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Mike, Master Electrician
Category: Electrical
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Experience:  Licensed Master Electrician - OnQ Certified Data Voice Audio Video Installer
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We have a 60 amp electrical service to our home, plus a

Customer Question

We have a 60 amp electrical service to our home, plus a separate meter for an electric hot water heater no longer in use. There's not a lot of draw in the home--gas stove, gas HW heater, no toys. Then there's an overhead 4-3 alum line that runs about 75' to our well house with a small breaker panel that powers the 1 1/2 hp pump, along with a 6-3 copper line that carries 230 to the barn (shop), and another 10-3 copper line that carries 115 to the shop.
What I'd like to do if possible (without spending the $2000 right now needed to do an upgrade for all this), is to install my 20 hp RPC in the shop. It's static converter/idler motor combo. It draws 20 amps at idle on three phase, and a guess I got was 30 amps at idle on single phase 230. I also have a 3 hp dust collector, 5 hp table saw, 4 hp jointer, 3 hp jointer, 4 hp air compressor, and I think 2 hp air filter cabinet. There are all three phase. What I was thinking is to start up the RPC with an idler motor where it will draw approx. 30 amps, then have a separate line to run the dust collector, and another to run either the table saw, jointer, or bandsaw--just one at a time. The air compressor can wait to run by itself, as can the air filter, but the dust collector would need to run with the table saw, or jointer, or bandsaw. Is this doable with what I currently have? and how would I set up the circuits? Normally no one is home during the day when I'm using the shop, so theoretically the full 60 amps would be available for use, and I believe the lines are big enough to carry these loads. If I blow the main fuses on the odd occasion I'm not worried about it. Just don't want to be stupid about it.
Submitted: 1 year ago.
Category: Electrical
Expert:  AssuredElectrical replied 1 year ago.

Welcome. My name is ***** ***** would be glad to assist.

Based on the information and all that equipment, I really see only have 2 choices.

1. You can install a disconnect for each piece of equipment and the feed to all of those from the RPC

This is very expensive

2. Install a 3 phase panel and then feed the panel from the RPC and have individual breakers for each. piece of equipment

then you can control what is on or off based on the needs.

If you go to the secondary used market, you can pick up 3 phase used equipment at some really low pricing.

Buying all new will get costly also with panel and breakers.

Not sure if this is the type of situation you were looking for or not.

Let me know your thoughts and we can kick it around some if needed to see.


Customer: replied 1 year ago.
Thanks Tommy, that's helpful and option two sounds like it will work well, and I hadn't even thought that far ahead. My real question was whether the overall limit of 60 amps is enough to make all this work safely, and then, what size breaker should I install for the RPC (running on the 6-3 copper line with 230v single phase from the well house) to still have enough amps left to run the equipment. As mentioned, I'll probably start the 20 hp idler with a pony motor so I can use a smaller circuit and have some left for the machinery. Seems like the RPC alone will use 30 amps on single phase, though I'm waiting for verification on that from another source--unless you know. Then from what remains available in amps, the dust collector (3 hp) and table saw (5hp) running concurrently comprise the largest draw. Is the remaining 20 or maybe 30 amps enough to run both of those?
Expert:  AssuredElectrical replied 1 year ago.

The company you are looking at to get the RPC from, should have something online on the FLA (full load amperage) on single phase

But to see, need the manufacturer and model number.

From you post, all the equipment was 3 phase, so I must have missed something.

"There are all three phase. "

If all 3 phase nothing else will take any other power except through the RPC.

Let me know what I missed?

Please elaborate on the need for an idler motor to start the RPC? It doesnt operate on its own?


Customer: replied 1 year ago.
Thanks Tommy. The 6-3 wire from the well house is delivering 230v single phase to the shop and this will power the RPC. All of my larger machines in the shop are three phase, hence the phase converter. I already got the info on the motor from Marathon Electric, just waiting on them to confirm what the draw will be on the 20 hp idler with no load, but running on single phase. At idle with three phase it draws 20 amps. I had one educated guess that it will be at least 30 amps if powered by single phase. The idea with the pony motor is to get that big 20hp motor up to rpm before switching on power to it so it doesn't draw as many amps on startup and require say, a 50 amp breaker. But you're saying that all the three phase machines (dust collector and table saw for instance) will be drawing their power solely from the one circuit the RPC is running on? And what will be the total balance due if I call?
Expert:  AssuredElectrical replied 1 year ago.


First, you would have to look at each piece of equipment you are wanting to run on the converter.

They will all have nameplate data, so you need to record all of them so you will have the FLA that each

one will use, then you can any 2 together if they must be running at the same time or look at all your combinations.

The specifications should easily be available to know what the RPC will draw at start with no load.

They should also have a HP to Amperage on the 240 single phase side and the same for the 3 phase side.

They publish charts on these, or should anyway.

That makes it easy to look at and know if you will be able to run one.

20 hp probably could go as high as 60 amps on the input side if fully loaded and that is too much for your situation.

The 6/3 cable is only capable of 55 amps maximum

Can you provide the Model number and I should be able to find all the information hopefully.

Customer: replied 1 year ago.
Tommy I spoke with Marathon earlier and gave them the model # ***** the 20hp motor. They referred me to one of their engineers who deals with three phase, so I'm waiting to hear back. If you'd like to pursue that as well I'd be happy to give you the number.I also need to collect the amperage info off each motor and see what the combined draw will be--got that.What I'm trying to do with my limited 60 amp service at this point is to make a 'less-than-ideal' situation work safely for a year or two. I realize the 20hp motor should normally be on a 60 amp circuit. But if I were to use a pony motor to get it up to it's rated rpm (1760) and then kick the power on to the motor it won't draw nearly as much amperage but will sit there idling the entire time. There are a lot of guys out there who are doing this with just a bare three phase motor. I actually have a complete RPC which doesn't require a pony motor to get it going, but it does require a lot of amps--unless I bring it up to speed first before delivering power to it. This potentially allows me to install it on say, a 50 amp circuit, maybe 40 amp (since no-load draw is 20 amps on three phase input and around 30 amps with single phase input--waiting to confirm with Marathon).I'm hoping by doing this it will leave enough power available to run the machines. But do I understand you correctly that each three phase machine would be drawing power from the circuit the RPC is on? In that case perhaps I should install the RPC on a 50 amp circuit which should be plenty for a maximum total of 8 hp running simultaneously with some load, and the 20 hp just idling. My thinking is that it might work, and of not, the main will blow and I'll have to come up with a better plan. Am I missing some pieces, is this making sense, and is it a fairly safe recipe?
Expert:  AssuredElectrical replied 1 year ago.


If you could just post the Model number we can see what is available for the specifications and operation of the RPC.

And the Manufacturer if it is not Marathon.

Until we can see the specifications, there is no way to know if this extra start up motor is a need or not. It may pull more

than the RPC at idle , in which case you have a larger load than necessary.

We need to deal with facts and not guesses at this point.

There are some rotary converters that start with a very low input current at idle due tot he design of the controller and ramp speed.

Please post what you have and we can move forward to evaluate.


Customer: replied 1 year ago.
Thanks again Tommy, the RPC is a combined Ronk static 'Add-a-Phase' model 80A, serial # ***** and a Marathon 'E Series' 20 hp three phase motor, model #6VK256TTFC4026AAS. They have a place to look up model numbers on their site but this didn't come up. I had to call to get any information. I doubt that it's a soft start motor.As for the pony motor to bring it up to speed and create a 'soft start', those are normally just small single phase motors connected with pulleys and a belt or a coupling which can disengage once the big motor is running on it's own power.
Expert:  AssuredElectrical replied 1 year ago.

Ok, thanks.

Give me some time to see what I can find out.


Customer: replied 1 year ago.
Tommy, just checking to see if you have any additional information on this.
Expert:  AssuredElectrical replied 1 year ago.


Looking for the information does not bring anything up on the RPC.

They use models and the only one they show for multiple motors, is for 2 motors.

I am unsure where this 20 HP motor you are referencing is coming from?

Who is supplying this motor and why? It is not in the RPC design.

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