Subpanel / RPC panel / 3 Phase Load Center Wiring
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  1. #1
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    Default Subpanel / RPC panel / 3 Phase Load Center Wiring

    I know I'm new here so I apologize for asking help already. I've looked at about every RPC thread I could find and decided it's in my best interest not to attempt to build one myself. Instead I'm leaning towards purchasing an American Rotary 20 HP ADX to power a Quincy F340 I purchased that has a 7.5 HP 3 phase motor. American Rotary actually says with the ADX RPC I only need the 15 HP RPC but for the difference of slightly more than $100 I'm opting for the 20 HP. But I want to wire this thing up right and I've attached what I understand I need to do. If you could critique it I'd really appreciate it. I'm starting with 200 amp service in a CB panel that is full and will need to put in a couple of tandem CBs to allow me to fit in a 100 amp CB to power a subpanel and from there the converter panel which will feed a 3 phase load center.

    I also plan on adding a 220 circuit off the 100 amp subpanel for a welder and 110 circuit for misc stuff.

    Thanks,
    Daveslide1.jpgwiring-diagram.jpg
    Attached Thumbnails Attached Thumbnails wiring-diagram.jpg  

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    Hi Dave,

    I went down this road about a year ago and reached the same conclusion you did... with one exception. Rather than buy a complete "store-bought" RPC I bought the control box from a company called "Phase-Craft". I bought 3 phase motors (one 20 and one 20HP) for a total of $300. So that's $150 per motor. I then bought a Phase-Craft 20 hp control box from the seller on ebay. I don't remember the exact price but to my door in Texas (Phase-Craft is in California) the price was less than $500. The components were elegantly and efficiently arranged in the box.

    The guy's customer service was out of this world. He is genuinely a prince of a guy. My RPC runs beautifully. Total cost - including wiring - was less than $700.

    I would buy another of his units in a new york minute.

    Vernon

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    I thought about that option as well. But since I'm trying to start a 7.5 HP air compressor all of the research I found said you'd need 2.5 - 3 times the starting HP in the RPC. So I'd need to source a 25 HP motor and the panel. The panel on ebay is $620 shipped and the motor would be at least at least that for a 25 HP unless I happened upon one at a scrap yard. The ADX20 is $1500 on ebay and comes with a Baldor motor and free shipping.

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    My setup is pretty much exactly as you have except I have a 10hp phase converter. I couple things I noticed... where's your ground from the main panel to subpanel? Also, a 20hp phase converter at FLA is going to pull way more than 60A. Probably more like the 100-120A. Remember it's something like 1.8 single phase amps per 1 3-phase amp. I have a 60A single phase breaker feeding my 10hp converter.

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    Good catch on the ground...forgot to add the line.

    The wiring info from American Rotary says I can us 10 gauge wire from the panel to the 20 HP RPC motor

    Just got off the phone with John at American Rotary. He said the RPC motor is a generator (no shaft) and only uses about 3 amps to run. He did say that at startup it would pull around 100 amps but the 60 amp CB I'm planning would be more than sufficient to handle that momentary load.

    I'm unsure about the gauge of the neutral and ground. I suspect they can be less but am not sure.

    And I'm a bit uncomfortable with #10 from the air compressor up to the 60 amp 3 phase circuit breaker (John said I didn't need the safety disconnect between the air compressor and the load center)

    Thanks,
    Dave

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    I'll agree on the single phase side. Don't worry about 10 awg to the compressor motor. 7 1/2 HP is only 24 FLA, 10 awg can handle 40A. But I am wondering why he says that should be on a 60A breaker. Maybe because it's a hard starting load. I'd still try with a smaller breaker first. You are going to need a motor starter on the air compressor with properly sized heaters.

    FYI, my 5 HP compressor is on a 30A breaker and has never tripped it.

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    OT - What was your reason for choosing a RPC over a VFD?

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    I actually have many 3 phase woodworking tools and eventually will have a full up workshop instead of a 2 car garage and this is planning for the future

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    Quote Originally Posted by daviddavies View Post

    Just got off the phone with John at American Rotary. He said the RPC motor is a generator (no shaft) and only uses about 3 amps to run.
    I'd not be too sure about that......

    In the first place, as an unloaded motor (no 3 phase load) the idler will pull 30 to 40% of its nameplate FLA, just because it's an induction motor (actual power is low due to power factor).

    Then, under load, the idler will pull current equal to the load power on the output line, plus losses. That is about 1/3 of the LOAD motor power, plus about 25% extra for losses (to be conservative).

    You can add those up.

    Unless the guy you talked to has a perpetual motion machine, there isn't any way to get away from a current draw significantly over 3A for a 20 HP idler.

    I think the number for the start surge is low also....

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    As I recall, our setup with no load and 27 hp of idler motors draws 20 amps 240 volt as measured by a clamp on meter, so 3 amps for 20 hp does seem very low.
    Under load I am not sure of the draw, but when standing by the phase convertor you can hear the increased electrical load humming thru the system when someone starts a large motor out in the shop.
    Glen
    Last edited by worntorn; 09-09-2013 at 10:38 AM.

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    Sounds like I need to call them again tomorrow. Thanks for the sanity check.

    This weekend my son and I got the 100 amp subpanel built and ready. Ended up going with 2-2-2-4 aluminum just because of the cost savings over copper.

    Thanks,
    Dave

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    Please post some pics of the completed setup.

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    Kim at American Rotary responded to my email and...

    Good morning Dave,
    You have really done your "homework" in detail. Gresat job.
    Looking at the threads these are the questions I picked-up on:
    1.) From National Electric Code (NEC) the min. gound wire size for #2 gage or smaller conductors need to use min. of #8 copper ground wire. Ref. Section/Table 250.66

    2.) A 60A 2-pole breaker feeding the 20 Hp RPC? Need to size the 2-pole breaker based on the 3-phase load. In this case the 7.5 Hp compressor motor has a full load amp (FLA) of 20A. So that 3-phase 20A load is reflected to the 1-phase side at 35A at 230V. (20A x 1.73 = 35A) The 1.73 factor is the square-root of 3. Most electricans recommend the breaker should be at least 1.25 times the FLA. So at 35A x 1.25 = 43A. The20 Hp Idler generator will only pull FLA 2.48A at 230V. That is all the power the 20 Hp Idler requires. S now have a total 1-phase amp requirement of 35A (Comp. motor) + 2.5A for Idler = 38A. Then take 38 x 1.25 = 48A, so a 60A 2-pole breaker is what I would recommend.

    This 60A 2-pole breaker is large enough or suited to handle the in-rush amp demand when the 7.5 Hp comp. motor starts.

    3.) The 60A 3-pole breaker in the 3-phase load-center, is OK. For a 7.5 Hp motor has 20A FLA, then using he 1.25 factor equals 25A. You could use a 40A 3-pole breaker feeding the comp. motor. If yhou already have the 60A 3-pole breaker I would use that, if you have to purchase a 3-pole breaker, then down size to a 40A.

    4.) Using #10 gage wire for the 3-phase connection between the RPC control panel and Idler generator? This 3-phase connection between the control panel and Idler only has to carry 1/3 of the total 3-phase amp load. Since we "pass-through" the incoming 1-phase connections, 2/3 of the total 3-phase FLA flows through output connections T1 & T2 that wire size is min. of #6, the wire size for T3 (Mfg. phase) can be down-sized to the #10 size.


    He said since my machine is right next to my subpanel and RFC panel I wouldn't need either of the safety disconnects. So that'll save a few dollars.

    RFC ordered today. Hope to have it installed by next weekend. Also ordered an aftercooler and need to plumb that into the air compressor as well

    Dave

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    You could use a junction box with a set of terminals like a distribution panel and use fused diconnects at each machine with time delays nothing beats a fuse and I would definitley use a magnetic starter for your air compressor or any heavy inrush load your square-d breakers and buss will last about 2 years before they arc themselves out. Also look in the nec for minimum grounding electrode conductor for connected loads i believe it is. there are lots of good tables to use in there. As far as wire size goes be carefull wich colum your reading from on 310.16 its not the clearest info in the world #14 for 15 amps #12 for 20 #10 for 30 and #8 for 45 and #6 for 60 I beleive that lines up wiith the 60 degree celsius colum.

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    Dave, it's nice to see the detailed response. One thing I would like to mention is that you gave her the 7.5hp air compressor as the load. She replied with around 48A draw and a 60A breaker would be fine. I agree but you have a 20hp phase converter and could have several other large motors running along with that compressor. Maybe someone with more experience can chime in but what happens when you have a couple 5hp motors running along with that 7.5hp compressor? Your FLA is going to be more like 80-90A. You can always change a breaker but you may want to size your wire between the subpanel and converter for the max possible load the converter could see.

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    Good point and since the RFC panel will be about a foot away from the subpanel the cost of the copper is insignificant so it wouldn't hurt to go up in size.

    Now I have to confess that you lost me Nick Z "You could use a junction box with a set of terminals like a distribution panel and use fused diconnects at each machine with time delays nothing beats a fuse and I would definitley use a magnetic starter for your air compressor"

    Junction box?
    Set of terminals like a distribution box
    fused disconnects

    I had safety disconnects in my original plan but was told I didn't really need them since the air compressor and RFC panel and subpanel are all within sight of each other.

    Magnetic Starter...Isn't the starter on the air compressor a magnetic starter?

    Hate to admit it when I'm so obviously ignorant in an area but there you go.

    Thanks,
    Dave

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    RFC came in yesterday and will be hooking it up tomorrow and going to plumbing supply to get black pipe air lines. 1" main and 3/4" take-offs


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