That's what I would do. The motor should never need the neutral, so their saying it needs it should be a red flag. It must be a controller or something that needs it, but be very careful with that because if this is for a foreign piece of equipment, they might be saying they need the neutral because for them, line voltage is 415Y240V, so to get 240V they do need the neutral and if they are using it to run controls, they would be 240V single phase controls.
Never mind, after typing a rather longwinded response I realized that I was simply restating the obvious.
If you cannot change anything inside the panel, then you could still provide a zig-zag wound grounding transformer or a delta-wye isolation transformer external to the device to supply a synthetic neutral to connect to their unchanged cabinet.
Or option 2, hang an outdoor rated potted transformer on the outside of the box and feed it from the 240V in the box. It would need fusing though, and if they don't want you putting a CPT in their box, they probably don't want the fusing either. That makes putting a CPT is a NEMA 3R box yourself a better option actually, you can get the type that have the primary and secondary fuse blocks already mounted to the transformer. just slap it on the side of their box and use close nipples to connect it. Probably cheaper and/or easier than trying to run a neutral conductor in an existing conduit 400'.
He said that the 4th wire was ground.
There is a neutral at the transformer and/or main panel, just not at this panel 400 feet away. He is trying to avoid having to pull a neutral 400 feet through an occupied conduit. I don't blame him.
That's what I was thinking, because he would be able to get away with using a tiny little zig-zag if he's only powering controls. But it looks like it'd be a violation of 450.5 since his system is already grounded upstream.
It would be a violation if you actually grounded the center point. But you would be creating a derived neutral and that neutral will not be bonded to ground, since the hot leads are already ground referenced.
