Utility specs large transformers for motors

[electrofelon]

Have a job where we brought in a 200 amp 480 service for two motors, a 20 hp and a 30 hp. I recently got the design from the utility and they are using 3x100 KVA transformers. Is this typical? I Was expecting them to install 3 15's or 3 25's. It seems like they sized the bank to the LRC of the larger motor. Just seems a little over the top to me. Opinions?

 

[Ingenieur]

sqrt 3 x 200 A x 480 ~ 170 kva
I would expect 3 x 50 kva
15 or 25 too small imo, they must have sized on service size, not load
probably used what they had
perhaps for 480 the smallest they stock is 100's
perhaps prim v determined what they stock
who knows

 

[electrofelon]

I guess they want a stiff supply to keep flicker and VD within target during motor starting. This might mess up my fault current calcs. IT was a catch 22 when I set this up: the utility asked me a few questions and then said just said build it and get it inspected, and after I did that they sent it to engineering :dunce: I assumed a full 200 amps for transformer sizing as absolute worst case. Its going to be close. Hopefully the transformer impedance is less than what the book says, probably is.

 

[augie47]

At 200 amp the service drop or lateral will probably negate the issue but you might want to check your available fault current.

 

[electrofelon]

At 200 amp the service drop or lateral will probably negate the issue but you might want to check your available fault current.

yeah that is what I am hoping. I remember that I was right there at max with 3 75's and not counting the service drop and SEC impedance.

 

[petersonra]

there is no telling why pocos do what they do. I suspect in your case maybe that is just the smallest 480V xfmrs they stock. When you buy 1000s of them at a time it may well be more cost effective to just get a unit that can handle most of your installations rather than stocking a bunch of different sizes.

 

[kwired]

Have a job where we brought in a 200 amp 480 service for two motors, a 20 hp and a 30 hp. I recently got the design from the utility and they are using 3x100 KVA transformers. Is this typical? I Was expecting them to install 3 15's or 3 25's. It seems like they sized the bank to the LRC of the larger motor. Just seems a little over the top to me. Opinions?

3-100kVA units? total 300 kVA bank?

Around here we typically only see 3-50 kVA maximum for service to 100 HP irrigation wells, and those can be loaded 24/7 at times in mid summer. Anything larger then that usually is more then they want to hang on poles and they will provide padmount transformer.

 

[JFletcher]

Wouldn't you want a higher transformer impedance than what is listed for a lower fault current?

 

[kwired]

there is no telling why pocos do what they do. I suspect in your case maybe that is just the smallest 480V xfmrs they stock. When you buy 1000s of them at a time it may well be more cost effective to just get a unit that can handle most of your installations rather than stocking a bunch of different sizes.

Unless they just never run into 480 volt services that are low capacity I find it hard to believe they would only stock 100kVA units as their smallest unit. 100 kVA is enough for ~100 hp of load at .85+ power factor.

We see 10 and 15 kVA (not just 240 volt but 480 volt also) pots all over the place on limited load services around here.

 

[petersonra]

Unless they just never run into 480 volt services that are low capacity I find it hard to believe they would only stock 100kVA units as their smallest unit. 100 kVA is enough for ~100 hp of load at .85+ power factor.

We see 10 and 15 kVA (not just 240 volt but 480 volt also) pots all over the place on limited load services around here.

I don't know how it is now but for decades around here the cost of a 240 V delta service was dirt cheap but a 480 V delta service was insane. The difference for the same kVA worth of service was so much that it was common to see 240-480 V transformer banks fed from the service. Weirdly, ComEd sold and serviced a lot of those 240 V transformers. I have absolutely no idea why that was cost effective. My understanding is the electricity costs were even lower for 240 services as opposed to 480.

These days if you want three phase it is either 208/120 or 480/277, unless you can convince ComEd you have to have a 480 V delta, and I have been told it is not easy to do. I have been told they still allow some 240 V delta services with one side center grounded, but they limit them to pretty small services.

My brother told me today that the new smart meters will actually pay you to use electricity during certain times of the day. Literally. It gets billed to him at a negative rate. What the heck is smart about that.

 

[JFletcher]

I don't know how it is now but for decades around here the cost of a 240 V delta service was dirt cheap but a 480 V delta service was insane. The difference for the same kVA worth of service was so much that it was common to see 240-480 V transformer banks fed from the service. Weirdly, ComEd sold and serviced a lot of those 240 V transformers. I have absolutely no idea why that was cost effective. My understanding is the electricity costs were even lower for 240 services as opposed to 480.

These days if you want three phase it is either 208/120 or 480/277, unless you can convince ComEd you have to have a 480 V delta, and I have been told it is not easy to do. I have been told they still allow some 240 V delta services with one side center grounded, but they limit them to pretty small services.

My brother told me today that the new smart meters will actually pay you to use electricity during certain times of the day. Literally. It gets billed to him at a negative rate. What the heck is smart about that.

Very simple... If more people use power at off-peak times, the power company does not have to upgrade its infrastructure. Virginia power used to pay us an exorbitant amount to run our generators at peak times so they would not have to bring additional equipment online ( or go with rolling brownouts). years later, after they had to upgrade anyway, they tried to get out of that contract. Tried... and failed. they did make it so that it was practically a National Emergency if the generator was not on and loaded by 2 p.m.

 

[electrofelon]

Wouldn't you want a higher transformer impedance than what is listed for a lower fault current?

Yeah oops, that's what I meant

 

[JFletcher]

Yeah oops, that's what I meant

Just making sure everyone is reading your post, right?:lol:

The surest way to get an answer on the Internet is to post something incorrect.:lol:

 

[wbdvt]

I guess they want a stiff supply to keep flicker and VD within target during motor starting. This might mess up my fault current calcs. IT was a catch 22 when I set this up: the utility asked me a few questions and then said just said build it and get it inspected, and after I did that they sent it to engineering :dunce: I assumed a full 200 amps for transformer sizing as absolute worst case. Its going to be close. Hopefully the transformer impedance is less than what the book says, probably is.

Instead of using the infinite bus fault current for your sizing calcs, if you are close, you may want to use the utility actual available fault current.

 

[kwired]

I don't know how it is now but for decades around here the cost of a 240 V delta service was dirt cheap but a 480 V delta service was insane. The difference for the same kVA worth of service was so much that it was common to see 240-480 V transformer banks fed from the service. Weirdly, ComEd sold and serviced a lot of those 240 V transformers. I have absolutely no idea why that was cost effective. My understanding is the electricity costs were even lower for 240 services as opposed to 480.

These days if you want three phase it is either 208/120 or 480/277, unless you can convince ComEd you have to have a 480 V delta, and I have been told it is not easy to do. I have been told they still allow some 240 V delta services with one side center grounded, but they limit them to pretty small services.

My brother told me today that the new smart meters will actually pay you to use electricity during certain times of the day. Literally. It gets billed to him at a negative rate. What the heck is smart about that.

They have plenty of 240 volt transformers available - they are the same ones they are using on 120/240 single phase services. They apparently aren't interested in stocking 480 volt transformers, but will still supply them - with additional cost. To build a 480/277 system they are 277 volt secondary. Rural POCO's around here do supply 240/480 single phase, most of the time it is in remote areas where there is only single phase lines running and customer will need to supply phase conversion equipment if three phase is needed. Usually for limited KVA loads - like irrigation machines. If three phase primary is available they will build an open delta system for such loads - then customer at least doesn't need phase conversion equipment. About 75 kVA and up they prefer to build 480/277 systems but still may see occasions with open delta when all three phases aren't readily available in the area.

 

[electrofelon]

This is a very populated area, therefore I find it unlikely that they don't stock 25's 50's and 75's.


Wbdvt, yes not assuming infinite primary would help, but getting that information may be a challenge.


It's just kinda funny, usually we are like, "ooo did they make a mistake by installing such a small transformer? " this time it's the other way around. Never thought I'd see the day!

 

[electrofelon]

It's just kinda funny, usually we are like, "ooo did they make a mistake by installing such a small transformer? " this time it's the other way around. Never thought I'd see the day!

So We just got that service hooked up finally. This is the same one I complained about in some other thread where I was told it would be $750 but turned out to be 16K. Sooooo, looks like they installed 3 167's! Based on the loads, I would have originally thought they would install 3 50's, the utility plans ended up showing 3 100's, and I end up with 3 167's. Thats a full 600 amps if you havent done the math. Its a 200 amp service. Well that blew my AIC out of the water :rant: Lets hope there is some good primary impedance.


Anyway, I took some pics. There are some interesting things going on. First, note that they install a redundant neuter to each can; I dont recall ever seeing that before. Then the parallel phase connections to the dinky quadplex is interesting. Finally, the use of mechanical L lugs is also somewhat odd to me. Oh and it doesnt like they tried to save any money on those terminal blocks......

 

[zbang]

"All's well that ends."
--Wm Shakespeare

 

[wbdvt]

Wbdvt, yes not assuming infinite primary would help, but getting that information may be a challenge.

I don't see why. I am usually able to obtain the available primary fault current (3ph & SLG), associated X/R, fuse data, transformer data, riser cable (if padmount txf) from the utility without a challenge. Larger utilities may take a month to turn the request around but only once did I have an issue. I ended up filing a complaint with the state PUC and the utility eventually relented.

Thats a full 600 amps if you havent done the math. Its a 200 amp service. Well that blew my AIC out of the water :rant: Lets hope there is some good primary impedance.

I think you mean secondary impedance. Assuming a 2% impedance on the dist pots, this is a fault current of about 30kA at the txf sec terminals. I don't think you will get a significant reduction in fault current with the secondary quadraplex. If you are using the infinite bus, I think your equipment should be rated for greater than 30kA

 

[kwired]

So We just got that service hooked up finally. This is the same one I complained about in some other thread where I was told it would be $750 but turned out to be 16K. Sooooo, looks like they installed 3 167's! Based on the loads, I would have originally thought they would install 3 50's, the utility plans ended up showing 3 100's, and I end up with 3 167's. Thats a full 600 amps if you havent done the math. Its a 200 amp service. Well that blew my AIC out of the water :rant: Lets hope there is some good primary impedance.


Anyway, I took some pics. There are some interesting things going on. First, note that they install a redundant neuter to each can; I dont recall ever seeing that before. Then the parallel phase connections to the dinky quadplex is interesting. Finally, the use of mechanical L lugs is also somewhat odd to me. Oh and it doesnt like they tried to save any money on those terminal blocks......

Around her you don't get a new bank that large on a pole, anything over 150 KVA will be pad mount transformer.

The secondary lugs used are just fine - if the capacity of the transformers was actually being used.