Transformer size

[electrofelon]

. I'm just baffled with the size of that service. Even if it is a building full of electric gerbil feeders, any rational math I got does not compute.

As i said, it is very plausible that those transformers are well sized. Dont know if you caught my example earlier in the thread of the 2000A service with a 360A demand. And remember thats max demand over 3 years, so ill bet the typically POCO would size to less than that, maybe even 2/3 to 3/4 of that.

 

[fastline]

As i said, it is very plausible that those transformers are well sized. Dont know if you caught my example earlier in the thread of the 2000A service with a 360A demand. And remember thats max demand over 3 years, so ill bet the typically POCO would size to less than that, maybe even 2/3 to 3/4 of that.

I think where I end up pissing people off is stating the obvious....like that is NOT a 1200A service, no matter how someone wants to pencil it. Just always baffles me what POCOs do.....bank on you bluffing. I've made them more than once. Warned them we need bigger, then they see the severe sag and get to fix it.

 

[electrofelon]

I think where I end up pissing people off is stating the obvious....like that is NOT a 1200A service, no matter how someone wants to pencil it. Just always baffles me what POCOs do.....bank on you bluffing. I've made them more than once. Warned them we need bigger, then they see the severe sag and get to fix it.

the power company doesn't care what your NEC service size is, they only care what you provided to them for load information.

There's a distillery in my hometown. I guess they added another still at some point and blew up a transformer. I was talking to their electrician and he was complaining how he was within his service size and how the Poco screwed up big time and weren't supplying him with his full service size. Clearly he doesn't know how things work. What were your loads you originally provided to the power company? Do you did you inform them you were adding another still?

 

[don_resqcapt19]

I think where I end up pissing people off is stating the obvious....like that is NOT a 1200A service, no matter how someone wants to pencil it. Just always baffles me what POCOs do.....bank on you bluffing. I've made them more than once. Warned them we need bigger, then they see the severe sag and get to fix it.

It is a 1200 amp service per the NEC. The size of the utility transformers has nothing to do with the size of a service installed under the rules in the NEC.

 

[tortuga]

It sure looks like a bank of three 25kVA transformers.

A true 1200 amp service would require three 150 kVA transformers...not even sure you could support three 150 kVAs on a pole. I think they are around 1800 pounds each.

The largest pole mounted transformer they do here is 100kVA.

It is a 1200 amp service per the NEC. The size of the utility transformers has nothing to do with the size of a service installed under the rules in the NEC.

They are probably looking at the largest motors and the continuous loads and figuring a 8-10% demand factor on the rest.

Something like this
1200A @ 208 = 432kVA

Take 80% of the NEC calc;
432 * .8 = 346kVA

Largest motor locked rotor amps and continuous loads =
50kVA

346-50=296kVA

8% demand factor on 296 kVA:
23.68 kVA
50kVA + 23.6 73.68 kVA
75kVA bank
With a 1/2 hour rating of 225kVA

 

[fastline]

It is a 1200 amp service per the NEC. The size of the utility transformers has nothing to do with the size of a service installed under the rules in the NEC.

I don't think you follow what I'm saying. Those 25s cannot serve 1200A, any way you want to look at it. Consider buying a 500hp car, but salesman tells you if you floor it, it will blow up..... Is it really 500hp?

 

[electrofelon]

I don't think you follow what I'm saying. Those 25s cannot serve 1200A, any way you want to look at it. C

I don't think anyone is saying they can. Again that's not the way it works. Transformers are sized to the loads you provide to the POCO.

 

[fastline]

I don't think anyone is saying they can. Again that's not the way it works. Transformers are sized to the loads you provide to the POCO.

Maybe this "1200A buildout" is for planned expansion or future proof, where the loads 'today' are much smaller so the POCO sized for the 'today loads'. We do that commonly.

 

[don_resqcapt19]

It sure looks like a bank of three 25kVA transformers.

The largest pole mounted transformer they do here is 100kVA.


They are probably looking at the largest motors and the continuous loads and figuring a 8-10% demand factor on the rest.

Something like this
1200A @ 208 = 432kVA

Take 80% of the NEC calc;
432 * .8 = 346kVA

Largest motor locked rotor amps and continuous loads =
50kVA

346-50=296kVA

8% demand factor on 296 kVA:
23.68 kVA
50kVA + 23.6 73.68 kVA
75kVA bank
With a 1/2 hour rating of 225kVA

I really doubt that this utility looks at any of that.

 

[don_resqcapt19]

Maybe this "1200A buildout" is for planned expansion or future proof, where the loads 'today' are much smaller so the POCO sized for the 'today loads'. We do that commonly.

It is a 100 year old school and there are no expansion plans.

 

[tortuga]

It would be interesting to see the load calc for the school.

 

[winnie]

I don't think you follow what I'm saying. Those 25s cannot serve 1200A, any way you want to look at it. Consider buying a 500hp car, but salesman tells you if you floor it, it will blow up..... Is it really 500hp?

You buy a 500hp sports car, and use it on a small island where if you floor it you go into the water. Is it really a 500hp sports car?

You are exactly correct, the transformers cannot supply 1200A. They have a continuous rating of 208A, and probably in short term overload the utility will allow them to supply 400-600A.

Very likely the 1200A was based on NEC load calculation rules and someone saying 'I'd like a bit of headroom', and applies only to the service equipment. The customer paid for that NEC sized hardware, and that hardware can handle 1200A. The customer got 1200A capable service equipment (maybe...remember the 80% continuous rating of most equipment). If you connect a 1200A load bank the service equipment will be just fine.

The utility probably estimated the power consumption by their own rules, and said that a 3x25kVA bank will handle it. If they get it wrong (because the customer has power quality issues or the transformers overload excessively and die) then the utility company will have to replace the transformers. If you connect a 1200A load bank to the service equipment, expect the utility equipment to fail. The customer almost certainly did not pay for the utility hardware.

Now if the customer has a contract with the utility company saying that they get 208/120V three phase power at 1200A, and the customer paid for the utility equipment, then I agree: the customer didn't get what they paid for. But dollars to donuts that this is not the agreement between customer and utility, and there is a high likelihood that the normal operation of the building will be properly served by the utility provided equipment.

 

[don_resqcapt19]

I don't think you follow what I'm saying. Those 25s cannot serve 1200A, any way you want to look at it. Consider buying a 500hp car, but salesman tells you if you floor it, it will blow up..... Is it really 500hp?

that has nothing to do with it being a 1200 amp service under the rules of the NEC...just because the utility did not install enough transformer to support a 1200 amp service has nothing to do with the NEC size of the service. When (if) the transformers fail, the utility will replace them.

 

[don_resqcapt19]

Very likely the 1200A was based on NEC load calculation rules and someone saying 'I'd like a bit of headroom', and applies only to the service equipment.

I have no idea of what the architect and his engineers did for the new design...I have little respect for the company that designed that project.

They should have started with the demand load from the old 600 amp single phase service and added the new AC load to that, to size the new service, but they probably just picked a value out of a hat

 

[tortuga]

Do do you know about how may square feed the school is?

 

[don_resqcapt19]

Do do you know about how may square feed the school is?

Just a guess, probably about 35,000 in the old 3 story part, and another 12,000 in a newer addition that has a gym and a few class rooms. The old part has very little insulation so the air conditioning load will be high. Between students and staff the people load will be about 400.