr (by the POCO here at least)- otherwise known as what zman most probably has, which won't be a good place to put his 72kVA of PV output! :huh:

If:
a = 100A/240V

b= 100A/120V
c= 100A/120V
It's a 200A service. (25kVA xfmr @120V)

If a b and c are 50A, it's a 100A service (10kVA xfmr for 12kVA/100A) max loads (which hardly ever happen in homes)

#### PVfarmer

##### Senior Member
All I can say for sure is that:
1. here, 400A (limited to 320A continuous) is the maximum "standard" single phase service. Or any other service- you can get 400A of 1ph 120/240V, or of 1ph 277/480, or 3ph 208/120, or 3ph 480/277.
But- 1 (Any) 400A service with a class 320 meter and socket is limited to 320 continuous amperes load capacity

2. If you want to install more than 50kVA of single phase PV, you have to ask.

Seeing as 50kVA of PV is = to 416A at 120V...these 2 things I know (400A /50kVA) as facts must be related somehow.
As in, a 50kVA/400A (so 200A at 240V) is the maximum standard service, and you can't overload the standard 50kVA xfmr with your PV output, because the xfmr doesn't belong to you!

I don't see how they'd sell you a 400A of 240V service which is = to ~96kW and provide a 50kVA xfmr that would waste the POCO's power not going through the meter when overloaded and eventually go kablooey.
Then I or the customer could yell at them!!

If the standard/50kVA service was 400A of 240V capacity, you could install 90kVA of single phase PV on that no problem.

#### wwhitney

##### Senior Member
I'm just not seeing how 400A on leg A *and* 400A on leg B is anything but 800A going through the breaker, which is a 400A breaker. Even if neutral is zero, there's still 2x the rated amperage going through the breaker.
Each pole of the two pole breaker can handle 400A independently. There is no single point on the breaker that sees 800A. Just different points that see 400A. No violation of the breaker rating.

100% of that 400A / 50kVA service would be 200A of 240 motors.
If you have a 400A /50kVA service, your service voltage is 120V, and you can't run any 240V motors.

if the meter is rated 400A, that means 200A on L1 and 200A on L2, so 400A / 120V rating means 200A of 240. It's rated for the total going through it in any direction.
Wrong! The rating is the current going through any conductor. 400A on L1 and 400A on L2 is the rating of a 400A meter.

Cheers, Wayne

#### wwhitney

##### Senior Member
All I can say for sure is that:
1. here, 400A (limited to 320A continuous) is the maximum "standard" single phase service. Or any other service- you can get 400A of 1ph 120/240V, or of 1ph 277/480, or 3ph 208/120, or 3ph 480/277.
OK.
But- 1 (Any) 400A service with a class 320 meter and socket is limited to 320 continuous amperes load capacity

2. If you want to install more than 50kVA of single phase PV, you have to ask.
OK.

Seeing as 50kVA of PV is = to 416A at 120V...these 2 things I know (400A /50kVA) as facts must be related somehow.
Nope, it is a coincidence. The POCO happens to undersize its transformers by a factor of 2, and 120V is half of 240V. Two totally unrelated things.

I don't see how they'd sell you a 400A of 240V service which is = to ~96kW and provide a 50kVA xfmr that would waste the POCO's power not going through the meter when overloaded and eventually go kablooey.
Then I or the customer could yell at them!!
They do exactly this. Your NEC sized 400A residential service is going to draw over 200A very little of the time. The transformer will spend very little time overloaded. This will shorten its lifespan very little. The transformer will not go kablooey. The POCO will waste less money in the excess heat generated when the transformer is occasionally overloaded that it would waste if it provided a full sized transformer. Most of the capacity of that full-sized transformer would sit there unused most of the time.

PV generation is different, in that if you have a system sized at 50 kVA, it will actually generate 50 kVA for at least a few hours straight quite often.

So your confusion boils down to the fact that the nominal size of a POCO service, as calculated according to the NEC is very conservative and can easily overstate the actual current demand by a factor of 2. While the nominal size of PV generation is quite accurate and not overstated.

Cheers, Wayne

#### PVfarmer

##### Senior Member
There is two 120 volt coils but they are connected in series and only rated 25 kVA each, or the 240 volt coil has two halves and the center point is made to be usable - either way you wish to look at it, still gives you same usable results.
Can't you get 25kVA of 120V (or 208A) from a single 1ph 25kVA xfmr LV side, but to get the same 208A/25kVA/120V from a 240/120V high leg setup 3ph xfmr, you'd need a 75kVA xfmr, as it's only using 1 of the 3 windings?

If you have a 400A /50kVA service, your service voltage is 120V, and you can't run any 240V motors.
Um...what do you mean there? If it's a 50kVA 120/240V single/split phase service, you can run 240V motors.
The question is just how many of those motors can you run without explosive or other issues, and what is the standard POCO name for that service.

I'm saying with 400A/50kVA you can run 399A of 240V motors (and one 120w 120V lightbulb) and overload the xfmr to 200%, but you really shouldn't. Sure, you could run 800 120w 120V bulbs.
But to do that you'd have to have an 800A panel, which sounds a bit large for a 400A service!

#### wwhitney

##### Senior Member
If it's a 50kVA 120/240V single/split phase service, you can run 240V motors.
Services size names don't involve the size of the POCO transformer. The POCO transformer might be anywhere from 10% to 100% (numbers I just made up) of the nominal service size.

So a 50 kVA 120/240V single service is a 200A service. The POCO might use a 25 kVA transformer for it, it doesn't matter, it's a 50kVA service.

Thus a 50kVA, 400A service must be a 120V only service. 50 kVA / 400A = 120V (with rounding).

Cheers, Wayne

P.S. Much of what I've said today I've learned by reading this forum for the last ten years. So while I'm 99% sure I have the terminology correct, it would be good if someone else verified that.

#### ggunn

##### PE (Electrical), NABCEP certified
1. With 300A of 120V load on the L1 to N leg A, and 150A of 120V on L2-N leg B, you then have 150A on N?
I'm just not seeing how 400A on leg A *and* 400A on leg B is anything but 800A going through the breaker, which is a 400A breaker. Even if neutral is zero, there's still 2x the rated amperage going through the breaker.
No, and I am painfully aware that you can't see it. That's why you really should get some training if you want to get into this line of work.

Maybe this will help. When you have a 400A line to neutral load on L1 and a 400A line to neutral load on L2, the neutral current is zero and the two loads are in series. During one half cycle 400A comes up through L1, through the first load, through the second load, and back out L2. During the next half cycle the current is going the other way but the magnitudes are the same. It's a loop and there's 400A everywhere in the loop.

This is AC. It's different from DC. As long as you insist on thinking of L1 and L2 to be positive equivalent additive supplies, you are going to keep falling into the same trap.

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#### electrofelon

##### Senior Member
PVfarmer,

1. Please ignore the size of the utility transformer. You continually talk about it, but it has no bearing on anything whatsoever. Even if you were doing a PV job and saw that the transformer would be definitely way undersized (even by utility sizing methods) what do you care? Are you going to start throwing out estimates to the client as to what the utility will charge them?

2. I think it would help greatly if you got away from using the "400 amps of 120V" type of thinking. Generally, when we calculate things, everything gets converted into VA, added up, and then at the end you convert that to current per line or phase. This figure would assume perfect balance, but imbalance is rarely an issue as the NEC load calcs are quite conservative and most designs include some extra capacity. For a single phase service, you will get the current on each pole of the two pole breaker. For three phase you will get the current on each pole of the three pole breaker. Having 200 amps on each pole of a three pole breaker is not 600 amps, not 600 amps of 120V, not 200 amps of 208V - it is 200 amps. There is a reason we dont do use the "amps at volts" method and that is because it is super confusing to keep track of how the currents "cancel" or "combine" across different phases, lines, neutrals, etc. I want you to never say "X amps of Y volts" again, ok?

#### PVfarmer

##### Senior Member
1 Services size names don't involve the size of the POCO transformer. The POCO transformer might be anywhere from 10% to 100% (numbers I just made up) of the nominal service size.

2 So a 50 kVA 120/240V single service is a 200A service. The POCO might use a 25 kVA transformer for it, it doesn't matter, it's a 50kVA service.

3 Thus a 50kVA, 400A service must be a 120V only service. 50 kVA / 400A = 120V (with rounding).
1 Maybe sometimes, that 10% thing sounds a bit radical, but there are definitely standard service sizes (100, 200, 400 amp basically) and standard sizes and configurations of service xfmrs that go with them.
They're on pages 71 (services) and 100 (xfmrs)
https://www.nationalgridus.com/non_html/shared_construction_greenbk.pdf

2 A "200 amp service" could be a 10 or 25kVA xfmr, or a 50kVA xfmr.
But it seems like a 50kVA unit would be called a 400A service, so if sized to the maximum load, at 96% of xfmr capacity rating it would supply either 400A at 120V and zero 240V or 200A of 240V and zero 120V.
200A of 120V loads and 100A of 240V loads also = 96% of 50kVA.
But 400A of 240V loads would be 192% of 50kVA rating- running the xfmr at 192% doesn't sound like "hey, you have 400A of service".

Maybe this will help. When you have a 400A line to neutral load on L1 and a 400A line to neutral load on L2, the neutral current is zero and the two loads are in series. 400A comes up through L1, through the first load, through the second load, and back out L2. It's a loop and there's 400A everywhere in the loop.
So how many kVA is that?

1 Even if you were doing a PV job and saw that the transformer would be definitely way undersized (even by utility sizing methods) what do you care? Are you going to start throwing out estimates to the client as to what the utility will charge them?

For a single phase service, you will get the current on each pole of the two pole breaker.
That has happened- the choices seem to be either don't do the job or get a bigger transformer! (Which seems to be where zman is at?)
Getting a bigger transformer is a better option than not getting PV as far as I've figured- the PV pays for it easily.

Each pole- that means zman's 72kVA / 300A combined inverter output is 300A on each pole of a 400A breaker.
All the other stuff aside- he can't put that 400A PV breaker on the load side of his 400A main disconnect meter main.
Not sure what the 320A continuous limit on the meter there is, but even if the meter/main has a 400A rating, that only leaves 80A of breaker for loads.

#### PVfarmer

##### Senior Member
That link to page 71/list of services offered in previous comment.
You can request 200A of:
1. 120/240V 1ph
2. 208/120V 3ph
3. 480/277V 3ph

There's no way those are anywhere near the same setup or kVA of xfmrs.
It is as far as I can tell:
1. 25kVA
2. three 25kVA on a pole or 1 75kVA on a pad.
3. not sure, 200A=166kVA, has to be three 50kVA on a pole or a 150kVA on pad, maybe a 225kVA on pad.

Isn't that sort of the first thing you want to know for a PV project?
Why would you get any PV inverters that weren't the same voltage as the customer's existing service? Or try to put PV thru at more than 100% of the xfmr's rating? (They won't let you, so why try?)
I know of one inverter that requires the total PV Output to be <=90% of the step up xfmr kVA- there's a darn good reason to know the kVA of the service.

There's one reason to get inverters that aren't the same voltage as an existing service- the service is not enough kVA, so you're getting a bigger and better service.

#### wwhitney

##### Senior Member
2 A "200 amp service" could be a 10 or 25kVA xfmr, or a 50kVA xfmr.
It could be "on" a 10 or 25 kVA xfmr. The service is not the transformer.

But it seems like a 50kVA unit would be called a 400A service
You mean to say that a 50kVA unit would be more likely to be used for a 400A service than to be used for a 200A service. Again, the service is the not the transformer.

But 400A of 240V loads would be 192% of 50kVA rating-
Indeed

running the xfmr at 192% doesn't sound like "hey, you have 400A of service".
That just goes to show that if you have a continuous load at 400A, you need a service bigger than 400A. The NEC requires you to use a 500A feeder (125%), so you'd probably get at least a 600A service. And then the POCO would likely use a transformer bigger than 50 kVA, so you wouldn't have that problem.

BTW, am I correct in thinking that POCO transformer ratings are continuous ratings? If so, that's a factor of 80% (which is 1 / 125%) right there, the NEC rating of the service would be on a non-continuous basis. So the POCO transformer would likely never be more than 80% of the service size.

Cheers, Wayne

#### electrofelon

##### Senior Member
1 Maybe sometimes, that 10% thing sounds a bit radical, but there are definitely standard service sizes (100, 200, 400 amp basically) and standard sizes and configurations of service xfmrs that go with them.
They're on pages 71 (services) and 100 (xfmrs)
https://www.nationalgridus.com/non_html/shared_construction_greenbk.pdf

2 A "200 amp service" could be a 10 or 25kVA xfmr, or a 50kVA xfmr.
But it seems like a 50kVA unit would be called a 400A service, so if sized to the maximum load, at 96% of xfmr capacity rating it would supply either 400A at 120V and zero 240V or 200A of 240V and zero 120V.
200A of 120V loads and 100A of 240V loads also = 96% of 50kVA.
But 400A of 240V loads would be 192% of 50kVA rating- running the xfmr at 192% doesn't sound like "hey, you have 400A of service".
:rant:

That has happened- the choices seem to be either don't do the job or get a bigger transformer!
OF course it has. But its the utility's call as to "how hot" they will run the transformer and what they will charge so I still dont see what good it does you to conjecture/obsess about it until you get the proposal from them. IF you are somewhat flexible with the system size, you could inquire what the max size PV without a transformer upgrade would be and weigh out the cost difference, customer needs, etc.

Each pole- that means zman's 72kVA / 300A combined inverter output is 300A on each pole of a 400A breaker.
All the other stuff aside- he can't put that 400A PV breaker on the load side of his 400A main disconnect meter main.
systems that are relatively large compared to the service size are usually supply side connected. In his case, taking that meter main out and throwing it in the river is probably where I would start

But 400A of 240V loads would be 192% of 50kVA rating- running the xfmr at 192% doesn't sound like "hey, you have 400A of service".
I think that is pretty typical, even a bit large actually. In my experience, I usually find an NEC load calc to result in about 2.5 times the actual load. System I have been working on the last few days is actually a near textbook example: 1000 amp 120/208 service (ampacity of service entrance conductors), 150 KVA transformer (~420 amps). Actual load hovers right around 115 KVA (~320 amps) during peak times, Peak demand from last two years is 156 KVA

#### PVfarmer

##### Senior Member
1 It could be "on" a 10 or 25 kVA xfmr. The service is not the transformer.

2 You mean to say that a 50kVA unit would be more likely to be used for a 400A service than to be used for a 200A service. Again, the service is the not the transformer.

3 so you'd probably get at least a 600A service. And then the POCO would likely use a transformer bigger than 50 kVA, so you wouldn't have that problem.

4 BTW, am I correct in thinking that POCO transformer ratings are continuous ratings? If so, that's a factor of 80% (which is 1 / 125%) right there, the NEC rating of the service would be on a non-continuous basis. So the POCO transformer would likely never be more than 80% of the service size.
1 Sure, but the service is provided by the xfmr, the same xfmr the PV goes thru, so the size of it has to be taken into account at some point- why not at the first point?
If a customer says "hey, I want some PV", you don't run out and buy inverters without asking "what voltage service do you have?"
And after that "how many amps is your main switch" seems like a logical follow up.
But the amps of the main switch doesn't always equal the amps of the service, so next I'd ask "what size is the xfmr", the customer wouldn't know, so I'd ask the POCO- don't want to oversize the PV inverter output.

2 I'm trying to say a 50kVA xfmr wouldn't ever be supplying a 200A service, because it would be oversized 2x, and the POCO doesn't do that.

3 Yep. If it was a 600A irrigation load that ran for a month a year for a few hours a day, the 50kVA would work for 600A.
If it was 600A of heaters running 50% of the time, 75kVA would be the one. That's why I'm saying 75kVA would Be "standard" for someone using 600A on a regular basis.
I think the limit for single phase is 1200A / 150kVA in fact.

4 Doesn't the NEC end at the service point? So it's not factoring in the service xfmr at all?

:rant:

OF course it has. But its the utility's call as to "how hot" they will run the transformer and what they will charge so I still dont see what good it does you to conjecture/obsess about it until you get the proposal from them. IF you are somewhat flexible with the system size, you could inquire what the max size PV without a transformer upgrade would be and weigh out the cost difference, customer needs, etc.

systems that are relatively large compared to the service size are usually supply side connected. In his case, taking that meter main out and throwing it in the river is probably where I would start

I think that is pretty typical, even a bit large actually. In my experience, I usually find an NEC load calc to result in about 2.5 times the actual load. System I have been working on the last few days is actually a near textbook example: 1000 amp 120/208 service (ampacity of service entrance conductors), 150 KVA transformer (~420 amps). Actual load hovers right around 115 KVA (~320 amps) during peak times, Peak demand from last two years is 156 KVA
Dude! All I'm saying is that:
If there's a 25 kVA xfmr, it will be loaded to 100% of capacity if:
There are 208.3A of 120V loads.
OR
There are 104.15A a 240V loads.

And that means a limit of 100A of input from 240V PV inverters.

---

They don't give me the proposal- I give it to them!

Yes! Throw it in the river, zman! Best advice yet!

Could you elaborate? Is that single phase or three ph 120/208V? And why is there a 1000A service when the load never gets close to 500A?

#### Iron_Ben

##### Senior Member
Can't you get 25kVA of 120V (or 208A) from a single 1ph 25kVA xfmr LV side, but to get the same 208A/25kVA/120V from a 240/120V high leg setup 3ph xfmr, you'd need a 75kVA xfmr, as it's only using 1 of the 3 windings?

I agree with the other posters here in that your lack of understanding of the fundamentals and your misuse of the terminology are real problems. I appreciate your curiosity and willingness to learn, but wwhitney and others have given excellent examples and explanations, which you seem to ignore. Focusing on the kva rating of the poco transformer is a waste of time for conventional power flow. (I grant that in PV applications it can matter.)

One more real life example: at my old house, my neighbor and I shared an overhead transformer. We each had a "regular" house - 2,000 sq ft, mostly electric, but with natural gas service. We each had a 200 amp service. 200 amps x 240 volts = 48 kva. 48 kva per home x 2 homes = 96 kva. The transformer that served us both was a 10 kva. So that's nearly a 10:1 ratio of service "size" to transformer rating. I sold the house earlier this year. At that time, the transformer had over 30 years of service on it. I worked for that poco, by the way.

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