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.

Tesla
I have no idea where you come up with those numbers but i am enforcing the code according to Ghosts numbers.

Tesla,

You are quite correct that the line-neutral current on the secondary of the transformer is 4X the line to line current on the corresponding leg of the primary delta.

However you are making the error that the 'line' current feeding the primary is equal to the current on each delta leg in the primary; this is not correct as the current on each supply leg must split through _two_ branches of the delta.

Given the example transformer, fully loaded (but ignoring losses).
75000/480/root(3)=
90.2A flows on each of the three supply conductors.

75000/480/3=
52.1A flows on each of the delta legs of the primary.

(75000/480/3) * 4=
208.3A flows on each of the wye legs of the secondary.

-Jon

I finally figure out 480 to 120 1 phase transformer the calculation offered by tesla might hold up. The transformer ratio is not 4 it is 2.3. Think of a 480/277 to 208/120 transformer the ratio is 2.3. I know this is a delta/wye configuration but the ratio is the same you take the primary voltage phase to phase and divide by the secondary line to line voltage. The only way to connect this transformer and get 120 volts is wye on the secondary. If you could connect the secondary at 120 volts Delta IE a 480 to 120 volt Delta / Delta then the secondary current is 75000/(120*root3)=360 amps but the current is 208 amps if it is a 120/208 transformer. The secondary OC is 208 amps or if protected on the primary at 100amps the seconday is 230 amps and a 225 amp panel is too small without secondary OC protection. If a 90 amp fuse holds then a secondary panel rated at 225 amps could be installed without secondary OC protection.

I know this is an old thread, but I see there is still some confusion as to the turns ratio of a delta-wye transformer (in this case, 480V x 208/120V).

According to mikesh, the turns ratio is 2.3:1 since 480V/208V = 2.3 (approximately).
While that seems like logical thinking, it is incorrect. You must remember that each ONE of the 3 coils of the delta-connected primary induces voltage onto only ONE of the 3 coils in the wye-connected secondary. (Look at a the schematic of a 3-phase transformer for reference.)

In a delta-connected winding, the line voltage is the same as the phase voltage (the voltage across a single coil). So, in this case, the voltage across each coil is 480V.

In a wye-connected winding, the line voltage is 1.73 times larger than the phase voltage. This is a result of two coils having voltages of 120V each, but being 120° out of phase from each other. In other words, 120V × 1.73 = 208V.

Since the voltage across each individual coil in the secondary is 120V (this is the phase voltage), you can clearly see that the turns ratio is 480V ÷ 120V = 4:1

What mikesh has calculated is the line voltage ratio (VL-Pri : VL-Sec). This is a valid calculation for the turns ratio of delta-delta and wye-wye transformers only (not wye-delta or delta-wye).

Just saw this thread, and have a quick comment;

Per the 75 KVA rating...

On the Primary (Delta) Side:

At Full Load, each of the (3) Windings will draw 52.08333- Amps at 480V, with a resulting 25 KVA per Winding.

Input Phase Terminations at Full Load will carry 90.208333- Amps, which is 52.08333- Amps x 1.732.

With a Balanced, Full Load output on the Secondary side, all (3) Input Phases (Phase "A", Phase "B" and Phase "C") will carry 90.208333- Amps.

Multiplying 90.2 Amps by the Three-Phase Voltage of 831 (480v x 1.732 = 831.36) equates in a total Apparent Power flowing in the (3) Primary Windings of:

74,956.2 VA
or simply 75.0 KVA

On the Secondary (Wye) Side:

At Full Load, each of the (3) Windings will carry 208 Amps.
The Voltage Ratio is 4:1, so each Secondary Winding is 120V.

With 208 Amps flowing per 120 V Winding, each Secondary Winding has an Apparent Power rating of 24,960 VA, or simply 25 KVA.

Connecting the (3) Windings together in a Star formation results in a Winding to Winding Voltage of 208 V (120 V x 1.732 = 207.84 V).

Multiplying 208 Amps by the Three-Phase Voltage of 360 (208 V x 1.732 = 360.256 V) equates in a total Apparent Power flowing in the (3) Secondary Windings of:

74,933.248 VA
or simply 75.0 KVA.

--Scott (EE)