# Rpc Question



## bpimm (Jan 19, 2016)

The new to me lathe and mill I bought a while back came with a 5HP rpc, it's still hooked up with a short 3' wire from the switch box to the idle motor. what I'd like to do is put the idle motor in another room off the shop so I don't have to listen to it. would a 25' run of 10 gauge wire from the switch box to the idle motor hurt the operation of the system?

 Thanks


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## John Hasler (Jan 19, 2016)

Use #8.


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## abrace (Jan 19, 2016)

#10 could meet code if you pipe it or use something other than type NM (romex), since motors do not fall victim to the small conductor rule 240.3(d). Romex, however, is limited to 30 amps because it has to be sized according to the 60 degree column in the ampacity tables...MC and THHN in pipe can use the 75 degree and go all the way to 35A. A 5HP motor has an FLA of 28A, after bumping it by 25% (motors have to be sized for continuous duty unless otherwise specified) this brings the required ampacity to 35A, just squeaking it in with 10AWG if not using romex.

This all said, I agree with John, go with 8 AWG. It will give you a 50A ampacity if you pipe it and use THHN or an MC cable, or 40A with romex. A nice margin, less voltage drop, a win all around.

Obviously this all assumes the use of copper conductors. If Aluminum, it is another conversation.


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## Cheeseking (Jan 19, 2016)

Any consideration of fusing, disconnect means and overload protection on the RPC?


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## bpimm (Jan 20, 2016)

abrace said:


> #10 could meet code if you pipe it or use something other than type NM (romex), since motors do not fall victim to the small conductor rule 240.3(d). Romex, however, is limited to 30 amps because it has to be sized according to the 60 degree column in the ampacity tables...MC and THHN in pipe can use the 75 degree and go all the way to 35A. A 5HP motor has an FLA of 28A, after bumping it by 25% (motors have to be sized for continuous duty unless otherwise specified) this brings the required ampacity to 35A, just squeaking it in with 10AWG if not using romex.
> 
> This all said, I agree with John, go with 8 AWG. It will give you a 50A ampacity if you pipe it and use THHN or an MC cable, or 40A with romex. A nice margin, less voltage drop, a win all around.
> 
> Obviously this all assumes the use of copper conductors. If Aluminum, it is another conversation.



Thanks John and abrace, I will go with 8 romex, piping would be a royal PITA.


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## bpimm (Jan 20, 2016)

Cheeseking said:


> Any consideration of fusing, disconnect means and overload protection on the RPC?



Cheeseking, I'm assuming you are talking about some form of protection between the RPC and tools, The way it was set up did not have any, both machines were hard wired into the RPC control panel which just has the disconnect for the single phase mains and a timer and disconnect for the start caps. I'll see if there is something I can add to the existing panel to accomplish this, it does sound like a good idea.


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## abrace (Jan 20, 2016)

Rotary phase converters are treated a little differently than motors. I believe the OCPD can be no more than 125% of the FLA listed on the nameplate of the rotary phase converter...if it has one 

As for disconnects, I don't have my code book here to see if there is special disconnect treatment for a RPC, but in general you need a disconnect within 50 feet AND WITHIN SIGHT of the device. In lieu of the disconnect the breaker (or disconnect not within sight) will often suffice PROVIDED it is capable of being locked in the off position. They sell locking attachments for breakers that are pretty cheap.

---Aaron


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## abrace (Jan 20, 2016)

Just realized Cheeseking was asking about overload protection and not overcurrent. Yes, the motors should have overload protection usually accomplished via either FUSES or magnetic starters with heaters. The OP should already have this stuff since the RPC is in place, he is just talking about relocating the idler motor to a different area. Standard rules regarding overload protection apply whether the idler is next to the lathe or 20 feet from it.

---Aaron


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## bpimm (Jan 20, 2016)

abrace said:


> Rotary phase converters are treated a little differently than motors. I believe the OCPD can be no more than 125% of the FLA listed on the nameplate of the rotary phase converter...if it has one
> ---Aaron



The RPC has no labeling on it, I think it was built by the PO. The RPC Motor is rated at 14.3 Amps.




abrace said:


> As for disconnects, I don't have my code book here to see if there is special disconnect treatment for a RPC, but in general you need a disconnect within 50 feet AND WITHIN SIGHT of the device. In lieu of the disconnect the breaker (or disconnect not within sight) will often suffice PROVIDED it is capable of being locked in the off position. They sell locking attachments for breakers that are pretty cheap.
> ---Aaron



The "within sight" requirement may be interesting... The control panel of the RPC and the idle motor  will not be "within sight" of each other. The CP will be on the wall beside the lathe and mill and the idle motor will be in the room with the breaker so each part is within sight of the breaker or the disconnect.

With the RPC disconnect next to the equipment do I need a separate disconnects for the lathe and mill after the RPC?


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## abrace (Jan 20, 2016)

bpimm said:


> The RPC has no labeling on it, I think it was built by the PO. The RPC Motor is rated at 14.3 Amps.
> The "within sight" requirement may be interesting... The control panel of the RPC and the idle motor  will not be "within sight" of each other. The CP will be on the wall beside the lathe and mill and the idle motor will be in the room with the breaker so each part is within sight of the breaker or the disconnect.
> 
> With the RPC disconnect next to the equipment do I need a separate disconnects for the lathe and mill after the RPC?



I just read the code section related to RPCs. Basically, as I see it, you technically need a total of 3 disconnects.

Disconnect 1) The RPC (your idler motor) needs a disconnect within sight of the RPC under 455.8(a). I can find no exception for this disconnect that allows it to be located remotely even if lockable. This disconnect is intended to disconnect the SINGLE PHASE conductors going in to the RPC. No disconnect is required in this location for the 3 Phase output conductors. Bear in mind that since this is running motor loads, the disconnect itself must be 'horsepower rated'. A 5 HP Square D general duty disconnect should work, you can get them for about $80.

Disconnect 2) The motor controller also needs a disconnect. Motor controller is whatever is used to fire up your three phase motor, and this disconnect would be for the 3-phase conductors as they come from your RPC and enter your motor controller. Motor controller would be defined as the box housing the contactor, or hp rated switch, that is used to fire the motor up. Often they are 'magnetic starters', but not always. A motor controller could literally just be a big manual switch, like would be the case for an exhaust fan. This disconnect has to be within SIGHT of the controller. This is covered under 430.102(a).

Disconnect 3) The motor itself also requires a disconnect under 430.102(B). However, the Controller disconnect directly above may serve as this disconnect provided that the controller disconnect is within site of the motor. In most cases this 3rd discrete disconnect is not needed. This disconnect is covered under 430.102(b)

If your RPC, motor controller, and motor are all in the same place, 1 disconnect should suffice for all 3 requirements. In your case where you are moving the idler motor outside and leaving its starter circuit where it is, I think disconnects 1 and 2 should work...I cannot find any special disconnect requirements for an RPC control circuit, but others may know of some.

For you, a lot of it will depend on what your electrical inspector defines as the 'RPC'. An RPC is the combination of the idler motor and its start/control box/circuit. Since the RPC disconnect needs to disconnect the single phase supply conductors to the RPC, if the first entry of these conductors from your building wiring into the RPC is the control box that you aren't moving, then an argument could be made that a disconnect at that location is meeting the spirit of 455.8(A), and a disconnect at that point, assuming that point is next to your lathe and your lathe's motor controller, could potentially serve for all 3 disconnects even if moving the idler motor outside...especially if it is lockable. That would eliminate the need for a disconnect next to your idler motor. That could be an easy sell to your inspector assuming he/she is reasonable.

These are always sticky areas up to a lot of interpretation by everyone. At the end of the day the only opinion that matters is that of your electrical inspector.


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## bpimm (Jan 20, 2016)

abrace said:


> I just read the code section related to RPCs. Basically, as I see it, you technically need a total of 3 disconnects.
> 
> These are always sticky areas up to a lot of interpretation by everyone. At the end of the day the only opinion that matters is that of your electrical inspector.



Thanks Aaron, that clears it up.


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## Cheeseking (Jan 22, 2016)

Hope I didn't start a hornet nest bpimm but ya just make sure you study and follow code requirements.   A plug/receptacle could satisfy as a disconnect under certain conditions.  

I set up my shop 3ph as follows:

60A 240 1ph breaker off my main load center 


6 AWG / 8 for EGC to fused disconnect (safety switch) for the 1ph


To a NEMA starter w/overloads for the phase converter.   The push button to start and stop works really nice.



To 7kw phase conv



To 30A 3ph fused disconnect.



To 3ph load center with breakers for each machine.



Twist lock receptacles/plugs at each machine.
	

		
			
		

		
	




All this stuff fit nicely in a corner not too far from the main panel.   



Now to add to the complete protection overkill I have yet another disconnect switch and manual starter w/overloads for the lathe motor. .   (It was on the lathe already)


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## Cheeseking (Jan 22, 2016)

Took the safety switch off the BP and went with only a twist lock plug/receptacle.


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## abrace (Jan 22, 2016)

No hornets nest,  your install looks really nice.

A plug and receptacle can absolutely serve as a disconnect, however both the receptacle and plug need to be specifically rated for the horsepower of the motor..you can't just look at the amp rating and think you are all set. The amp rating is only for resistive loads...motors are inductive. You need to see the HP rating and it needs to meet or exceed your motor. Otherwise they cannot serve as the disconnect.

Most common NEMA pairings max out at 3HP for a single phase feeder and that is with a whopping 50A plug. There is not much in the 5HP range, although some companies like Leviton have some proprietary non NEMA starndard offerings

As soon as you get into the 5HP range as the OP has, the plug/receptacle as a disconnect starts to get more difficult to do.

Below is a link to the Nema HP ratings for plugs. If you have a standard 240V 30A receptacle, that is a 6-30, and it is good for only 2 HP. Its a good chart to keep handy to see if your plug/receptacle combination can actually be considered the 'disconnect' under the code or if you need to do something else.

http://www.leviton.com/OA_HTML/SectionDisplay.jsp?section=66500&minisite=10251


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## Cheeseking (Jan 22, 2016)

Yep.  I think the verbiage is found in NEC article 430.109 (F) which permits plug as disconnect means.   Of course HP rated to match like you point out.


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## FOMOGO (Jan 23, 2016)

Cheeseking said:


> I set up my shop 3ph as follows:
> 
> 
> 
> Nice job on your converter setup. I see the in sight rule being served by an inexpensive video camera and monitor. That's just how a twisted mind works.  Mike


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## abrace (Jan 23, 2016)

Very true. There is 'safe' and then there is what meets code. Often the code requirements are silly, especially when it relates to our shops. The disconnect rules are written for big industrial/commercial areas where you may have a lot of workers, and you don't want to have someone turn a machine on or circuit on that is being actively worked on by a different worker. In our home shops, I think it is unlikely our wives will come out and fire up the lathe breaker on us while we are working on the circuit...yet the same rules apply.

Don't even get me started about all the stupid AFCI rules and tamper resistant receptacle rules.

There are a lot of manufacturers involved in various ways with the code making panels, and their incentive is to invent new rules so that we will have to buy more products.

That is what 90% of it is all about.


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## Cheeseking (Jan 23, 2016)

bpimm another option to consider is a relay of some sort.    That way you can run small gauge conductors (less $$) from the RPC starter to your switch in the shop.


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## andermma (Jan 31, 2016)

hi, you might consider using thhn #8 copper wire in flex or emt instead of romex - emt is not to hard to install and is a lot easier than hard pipe. EMT looks like pipe but is thin wall and can be used indoors and out easy to cut with hacksaw with a fine blade - mount a disconnect near the rpc unit on the feed side at the least so it can be shut off and locked out for service.  looks like in the pics above they are using watertight flex to connect to the disconnect. 

Mitch


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## JimDawson (Jan 31, 2016)

One thing to watch out for when plugging in a VFD is do it like you mean it.  Align the plug, then connect it in a quick motion to minimize arcing. The inrush current is insane on a VFD for a fraction of a second while the capacitors charge.  The best way it to kill the breaker, then plug it in, then turn the breaker back on.


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## abrace (Jan 31, 2016)

JimDawson said:


> One thing to watch out for when plugging in a VFD is do it like you mean it.  Align the plug, then connect it in a quick motion to minimize arcing. The inrush current is insane on a VFD for a fraction of a second while the capacitors charge.  The best way it to kill the breaker, then plug it in, then turn the breaker back on.



This is why I am not a fan of using a plug as a disconnect for motors of any significance (or capacitors, the opposite of an inductor)...and also why most plugs top out at around 2HP, or 3HP for the really big ones. You absolutely want to avoid connecting/disconnecting under load. Your idea about killing the breaker to plug in is a good one. However, molded case breakers aren't great at repeated connect/disconnect under load either, but they are going to be better than plugs. The prongs in a plug are not meant for a lot of repetitions with arcing. They won't hold up, they are too thin.


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