My apartment house has 120/208v Wye connected (4 wire plus ground) service from the utility. Apartments have 120/240 service. we can easily measure the 120v from phase to neutral - and I can comprehend the math that shows 208v between a phase and the imaginary point directly opposite the phase.
i can't see how this provides the 240v part of 120/240 to the apartments.
i am certain this has been addressed 100 times before, and would appreciate somebody helping this dummy for the 101'st time.
thanks!

Is there a room where they might have some 208 primary, 120/240 secondary transformers?

Are you sure it isn't 240/120 center tapped delta?

120/208 3 phase, 4 wire secondaries from POCO transformers is common for multi unit apt construction.

Did the OP actually see 240v? I know we always had this issue in computer rooms that were in light industrial area where they used Delta Vee. It was necessary to actually verify the voltage before you believed what the customer said. We have burned up some stuff when we assumed 208 wye.

Greg:

The OP lists his occupation as retired. I read his post twice and was going to reply with the usual comments.

Secondary actual voltage, load, lowest secondary voltage with 'heavy load'. Actual voltage at apt panel; etc.

I stayed silent on this

As you know the 240 volt items are AC, maybe cooling, maybe water heater. They tolerate <240 volts

I have not heard of any issues from the +/- 600 apt units that have been built in the last 2 years.

If he is really seeing 240v with a reliable meter, I think we can agree this is not normal 3p wye. It is more likely if measured that "240" is really more like 208. You are right that a lot of equipment has a wide enough "mouth" to take both but other stuff will need to be tapped to the available voltage.

He may be using a 'wiggy' type tester!

If the OP has 120/208V 3Ø feeding his building, then the individual units are 120/208V 1Ø, not 120/240V. Really quite common for larger multi-family buildings, electric ranges, clothes dryers are also rated for 120/208V at a lower KW rating, A/C equipment is 208/230V, with the only thing required is for the installer to change the tap on the control transformer. So there is really no problem.