Sizing a input breaker for a 40 HP RPC

I am in process of having a 40HP RPC built but I am getting confused about the size of single phase feeder breaker size needed. I originaly was looking for a 30HP but I was able to get a 40HP 3 Phase. 1750 multi tap motor for free. The RPC will be located less than 3' from the panel I have acess to to add another 200 amp panel to my service.
I have numbers from 125-200 amp breakers needed. My sever side of receiving this power is a Hydraulic press of 10 HP, a 10HP air compressor and a 15HP lathe.
Any help is appreciated, Thank you!

The NEC requirement for a phase converter is to set the overcurrent at not more than 125% of the "nameplate full load single phase input".

That's fine, except you don't know that rating.

The 40HP motor has an NEC rating full load current (3 phase) of 104A on 230V. You won't load it to full load as a converter, and so you won't draw the full load current.....

The 40 HP motor can supply about 25 to 30 HP worth of motors, with no single one bigger than 25 HP. Of that 30 HP, the phase converter supplies 1/3, or about 10 HP, since it produces output only for the 3rd (generated) leg.

Using that as a starting point, look at a 10 HP single phase motor. It draws (per NEC rating) 50A at 230V.

But, the no-load draw of a 40HP motor is larger than that of a 10 HP. Estimating the no-load draw of the 10 HP motor at about 35% of full load, it comes to about 18A. The idle of teh 40 HP, at around the same percentage, would then be about 35A.

Figuring the difference to be the actual load draw for 10 HP, then I would estimate the full load amperage of the nominal 40 HP motor used as a converter as about 35 + 32, or about 67A (35A no load, plus 50-18 = 32, total 67A). Then, 125% of that is about 85A. A 90A breaker may be the closest you find, so a 90A breaker is indicated.

In reality, the single phase no load input will be a bit more than the 3 phase no load input current, so it could easily be 40 to 45A. Then the same addition would give about 95A, or rounding to a breaker value, 100A.

This is very rough estimation, but the logic is defendable. If you know the no load single phase draw of the motor, you can use that instead of the no load estimate, and get a better number.

You probably can't get a better number unless you load the thing to a 10 HP motor output current and measure.

Full load amperage of your three phase motor x 1.732 will give you the single phase full load amps. Now do the 1.25 x to size your single phase breakers for the 40 hp motor. The smaller motors should not be starting while you are starting the 40 hp so you should be good.

The question is which "3 phase motor" you mean. It appears to be the idler, which wouldn't be correct. That assumes the idler motor will be working at full load.

it will not be.

First, the idler is always oversized for the load, typically by 1.5 :1.

Second, it does not need to provide any power other than the generated leg, which is 1/3 of the total 3 phase power provided.

Therefore the amperage will be lower, even compensating for the 3 phase to single phase conversion. Every RPC design is different that way, depending on the exact motor type, but I think the calculation above is reasonable in the absence of better input information.

If you were meaning the load motor, then you would still be "off" for some of the same reasons.

JST you are correct The 40HP is the idler I was not clear on that sorry. So am I correct from your formula that I need at least a 100 amp breaker?

What I come up with is a 100A for the idler, and really no higher. It would need to be a good time delay, because even unloaded the motor may take a bit to get it started.

I would not be surprised to find that a rough "gesstimate" like this was off by 20% or more.

So I need to wait before throw the I switch on any working equipment, That makes sense! Thank you.