I have searched the code book for a rule that requires that all 240 volt single phase circuits to be fed from the two 120 volt-to-ground phases without success. Can anyone steer me in the proper direction?

 

It doesn't exist.
You can use any two legs of a 3-phase service to get a single phase circuit. Even with a 120/240 3-phase delta service, that has a high leg to ground, you can use either of the three legs. They are all 240v line to line.

 

That is what I expected and what I have always done. The problem is that the utility sizes their transformer bank assuming all single phase load will be on the lighting transformer.

 

Why is this? In that case the only draw on the high leg would be from three-phase loads.

 

I smell 'design issue' rather than 'code requirement'

I've never worked with these systems, but my understanding is that it is pretty common for the utility to size the third leg for 3 phase loads _only_, rather than assume that the single phase loads will be balanced across the three phases.

Perhaps this is historic, with most of the single phase loads expected to be 120V line to neutral loads.

In any case, the transformers supplying the high leg are generally smaller than the center tapped transformer, and often the third leg will be supplied by a single small transformer in an open delta configuration. Totally fine for a few motor loads, where the motors themselves could generate the third leg.

I believe that modern practise would be to install 120/208 wye service rather than 120/240 delta with a center tap.

-Jon