Rpc Question

[bpimm]

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

 

[John Hasler]

Use #8.

 

[abrace]

#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.

 

[Cheeseking]

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

 

[bpimm]

#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.

 

[bpimm]

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.

 

[abrace]

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

 

[abrace]

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

 

[bpimm]

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.

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?

 

[abrace]

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.