75KVA or 112.5KVA transformer?

[Weezyvelli410]

I'm on a project that has an existing 300 amp 120/208 3 phase service. I am replacing this service with a 400amp 277/480 3 phase service. I am keeping the existing electrical panels and will be feeding the original 300amp main breaker with a transformer powered by the new 480volt panel. From the nameplate ratings on transformers it looks like a 112.5Kva transformer is needed. But by doing some research and calculations as well as finding a nice article on Mike Holt & ECM magazine it looks like a 112.5Kva transformer has a secondary 400amp OPCD after the 125% rule (215.3 & 240.6A) and the breaker round up. So is a 75Kva transformer only need? I'm used to just reading a print and installing what is provided for services. This is my first job on my own without a set of drawings for sizing transformers. Any help in the right direction would be much appreciated. Thanks

 

[JoeStillman]

A 400A breaker is the maximum you can use on a 112.5 kVA transformer. Nothing says you can't use a 300A breaker. That would be the only way to be sure you have a big enough transformer without calculating the actual load. But if you have peak load information from, say, a year's-worth of electric bills, you may find the load on that 300A service was way below 75 kVA. 300A is the maximum breaker you can feed from a 75 kVA. Be careful about the wires though, the next-size-up rule of 240.4(B) is over-ruled by the transformer secondary tap rule of 240.21(C). You wires must have an ampacity of 300A if that is the breaker size.

 

[jim dungar]

There is nothing in the NEC that limits the secondary side breaker fed by a transformer.
However, the primary side protection is limited if the secondary breaker exceeds 125%, which is why many people use that number as a maximum.

Start with your loads and work backwards to your source. Size or choose your secondary conductors and panelboards. Then size your secondary breaker. Now pick your transformer with enough capacity to feed your estimated loads. Finally select your primary side protection and conductors.

You may find you can use an 'on hand' 75kVA transformer today with plans to upgrade it to a new 112.5kVA and primary breaker if your load grows. But your oversized panelboard and secondary conductors wouldn't need to be redone.

 

[petersonra]

Read carefully what the sizing requirements are for both primary and secondary over current protection on transformers.

I think somebody else said that the sizing information for overcurrent protection on transformers is maximum rating. You are free to make it less.

Sometimes you have to do some prophecy and decide what somebody might decide to add as far as loads go down the road. These days sometimes you have to make your decisions based on what parts are actually available.

You should probably start by doing a load calculation.

 

[infinity]

Since the existing service is 300 amps I would go with the 112.5 kva and give them the full 300 amps that they already have.

 

[wwhitney]

300A is the maximum breaker you can feed from a 75 kVA.

300A may be the maximum load you can serve (didn't check the math), but it's certainly not the maximum secondary breaker you can supply. For example, my understanding is that for a transformer, the 75 kVA rating is a continuous rating. So if you have a 300A continuous load, you could supply it from the 75 kVA transformer, but barring 100% rated breakers, you'd need to use a 400A secondary breaker.

Cheers, Wayne

 

[JoeStillman]

300A may be the maximum load you can serve (didn't check the math), but it's certainly not the maximum secondary breaker you can supply. For example, my understanding is that for a transformer, the 75 kVA rating is a continuous rating. So if you have a 300A continuous load, you could supply it from the 75 kVA transformer, but barring 100% rated breakers, you'd need to use a 400A secondary breaker.

Cheers, Wayne

Thanks Wayne, I should clarify my assumptions. The 75 kVA transformer rated amps are 90A primary, 208A secondary. I would want to feed the primary at more than 125% because of the potential of tripping on inrush. That puts us on the second line of table 450.3(B) and our max secondary is 208A x 1.25 = 260A. Then the next-size-up exception gets us to 300A per table 240.6(A).

You could feed any size breaker on the secondary if the primary breaker was not over 125% of the primary current, 90A x 1.25 or 112.8A and a 125A breaker. My SKM software puts the 75 kVA transformer inrush to the right of a typical 125A breaker curve, meaning it could trip on magnetizing.

 

[junkhound]

What is the size of the poco xfmr feeding you?

Here, poco (PSE/Potelco) will used a 25 kVA xfmr (twenty-five) to feed 5 (FIVE) 200 A services of Habitat project of units having all electric kitchen and laundry but gas home heating and water heater.