Warehouse Options

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The Situation

We have a 3 phase air compressor, but our breaker box only has a single 3-phase breaker (it's for the air conditioning). The 3rd phase wire comes directly out of the wall and into that single 3 phase breaker. It's not a 3-phase panel. There is room for 1 more 3 phase breaker (delta type) in our breaker box.

We want to get a 7.5hp air compressor running and our lathe which currently has no motor (but the stock motor is 3 phase). We may have more 3-phase equipment in the future too.

Note: Our landlord will not help us pay to improve the 3-phase power situation in our building.


  • Buy a ~20 hp rotary phase converter for the compressor and lathe
    • Pros: We get to keep the 3-phase air compressor, we do not need to tweak the lathe, can run future 3 phase stuff, and we can take it with us to our next building.
    • Cons: Rotary phase converter will cost $1000+, and we can't run the compressor and lathe at the same time. Also consider additional wiring cost. Not as efficient as 3 phase power without the converter (but won't be running much anyway). Chart Explaining Phase Converter Requirements

  • Sell the compressor and modify (or sell) the lathe for 1 phase.
    • Pros: Likely the most cost efficient, because we won't need to buy anything to improve our electricity situation.
    • Cons: Not sure if a 1 phase motor on the lathe will work as well as a 3 phase motor. Requires hacking (or selling) the lathe, selling the air compressor, and buying a 5hp air compressor instead of a 7.5 hp.

  • Get a rotary phase converter (or VFD?) for the 1-3 hp 3 phase equipment and get a different compressor.
    • Pros: Rotary phase converters for 1-3hp motors are only $300-500, and can run several pieces of equipment at a time (Frank says $150 VFD is better). It's not a permanent investment, we can sell the converter or VFD.
    • Cons: Will need to get a 1 phase air compressor.

  • Install a 3 phase subpanel
    • Pros: We can run several pieces of 3 phase equipment at the same time without buying converters.
    • Cons: Will probably cost $400-1500 ($1500 was the first quote). It's not a good idea to make a large investment in property we likely won't be in for the long term. Doing this legally with a permit is expensive, and doing this without a permit carries a lot of liabilities (insurance doesn't cover claims that result from unpermitted electrical work, we could get a $600 tampering fine, and the city could require us to get a permit if they catch us - then require us the bring the rest of the building up to code).

  • Replace the air compressor motor
    • Pros: We would not need to get rid of the compressor that Robert has become emotionally attached to. We may be able to sell the old motor or keep it and use it later.
    • Cons: Will at least cost $200 for a less powerful motor. It's more cost effective to get a new compressor. Does not allow us to run other pieces of equipment, so a phase converter will be required for the lathe. This option has nearly the same results as selling the compressor and buying a single phase one.

  • Replace our main panel with a 3 phase main panel
    • Pros: We can run several pieces of 3 phase equipment at the same time. Also, we can fix the problems with our current electrical setup.
    • Cons: Probably our most expensive option, we have not gotten a quote on this yet. It's not a good idea to make a large investment in property we likely won't be in for the long term.


Ended up selling the air compressor for a profit to avoid the hassle/cost/liability of installing 3-phase, and we purchased an air compressor the same size/power as the 3-phase air compressor for only $100 more. We are planning to get a $150 VFD to get the lathe working.