If I have a 600A 480v service and I step in down to 120/208, will I have an increase of available amperes at the lower voltage? Or will the amperage remain the same no matter how I change the voltage? Thanks

The amperage is dependent on the KVA size of the transformer you install. If you are trying to get more than the 600 Amps at 480 volts by going to 120/208, this is not practical.

There are many variables involved, can you explain further what you are trying to achieve?

The area of concern has a 3-phase, 480v, 600a unused switch from the main service. The customer wants 120/208v 3-phase power to his suite but needs 800 amps. Sounds like 600a max is what he's going to get unless an additional feed is brought in. Is that right? Thanks.

OK quick lesson on capacity to help you see the relationship:

The benchmark for power is watts, or, with AC power,and more accurately, volt-amps (VA).

600 Amps at 480 3 phase=498 KVA of power.

600 Amps at 120/208 3 phase = 215 KVA of power.

My point is not to compare amps at one voltage to another voltage. All depends on the load in power, and the voltage available. 600 amps of 480 volt power is much greater than 600 amps of 208 volt power.

Not sure if this is clear enough to help you undestand, but hard to teach electrical theory is a few sentences.

So, to get 800 amps of 120/208 volt power from a 480 volt 3 phase service you need to install a 300 KVA transformer, which will draw a max of 360 amps from the 480 volt service.

This is practical, and done when necessary. If purchased new, a transformer can cost in the range of $3000.00.

Here is a page from EBAY,
for a cost of $1500.00.

Along with the transformer, also is needed a 350 or 400 Amp 480 volt distribution circuit breaker, assuming you have a circuit breaker distribution switchboard, the high and low voltage feeder cables, and an 800 Amp 120/208 volt distribution switchboard.

Depending on many variables, the costs for this can be $15-$20K. This should also be designed by an electrical engineer or a contractor capable of design/build projects.

Hope this helps. Get back if you have any other questions.

Can you show me the formula that you used realitive to the voltages in converitng the KVA to amps?

The Ohm's Law is my aviatar, but may be to small to read.

The basic power formula is P=EI.

For 3 phase power the formula is:

KVA= Volts x 1.73 x amps / 1000

So hypothetically instead of 600amps...

If I had a 3PH 480v, 400a service or 332 kva, could I convert that into 3PH,800a at 208 with a 300 kva step-down transformer? Would this satisfy the NEC? Is 32 kva enough "cusion" to do this or would the lower voltage need to be fused at max 80 percent of my 332 KVA supply thus scaling back my 208v main fuse to less than 800 amps?

You are on the right track. Yes the 300 KVA xfer can be protected by a 400 Amp breaker/fuse.

The "cushion" you refer to is irrelevant. Again you are on the right track, "cushion:" is done by derating, which is done by sizing equipment to handle the calculated connected load.

Any transformer is protected by a max breaker sized 125% of the Amps/KVA rating. This is why the 300 KVA xfer can be protected by either a 350 or 400 amp breaker/fuse on the primary side.

The 800 amp service needs to be derated down to 640 amps of connected load.

So, to make things more confusing, if the tenant actually needs a full total of 800 amps, then the system feeding this connected load of 800 amps must be sized at 1000 amps. If this the case, then the transformer needs to be a 500 KVA transformer.

Keep in mind, the tenant still has the 480 volt available, and his space should be designed to take advantage of this voltage as much as possible.

Lighting, HVAC, and any other load that can be at higher voltage than 208, should be used.

What type of business is this the tenant will have?

The tenant just wants an 800 amp 208v hand off to his space. His actual load will be in the 600 range. So, it sounds as though I can squeeze this out of the 400a, 480v existing service via a 300 KVA step-down transformer. So, technically there won't be much (if anything) left over to derive from the HV source for lighting etc.

Sounds good, 600 Amp connected load connected to a 800 Amp 120/208 Volt service.

600 Amps of 120/208 Volt power will draw 260 amps from the 480 volt service. The 600 Amp 480 volt service must not be loaded more than 80%, or 480 Amps, minus the 260 amps for the tenant, leaving 220 amps of capacity on the 480 Volt 600 Amp service.

Still a great deal of capacity of 480 volt power that should be used as much as possible.

My point was that the tenant may be able to use a smaller 120/208 volt service if some of his equipment can use 480 Volt.

Thanks for the great communication!