Re: changing 3wire 240v to 4wire 240v
You may connect a range or dryer using and EXISTING 3-prong cord-and-plug disconnect (in the past this was an acceptable method). In this case, the neutral in the appliance is bonded to the frame and a single grounded conductor is used for both ground and neutral. However, the grounded conductor in the cable must be the same size as the current-carrying conductors.
Originally Posted by TomF
In any NEW installation, you must use a 3-wire-plus-ground cable and a four prong receptacle. When installing the pigtail cord, you must ensure that the neutral and ground/frame are NOT bonded together.
In the case of a built-in range, stovetop, or oven unit, the principle is the same; ONLY if the existing wiring only has two hots and a ground may the neutral and the ground/frame be connected; otherwise (existing 3 wire plus ground or new installation) you must have separate ground and neutral running to the appliance.
In no case in current code is combined neutral and ground acceptable EXCEPT for existing wiring and service entrance conductors.
In the case of the hot tub, I suspect that the existing wiring either was not originally for a hot tub or was incorrectly installed. It should be supplied with a 3-wire-plus-ground 120/240V service and GFI protection.