This is possible within Code -- you just need to get the right stuff
First off -- what you want to do is quite possible to do within Code. Phase converters treat the 240V supply coming in as a single phase, ignoring the neutral. However, the L1-L2 (in their terms) coil on a phase converter is effectively in parallel with the center-tapped secondary on a North American single phase service transformer. This means that electrically, this configuration is equivalent to one, center-tapped coil, which makes the overall situation come out looking exactly like a high-leg delta service. While the power company no longer gives these out, they're still legal by Code, although a few unique requirements kick in.
First off, you'll need a three-phase, four-wire, 120/240VAC panelboard fitted with main lugs and a separate ground bar, wired as follows:
- Feeder ground + GEC (if present) to the ground bar
- Feeder neutral to the neutral bar
- Feeder hot 1 to the 'A' phase busbar
- Feeder hot 2 to the 'C' phase busbar
Second, if this workshop is a separate structure, you'll need a feeder disconnecting means. Normally, a main breaker in the panelboard would be used, but three phase panelboards are designed for three pole main breakers, most of which are only listed and labeled for use in three phase applications. There are three pole breakers (such as the Eaton type CHH) that can be used for single phase service, but they are rarely seen as main breakers in three phase panelboards.
Instead, you can use a two pole, 120/240VAC breaker or disconnecting switch in a separate enclosure if a disconnecting means is necessary. This also allows for more flexibility with placement of the panelboard within the workshop.
You can now wire up the rotary phase converter. L1 and L2 on the phase converter connect to the A and C phases on the three pole breaker for the converter circuit, while L3 on the phase converter connects to the B phase on said three pole breaker. The breaker and wiring should be sized to 125% of the nameplate input full load amps of the converter as per 455.7(A) and 455.6(A)(1), respectively; provided the breaker panel is in sight of the RPC, this also provides the disconnecting means required by 455.8(B) and (C). If the panelboard isn't in sight of the RPC, a separate breaker enclosure or HP-rated switch is needed; use the same amp rating for this disconnect as the breaker. Finally, the breaker in the panelboard for the RPC should be held down with an appropriate hold-down kit as it is being backfed on its B phase when the RPC is running.
A /3 cable should be used to wire this circuit, with the white wire used for the B/L3 phase and retagged orange with orange electrical tape such as Scotch #35 -- this meets the requirements of 455.6(B) for a marking system for the manufactured phase that is consistent throughout the system and premises, and also is standard for high/wild legs in high-leg delta services. If you're using conduit instead of cables, you can simply use orange THHN instead wherever I specify to tag a white wire with orange tape.
Of course, you'll need branch breakers for your panelboard. Single pole breakers can be placed on the A or C phases, but not the B phase as that's the "wild leg" and runs at 208V relative to the panel neutral. (Putting single phase loads on a manufactured phase is a violation of 455.9 atop the fact that there are very few single pole breakers in existence that are straight rated for 240VAC service.)
120/240VAC or 240VAC-only circuits should use a two pole breaker connected to both A and C, but not B. You can also use a three-pole breaker for this provided it is rated for single phase service, if you can find such a critter that is. You cannot use A and B or B and C for a 240VAC only circuit, though -- not only will it not work unless the phase converter is running, but most two pole breakers are 120/240VAC slash rated, which means they can't be used with the 208V-to-neutral B-phase at all. Furthermore, doing that with a 240VAC load would still be a 455.9 violation no matter what else was done..
Three phase circuits, of course, use a three pole breaker connected to all three phases in the panelboard. Note that a three phase circuit will need /3 in the no-neutral case or /2/2 if it requires a neutral as well for 120VAC controls -- tag the white wire orange with the aforementioned tape, and use the "candy cane wire" for the actual neutral on the /2/2.
Finally, you'll need a means of complying with 455.21 and 455.22 so that you don't have three phase motors trying to start sans a phase or continuing to run if the B phase drops out. The easiest (albeit likely not cheapest) route is to use three pole contactors in the panel, 1/three phase utilization circuit, with 208VAC coils connected from the B phase of that circuit to the neutral. This way, the B phase must have some power supply in order for three phase power to make it to three phase machines. Obviously, a more sophisticated relaying approach could be devised that provides sensitive undervoltage protection for the B phase, but that's beyond the scope of this answer.
