Bad to use only 2 legs of 208 wye 3 phase?

[Moosifer]

I hope this makes sense.
I have a customer with a conex that has a 100a panel and needs a 100a panel but will pull less when the AC or heat isn't cranking hard.
They want to be able to feed it with either 208 wye three phase or 240v single phase.
I am wondering if wiring it up with a 208 to 240v transfer switch that will take 208 or 240 and feed a single phase panel will be a bad idea with that kind of load imbalance when it is only connected to 2 legs of three phase. This is an industrial type portable unit so a 50a load or whatever normal load it would pull on only two legs should not be a giant imbalance percentage wise, but I don't know how many different places they may plug it in to three phase.
Not sure if I am being paranoid but I'd rather get some educated opinions.

 

[ron]

You can feed a 3 phase transformer with single phase, but I'm not sure you will get the voltage on the secondary that you need. ranging from 208V or 240V is a bigger spread than typical transformer taps can accomodate.

Consider the loading. A 3 phase transformer is essentially (3) single phase transformers, so if it is 75kVA 3 phase, using only 2 windings of the 3 would be 50kVA

 

[Moosifer]

You can feed a 3 phase transformer with single phase, but I'm not sure you will get the voltage on the secondary that you need. ranging from 208V or 240V is a bigger spread than typical transformer taps can accomodate.

Consider the loading. A 3 phase transformer is essentially (3) single phase transformers, so if it is 75kVA 3 phase, using only 2 windings of the 3 would be 50kVA

I think I did not explain that very well. Internal components will all be 120v other than the mini splits. So they could use 120v to neutral off 2 legs and 208 off the same 2 legs and then flip the transfer switch to 240v and send 240v to the mini splits. My concern is the 3rd leg would not be used when on 3 phase and cause a load imbalance.

 

[infinity]

My concern is the 3rd leg would not be used when on 3 phase and cause a load imbalance.

What other loads are on this panel?

 

[roger]

Using 208 single phase is common practice in a lot of multi family applications, I would bet there are many cases where two phases are heavily loaded compared to the third

 

[Lioneye]

No Prob, if done right. I would full size the neutral tho

 

[kingpb]

it's called a 120/208Y, single phase 3wire system with open Y.