ECN Forum Forums General Topics of the Trade General Discussion Area 75kva transformer

Transformer is 75kva 3 phase 120-208. The panel does not have a main breaker just (4) 100 amp breakers. I'm trying to figure out if setting a 200 amp single phase subpanel is going to be to much for the transformer to handle. I am calling the local power company to see what the usage in Kwh's. What is the best way to determine if the transfomer can handle the additional load? How do I turn Kwh's into useful information? Thanks!

More information is needed.

kWH is the wrong physical quantity.

75 kVA 3 phase normally feeds a 250A 3 phase panel with a Main breaker of 225A.

I can't figure out what you've got.

First is this the service panel to the building?

Is the transformer owned by the power company or is it customer owned?

400 amps of secondary protection is already to much for a 75 kva transformer @ 208 volts.

A 75 kva transformer has a rated output at 208 volts of 208 amps. (Yes thats 208 amps at 208 volts)

How where you planing on getting power to this new panel?

You can not 'tap' the existing secondaries, you have to go right back to the transformer terminals using double lugs.

Also one last thing.....please do not set a 200 amp single phase panel off a 3 phase transformer....use a three phase panel.

It is important to keep the load on the transformer balanced and that will be all but imposable with a 200 amp single phase panel connected to a 3 phase transformer with an 208 amp rating.

iwire,
You are right about not using a single phase panel. A 3 phase panel will be used. Here is what I see when I go there. A transfomer that has the meter and a lock that has the power company name on it. A main panel that has 3 (not 4, I looked back at my notes) 100 amp breakers on it. The owners showed me 3 sub-panels inside the building. What useful info. can I get from the POCO? According to UGLY's a 3 phase 75kva transformer is capable of producing 180 amps per leg. Even though each leg has to be protected by overload protection at that amount, does that mean the transformer is capable of producing 540 amps overall?

A 75 KVA 208 V transformer is capable of supplying 208 amps _per leg_.

A 100A breaker is 100 amps _per leg_. Doesn't matter if it is 1, 2, or 3 poles; each pole can take 100A.

-Jon

No, you cannot take the amps per leg and added them to get a single total amps. Each leg is limited to its maximum amps, you cannot overload one leg even if a different leg is underloaded.

A 75kVA delta/wye transformer is capable of ( 90.2 x 4 = 360.8 line to neutral amperes at 120 volts per each phase.

The 90.2 amps is the line to line draw of a delta connected 480 volt source. The turns ratio is 4:1 going from delta to wye. ( 480/120 )

Typical practice is to only partially load these transformers, ie max the draw at 225A line to neutral -- each phase.

Even if the delta voltage were higher... the load side calculations would remain the same for a 75kVA transformer.

Your latest info indicates that you could double tap this transformer. Do some load calculations. If the transformer belongs to the utility present them and the AHJ these load calculations in your submission for increased service.

I sure hope that the three 100 amp breakers have a common handle tie; in which case it's a single three pole breaker ... not 3 breakers.

This must be the first time through for you. Hook up with someone more experienced as soon as possible. Farm it out.

A 75kVA transformer is designed to change voltages, but the total power involved is the same on both sides (ignoring losses).

On the primary side; 75,000VA/(480V)(1.73)=90.2Amps
On the secondary side: 75,000VA/(208)(1.73)=208.3Amps

And at 120 Volts 360.8 amperes LINE to NEUTRAL.

You guys are using line to line; not line to neutral.

The vast bulk of the loads in the panel will be at 120V. These are line to neutral loads.

When you inspect a dry type transformer you will note that the high side is delta connected -- line to line -- 480 volts. The load side windings are connected wye with a turns ratio of 4:1.

The line to line of 208A at 208V would be used if you had a delta/wye transformer running 208 delta to 208/120 wye. They are made typically as isolation transformers when the Service is 208/120 wye but isolation is desired. Such a transformer would still produce 360.8 line to neutral amps if it was 75kVA.

Fine...you go argue with the folks at GE, Seimens, Square D, Eaton, Niagara and everybody else that makes transformers and tell them that all of their tables are totally wrong.

I give up.