I am renting a small warehouse which currently has single phase power. All of the machines I use require three phase power. I called the electric company and asked about a new installation of three phase power, but it was incredibly expensive. The building next door does have three phase power and they have offered to provide service from their transformer bank.

Upon calling the electric company, I was told that the wire had to be buried and it must be copper, not aluminum for three phase commercial service. I looked at Lowes' website and I have no idea what cable is needed. Here are the specifics for my project:

  • Distance: About 200-250' from my breaker box to the remote 3 phase connection
  • Power Needs: I have a 40Hp screw air compressor and run a few smaller machines that have 5 - 10hp electric motors. In addition to this, I test other machines in order to sell them, and they can have motors up to 100 or 120hp. I can avoid running multiple motors at the same time, if needed. Also, I have an electric pressure washer with two 2.5 HP electric motors and an electric water heater. Its electric plate states 20kw as total electric needs.
  • Conductor: Must be copper per electric company
  • Location: Must be buried, in a conduit
  • Line Voltage: I have no idea what voltage comes off of the powerline at 3 phase.
  • Machine Voltage: Some machines I have say 480 Volts, others say 240 Volts and, I believe, some say 208 Volts. I can't remember on the last one, but I know 480 and 240 are used on some.

Can you recommend what size of cable I need to specify for this distance and power usage? Also, how many amps should the 3 phase breaker box I specify be rated at, and what else should I know before signing on the dotted line? I will hire an electrician to install the box, but I am trying to educate myself on the matter before spending any money.

  • Harper, this comment is not constructive in any way. I clearly said that I am going to hire an electrician, but I want to educate myself on the matter before spending ANY money (yes, it costs money to hire an electrician.) Without asking questions online and reading literature from my electric company, I can't even have an informed conversation with the electrician on any level. Part of my reason for doing this is to set a budget up for the entire process, even if it's a very rough number. Please, don't get a job in customer service as it is far beyond your skill and knowledge. – dingalingchickenwiing Apr 27 '18 at 18:43
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    Only at the end you said you'd hire an electrician to install the box, but throughout, you kept mentioning you purchasing your own cable. (at Lowes). Keep in mind this is a DIY site, so posting here implies you intend to DIY. For now, my goal is stopping you from wasting money on the wrong cable. If you want to school up, we're happy to help. – Harper - Reinstate Monica Apr 27 '18 at 19:10
  • Harper - This is a DIY project, but no matter how small or big the project is, I am going to hire a licensed electrician to ensure everything is done correctly. Installing cable in the ground is DIY, and this conforms with site rules. My question is appropriate (and thankfully you gave a good answer below.) – dingalingchickenwiing Apr 30 '18 at 18:38
  • Speedy Petey - this is not about a commercial electrical job and no one would use Stack Exchange if 100% prepared for the job. This shop, or 'warehouse' as I call it, has my machines and I want to run three phase power without a generator. I am installing a sub-panel for this purpose. Being unprepared for a project is a silly reason for closing the question as off-topic. This question is in compliance with the Stack Exchange DIY rules. Please go away as you have not offered any benefit to this thread. – dingalingchickenwiing Apr 30 '18 at 18:43
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    "If you want to school up, we're happy to help." - I still just don't understand this. By asking a question on this site, I am 'schooling up', yet you chide me for not wanting to 'school up.' That doesn't even make sense. I am meeting with an electrician this Thursday for further discussion, but it appears that you guys don't even want me asking questions about this. By asking a question, I am 'unprepared' for the job per your comments. Well, duh. I am trying to 'school up.' This is ridiculous and you guys are acting in a way that does not follow the diy stackexchange policy. – dingalingchickenwiing Apr 30 '18 at 19:04

This answer will confine itself only to the size of the conduit you should lay, and as you'll soon see, even such a simple question has layers of complexity.

This work will surely need to be fully permitted and that means it will need to be done to the spec of your AHJ (generic term for your electrical inspector: Authority Having Jurisdiction.) That will be the reason for the engineering review.

Metering and fusing at the supply

The other company certainly isn't going to give you 30kw of power for free. There will need to be a metering arrangement at that site, whether that's submetered and rebilled by the other company, or the utility puts a meter there.

There must also be overcurrent protection for the long run from this supply transformer to your site. This may not be a simple household style "breaker box" as you say.

This stuff all needs to be figured out before you can really advance further. What you can do is get the conduit laid in the ground. Conduit is mandatory, which means cable is impractical. Into the conduit will go several single-conductor wires, of a rating fit for outdoor/wet location use, such as THWN-2.

Conduit type

There are a variety of conduit types, such as PVC plastic, EMT metal, IMC metal, or the heaviest Rigid metal. Rigid is pipefitted, with screw-in fittings. All conduit is presumed to be full of water at all times. The defense against water is in careful selection of wire.

You may have a maximum of four 90 degree bends (or rather, 360 degrees total) of bends. However even this will be quite tough to pull, and will require a professional with his truck full of special pulling gear. It's best to keep bends to an absolute minimum for ease of pulling, but typically you need two 90s just to enter and exit the ground.

Trenching depth is a big deal in some cases. Rigid conduit (the most expensive) needs only 6" of cover atop it; 12" if it's under a driveway or parking lot. Other conduit needs 18-24" of cover.

Figuring out conduit size is pretty simple, the conduit size is decided based on the number of wires and the size of those wires. Look it up in this table or this calculator and you're done. Easy peasy

Wait. How many wires?

First, metallic conduit can often be a grounding path, in which case a ground wire is not required. I am skeptical of metal conduit outdoors being an effective grounding path if it isn't Rigid conduit. You really want a ground wire, so don't mess around here. If you need a ground wire, it's allowed to be smaller than the conductors.

Ground aside, the number of conductors depends on the configuration: 3-phase "delta" or "wye". Delta has 3 conductors and no neutral. Wye has 4 conductors and a neutral. Most motors do not use the neutral. In most cases the neutral can be downsized when the loads are expected to be balanced (which motors will be).

A wye supply can drive delta tools. In some cases, if the supply is wye, and you only need delta, you can simply take the 3 wires of delta.

This brings us to 3, 4 or 5 wires.

How large should the wires be?

You want to size your conduit for the worst-case wire size that you might ever need. Conduit is more expensive than wire when you account for installing the conduit, repaving and the like.

You have provided a power requirement (120hp machine under test + 40hp compressor that someone will someday forget to turn off while testing). That's 160hp or roughly 120kw.

You don't have a choice whether you'll be getting 208V, 240V or 480V, unless you intend to provision your own transformers.

Running the numbers for 480V... 120,000 watts divided by 480 divided by sqrt(3) = 144 amps. That's enough infomation to head over to the Voltage Drop Calculator, using 5% as the tolerable voltage drop. We get less than 2% drop with 1/0 Copper wire, which is a better answer than I was expecting. 480V really is magical stuff.

At 240V it's a bit more frightful: 289 amps. Calc says 350kcmil copper wire - yikes. "Kcmil" aka "MCM" is for large size wires too large for the AWG sizing system. At these large sizes, you should really have a talk with the AHJ about using aluminum. Copper is not for this.

At 208V, we're talking 333 amps. Calc says 500kcmil.

Trying to run a 120hp machine on anything less than 480V 3-phase seems like madness.

Back to conduit size

In the 480V case (#1/0 wire), you could skin by with 1-1/2" conduit, but I would upsize to 2" instead, for ease of pull and future expansion.

240V (350kcmil) could get away with 2-1/2" conduit but I would go 3". 208V (500kcmil) will require 3" conduit.

There's no penalty for installing too-large conduit, except the cost of the conduit itself, which is negligible in the scope of total project cost.

Fitting conduit is a huge amount of the total work, which is why it's a good thing to DIY. That done, pulling the particular wires required is straightforward work for your electrician when she has determined the right wire size.

There's also no penalty for installing too-large wire (except for the cost of the wire) however expect that cost to be considerable.

Adapting your machines

480V machines can often be re-jumpered 240V, or vice versa. There will be a label on the motor indicating how to change jumpers. It's not hard to design a motor to be 240/480, just not every motor is.

Some 240V machines can be run on 208V, or vice versa, but it's not as common. Thermal machines like ovens will produce less power (208/240)^2 - and motors would need to specifically be designed to be dual voltage 208/240. You have to consult the documentation.

If you need a voltage you do not have, you will need to provision either a 3-phase transformer, or three single-phase transformers connected correctly. You could have them right in your shop.

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  • This answer is useful and has a good amount of information for me to research and learn further from - thank you. – dingalingchickenwiing Apr 30 '18 at 18:35
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    "You don't have a choice whether you'll be getting 208V, 240V or 480V, unless you intend to provision your own transformers." I am going to provision my own transformer so the 'Adapting your machines' section won't apply. This answer wasn't overly complicated and I understand everything you are saying. You should have initially posted this answer without leaving the off-topic and rude comments on my initial question. While I don't appreciate your initial comments, I do thank you for finally submitting a useful answer. – dingalingchickenwiing Apr 30 '18 at 19:07
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    @dingalingchickenwiing Harper, ThreePhaseEel and the other regulars are VERY helpful. Sometimes things don't come out sounding as nice as they should and it takes a little while for new people to get up to speed with how things work on StackExchange - e.g., how to phrase a question so that it can be answered well, knowing what kind of basic research to do before asking the question, etc. Stick around - you will learn a lot. I know I have - I participate only minimally on HomeImprovement and am amazed at the quantity & quality of answers provided by Harper & others. – manassehkatz-Moving 2 Codidact May 1 '18 at 4:12

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