Pre-Made RPC Panels/Amperage Calculating

I have been collecting woodworking tools in hopes that I will be able to start a woodworking business. I now have three 3 phase tools, all purchased within the last month. I obviously plan on buying or making an RPC, but the electrical run to my garage is only 50 amps (when I first ran the electrical I had no plans to include an RPC in my shop). I have been told that 50 amps should be enough, but most 10 HP RPCs have a max. circuit rating of around 60 amps. (Yeah, I know it's a MAX rating)

I know that I probably won't ever get close to that kind of draw, but how can I be sure that when running the RPC and starting one of the machines I won't brown out the lights? I may get to a point where I am trying to run flourescent lights, a fan or two, maybe even a window air conditioner, a radio, the RPC, and one tool (either 3 phase or 1 phase) concurrently. I know this isn't very much, but I was hoping to get an idea of how to calculate the maximum draw.

Next, I have found a pre-made 10 hp RPC panel on e-bay (from WNY) for $175. I am confident I could make my own, but I kinda don't feel like tackling this project from scratch. Does a panel like this sound like a good option? Of course, I would still have to find a used 10 HP TEFC motor, but are there any signifant problems with trying to go this route? What other expenses may be involved?

I know this is a lot for one post, but I am hoping the answers aren't nearly as long-winded. Thanks all!

Hutch

Posting more info about the motors and equipment to be run off the RPC would be helpfull. Hp, full load amp ratings, operating voltages, specific machines and likely patterns of usage would be a good start.

From the information you've given so far, it seems doable and with more info we'll be able (hopefully) to give you better advice.

neil

The RPC draws the highest current while starting. It will draw more than 50 amps, but only for a short period of time so it will not blow a 50 amp breaker.

Once idling, a 10 HP RPC will draw about 5 amps so it will not be the source of your currnet limitation.

The limitation will come from your machines. If they are all under 3 HP you will be fine.

If they are in the 5 HP range, you will most likely be fine, but it will depend on the other loads as well as how hard you are working the machine.

If they are in the 10 HP range you might get away with it, but you would have to make sure you minimize other loads (shut the A/C, fridge, etc) and take less than full load cuts.

50 amp service is rated for 40 amps continuous use and 50 amps peak. Take the 3 phase FLA of the tool motor an multiply by 1.73. Add 5 amps for the RPC and you will have an approximation of how much current that particular tool will draw at full load. Take into account the current draw of the other shop items and see where you are relative to 50 amps...

If you are going to buy a WNY box, get the gold series. It has nicer internals.

-Joe

Fla

Is the FLA what's listed on the motor plate? I don't have the other tools at my shop yet, but I will give as much info as I can.

1.) 5 HP Radial Arm saw, currently 440v but I'm going to re-lead it. 6.7 amps@440V - medium start motor according to the motor shop.

2.) 3 hp tablesaw motor, 200V. Unknown amps. Sorry, I know this isn't very helpful. It's currently still at a local school waiting for me to pick it up.

3.) 8" jointer, 1/2 HP 200v. Unknown amps, also at the school.

I will specify more info as soon as I can. Also, I don't think any of these tools will be running concurrently. To try to reduce startup amps, is it worth trying to use a 7.5 HP idler, or is that cutting it a little close for the RAS motor?

Hutch

P.S. There is a commercially available 10 HP RPC whose max. breaker rating is only 40 amps. Why would most other 10 hp RPCs be so much more?

The commercial 10HP unit that only "requires" a 40 amp service will not allow you to run a full 10HP 3 phase tool. It will probably work just fine if you don't approach the limit of the tool, but it just can't generate the full 10 HP. I would say the suggested feeder is too small.

A 3 phase motor draws about 2.8 amps per HP, so a 10 HP 3 phase motor will have about a 28 amp FLA. (The number on the motor plate) If you multiply that by 1.73 to get from three phase to single phase, you get 48 amps. You have to add some current for the idler motor (about 5 amps) which brings you to 53 amps. Most people size 10 HP RPC feeders at 50 - 60 amps.

The 40 amp service has only 8800watts (220V * 40 A) available, which equates to 11.8 HP. The idler itself will draw about 1.5 HP (220V * 5 A) so that leaves 10.3HP. If your tool motor was 100% efficient, you could just squeak out 10 HP, but more likely your tool motor is 80% efficient or so, so you just will not be able to get 10 HP out of 40 amps single phase.

Going to a 7.5 HP idler will not really help your situation. Your idling current will go down, but you do not change the current demand of your 5 HP saw.

6.7 amps @440V * 2 = 13.4 amps @ 220V three phase, which is about 23 amps single phase, plus the 5 amps for the idler brings you to 28 amp draw if the saw is taking a full HP cut. If you have 50 amp service that leaves you with 22 amps @ 220V for the rest of the shop. You will probably be fine with that, depending on how big your A/C unit is.

I would build or buy a 10 HP RPC, fed with a 50 amp circuit.

A lot of the eBay grade RPCs are junk, the WNY gold series is decent.

-Joe

50 amps it is....

Thanks for all the information, Joe. I really appreciate it.

At least now I know that the 40 amp rated RPC is probably rated too low. Granted, it would be nice to be running 60 amps in my garage, but it's not worth the digging and $225 to get new wire at this point.

This may be a stupid question, but if I were to put a pulley on the idler shaft, and wrap a rope around it and pull it (like a sideways top) to start it spinning before throwing the on switch, would this help it draw fewer amps on startup? I only ask in case it actually pops the circuit breaker on startup. Also, does it matter whether the idler is 3450 rpm or 1725 rpm? Thanks.

Hutch

P.S. WNY hase a sizing chart that says the rotary fuse size is 30 amps. I am a little confused. They also don't say if a panel is in the Gold series or not. Hm....

Matt that might actually make it draw more amps on startup

What size is the wire you got buried now?

Based on the information that you posted, your total single phase current draw would be around 45amps if all your piece of equipment was running at one time.
The prices on the control panels on wnysupply website are for standard series panels, the gold series is not listed as of yet. The 10Hp gold series would sell for $320 minus a 15% discount for PM members.

Just keep a flashlight in your back pocket.

If all your equipment was run at full load simultaneously, you would draw about 45 amps as Macplus stated.

Since you said you would never run two pieces simultaneously, I calculated based on your largest piece of equipment being the 5 hp saw. For that and the RPC I get 28 amp single phase draw if it was taking a full 5hp cut.

30 A is what you want to fuse the three phase output of your RPC at. This will allow you to use the full potential of the RPC, and only run #10 wire as your output wire. 30A three phase is equal to about 52A single phase. (30*1.73)

The gold series WNY is what you want. It will be more expensive than the standard WNY, or other "eBay grade" RPCs, but it has much more appropriate sized components. I'd have to find the thread that showed the internals, but I think the only nitpick I had was that it did not have a fuseholder for the three phase output. You can buy a fuseholder surplus for a few bucks so it is a very easy and inexpensive addition.

-Joe

"30 A is what you want to fuse the three phase output of your RPC at. This will allow you to use the full potential of the RPC, and only run #10 wire as your output wire. 30A three phase is equal to about 52A single phase. (30*1.73)"

I would run 2 x #6 to the panel, and to the A and C loads, then tap off 3 x #10 to each three-phase machine.

This way, you could still use the full-rated single-phase capacity to single-phase loads, and for three-phase loads you'ld still be OK.

If you want to mimimize the starting inrush for your converter, you make is with a
pony motor, belted to the idler. The pony motor brings the idler up to synchronous
speed before the power is applied to the idler. Lowest inrush possible that way.

Do you want the photo?

Jim

Some say 1725 RPM starts easier, some argue that 3450 RPM can keep better phase relationship, I dunno. I like 1725 RPM personally, but it might not matter much.

If you start the motor spinning with a rop or otherwise you will reduce inrush amps.

-Joe

Ignorance isn't bliss!

Well, I have a couple things I am wondering about. Woodworking machinery is quite forgiving, in terms of how accurate/balanced the 3 phase is, correct? I don't know how nice of a panel I actually need, and I don't plan on doing major production work. Why do you think the cheaper panel might not be a good idea?

Also, is it pretty much a certainty that the 10 hp idler would draw 50 amps on startup? I already have a 40 amp fuse, and was curious as to whether I would have any hope of that working.

Thanks all for the input!

Hutch

The quality of the three phase output will be the same between the regular and gold series panels.

The gold series uses components that are better sized to handle the inrush currents, and adds features such as overload protection.

The 10 HP converter will draw well over 50 amps at startup, but it will not blow a 50 amp breaker since the inrush current only lasts a very short period time. It probably will work fine with a 40 amp breaker as well, as long as you do not run more than 7 HP.

I would go with the gold series WNY and a 50 am breaker feeding #6 wire to the single phase side.

There are a lot of eBay panels out there that will be cheaper than the WNY gold, but most of them are built with undersized components. Take a look at this thread and this thread for more info. Except for the lack of a fuse block for the output, the WNY gold looked very good inside.

I have no affiliation with WNY, or any of the other converter makers, I just look at the pictures that people post.

-Joe

I already have a 40 amp fuse, and was curious as to whether I would have any hope of that working.
If you are using fuses, make sure they are dual-element. As they are dsigned to take in account for motor inrush current.

Words

I meant to say that I already have a DP 40 amp breaker.

Next up:

I now feel like I have a few viable options.

1.) Buy a cheap panel and try to improve it with your help.

2.) Buy a Gold series panel, (and maybe add the component you mentioned)

3.) Make an RPC and know that all the components were properly sized.

I feel that if I had a good diagram or a well labeled schematic, a list of parts, and where to get them I think I might be able to make one. Is this an inexpensive thing to do? Do you think it's a project worth tackling? Thanks.

Hutch

P.S. I have access to an inexpensive multimeter. Is this ok for this kind of project? I certainly won't need a good MM for anything else.

Only thought I have is for you to eliminate the "buy a cheap panel and upgrade it" This will cost more than buying the gold series to begin with.

Cost of building your own will be about the same as the gold series you are looking at. If a person has a lot of the parts lying around, costing nothing, then one can build cheaper but not if you have to buy from eBay or buy new. I have a 7-1/2 hp and a 10 hp capacitor start RPCs under construction now. Here is a list of components I use for a 10 hp and what I paid for most of them on ebay.

16 x 20 electrical enclosure $80, used good condition
60 amp, 3 pole, contactor, $20, New old stock, main contactor
40 amp, 3 pole contactor, $15, New old stock, start contactor
overload relay, $25, Used good condition, for idler motor protection
distribution block $20, New unused condition, to make all the connections easily
2 ea, 3 pole, 30 amp fuse blocks, $15 new old stock, output circuit fusing
Run Caps, 280 mfd total, $50, used
Start Caps, 3ea, 324 mfd caps, $30, new, for start circuit
Potential Relay, $30, new, for start circuit
30 amp fuse block for control circuit fuses, $10, new
Start / Stop push buttons, $10
Throw in about $20 for wire to wire it and I have over $300 invested and I still need to buy some fuses.

These prices include shipping costs.

What I have is a good enclosure, good contactors and good overload protection. I do not have to buy anything else to install the RPC other than wire and conduit.

Idler RPM...

In my home-made RPC, I ran a 10hp 3600rpm TEFC motor initially... it did start slower, drew more current, for a longer amount of time. I switched to a 10hp 1725rpm ODC, which starts a whole lot faster, with less start capacitance, and is a whole lot quieter. My experience is that the faster idler makes the output 'stiffer'... but running my machines (Monarch 10EE and two Bridgeports) never seemed to make a substantial difference other than a little bit of startup time.

Of course, if I want a stiffer supply, I can switch on another idler... or start up (and not load) another machine. I HAVE spun up both bridgeports to see how much of an effect it has on the Monarch's MG-spin-up time, and the Monarch DOES like it... and of course, I've let the Monarch idle while starting the BPs, and they're happier that way...

But it works fine either way.

I have my RPC fitted with 60A breakers, and the only time I ever tripped 'em, is when I was 'trimming' the start-cap value. Do note, however, that my RPC has a 'load' contactor, which doesn't close until after the RPC has started (detected by the 'generated phase' being above a certain voltage, and the start-capacitor contactor being disengaged... both for more than about five seconds)... so there's no way I could ever engage a load while the RPC was starting. Furthermore, my machines are all fitted with contactors on latching circuits, so IF there's a power-blip, the machines will shutdown, and not restart until I've come over and re-pressed the START button on the RPC, THEN on each machine.

Option 1, "Buy a cheap panel and try to improve it" is a bad idea. It will cost more than buying a good panel.

Option 2 is the easiest.

Option 3 may be the best, but it will take the most time and could cost more or less than option 2 depending upon parts availability.

-Joe