The transformer receptacle's OCPD will be after the receptacle, meaning there is not OCPD between the receptacle and transformer. I have sized the fuse and include the size (25A) in the user manual and electridal schematics. The plug's chord is sized for the 25A fuse.
Your first sentence doesn't make sense. You say the ocpd is after the receptacle but yet say it is not between the receptacle and transformer...???
Are you using a primary only protection schema for the transformer? Under the NEC, you can only go to 125% of the rated primary current (19.5A) and round up to next standard size (20A). Using primary and secondary protection schema, you can size the primary ocpd up to 250% (39A), but not permitted to round up to next standard size (35A max standard rating).
You can size primary protection at 25A, but because it's greater than 20A, you will have to implement secondary protection also, at not greater than 125% of rated secondary current (31.25A, next greater standard size of 35A permitted). Refer to 450.3(B).
The equipment normally wires directly to a service disconnect. The transformer is a rarity, as well as the switch rated plug. I believe the plug should be able to disconnect under load if needed, but it will not be it s normal operation. The meltric has a pushbutton pawl on the receptacle that will break the circuit before the plug can be removed. The plug then has to be rotated before it can be removed.
Using a switch rated plug/receptacle combo is, IMO, purely a design decision. If plugged in directly to an immediately adjacent disconnect (service or otherwise), it would simply be a matter of opening the disconnect before disengaging a standard duty plug.
In my honest evaluation, the 60A single phase plug/receptacle is the better choice than the three phase derated for single phase use.
I agree the single phase plug is the more appropriate choice... but if you are only using 25A, or even 30A xfmr primary protection ocpd, all that is necessary is a 30A rated plug/receptacle combo. The only doubt on my end is whether or not the 25A (or 30A) fuses will survive under all nominal operating conditions. I'm not saying they won't... I simply don't know if they will handle all non-fault surges without blowing. I know you said there is a delayed start... are the motor controls are configured so neither motor starts at the same time? If yes, then I'd feel pretty comfortable fusing at 30A and only slightly more skeptical at 25A.
