our system is a 480V-Y/277 grounded system. An OEM specifically requires to use a 480-Y/277V with live neutral to power an electric heater. With I-neutral = 0, and I on high resistance ground = 0, the two systems can deliver the 277V requirement but OEM specifies to use live neutral instead and will not allow the 480V-Y/277 grounded system to be used. Any thoughts why?

The primary reason for ungrounded neutral is for eliminating unscheduled opening of overcurrent devices. I am unable to conceive a reason that would benefit the operation of an electric heater.

Can you provide make and model number of the unit in question?

If you are bound to this requirement, you may need to install a 480 to 480 V transformer to create a separately dereived system, that the neutral can be ungrounded, and isolate from your grounded system.

Assuming the heater elements are 480 volt rated, what does the unit need 277 volts for? Control? Fans?

Hi tkrussell, the heater is Eemax EX630, pdf of spec and install instructions available on EEmax Tankless Electric Water Heaters. I am not the designer of the heater but the activation controls is the close answer. I have considered a 480 to 480V transformer but this option is not acceptable because of known problems of harmonics generated of this option. I am inclined to use our 480Y-grounded system which is capable of feeding 3ph 277 volts. Eemax tech support does not recommend this and were specific on having a live neutral, not 480VY-grounded system.

Well that is a lousy website. I could not find your model number, but did find one 3 phase unit, and they state a ungrounded neutral.

I cannot imagine why. And most systems across the globe use a grounded neutral, so I would be looking for another unit that would match my typical system rather that spend money to buy their unit AND spend more money to change my system to meet their specs.

As far as using a transformer for your application, this is a pure resistive load, and harmonics's will not be an issue.

Have you spoken to their engineering dept, to learn why they insist on an ungrounded neutral? I would be very curious.