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Al Baker(pumpman) · Sep 30, 2021

If I back feed a 3 phase transformer, how do I know if it is heavy enough to handle the amps needed?? I know how to figure out fuse sizes to run a certain size motor, but how about amp sizes of transformer?? One size cant fit all? Are they rated at the step down wiring, or are listed either way? Sorry, I feel like a kindergartener here. Al

Steve@Advance · Sep 30, 2021

I would convert the motor load to KVA, then size the transformer accordingly, plus at least 125%.

So, a 5 HP motor rounds out to about 4.5 KVA x 1.25 = 5.625

Round up to a 6 KVA, or a 9 KVA for extra cushion.

At least that's my take on it, best seek better qualified input before buying anything!

MarkB_MI · Oct 1, 2021

Steve is right about the kVA rating; the volt-amps will be the same for primary and secondary windings.

An important point is that you need to know how each component of a three-phase system is configured. Is it wye, where the voltage is measured phase to neutral or is it delta, where the voltage is measured from phase to phase? If your shop power is 'high-leg', then it's delta. It's possible your motor can be configured wye or delta, which would change its voltage rating.

John T · Oct 1, 2021

Al yours are good questions, better to ask then fry something. The Transformer obviously needs to be rated higher power then the power drawn by the motor. Due to Power Factor VA instead of Watts is often labeled but regardless due to heat losses and inefficiency so long as the transformer is rated say at least 15% to 20% higher then your motor at full load, you should be fine. Your transformer is likely an isolation transformer NOT an Auto Transformer but should work as you have indicated as step down or step up.

Best I recall but its been yearssssssssss lol: You first compute the Maximum Continuous Current,,,,,,,,Size the conductors to have a Minimum ampacity of 125% of the MCC, size the overcurrent protection to protect the conductors, provide thermal overload protection for the motor. Back when and where I practiced a disconnect means was required within sight and within 50 feet and I often specified a Combination Starter which did BOTH.

I guess your motor isnt dual voltage (wiring diagram and number of wires under the cover is where to look) because it would be so much simpler grrrrrrrr

John T

More 3 phase

Al Baker(pumpman)

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Steve@Advance

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MarkB_MI

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John T

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