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I have old-school delta 3 phase (no neutral 240v) service to my shop that has 60 amp fuses on the main. I'd like to hard wire a new machine (max 30amp) about 40 feet away by running #6 wire in conduit to a non-fused 60 amp safety switch that would sit next to the machine. Would this be a proper approach or Should I consider a 60 amp safety switch installed with 30 amp fuses? Should I do something else entirely? Is #6 AWG the correct size wire for this run? I need a little bit of voltage drop because this is a 220v machine with a top end limit of 242v and im measuring 243v at the panel. I'd rather not have to add a buck boost transformer for barely being over voltage limits on the machine. Thanks, Max

Edit: It's high leg delta with no neutral (this property is a 100 year old egg ranch). Machine is 220V / 60 Hz 3 Phase / 15 KVA (7.5hp). A disconnect would be nice because I'd like to leave it disconnected when not in use to protect the electronics. I'd also like it in case something happens with the machine and I can't stop the machine at the control panel

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    Are you sure it's truly an ungrounded delta service, or do you have a corner-grounded 240V delta, or is it a high leg delta with the neutral omitted?
    – ThreePhaseEel
    Aug 9, 2018 at 11:47
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    Do you want a disconnect near the machine for convenience or to satisfy code requirements? Can you post the nameplate info?
    – batsplatsterson
    Aug 9, 2018 at 13:17
  • What is the horsepower of your machine or nameplate rating. Maybe post a picture of the nameplate?
    – Retired Master Electrician
    Aug 9, 2018 at 15:45
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    Hello, and welcome to Home Improvement. I'd be really concerned at depending on the voltage drop of that wire to adjust the voltage. In any case, this really isn't on-topic here.
    – Daniel Griscom
    Aug 9, 2018 at 16:09
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    I'm voting to close this question as off-topic because this isn't about home improvement.
    – Daniel Griscom
    Aug 9, 2018 at 16:10

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OK at 220V your RLA (run load amperage) is 39.2A, NEC Table 430.250 says you can calculate 22A for a 7.5 hp @ 230V. You are saying that the new machine would run on 30A so that would fit the RLA for a 7.5 HP motor. Using that as our guide your minimum circuit amps would be 27.5A. If you machine is actually 15KW then your RLA is 39.2 and your minimum circuit amps would be 49A.

So you either need a circuit that can be fused at 30A (7.5HP) or 60A (15KVA). Your #6 conductors would work just fine. Running a quick voltage drop calculation you are under 3% but you will drop a volt or two. Besides most motors prefer to run a little hot (higher voltage)rather than under voltage.

Because of the discrepancy of the information, I would install a 60A disconnect for the machine, since you can always fuse down, while fusing up could be a problem. Also your machine disconnect must be within line of site, maximum 50' away in an unobstructed pathway and lockable. Whether you want to fuse it at your disconnect is up to you, but never fuse it higher than the main or any upstream over current protection.

Also pay particular protection to @ThreePhaseEel. If it's a 100 year old facility it could very well be a corner grounded delta. You might verify that with a local professional before proceeding.

Stay safe and good luck.

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