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Help with back feeding 440/460 V transformer


Aug 11, 2018

I bought an older Oliver table saw that is 440V only and also have a Doall saw that is 440v. My shop is wired 220V delta 3 phase.
Found this 25KVA transformer on a local sale and wired it from panel into bottom of breaker with SO cord shown below.
When I first powered it from breaker it "groaned" and in research here I found that I could not ground transformer to panel. Hence the ground wire on SO removed from case ground lug.

Using the 440V taps, I am reading about 445V across 2 legs uniformly. With ground wire to saw tied to ground in transformer case, I am reading about 60V from saw frame to building ground.

Transformer is mounted on a wheeled dolly, ungrounded.

I have since removed the ground wire from transformer case to saw.

My question is:
Can I run ground from panel (isolated) through transformer to the saw grounding to contactor?
The transformer would be isolated from the ground.


Jun 16, 2001
St Louis
You should be able to run the equipment grounding conductor through.

But what is it connecting to on secondary? You are reversing a 480:240 transformer that is delta on the 480, correct? So you have no ground on the 480 side.

Since it is not "part of" the machine, your best bet is likely to "corner ground" the 480V, so that a fault from a 480 wire to chassis on the machine will open the breaker and alert you to the fault. Otherwise, one fault would not do anything until a second fault occurred.

If you have it powering both machines, you have created a "branch circuit", and you do need to re-create a ground reference on the secondary, which a corner ground does.