What's new
What's new

Question about using #6 THHN wire for a 30HP RPC


Nov 23, 2020
Franklin, TN
I am wiring up my new 30 hp RPC and my new-to-me VMC this weekend. I need to run 36 feet of wire from the breaker panel to the RPC (up a wall, across the ceiling and down to the RPC).

The RPC requires a 70 amp breaker. It can draw up to 70 amps on start up and idles at 15 to 20 amps.

According to the VMC's manual, the Sharp SV-2412SX 3 phase power requirement is 18 KVA or 44 FLA according to this calculation (18*1000)/(1.732*.98*240).

Various wire charts show #6 THHN is rated at 75 amps. So I would like to run #6 stranded copper THHN from the panel to the RPC.

Anyone have any concerns with using #6 stranded copper THHN for this setup?
Last edited:
I think code says it is good for 60, as I recall my old sub to my garage was wired and breakered as such, but the electrician told me 70 would be ok when the shop started blowing breakers. If it is a long run the extra size would not hurt, and if it is a short run the cost is not that high.....it is not going to melt the wire and the duty cycle on the machine it will not be a problem, but I like big wire myself
I just heard back from Croman Converters (who, by the way, is super helpful and offers amazing support).
Jamie said ...
"You really need to go with #4 on a 30hp or you risk having a voltage drop and won't get full performance from the converter. You don't need solid just #4 copper. You only need the 2 feed wires that size. The ground can be #8."
Hopefully this thread will help others who might have a similar question.
Last edited:
The RPC requires a 70 amp breaker. It can draw up to 70 amps on start up and idles at 15 to 20 amps.
The current inrush on my 30 hp Phase a matic RPC is significantly greater than 70A. It will peg the meters on the transfer switch that feeds it (100A meters).

I recently increased the wiring size to 1/0 in order to better reduce wiring losses. One thing that may be different about my setup from yours is that I'm also feeding a 240 3 phase load center through the same wiring, so my larger wire is carrying both the current for the phase converter output along with the equipment operating off of the panel. I did it this way because when I kill the power to the converter I'm also killing all three legs to the panel.
Yeah, I'm not running any load on the line from the panel other than the phase converter.
L1/L2/L3 from the phase converter powers the VMC only.
Jamie did say this about his 30 HP converter ...
Basically the 70 amp breaker is to start it. Those usually idle at about 15 to 20 Amps.
Depending on the load you are applying the amp draw will go up but never over 70 Amps.
So the VMC is going to suck 44 amps, plus the load of your converter under full load. Basically the wiring from your breaker to your RPC is also supplying the current for your VMC, so you have to factor all of these loads into your wiring sizing.

My R 30 Phase-a-matic has a recommended breaker size of 200A, and a 12 amp idler current draw. But on top of that you have the current draw when the converter is under load. Sizing your wire for 70A, and subtracting the 44A of current to your VMC that is traveling through the same wires to the converter, only leaves you with 26A of available current to power the RPC at full load. That's not enough to generate a 3rd leg for a 44A current draw.

Typically you would need to run at least a #3 wire with a 30A RPC. Depending upon which manufacture you go with, they recommend breaker sizes between 100A to 200A. I found that a 100A breaker was insufficient for my 30HP RPC, and ended up using a 125A breaker.

What is the rated motor HP on the VMC?
@scsmith42, the spindle is 15 HP. Jamie and I have had several discussions regarding the loads. He's aware of the VMC power requirements and he told me that a 70 amp breaker, #4 wire with his 30 HP unit would be more than enough to run the machine. Before I install the conduit and do final routing of all the wires, I'll get the mill under power and air to test the setup this weekend

RPCs are addressed in a specific article in the code - specific rules apply to them.

Read NFPA-70 Article 455: Phase Converters. In particular, Article 455.6.

Read Article 310 for the conductor sizing rules and size them based on Table 310.16.

Realistically you're probably looking at conductors sized #3 or larger, assuming 75*C rated conductors and terminations. #1s if the RPC terminals or upstream breaker are rated 60*C. Most are 60/75 dual-rated these days but old school stuff in an existing installation can bite you if you don't pay attention. Crispy, overheated fuses and circuit breakers are not fun.
Last edited:
30 hp I would personally be running 1/0 copper minimum as 18kva single phase is just under the 200A range
18 kVA / 240 V = 75 A

Code requires conductor ampacity not less than 125% of the RPC single phase nameplate amperage. This is typical of all motor circuits throughout the code as an allowance for the possibility of sustained motor overload conditions and as headroom to allow for nuisance-free motor overload protection. If the motor overload protection were only sized at 100% of nameplate it would nuisance trip constantly.

Assuming 75 A is said nameplate value for the sake of this hypothetical, that would come out to 93.75 A. That would be your minimum circuit ampacity. Max OCPD would be 100A if using 455.7(A). IOW, you would need a 100A branch circuit for a 75A RPC. No more, no less. Installing an OCPD on the three phase output sized at 100% of the three phase rating would then provide satisfactory supplemental overload protection in light of the input OCPD being sized at >125% of the RPC's input rating.
Last edited:
read that wrong, his machine is 18 kva, 44FLA.
the 30 hp motor would have to be more amps then that, im thinking from the input side from the panel like stated.
As my 20HP has 49A at 3 phase which works out to about 83A single phase needed for the input and trips a 60A Breaker easily on start up(ask me how I know).
Just rough half ass guess would put 30 hp in the 80A range 3 phase X the 1.73 from 3 to single phase is about 138.4A single phase input max to drive it is my thought and will easily 150% on start up and size up conductors to be safe.
The whole issue of RPCs is slightly complicated........ even with the NEC.

The general assumption is that folks will run the RPC on it's own single phase branch circuit in a sub-panel, and then combine the RPC generated leg with the single phase input to form 3 phase branch circuits, each with its own 3 phase overcurrent. That way, the wires going to the RPC do not carry any load current directly to the various motors. For that, the 125% rating makes sense, and protects the RPC.

What a lot of people end up doing is to wire the branch circuit to lugs on the RPC, and then continue the wires from those lugs to the various 3 phase loads. Then there is no dedicated motor protection on the RPC, the wires to it carry the load currents also. It is not possible to have a motor protector on the RPC unless it is integral inside the RPC, and the overcurrent sizing is a problem.

That second method is not correct, except in some specific cases.

If the wires coming from the panel power everything, RPC and loads, then they must be sized for the loads like any branch circuit. The RPC section does not affect that rating, it only affects the wiring to the RPC itself, which would need a motor starter/protector of a proper rating.
The first scenario you've described is illegal:

455.21 Start-Up
Power to the utilization equipment shall not be supplied until the rotary-phase converter has been started.

In the scenario described, AC power will remain on 'A' and 'C' busses regardless of whether the RPC is running or not. An output contactor interlocked with the RPC motor starter's overload protection and/or a monitoring relay on the manufactured phase is required to satisfy the code. Can't just backfeed into the same panel supplying the RPC. A dedicated output panel load-side of these control elements is necessary.

Any commercially manufactured RPC control panel worth it's salt ought to have all these functions integrated into it. Power distribution block to separate motor circuit from output circuit, motor starter with overload protection, monitoring relay, output contactor, ancillary contactors for starting and balancing capacitors, etc.

This is why an idler motor on it's own does not comprise an RPC. You need a suitable control panel and an idler motor together to form a complete RPC device.

When in doubt, read the manufacturer's instructions.
Last edited:
Interlock the RPC output so that the generated leg must be present in order to energize the sub panel at all. That positively avoids ever having the single phase output, if the generated phase is not present, even for the startup time of the RPC.

That's more or less a given, although I know a lot of places where the RPC is not wired that way.
although I know a lot of places where the RPC is not wired that way.

Hence state requirements for licensure, a big fat five digit bond and liability insurance to the tune of a few hundred thousand dollars at minimum. And enormous fines for doing electrical work without those prerequisites.

Moral: Homeowners and handymen ought to leave well enough alone and call a professional. There's a reason you have to go through years of training and pass a lengthy exam before you can legally touch this stuff out in the wild without immediate supervision. And even then you still need to earn both a Master 'A' and a Contractor license on top of that before you can truly do it on your own.
Last edited:
Too big a breaker, and you will not kick the breaker when the RPC fails to start. I really think some are overthinking. The breaker protects the wiring, and to some extent the device. A VMC runs FLA when it bogs the spindle at full acceleration, basically never. Oversize the wire, sure. But understand you are oversizing the wire over inductive issues, voltage sag under load, but not safety.
That VMC will have a constant load[a load that the breaker cares about, not an instantaneous load] of, what, less than 30 amps 3 phase.
So 45 amps plus losses, maybe 50 amps.
Oversizing everything is not good design.
The OP said the RPC requires a 70 amp breaker
6GA might be a little small, depending on length
so 4 GA is CODE and more than sufficient
unless you own stock in the wire company
To be fair, 455.21 (and 455.22) do not specifically require the contactor in so many words. The actual requirement under 455.21 is "Start-Up: Power to the utilization equipment shall not be supplied until the rotary-phase converter has been started."
Technically, with that wording, it is sufficient to start the RPC at the same time that power is supplied through to the loads. If the writers of the Code had intended it to be a requirement to avoid single-phasing, they could have specifically required the generated leg to be "up" before connecting loads. They did not.
What a local AHJ interprets that to mean, may vary, and they might require a contactor triggered from the generated leg. That is potentially problematic, however, due to the common issue of a short drop in voltage when a near-maximum load is started. The triggering may need to be carefully done, simply runnng the contactor from the generated leg would interlock, but might drop out.

I agree that the output contactor is a good plan, and may be required in order to start the RPC, since any load applied before starting might affect the self-start. But the Code does not specifically require that solution. A number of RPCs (not all) include an output contactor in the unit, which would cover the requirement.

455.22 requires the loads to be disconnected from power if power fails (and not automatically reconnected). That can be satisfied by powering the RPC input through a contactor with a "seal" contact, so that it cannot be powered after a power fail, unless intentionally started. 455.22 thus does not directly address the single-phasing issue, but could be satisfied by a properly controlled output contactor.
Thanks everyone. There's been some great posts here and I learned a lot.

After confirming all the specs on the phone with the RPC manufacturer, I went with his wiring recommendations ...
  1. 70 AMP breaker.
  2. #4 copper THHN from the PANEL to the RPC.
  3. #6 copper THHN from the RPC to the VMC.
  4. #8 copper THHN for the ground from PANEL to RPC and RPC to the VMC.
I confirmed all the specs with the RPC manufacturer a second time, and he said the RPC can peak at 70 amps during startup, but it will never draw above 70 amps, even under load. I will be pick up a clamp meter and will watch the circuit load before I run all the wiring in conduit.

I finally got to test my VMC under power and air on Sunday and so far, so good. I will be installing the conduit this weekend and get the wiring up off the floor. 😂 :nono:

Last edited:
I see comments about using #1, #2 and #3 wiring here which I find confusing when, according to the NEC 310-16, the allowable ampacities for #4 and #6 THHN are 95 amps and 75 amps respectively.

Based on the chart below, the #4 AWG coming from the panel is well within the range of 125% of circuit size (70*1.25 = 87.5 amps). And so is the #6 AWG between the VMC and the RPC, considering the VMC's rating of 44 FLA (44*1.25 = 55 amps). And knowing that the RPC and VMC are the only two loads on this circuit, if I combine the RPC's idling 20 amps and the VMC 44 FLA, I'm still at 64 amps (if that is the correct math).

So considering all that, and knowing that the manufacturer of my RPC said "the RPC can peak at 70 amps during startup, but it will never draw above 70 amps, even under load," why would I need to use #1, #2 or #3 wiring? This seems pretty straight forward to me, but I'm not an electrician and I'm always willing to learn from people with more experience than me. Am I missing something here?

Last edited: