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Approach for extending 3-phase in a Shop

squeeze

Plastic
Joined
Jan 20, 2012
Location
Connecticut, USA
I have several 5.0 - 5.5 (16.2 amp max) hp loads on a rotary phase converter (RPC - Phasemaster MA1-B) whose nameplate states that the "largest motor" hp is 7.5, with total of 22.5hp. I have it powered (single phase) by a disconnect switch with a 40 amp two-pole breaker and 35 amp two-pole time delay fuses. Wired with 8 awg single-phase input and 10 awg 3-phase output. The machines are not run concurrently - though nothing prevents them from being run. Technically, any of these motors could be run on a 12 awg wire, but then that wire would be protected by a 35 amp time delay fuse which ... is likely not safe.

I'd like to extend the 3-phase to some smaller machines - tapping off the disconnect box with conduit and running to several outlets. The first outlet should support that 16.2 amp max load, and then the second outlet should support a much smaller motor in the 2hp range (5.6FLA at 220volts).

I'm confused about a logical approach. My gut tells me I should have a 3-phase breaker panel between the RPC and the machines with at least two 3-pole breakers. A 30 amp 3-pole breaker for the 5.5 hp with 12 maybe 10 awg, and 15 amp 3-pole breaker with 14 awg. Since a 3/4 conduit can hold up to 11/10 awg thhn, I should be ok putting the four 10 awg wires and four 14 awg wires in the same conduit for the less than 40 foot run? I'd also be ok using time-delay fuses instead (probably a 12amp and 25amp respectively)

Question
Is this a rational approach and if so, is there a small 3-phase load center type panel box (two, 3-pole circuits as described above) that people have used? Is there some sort of in-line breaker alternative (I have a space issue).
 
If you have any intention of using the shop for a "long time" get the load center. Really simplifies things.
I was lucky, my lineman neighbor dropped (used) one off with a selection of mostly useful breakers. I bought a few more for specific loads or nused machines over the years.

Most areas/decent size cities have a surplus electric industrial outlet of some sort. Some junkyards are known for having electrical motors and apparatus. Even industrial (faqctory) auctions can be a source. Any large enterprise tends to accumulate maintenance and removed electrical "stuff". Check around and it might not be necessary to buy new.

If you buy used, go for something for which new breakers are commonly available and cheap (relative) such as Square D.

smt
 
I went back to look at the manufacturer's installation instructions on the RPC (when all else fails - read the instructions) and what caught my eye is their chart detailing the breakers, wires, etc. On the 3-phase wire they specify 8 AWG ... which seems oversized for the loads, though on target for the capability. One thought would be to create a raceway at full capacity (the 35 amps) and then branch using fused disconnects which are reasonably cheap especially if I only need two of them. That would have the advantage of reduced voltage drop through reduced resistance, and greater flexibility. Conduit fill recommendations, including derating, would not allow the four 8/AWG conductors in a 3/4" emt, which is of course what I put up some months ago. It appears I could get 8/10AWG THHN conductors in that conduit with derating.

I already have the single-phase load center, and adding a three-phase would require some juggling to get the room, but it looks like I've boxed myself into a corner, and that's the only solution available.

WireSelection.jpg
 
You would need 8 AWG wire only if you had multiple machines that were in simultaneous operation (each machine adds to the generated 3phase power). A more reasonable approach, in my opinion, would be to estimate the max load the converter will see and size the wire and output breaker for that load. Unless one of the motors is pretty darned big, you may find that 10 AWG and a 30A-40A breaker (depending on the temp rating of the wire) is all that is needed. Smaller tools will need 10 AWG power cords, even though their current draw is way below the capacity of the wire.

Remember that the breaker is solely there to protect the wire. Overload protection for a tool needs to be done in the contactor for that tool.
 
There are a lot of things at play here.

First, every machine has to have a disconnect. That can be a plug or a disconnect switch, or a breaker. I'll assume your shop is not big enough to worry about line of sight or derating for long wire runs.

Machine tools are motor loads. Motor loads are handled very strangely by the NEC. Basically, a motor can take a large breaker to get started, but once running is much lower in amps. The NEC allows you to use much larger breakers for motors than the wire size would normally require.

Running multiple motors on the same circuit has very special rules. I suggest you avoid doing that if you can. Get a 3 phase panel and run a circuit for every machine.

Also, you need 3 conductors for every machine. If you need #10 wire, you can get 3 machines on one conduit. Remember that all the circuits in one conduit can share the same ground as long as it's large enough for the largest load. Ground size is based on the breaker size, not the conductor wire size.

You need to read the NEC and/or consult an industrial electrician. Don't bother with residential electricians, they're usually our of there element in a machine shop.
 








 
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