Breaker size required for 7 1/2 HP vs 20 HP RPC
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  1. #1
    Join Date
    Oct 2008
    Brenham, Texas
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    Default Breaker size required for 7 1/2 HP vs 20 HP RPC

    I have a couple of 3 phase motors available to use to build a bare bones RPC.

    One of these is an 1155 RPM 7 1/2 HP motor. I had it reconfigured for 230V 3 phase and had the motor shop install new bearings while they had it.

    Then, I happened upon a deal in a couple of identical Dayton 1755 RPM 20 HP motors.

    I don't remember the name plate amps for the 7 1/2 HP motor. But the 20HP motors are rated at 53 amps.

    I seem to recall somebody in here explaining that the single phase breaker that feeds the RPC has to be significantly larger than the nominal amperage stated on the data plate.

    Could somebody give me a rule of thumb here? I have a 200 amp single phase service available. I would like to start with the 20 HP motor but can't afford a 3 phase breaker distribution panel at the moment. So, I might play with the smaller motor first.

    In any case I would appreciate some guidance on how to properly size the breaker that feeds the RPC. The load motors will be a 2HP spindle on a Gorton tracer mill plus a 3HP motor that drives the hydraulic power unit. So that's a total of 5HP.

    Both the spindle motor and the HPU motor appear to be wired to a common bus in a fuse box mounted to the side of the machine. What I hope to do is wire the 3 phase output from the RPC directly to the fuse box. Of course I will start the mill and the hydraulic motors separately.

    Anyway, I'd like to know what the appropriate size single phase breaker would be for either motor. I will spin the motors to their design RPM with a pony motor before flipping the breaker.

    Also, on a slightly different topic but in regard to the same project: I have 3 phase magnetic starter on a Bridgeport mill that I no longer need as the Bridgeport is now powered by a 3 HP Hitachi VFD.

    Jim Rozen has mentioned the hazards of locked rotor faults resulting from power failures and then the power coming back on.

    Can a 3 phase magnetic starter be wired into single phase 220? The idea would be to salvage it from the Bridgeport and place it between the single phase feeder panel and the RPC for the purpose of preventing it from automatically starting back up after a power failure.

    So, in this regard, my questions are:

    a) Can I use a 3 phase starter on single phase? and,
    b) Does it have to be "sized" to the motor? The magnetic starter controlled a 2HP Bridgeport spindle motor. In this new application it would be starting either the 7 1/2 HP or the 20HP RPC, which in turn would be driving 2 motors totaling 5 HP.

    Is this starter an er... "non-starter?"



  2. #2
    Join Date
    Aug 2005
    Kingsport, TN
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    It is hard to answer all your questions without knowing how your RPC is designed.
    For instance:
    most commercial RPCs do not include a mag contactor or mag starter so they can sell them cheaply.
    most homebuilt designs discussed and plans shown here do have a mag contactor as the first component of the RPC. On this design, all RPC power passes through the mag contactor and any power blip will shut down the RPC. I install an overload relay to shut down the RPC in case of an overload. It is also desireable to use a full voltage starter/contactor coil or use a control voltage transformer to get the control voltage. When this is done, the rpc will shut down in the event one incoming single phase wire loses power, such as occurs when a breaker fails.

    Please post the rpc make and model so we can look it up or else post the wiring diagram for it.

    As to a breaker size. A resonable approximation will be at least 5 amps per idler horsepower and you must size the wiring accordingly. You can go larger if you design all the components to match the breaker size.
    for example: a 7-1/2 hp will need at least 40 amps. I use 60 amps on my designs just to get increased load motor starting capability. A 7-1/2 hp will start just fine from capacitors
    A 20 hp would need 100 amps for full output. You state you are going to use a pony motor start and do not need 20 hp. It should be possible to run on a smaller circuit as long as you do not overload it. I have a 20 hp idler, pony motor start RPC, on an 80 amp breaker and I have an overload relay to protect the idler from overloads. On my designs, all the power passes through a mag contactor first, then onto the idler. All have run caps on both "generated" phases.

    The conversion from 3 phase amps to single phase amps is 3 phase FLA x 1.73 = single phase amps.

    Size 0 mag starters are rated for 15 amps
    Size 1 mag starters are rated for 27 amps
    Size 2 mag starters are rated for 45 amps
    Size 3 mag starters are rated for 90 amps
    Size 4 mag starters are rated for 125 amps
    I think this covers any that you might have. IEC designs are different and one must read the amp rating on the starter to determine the capacity.

  3. #3
    Join Date
    Jun 2001
    St Louis
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    If, as I suppose, you are running one or two specific loads with the RPC, the NEC covers that.

    1) protection at not more than 125% of the nameplate full load single phase input amperes.

    You don't have a nameplate, so no go there.

    2) For specific fixed loads, protection at not more than 250% of the sum of the full-load 3 phase current rating of the motors served, where the input and output voltages are equal.

    This seems to be your situation.

    Wiring must of course be at sufficient size to be protected according to the usual ampacity tables.

  4. #4
    Join Date
    May 2007
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    Basic calc we use for 1p motors is HP * 746(W) / .8(PF) / .8 (eff) / Volts. This will get you within a couple amps on motor amps.

    For 3P, divide the above amperage by 1.73. Make decision on on closest breaker.


    Again, remember that breakers are NOT motor protection devices.

  5. #5
    Join Date
    Oct 2008
    Brenham, Texas
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    Thank y'all for your very useful information. It's somewhat over my head but I will attempt to understand it nonetheless. Hopefully, it will lead to my future questions being at least a little bit more intelligent.

    Meanwhile, to put us all on the same page, I wish to define the word "bare bones" as I used it in my original post.

    At least a couple of senior members here have described RPCs that were almost totally devoid of the niceties such as start capacitors, run caps, and such. Some have even operated RPCs for years with only a rope start.

    This is the approach I wish to emulate - at least for the moment. But to clue you in as to my skill level:

    I have successfully built several 200 amp single phase meter loops, panels, and sub panels. I have overcompensated for my lack of skill and experience with lots of overkill. By this I mean that I've bought the best available components and have generally used wire far in excess of what was needed.

    It is fitting to explain here that our project involves converting a dinky little wood frame house into a commercial business location. We are outside the jurisdiction of any municipality, so we're at liberty to pretty much do what we want. However, lucky for me, I WANT to do everything to the highest possible standard. The challenge is to learn what that standard is.

    The entire house measures about 30' x 30'. Originally, the house had typical residential wiring (romex run through the bare, uninsulated wall spaces) fed by a dinky, ancient, 75 amp, single phase 220 amp service. That service entered the house at the westernmost edge of the north wall of the house, which happens to be the "kitchen".

    Some years ago, I upgraded the service to 200 amps. At that time I paid the utility company to re-arrange the service, dropping it down in a much shorter direct path from the transformer pole to the MIDDLE of the SOUTH wall of the house. There, I installed an outside box affixed to the house, with an especially tall mast. The idea for the tall mast was to give extra vertical clearance for a forklift to crawl underneath. But I digress.

    The main service panel is a Square D QO 200 amp "feed through" box such as mobile homes use. The back side of the service entrance is the inside of a long, narrow closet. Since the NEC frowns on placing an electric panel inside of a closet (unless it's an "electrical closet") I removed the closet door and wall section. Hence, the closet is now an "alcove" in the front bedroom.

    Both the ceiling and walls of the closet are sheathed in 5/8" plywood and 5/8" "X" ("fire") rated sheetrock.

    Plus, as mentioned in an earlier post, where the service feeds through the exterior wall, I widened the wall in the end of the closet. The original wall was 2x4 stud construction. To this I added some 2x8s so that the wall in the end of the closet is 12" thick. To this widened, end wall, I mounted an identical feed through panel. So, the entire 200 amp service feeds through the back of the service entrance panel and into the interior box. Again though, both are "exterior" boxes with 200 amp main breakers. The power is fed to the subpanel with 4/0 wire. So I have a decent safety factor.

    The NEXT step is to run 200 amps out the top of the sub-panel, through thin wall 2" conduit, into the attic. The conduit will run not more then 20 feet and drop down to the FRONT PORCH by the front door. There, I will install an exterior 200 amp main breaker panel.

    I will then run the full 200 amps through THAT wall. Mounted just inside that wall, will be a 40 space 200 amp deluxe QO panel. This panel will be the feeder panel to our shop machines (2 mills, a lathe, and a drill press) all mounted in the "living room".

    So, what I'm trying to understand is: What safety devices do I need between this 200 amp single phase box and the RPC and between the RPC and the machines it powers?

    Because I'm newly retired, and living on a fixed income, I need to do it myself, and I need to do it cheap. But that doesn't mean I don't want to do it right.

    At present, the tentative "plan" is this:

    I will mount the 7 1/2 HP idler motor somewhere near or behind the Gorton mill.

    In a convenient location I will mount the magnetic starter (once I determine it's the appropriate size and rating) that I will rob from the Bridgeport mill, which runs from a 3 HP VFD. So I don't need the magnetic starter for the Bridgeport.

    I will run two appropriately sized and and protected single phase hots plus a ground conductor from the panel (just a few feet away) to the magnetic starter. I will wire the starter to two of the hot legs on the idler motor beneath the motor cover. Both of these single phase hots and the three phase "wild leg" hot will run from the idler motor to the three fused busses on the original Gorton fuse box. Both the spindle motor and the hydraulic power unit for the tracer are wired into this box. Since I'm running at half the voltage (previously 440 now 220) I assume I will need to double the ampacity of the three (3) fuses. Yes?

    I will either rope start the 7 1/2HP idler or spin it up to its 1155 rpm design speed with a pony motor. Once at or near design speed I will flip the appropriately sized single phase 220V breaker and push the button on the magnetic starter.

    Once the RPC is spinning at full speed I will engage the fuse box on the mill. I will then start one motor or the other - starting the bigger one first.

    I'm not asking any of you "Will this work?". Rather, I'm just asking you...

    "Does this sound like a plan?"

    Or have I totally failed to comprehend something I need to know?



  6. #6
    Join Date
    Oct 2011
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    Hi Vernon. I sure hope that is a slab house...Also not sure why you don't want any pf correction?? Caps are cheap. By the time you dink around trying to "pull start" your motor you could make it self starting.
    I made a video on how to build a simple self starting rpc that drops out the start caps. How to build a rotary phase converter - YouTube
    Ended up paralleling a 5hp motor w the existing, also ended up putting a pot between generated leg bus and the coil on the contactor to control coil voltage.
    After burning up coil....
    Good Luck!

  7. #7
    Join Date
    Oct 2008
    Brenham, Texas
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    Hi Linehand,

    Thanks for your response. The house is indeed on a slab. The great thing about it is it's barely more than a shack. If the machinery weight cracks the slab I will simply rent a jack hammer, chisel out the concrete, dig a hole, fill it with steel, and build a machinery base.

    Up to now we've had no such trouble. However, after a record drought in Texas last year a lot of people have started having foundation problems where there were none before.



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