480v 3ph to 240/120 singl. ph

[MstrCarpenter]

We've got a 70kva gen set we use for 480v 3ph loads. We also need to supply 240/120v loads at the same time, which we are accomplishing with a second gen set. Now we (I)need to move one gen set to another job so we picked up a transformer that will step down the 480v (+ or -) 3 ph delta to 208 3 ph. wye /120. We have our own 100a single ph dist. panel with the receptacles we use, so I'll only be using two of the three phases and the neutral. My logic says the supply voltage for my 240v-250v loads will be too low. I've been told that 208v 3 ph panels just get a double pole breaker and feed 240v loads all the time. Is this correct, or will I let the magic smoke out? Should I use a 3 ph panel to keep the load per phase closer to equal? Or does it really mater? I'm also assuming I won't be using the 480v neutral from the gen set to the transformer. It's been close to forty years since I've wired transformers like this. Apparently I didn't do enough to remember how. Any advice or knowledge is greatly appreciated.

[TDKPE]

If you have a lot of single-phase load, it would be prudent to use a 208/120V three-phase panel to keep the loads balanced.  Unless it's a large generator with small loads on it.  But the 208V could be a problem.  Magnetic controls may or may not pull in, and motors not rated for 208V are going to be operating under low voltage conditions, which means higher current for a give load. 

If motors are lightly loaded, it may not matter, but if they're working to their rated output, they'll be running hotter and life will be shortened.  Unless they're rated 120/208-230V, in which case they're designed for 208V at full output.

Kitchen appliances like ranges and cooktops and such are usually rated for both 208 and 240V, and simply output less heat on the lower voltage (Ohm's law), as well as water heaters, but there's no harm in that. Apartment buildings are often wired for 120/208V.

Your generator has a neutral?  480/277V output on the nameplate?

[MstrCarpenter]

If you have a lot of single-phase load, it would be prudent to use a 208/120V three-phase panel to keep the loads balanced.  Unless it's a large generator with small loads on it.  But the 208V could be a problem.  Magnetic controls may or may not pull in, and motors not rated for 208V are going to be operating under low voltage conditions, which means higher current for a give load. 

If motors are lightly loaded, it may not matter, but if they're working to their rated output, they'll be running hotter and life will be shortened.  Unless they're rated 120/208-230V, in which case they're designed for 208V at full output.

Kitchen appliances like ranges and cooktops and such are usually rated for both 208 and 240V, and simply output less heat on the lower voltage (Ohm's law), as well as water heaters, but there's no harm in that.  Apartment buildings are often wired for 120/208V.

Your generator has a neutral?  480/277V output on the nameplate?

Yes, it has a neutral and a selector for 480, or 250 3ph, or 240/120 outlets w/ breakers on the gen set itself. However, the transformer doesn't have a high voltage "HO" terminal, and the wiring diagram doesn't show it being used. I have H1, H2, H3, X1, X2, X3, and X0. and a grounding lug. Would the two neutrals be in sync with each other? Because I've connected this gen set to a 3ph panel, I know I don't have "wild leg", which if ircc means it's wye. Correct? Does it matter that the transformer shows Delta input, and/or is that why it doesn't have an "H0". Because I don't necessarily need it?
A couple of the loads have VFDs, which I can only assume also have P.C.s because they power 7hp 3ph motors with only two "hots" connected from the factory.

[MstrCarpenter]

The loads I'm most concerned with are on HEPA vacs. They each have three small motors for the three HEPA cartridge filters. On some units they're 120v, on others they're 240v. All the units can/do serve as a connection point for the grinders they serve. So yes, we also have a couple of 480v HEPA vacs for the biggest grinders.

[TDKPE]

Yes, it has a neutral and a selector for 480, or 250 3ph, or 240/120 outlets w/ breakers on the gen set itself. However, the transformer doesn't have a high voltage "HO" terminal, and the wiring diagram doesn't show it being used. I have H1, H2, H3, X1, X2, X3, and X0. and a grounding lug. Would the two neutrals be in sync with each other? Because I've connected this gen set to a 3ph panel, I know I don't have "wild leg", which if ircc means it's wye. Correct? Does it matter that the transformer shows Delta input, and/or is that why it doesn't have an "H0". Because I don't necessarily need it?
A couple of the loads have VFDs, which I can only assume also have P.C.s because they power 7hp 3ph motors with only two "hots" connected from the factory.

Normally, there would be no neutral into a step-down transformer of the type you need (and already have, apparently).  X0 is the neutral on the secondary, with none on the primary.  Sounds like a typical dry type transformer, with X1-X3 being the ends of the wye, and X0 the center.  

The generator's neutral may not even be connected when 480V is selected if the switch is manually connecting windings or winding halves (for delta output), but if it's wye connected, you'll still have 480V into the transformer.

[MstrCarpenter]

Thank-you! I feel much more confident now. As a side note I can bump up the voltage a little to bring the 208 up some while checking the 120 at the end of the extension cord under load so I don't go too high.