If it is your ONLY idler? Have a look at the nameplate "FLA", then "do the math" (online) for the higher Amperage you'll need off the input feed of just ONE phase where it expected three.
ISTR my Phase-Craft operated 10 HP Weg wants 55 A out of the single-phase panel, wudda been happy with 30 A if fed utility-mains 3-Phase for use as a motor-not-idler?
ISTR the 10 HP Phase-Perfect is almost the same. Around 55A single-phase feed for around 29 Amp three-phase out to the load side.
250 Amp single-phase, 2-pole breaker for your one? Only 200 A?
Check
your actual figure.
Then-also "fudge upward" for box space, conduit size, and max wiring Ampacity, but breaker it for the lesser actual it will need.
"Just me" but whether 2 -pole for the 1-P feed, or 3-pole for the 3-P loads, it is
never fuses,
always "common trip" circuit breakers, and always separate, and NOT fused disconnects.
I like the lockable "rotary" type for the lighter goods, big old 600-Volt-class Square-D lever-operated for the heavier stuff.
My only fuses go inside the equipment cabinets - "semiconductor" type, and fast-acting where a DC Drive "might" benefit.
Any fusing where there is more than one leg, one trips, the other(s) are still a shock-hazard at the least, and potentially a motor going in harm's way running on dropped-phase as well.
"Common trip" is what it says it is - takes them
all out of play when any one - or more - hits bad news.
CB's are not really "forever" items, nor meant to be convenience switches, but "usually" once the fault is cleared they can simply be reset.
Fuses yah may have to go-fetch or even ORDER and await replacements. Especially if you've had two hits in a row and just used up yer spares!
Yah don't have to buy a load center with buss bars to utilize CB's.
Stand-alones in decent NEMA box, yah use the "unit" mount ones. Square-D's are "QOU Series", for example.
One presumes you'll have a pony-start rig to deal with as well?
One of the reasons my rig is multiple idler, staged start, is that I only have 200A service.
Starting even a 20 HP in one "chunk" would be more grief than I care to be bothered with. The 10 HP or any of the lesser ones - no big deal. Basic remote-control button, cap-start, of any ONE, add the other(s) later, once it is up to speed.
And I can still "get up to" above 20 HP - or down to 3 HP - if need be.
Unitary 40 HP idler in one, single, bucket?
That's a tad more "dramatic" at start-up, ain't it?