277/480 Y 3p step down transformer120/208

[Cattleman22]

I am installing EV chargers which require 208 or 240v. And with amount of them I am installing will require 200 amps
My feed is a 277/480 Y 3p panel with a 400 amp feeder that panel is basically unused. . I'll be honest i do electrical work everyday but rarely need a step down xfer. So my question is what size 480 breaker do I need to feed the primary side of the transformer in order to get 200 amp 208 on the secondary. And the size of the xfer itself is needed.
I was thinking possibly a 75kva xfer with a 100 amp primary 480v feed. Like I said not very often I install large transformer. Thanks all for help.

 

[GoldDigger]

Are you going to feed the EV chargers in a balanced arrangement from three phase or use a single phase transformer and feed all of the chargers from one phase?

The ratio of input current to output current (when you ignore magnetizing current in the primary) is simply the inverse of the turn ratio.
The voltage ratio is simply the turn ratio.

That or "power out equals power in" tells you how input relates to output.

But your complication is that you appear to be wanting to see 200A at 208V (the line to line current) while your breaker will be seeing the line current instead. And are you talking about 200A per phase (line to line current) or per leg (line current) or a total of 200A when you simply add up the input current to each of the single-phase chargers?

Can you instead tell us the exact wiring arrangement you plan to use for the EV chargers and how you will balance them over the three phase secondary side?

 

[Jraef]

If it's 200A maximum load at 208V 3 phase, that's a 75kVA transformer (with near zero head room), but the only alternative would be a 112.5kVA and run it at <70% loading, which has some efficiency issues. Since it's unlikely that every charger will be at 100% load 100% of the time, I'd stick with 75kVA. Primary (480V side) current will be 90A. Use Article 450 to determine circuit sizes, OCPD etc. for you specifics of the installation.

If, as GD said, each EV charger requires 208V single phase, it's a little more complex than that.

 

[dionysius]

I am installing EV chargers which require 208 or 240v. And with amount of them I am installing will require 200 amps
My feed is a 277/480 Y 3p panel with a 400 amp feeder that panel is basically unused. . I'll be honest i do electrical work everyday but rarely need a step down xfer. So my question is what size 480 breaker do I need to feed the primary side of the transformer in order to get 200 amp 208 on the secondary. And the size of the xfer itself is needed.
I was thinking possibly a 75kva xfer with a 100 amp primary 480v feed. Like I said not very often I install large transformer. Thanks all for help.

Cattleman.......Sir, please sketch on a sheet of paper a schematic of what you are describing. Then with your cell phone camera take a pic and post it on here.

 

[Electric-Light]

When you're talking about electric vehicle charging level power, three phase chargers are better if you have the choice between single or three phase units. You should opt for single phase unit only when you don't have three phase service. To get comparably low charging ripple current, single phase chargers require capacitors to fullly handle the load through each zero crossing.

Do you have the option to get 480v chargers?

 

[Besoeker]

When you're talking about electric vehicle charging level power, three phase chargers are better if you have the choice between single or three phase units. You should opt for single phase unit only when you don't have three phase service. To get comparably low charging ripple current, single phase chargers require capacitors to fullly handle the load through each zero crossing.

Do you have the option to get 480v chargers?

I agree.Three-phase has advantages. Lower ripple and lower supply harmonics. But the rectified 208Vac will be higher than with single phase by almost 100V.