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Discussion Starter · #1 ·
Hello,
I am in need of some guidance. I have to get power to two machines that have a nameplate that is, 220v, 43KVA, 3 phase, 125a.

I have 480v in the building so the manufacture recommended transformers I bought are, 75 KVA transformer, 480v to 240v, I have changed the taps to get about 225v out of the transformer, the lowest I can get out of the taps.

These machines are supposed to be power from a 200 amp panel, along with about 55amp worth of other equipment.

Now I know the math doesnt add up but the engineer says that at 480v the amperage for the two machines is only 63 amps each. Those two added with the 55 amps should only be 180 amps.

Am I missing something? Dont we need to account for the 125a at the breaker for the machines?
 

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animal lover /rat bastard
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225v x 125a x 1.73 = 48kva

sounds ok as far as that part goes.

you haven't really provided a lot of info about the equipment, but you have to follow nec rules regarding the transformers (450) and the equipment (conductors protected at their ampacity/tap rule, and equipment rules for ratings/disconnects as they may apply)
 

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Discussion Starter · #3 ·
Yes, i have two transformers, one for each machine. the transformers will provide 220v to the machines, both are CNC machines
 

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These sound like ancient CNC machines if they are running at 220VAC.

Are you absolutely certain that they can't be wired to accept 480 3-phase?

Are these 2nd hand European imports?

BTW, the typical dry-type NEMA transformer has SIX taps. Playing with them, 2.5% bumps, is usually enough to get you down (2.5% x 6 ) 15% from nominal. I'll leave it at that until you come back with the name-plate tap readings.

Where did you get those transformers? 480 to 208Y120 are ten-a-penny. ( especially second-hand ) These days, 480 3-phase to 240 3-phase is actually a 'rarity.'

Cobbling up the owner's second-hand buys into a workable scheme is usually fruitless. These fellas usually buy without knowing the first thing about real-world electrical engineering.
 

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Discussion Starter · #5 ·
The CNC's are actually brand new, they are Doosan's built in Korea. The transformers have 7 taps and I am on the 7th, they are delta to delta transformers.
what I need to know is how to size the breaker feeding each transformer
 

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animal lover /rat bastard
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read 450. it tells you everything you need to know.
 
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Just be aware if you drop the voltage from 240 to 220 volts just remember the amparge will go up some so be aware of it.

but for the NEC code please read art 450 also check the art 440 and 430 if that do cover that part if need to be.
 

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Chief Flunky
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Equipment is rated in utilization voltages, transformers in supply. So 480 V supply, 460 V motor. Those are designed for 230 V supply, 220 V load. The extra 10% voltage is for voltage drop.

Sent from my SM-T350 using Tapatalk
 
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