Transformer sizing for step up and step down transformers

[wwhitney]

Skip the whole PV thing for a minute- a farm here, before PV, had a 25kVA xfmr, (200A service, 120V * 200A = 24kVA).

No, no. The "service size" is not determined by the POCO transformer size. Stop using the wrong terminology.

The main switch was 400A, because well... I don't know.

It was a 400A service. That's the terminology.

This makes sense to me- a 200A service means you have a service xfmr which delivers ~200A at 120V when running at near 100%.

That may make sense to you but it is wrong. A 200A service means the main switch is 200A. The service xfmr may only be able to provide 100A continuously, or 50A continuously, or 150A continuously--it is up to the POCO.

PVfarmer, I'm out--your stubbornness in using the wrong terminology has exceeded my patience.

Wayne

 

[jim dungar]

... a 200A service means you have a service xfmr which delivers ~200A at 120V when running at near 100%.

No, a 200A service tells you absolutely nothing about the size of the utility transformer.
Service voltages are usually listed as L-L or l-L and L-N, I have never seen one listed as L-N only.

I have personally experienced a utility feeding a 400A service, employing a '320A' meter socket, with a 15kVA 120/240V 1-phase 3-wire transformer. 5 years later, the machine shop had finally added enough new machines that the voltage drop was pretty severe, so the utility installed a 25kVA transformer instead. It has been almost 10 years now without a problem.

 

[PVfarmer]

There was not a single main switch. That service has 6 service disconnects, so you go bythe ampacity of the service entrance conductors

Ok. Still confused- you mean the customer's conductors there? As in the conductors from the service point to the main disconnect (fused switch or breaker and/or thru the meter?) which is feeding customer loads?
Or even more simply- when I'm in my driveway looking at the lines, the service drop is from the pole to the house, the service point is where it hits the house, and the entrance is from there to the meter (and then main panel).

Those are the entrance conductors as far as I call tell and the conductors going in the opposite direction from there (as in, connected to the service xfmr) from the service point are the service *drop* conductors.

So for the entrance version, that's your/customer's calc to do, and you "go by" the ampacity of the load for conductor sizing?

Anyway, going with "400A of 208/120V 3ph service..."- that means one has ~144kVA available of service (below 100% xfmr capacity), coming from a 150kVA xfmr.
That's ~400A of 208V available. It's also ~693A of 120V. :roll:
Please don't yell at me!

693A * 1.25 = 866A. Hence the 1000A conductors.

It could be 1400A conductors instead, in case expanding it to "600A of 208/120V 3ph service" in the future was a factor...correct?

 

[jim dungar]

That's ~400A of 208V available. It's also ~693A of 120V.

No it is not. Stop adding amps.

(1) 'system' (e.g. circuit or set of conductors) of 400A at 208Y/120V 3-phase 4-wire is equal to (3) 'systems' of 400A at 120V 1-phase 2-wire or (2) 'systems' of 400A at 208/120V 1-phase 3-wire.

You need to get some actual training on Electric Circuits. This forum is not intended to instruct DIYers. Have you looked at any of the training material offered by our host?

 

[PVfarmer]

A 200A service means the main switch is 200A.

Sorry, Wayne. Let me try again here with what Jim said.
Very briefly- if the shop has three 200A subpanels, there has to be a 600A main...something, somewhere, because of code. Or am I wrong about that (again)?
The load on subpanels could be 133.3A each, so 400A total, which would still be a 400A service.

400A service, employing a '320A' meter socket, with a 15kVA 120/240V 1-phase 3-wire transformer. 5 years later, the machine shop had finally added enough new machines that the voltage drop was pretty severe

I'm saying "HE" here as the customer you mentioned, not he as in you yourself, Jim.

Ok. Machine shop means lots of motors (mostly 240V?), and too many motors causing too much voltage drop because the ... this part is a question ... xfmr is overloaded?
Does that mean voltage drop becomes a problem before the 3 hour continuous load limit does?

Anyway....15kVA xfmr =15,000w = 62.5A @ 240V. Not a whole lot when the xfmr is at 100% of rated capacity. 94A is 150% and 125A would be 200% of xfmr rating. (also, 75A is 120% of xfmr)

If he's got 62.5A of equip in the shop, that 15kVA xfmr can run 24/7 at 100%, maybe even 24/7 at 120%. I'm not sure about that.
But maybe *not* 24/7 at 150%, and for only 30 minutes at 200%. The neighbors' lights might start to flicker.

Now, if that 15kVA xfmr was supplying *400A* or 320A of loads at 240V, that's 96kVA or 76.8kVA of loads- that 15VA xfmr will for SURE not work so well at 640 or 512% of it's rated capacity!

edit:

400A at 208Y/120V 3-phase 4-wire is equal to (3) 'systems' of 400A at 120V 1-phase 2-wire or (2) 'systems' of 400A at 208/120V 1-phase 3-wire.

Thanks for your help first of all.
I'm not DIYing at all here- I just met with the engineer I'm working with yesterday. It went very well.
So how many amps in total of 120V are available from a 400A 208/120V source if there are no 208 loads?

 

[wwhitney]

Please don't yell at me!

Why do you persist in doing calculations that you have been told are wrong or meaningless, when you know they are wrong or meaningless, as your comment above shows?

You have been looking for a forumula relating the size of the service transformer and the size of the service. There isn't any, it's the POCO's choice.

Wayne

 

[wwhitney]

Very briefly- if the shop has three 200A subpanels, there has to be a 600A main...something, somewhere, because of code. Or am I wrong about that (again)?

Yes, you are wrong. Subpanel sizes don't add that way. You could have a 100A main feeding three 200A subpanels, as long as the NEC calculated load for all three subpanels is under 100A.

Wayne

 

[jim dungar]

If he's got 62.5A of equip in the shop..

The amount of current actually drawn by the load is of little concern to sizing a service.
There are NEC methods for determining the minimum size service that must be provided. The designer is free to pick a larger size, in fact most people seem to round up to the next common rating like 100, 200, or 400A.
The utility then advises which metering equipment is needed based on the requested size. It is common for residences, and similar installations, to be allowed the use of a 320A plug-in meter on a 400A service.
The utility then provides a standard size transformer, often based on what is available in their storage yard or on their truck.

 

[electrofelon]

Ok. Still confused- you mean the customer's conductors there? As in the conductors from the service point to the main disconnect (fused switch or breaker and/or thru the meter?) which is feeding customer loads?

Yes. I have nothing to do with sizing the utility conductors - the conductors on the utility side of the service point.

So for the entrance version, that's your/customer's calc to do, and you "go by" the ampacity of the load for conductor sizing?

Yeah pretty much. Or sometimes there isnt a load calc and we just pick a service size, and size the conductors to that service disconnect size (think basic residential for example). Either way, the service conductors must be protected from overload by the OCPD integral or adjacent to the service disconnect. Note we get to use the "next size up rule" here. Also note that if there are multiple service disconnects, we can size the conductors purely to the load, even if the sum of the 2-6 disconnects way exceeds it.

Anyway, going with "400A of 208/120V 3ph service..."- that means one has ~144kVA available of service (below 100% xfmr capacity), coming from a 150kVA xfmr.
That's ~400A of 208V available. It's also ~693A of 120V. :roll:
Please don't yell at me!

693A * 1.25 = 866A. Hence the 1000A conductors.

It could be 1400A conductors instead, in case expanding it to "600A of 208/120V 3ph service" in the future was a factor...correct?

I am confused. Are you discussing that 1000 amp service I mentioned previously? Your numbers are off and besides you are going about it the wrong way. I really dont understand why time after time after time, you continue to talk about the utility transformer and use its size in your calculations. We have said over and over to forget about the utility side of things. Everybody had to start somewhere and didnt know this stuff at the beginning - thats fine - but are you reading or paying any attention at all to the responses we have posted?

The example I gave is not such a great example because it was partly existing, and its more complicated in that it has multiple service disconnects, multiple sets of service entrance conductors, and conductors in parallel. But, say this was a new facility from the ground up an I did a load calc and got 324200 VA (do you remember that: convert everything to VA?). Now we have decided to go with a 120/208 3 phase service since it matches our equipment and we have a few three phase AC compressors - besides a single phase service of that size both isnt practical and our utility wont supply it anyway. Now I need to find out how many amps that is so I use the 3 phase power formula which is P=I*V*1.732 and get 900 amps. I decide to go with a 1000 amp service since there isnt 900 amp equipment. So I have a 1000 single service disconnect and to size my conductors I decide to use 4 sets of 350 Kcmil AL XHHW (each set is 250 amps per 75 degree column). Remember no next size up rule above 800 A so I need the full 1000 amps for my conductors. Now note that if I had multiple service disconnects, say, 6 225 amp service disconnects because the design is such that there are 6 sub panels in the building, I can size the conductors to THE LOAD so I could use 4 sets of 300 Kcmil AL. In the single service disconnect case its a 1000 amp service. with the multiple service disconnect version where I sized the conductors to the load (I could have sized them the full 1000 I was originally thinking if i wanted of course), I have 920 amp service. So finally, the utility hooks me up and I have no idea what size transformer they are using - I dont care or need to know.

Please take some time to read through this response and others. We have invested time and responding so you should do the same. Ill even do a quick recap:
1. No using X amps of Y volts. convert everything to VA.
2. The utility transformer and its size has nothing to do with us.

Homework: Considering #1 and 2 above, if I have a 1000 amp 120/208 three phase service, how many 120V, 1500 watt electric heaters can I use assuming they are balanced equally across the system and they are all on for 1 hour, then all off for 1 hour

 

[PVfarmer]

=electrofelon;1700584

1 I am confused. Are you discussing that 1000 amp service I mentioned previously?

2 I did a load calc and got 324200 VA (do you remember that: convert everything to VA?).

3 So finally, the utility hooks me up and I have no idea what size transformer they are using - I dont care or need to know.

Ill even do a quick recap:
1. No using X amps of Y volts. convert everything to VA.
2. The utility transformer and its size has nothing to do with us.

Homework: Considering #1 and 2 above, if I have a 1000 amp 120/208 three phase service, how many 120V, 1500 watt electric heaters can I use assuming they are balanced equally across the system and they are all on for 1 hour, then all off for 1 hour

1. Yes
2. 324.2kVA load calc meaning...all the equipment turned on at once? We're still talking about a 150kVA xfmr?
3. That's fine- there's no way for me to know either.
These below are the only options for 208/120 3 phase service from the POCO here. 3 pole mounted or single pad mounted 3 phase setups. The (( XXX )) is what you request when you want new service. You have to fill out the load sheet for the POCO of course.

I will guess however if i had your 324kVA load example, and it was "continuous", I'd have 1000A service, and a 500kVA xfmr setup.
If it was 324kVA for less than 30 minutes every once and a while and ½ of 324kVA (or 162kVA) 90% of the time, I'd have the 400A / 150kVA xfmr setup/service.

Options:
3-25kVA or 1-75kVA (( 200A ))
3-50 ..... or 1-150 (( 400A ))
3-75 or ...(no 225kVA pad mounted option- 3-75kVA pole only) (( 600A ))
3-100..... or 1-300 (( 800A ))
3-167..... or 1-500 (( 1000A ))

Well that homework leaves me in a bind.
I simply can't convert the 1000A to VA without...using the number for volts.
I also can't say that a 120V 1500w heater = load of 12.5 amps.

If I could, the answer would be 80 heaters.

80 heaters * 1500VA each = 120,000VA divided by 120V = 1000A.

I'd get (( 200A/75kVA )) or (( 400A/150kVA )) service at 208/120V 3ph for those 120kVA of heaters if they were the only load, depending of course on how often they ran.

Or I'd get 277V heaters and get (( 200A /150kVA )) of 480/277V (smallest standard option) to save on the electric bill. Or even 100A of 480/277V service (75kVA of xfmrs) if the heaters didn't run all at once.

 

[iwire]

2. 324.2kVA load calc meaning...all the equipment turned on at once?

Its is an NEC load calculation, nothing is tuned on it may not even exist yet.

See article 220 for the details.

We're still talking about a 150kVA xfmr?

Forget about the power companies transformer. That is their issue not yours.

3. That's fine- there's no way for me to know either.
These below are the only options for 208/120 3 phase service from the POCO here. 3 pole mounted or single pad mounted 3 phase setups. The (( XXX )) is what you request when you want new service. You have to fill out the load sheet for the POCO of course.

Again, nothing to do with the transformer is any of the home / business owners, contractors concern.

I will guess however if i had your 324kVA load example, and it was "continuous", I'd have 1000A service, and a 500kVA xfmr setup.

If the main breaker is 1,000 amps it is an NEC 1,000 amp service even if the power company installs a 75 KVA transformer.

If it was 324kVA for less than 30 minutes every once and a while and ½ of 324kVA (or 162kVA) 90% of the time, I'd have the 400A / 150kVA xfmr setup/service.

Options:
3-25kVA or 1-75kVA (( 200A ))
3-50 ..... or 1-150 (( 400A ))
3-75 or ...(no 225kVA pad mounted option- 3-75kVA pole only) (( 600A ))
3-100..... or 1-300 (( 800A ))
3-167..... or 1-500 (( 1000A ))

You really have no clue at all but you are persistent.

Well that homework leaves me in a bind.
I simply can't convert the 1000A to VA without...using the number for volts.
I also can't say that a 120V 1500w heater = load of 12.5 amps.

If I could, the answer would be 80 heaters.

80 heaters * 1500VA each = 120,000VA divided by 120V = 1000A.

I'd get (( 200A/75kVA )) or (( 400A/150kVA )) service at 208/120V 3ph for those 120kVA of heaters if they were the only load, depending of course on how often they ran.

Or I'd get 277V heaters and get (( 200A /150kVA )) of 480/277V (smallest standard option) to save on the electric bill. Or even 100A of 480/277V service (75kVA of xfmrs) if the heaters didn't run all at once.

How can you make it this complicated? :lol:

if I have a 1000 amp 120/208 three phase service, how many 120V, 1500 watt electric heaters can I use assuming they are balanced equally across the system and they are all on for 1 hour, then all off for 1 hour

1,000*208*1.73=359,840

359,840/1,500=239 and change so I am going with 237 to keep things balanced.

 

[PVfarmer]

1 Its is an NEC load calculation, nothing is tuned on it may not even exist yet.
See article 220 for the details.

2 Forget about the power companies transformer. That is their issue not yours.

3 Again, nothing to do with the transformer is any of the home / business owners, contractors concern.

4 If the main breaker is 1,000 amps it is an NEC 1,000 amp service even if the power company installs a 75 KVA transformer.

5 1,000*208*1.73=359,840
359,840/1,500=239 and change so I am going with 237 to keep things balanced.

1
Um...why would anyone would put a *potential* 359,840VA or 360kVA of loads on a 75kVA transformer? That would potentially be 460% overloaded. Unless of course no more than 25% of loads were being supplied at any one time.
The customer's NEC load calc is the exact thing the POCO uses to decide what size xfmrs they are going to provide, and they also say "this service we are providing is XXX amps". They aren't going to guess- they want to use the smallest xfmr which is practical.

2
Would you want to design a PV system and find out the POCO xfmr has to be changed because it's too small at the last minute? I wouldn't.

3
Again, sure it is. When the PV inverter manual says "total PV inverter output (kVA) must be <=90% of grid interconnection xfmr"- I'm not going to guess.

4
The NEC service rating doesn't tell you what size the service xfmr is, therefore you don't know your limit for how much PV you can add.

5
Sure- you could put 474 heaters on that "1000 amp" 208/120 service too. Or you could put 711.
But if you didn't now what size you service xfmr was, 711 heaters 1500w each would be getting into uncharted and dangerous territory- what if it's only a 75kVA xfmr? :blink:

 

[don_resqcapt19]

1
Um...why would anyone would put a *potential* 359,840VA or 360kVA of loads on a 75kVA transformer? That would potentially be 460% overloaded. Unless of course no more than 25% of loads were being supplied at any one time.
The customer's NEC load calc is the exact thing the POCO uses to decide what size xfmrs they are going to provide, and they also say "this service we are providing is XXX amps". They aren't going to guess- they want to use the smallest xfmr which is practical.

Exactly. They will use the smallest transformer that they can....often 1/2 or less than the value that you get when you do the Article 220 calculations. The pole in front of my house has a 50 kVA transformer on it. It supplies power to 7 houses, two with 200 amp services and the rest with 100 amp services.

2
Would you want to design a PV system and find out the POCO xfmr has to be changed because it's too small at the last minute? I wouldn't.

That is why, around here, you have to start with the power company, and not the size of the existing service.

3
Again, sure it is. When the PV inverter manual says "total PV inverter output (kVA) must be <=90% of grid interconnection xfmr"- I'm not going to guess.

Again a reason to start with the power company.

4
The NEC service rating doesn't tell you what size the service xfmr is, therefore you don't know your limit for how much PV you can add.

Same as #3.

5
Sure- you could put 474 heaters on that "1000 amp" 208/120 service too. Or you could put 711.
But if you didn't now what size you service xfmr was, 711 heaters 1500w each would be getting into uncharted and dangerous territory- what if it's only a 75kVA xfmr? :blink:

Also a power company issue. You cannot guess at what size transformer that the utility has used to supply a given service...you have to get that information from them.

 

[iwire]

1
Um...why would anyone would put a *potential* 359,840VA or 360kVA of loads on a 75kVA transformer? That would potentially be 460% overloaded. Unless of course no more than 25% of loads were being supplied at any one time.

Because as an electrician or a designer the power company transformer is not my problem.

If it turned out to be inadequate the power company would replace it.

Not my problem or the customers problem, the power company will size the transformer based on their own methods and what you have told them you are doing / need of their service. They will tell you if they can provide that.

The customer's NEC load calc is the exact thing the POCO uses to decide what size xfmrs they are going to provide,

No, the POCO does not use the NEC load calculations to size their transformers.

They may use it as a list of what you are doing but they will size it based on their own methods and history of what similar installations required.

and they also say "this service we are providing is XXX amps". They aren't going to guess- they want to use the smallest xfmr which is practical.

I have never had a power company tell me how many amps they are providing. Never.:happyno:

But you are right, they will use the smallest unit that their history shows will do the job.

2
Would you want to design a PV system and find out the POCO xfmr has to be changed because it's too small at the last minute? I wouldn't.

Assuming I provided the power company with the correct details of my installation that would be the power company's issue and it would not concern me in the least.

3
Again, sure it is. When the PV inverter manual says "total PV inverter output (kVA) must be <=90% of grid interconnection xfmr"- I'm not going to guess.

You will either guess or ask them but that is not an NEC requirement that is an item you deem necessary because of one inverters instruction.

4
The NEC service rating doesn't tell you what size the service xfmr is, therefore you don't know your limit for how much PV you can add.

Again, the size of the transformer is not my concern. I fill out the co-generation applications with the size of the PV system. The power company tells me if they can handle that or not. I do not care what transformer they use. That is why they have an engineering dept.

5
Sure- you could put 474 heaters on that "1000 amp" 208/120 service too. Or you could put 711.

No, you can only put on what the NEC allows which would be 237.

But if you didn't now what size you service xfmr was, 711 heaters 1500w each would be getting into uncharted and dangerous territory- what if it's only a 75kVA xfmr? :blink:

It might fail, the power would cease, the power company would replace it. Life goes on. Not my problem, or the homeowners.

 

[PVfarmer]

1
I have never had a power company tell me how many amps they are providing. Never.:happyno:
2
But you are right, they will use the smallest unit that their history shows will do the job.
3
You will either guess or ask them but that is not an NEC requirement that is an item you deem necessary because of one inverters instruction.
4
No, you can only put on what the NEC allows which would be 237.
5
It might fail, the power would cease, the power company would replace it. Life goes on. Not my problem, or the homeowners.

1
Of course, they have no idea how much they'll be *providing* at any given time. That's a completely different issue than what the kVA rating of the xfmr (that they own) is, and of course if your combined loads are too large (over 200% for 30 minutes for instance) and/or if your proposed amount of PV output is too large (probably over 100%, or maybe 90% like I said) of that same xfmr kVA rating....they will say NO!

2 me previously:
The customer's NEC load calc is the exact thing the POCO uses to decide what size xfmrs they are going to provide, No, the POCO does not use the NEC load calculations to size their transformers.and they also say "this service we are providing is XXX amps". They aren't going to guess- they want to use the smallest xfmr which is practical.

They won't let you run 48kVA of 3ph PV output into the grid through a 25kVA transformer. History would show "never" on that one. If the inverter manual says 90% or less, I don't see how exceeding it is practial.

3
No- I will spend a lot of time guessing while trying to get in touch with someone who can answer the simple question. *Before* designing a thing. Why assume that you can install two 7700w inverters on a house and find out no, you can't it's a 10kVA xfmr for that account?

4
I don't see how that works. I was wrong also- I meant 192 heaters. I think.
You said:
1,000*208*1.73=359,840 -- 359,840/1,500=239 and change so I am going with 237 to keep things balanced.
You made it into a 3000A service. You used 208V on one side of the -- and 120V on the other side!

239 heaters * 12.5A per heater = 2987.5A.

If the service conductors (3 hots) are *rated* 1000A, I'd de-rate them. So that's 800A.
800 * 3 = 2400A
2400A / 12.5A per heater = 192 heaters, 64 on each phase. And no 208V equipment at all while all heaters are running.

192 single phase heaters, load = 2400A at 120V.

Load also = 288kVA. (2400* 120V)
Load of 288kVA ALSO = 161.8kVA of demand when supplied by 208V 3 phase.

(161.8kVA * 1.73) /120V = 2332.8A at 120V.

 

[iwire]

1
Of course, they have no idea how much they'll be *providing* at any given time.

You had stated that the poco will tell me the size of the service in amps.

They will not. Now you agree.

That's a completely different issue than what the kVA rating of the xfmr (that they own) is, and of course if your combined loads are too large (over 200% for 30 minutes for instance) and/or if your proposed amount of PV output is too large (probably over 100%, or maybe 90% like I said) of that same xfmr kVA rating....they will say NO!

First off who is 'they' is 'they' the power company? If so you need to understand what each power company will and will not do is diffrent for each of the many power companies in the US

Secondly again the size the power company transformer has nothing to do with NEC service size or capacity.

If the transformer ends up being smaller than needed the power company will replace it.

They won't let you run 48kVA of 3ph PV output into the grid through a 25kVA transformer.

Again, you are making broad statements about what a POCO may or may not do.

History would show "never" on that one. If the inverter manual says 90% or less,

I highly doubt any power company cares what your inverter manual says. The power company will provide the transformer they determine is necessary. Unless you want to foot the bill to own it yourself. I would not, that would be stupid.

I don't see how exceeding it is practial.

Add that to the list of things you don't see.

3
No- I will spend a lot of time guessing while trying to get in touch with someone who can answer the simple question. *Before* designing a thing. Why assume that you can install two 7700w inverters on a house and find out no, you can't it's a 10kVA xfmr for that account?

Because I do not care in the least what size transfomer is there. It does not matter to me.

It is a simple thing to fill out the co-generation application and wait for approval before designing a thing. And I still wont need to know the transformer size. It is irrelevant.

4
I don't see how that works. I was wrong also- I meant 192 heaters. I think.
You said:
1,000*208*1.73=359,840 -- 359,840/1,500=239 and change so I am going with 237 to keep things balanced.
You made it into a 3000A service. You used 208V on one side of the -- and 120V on the other side!

239 heaters * 12.5A per heater = 2987.5A.

You really need to look into basic electrical theory. You are trying to add up amps in a three phase system. It cannot be done that way.

I posted the simple calculations that I used. Show us where you think I was wrong.

If the service conductors (3 hots) are *rated* 1000A, I'd de-rate them. So that's 800A.

Given the parameters of this question why would you derate these conductors.

Show us the code section requiring that.

800 * 3 = 2400A

No, that is absolutely wrong and for you to continue to keep trying to add amps at this point certainly seems like trolling.

Load also = 288kVA. (2400* 120V)
Load of 288kVA ALSO = 161.8kVA of demand when supplied by 208V 3 phase.

(161.8kVA * 1.73) /120V = 2332.8A at 120V.

All I can say to that is :?:?:?

 

[electrofelon]

They won't let you run 48kVA of 3ph PV output into the grid through a 25kVA transformer. History would show "never" on that one. If the inverter manual says 90% or less, I don't see how exceeding it is practial.

they might. If that xformer is serving 6 other services, they might figure a baseline minimum for what the other structures are drawing, or they might figure the inverters will be at max output for a relatively short period of time or only when its cooler out, thus the transformer can take more - I dont know, I dont size utility transformers.

No- I will spend a lot of time guessing while trying to get in touch with someone who can answer the simple question. *Before* designing a thing. Why assume that you can install two 7700w inverters on a house and find out no, you can't it's a 10kVA xfmr for that account?

I guess perhaps we are just arguing about the specific steps in designing a PV system. I think most designers will spec a system to what the customer wants first and go from there. IF the utility comes back and says we can only do X KVA, or we can do what you spec'd but it will cost XX more than if it was Y KVA, so be it Ill discuss with the customer and go form there......If you want to take the size of the utility transformer into account and make guesses as to what the implications are, that is your prerogative. If you have lots of experience with a utility and know their sizing rules well thats great. Generally however, I think most of us just have to submit the plans and see what happens.

I don't see how that works. I was wrong also- I meant 192 heaters. I think.
You said:
1,000*208*1.73=359,840 -- 359,840/1,500=239 and change so I am going with 237 to keep things balanced.
You made it into a 3000A service. You used 208V on one side of the -- and 120V on the other side!

239 heaters * 12.5A per heater = 2987.5A.

But that 3000 amps is divided evenly across the three phases for 1000 amps per phase. If it was all on one phase, yes you would need a 3000 amp service and two phases would have nothing on them.

If the service conductors (3 hots) are *rated* 1000A, I'd de-rate them. So that's 800A.

Why would you do that? The "80% rule" is for continuous loads (perhaps my example was bad in calling them heaters as we usually think of them as continuous loads, but I did say one hour on one hour off).

Do a little research on three phase delta and wye connections. Draw them out and play around with some numbers for while so you can figure out how the currents and voltages "combine" and "change" on the serving feeder or service. Homework: consider 3 1000 single phase inverters connected symmetrically to a 120/208 three phase system. In the first case, say they are connected line to neutral, and the second case say they are connected line to line. Play with the currents in each case and compare how the inverter output currents compare with the current on the feeder/service.

 

[electrofelon]

No- I will spend a lot of time guessing while trying to get in touch with someone who can answer the simple question. *Before* designing a thing. Why assume that you can install two 7700w inverters on a house and find out no, you can't it's a 10kVA xfmr for that account?

Ok so what exactly would you do in this situation? Are you saying you contact the POCO and ask them if you can put 15400W of DG on their 10KVA transformer? Have you had luck with that? Every utility I have worked with you have to submit an application complete with drawings and calculations before you can talk to anyone at all about anything.

 

[roger]

PvFarmer, with every post you are showing how far over your head this topic is and the thread has become stagnant. The members participating have been patient but you are taking them to their limits so, since I have just been an observer I am closing it.


Roger