Powering a Bridgeport mill and Victor 1440G lathe?

After finally working out the kinks on a used 20hp rotary phase converter I'm now working on wiring up a 2hp Bridgeport mill and 3hp Victor 1440G lathe. The rotary is in the opposite corner of my shop about 80' away from the machines. I plan to run 4 gauge wire with a 50 amp breaker to the rotary, which is right next to a 100 amp service panel. From there I plan to run 6 gauge wire to a 3 phase load center, which is fairly close to both machines. Each machine will be on its own circuit with a 20 amp breaker going to the mill and a 30 amp breaker going to the lathe. Each machine will be plugged into a twist lock receptacle using 10 gauge wire. The Bridgeport motor apparently draws 5.8 amps at 240V and the lathe 3.4 amps (coolant pump included). Am I on the right track? Suggestions?

An RPC with a 20hp idler will likely have a usable output of around 10-12 horsepower. That would put you at an input around 38 amps and an output about 22 amps of three phase before things start to get toasty/unbalanced (three phase amps = single phase amps / 1.73). For an 80' run I don't see any reason to use anything above 8 or 10 gauge.

I think the RPC being fed with a 50 amp breaker is a good plan, though 6 gauge is fine unless you have a long run.

You should look at the smallest wire gauge that can handle the current, then calculate the voltage drop for your runs to see if you need to go up a size or not. That is unless you plan on upgrading the RPC later.


One important note: When shorted RPCs can supply up to the single phase breaker input rating to two of the output wires, so if you want to do things absolutely properly you need to size those two legs for the 50A breaker or add a breaker right after the RPC for 30 amps. I don't see anybody actually doing that very often though.


It seems what you want to do will work fine with no issues, but could be more expensive than is needed.

Thanks Strostkovy! Yes, I'm trying to do the best I can without going too overboard. I also want to leave something for future additions if the need arises. I like the idea of using 6/3 wire (same as the feed to the 3 phase load center) to the RPC panel as it will allow me to pick up a neutral from the main panel. I have a contactor that uses 120V to bring in power to the RPC. The 6/3 to the 3 phase load center seemed like a good idea considering the minimum wire size feeding it recommended by Square D is 6 gauge. It also leaves some room for future expansion. I like the idea of the extra 3 phase load center as it allows for a neat distribution to the 2 machines.

A few things I'm confused about though are; 1) If the lathe only uses 3.4 amps why does the manufacturer have 30 amp slow fuses built into it? 2) The Bridgeport has a 2hp motor but draws 5.8 amps, almost double what the larger 3hp motor draws on the lathe. It doesn't make sense. Regardless, is there anything wrong using a 20 amp breaker serving the Bridgeport and a 30 amp breaker for the lathe? And is 10 gauge wire to the receptacles appropriate for both cases?

Make sure you are careful with your neutral, and don't try to connect a load between neutral and the generated leg. I think you get something like ~177 volts, but don't quote me on that. What is the amperage of the load center? 6 gauge recommended feed wires makes me think it's a 50 amp panel, in which case I would stick with 6 gauge all of the way up to it as you intend.

The three phase load center should work great. That's what I am doing with my 30hp capacity RPC.

1) Lathe motors can draw a very high startup current to start spinning the potentially heavy stock, and will have to have large fuses to accommodate the resulting high currents. It's possible the run up time for the lathe with something very heavy in it is very close to or slightly longer than a slow blow fuse will pass the current for, so they have to uprate it.

2) Wait, that lathe is 3 horsepower and only draws 3.4 amps? That FLA seems correct for it to be wired for 460V. Hopefully it can be rewires to 220V, which could also explain the higher fuse rating because it would then be drawing 6.8 amps.
The two horsepower motor sounds about right.

I see no reason not to use the breakers and wire that you are suggesting.

I'll pick up the hot and neutral from the single phase main panel. It will only be used to operate the contactor in the RPC panel. I am aware of the limitations of the generated leg on the RPC. The 3 phase load center is rated for 125 amps but I won't be using anywhere near that limit. According to the manufacturer, Square D, the minimum incoming wire size is 6 gauge.

I checked the specs again on the lathe motor and the 3.4 amps is correct at 220V. Still doesn't make sense.

beeser said:

I checked the specs again on the lathe motor and the 3.4 amps is correct at 220V. Still doesn't make sense.

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Maybe they are small horses?