What size single phase circuit breaker to use for a 20hp RPC?

I have a 20hp American Rotary RPC. Been running it with a 40 amp breaker for years. Not loading it up as I can only use one machine at a time.

I am about to get a CNC, so there will be times it will run while I use some of my other manual machines so would like to get the breaker size right for the possible loads. I looked at the specifications on their website and it said it can handle an 80amp load.

I called them to confirm and they asked a field tech real fast and he said to use a 60amp circuit breaker.

In searching on the net others are saying 6amp per hp. That would be 120amps so a little confused as to what circuit breaker to buy to properly use the whole capacity of this 20hp RPC.

Thanks for any help.

Are you running 220V, or 440V or ?? ?

CarlBoyd

Sorry for not saying. It is a residence at 240v.

Jaxian said:

I have a 20hp American Rotary RPC. Been running it with a 40 amp breaker for years. Not loading it up as I can only use one machine at a time.

I am about to get a CNC, so there will be times it will run while I use some of my other manual machines so would like to get the breaker size right for the possible loads. I looked at the specifications on their website and it said it can handle an 80amp load.

I called them to confirm and they asked a field tech real fast and he said to use a 60amp circuit breaker.

In searching on the net others are saying 6amp per hp. That would be 120amps so a little confused as to what circuit breaker to buy to properly use the whole capacity of this 20hp RPC.

Thanks for any help.

Click to expand...

I have a 100A breaker on my 20hp American Rotary converter. I run 2 cncs on it all the time + the band saw and misc. other stuff.

My vote would be 80 amp but it really depends on the FLA of the idler I think. This should be confirmed by the codes.
55A for a 20HP FLA divide by 0.8 = 70amp.
Wires to suit of course.

Rules for breaker sizing is found in article 430 of the national electrical code. The breaker is sized to protect the wiring. Any electrical supply like Steiner Electric or Graybar have condensed versions of 430 with application notes. Well worth them.

Tom

Per the NEC:

"455.7 Overcurrent Protection. The single-phase supply
conductors and phase converter shall be protected from
overcurrent by 455.7(A) or (B). Where the required fuse or
nonadjustable circuit breaker rating or settings of adjustable
circuit breakers do not correspond to a standard rating or
setting, a higher rating or setting that does not exceed the
next higher standard rating shall be permitted.
(A) Variable Loads. Where the loads to be supplied are
variable, overcurrent protection shall be set at not more
than 125 percent of the phase converter nameplate singlephase
input full-load amperes.
(B) Fixed Loads. Where the phase converter supplies specific
fixed loads and the conductors are sized in accordance
with 455.6(A)(2), the conductors shall be protected in accordance
with their ampacity. The overCUlTent protection determined
from this section shall not exceed 125 percent of the
phase convelter nameplate single-phase input amperes."

You would have variable loads so 125% of the nameplate amps, max is your number. wiring to match.

I would run a 100A and you well want to get a buck transformer from them to bring it down to about 220V from the 240V ... the last one I got from them was like $350 or so ...

RPC are funny how they work ... you start one machine and after its running "its" motor also acts as a RPC .... so well you can start a light 20hp load with a 20 hp RPC you can run a LOT more than the 20HP after you get some of the other motors spinning.

years ago I was running a large cnc lathe off my old 20 HP RPC and the only way the lathe would run good was if I also ran a old 10HP crank shaper. you could watch the crank shaper slow a little when the lathe was getting up to speed.

Thanks guys great information. Looks like I will pick up a 100 amp circuit breaker. This will be at least enough to meet the "Pre-Installation requirements" list that preceded my new VMC so they can get it assembled and set up.

Still fighting with the local power company to treat me like a individual and not damn Walmart with what they are trying to charge me for 3 phase. They want $5000 just to submit an application. They figure $15k to $30k when done. The lines are running literally right over my workshop. I said I am paying for the new breaker panel, the electrician to put it in, all you have to do is run the wire 30ft down to the mast. She said "oh, and it will probably be a year if you get the forms in now..". These people are driving me nuts.

100 A is the easiest & cheapest Circuit to do.

At one location we had 208 across the street from us,and actually had 2 legs and neutral running in. took 4 months to get it converted to 208 3P.

And when they did do it, they messed up the drip loop so water was running under the insulation of the drop wires, down the pole, underground, up into our meter box, over the top, and dripping through the meter connections, re-entering the next wire, and ended up dripping through the master 3 phase breaker.....

THAT took 3 years (and 2 breakers) to get taken care of.

What does the data plate on the RPC state for amp draw?

You actually have TWO amperages. One is the RPC draw, the other is the load draw on the other two wires, which may require a separate breaker..

Two breakers

I just got my new 25hlp RPC 240v from southern phase converter out of Texas and these guys say put a 70amp to the rpc and a separate 50amp for the machine (23.5"x118" Tuda lathe) the lathe has a 10hp 440 motor but when wired at 220v it's 7.5hp since the lathe has no clutch it's a very hard to start machine....

Is it ok if I put it all on an 80amp breaker? And only start one at a time?

I have 6awg wire and the machine has over load protection and reverse and alot of safety features it's from the mid 1980's Japanese made Mitsubishi contactors.

Hi. I have a 20 hp engine. and I would like to build my own rotary generator. .. could you provide me with the data that your rotary has?