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Let's say I have a single-phase facility and I plug in a piece of 3-phase equipment, what happens?

  1. Is it dangerous?
  2. Is it less efficient (if so, approximately how many percent less in efficiency)?
  3. What kind of adjustments, if any, would I have to make to make this setup work?
  4. Would it increase or decrease the number of amps I use compared to a piece of single-phase equipment?

Likewise, if I have a 3-phase facility and I plug in a piece of single-phase equipment, what happens?

  1. Is it dangerous?
  2. Is it less efficient (if so, approximately how many percent less in efficiency)?
  3. What kind of adjustments, if any, would I have to make to make this setup work?
  4. Would it increase or decrease the number of amps I use compared to a piece of 3-phase equipment?

The power company charges based on wattage, so how would I calculate to find out which setup would be most cost-effective?

Are there any basic ground rules that I should follow when trying to work out these relationships?

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  • \$\begingroup\$ Your question is too broad. Provide more specifics on specific equipment you are considering. Provide specifics on the facility infrastructure you are working with. eg you want to run a 240VAC 5HP 3 Phase motor on in a Facility with single phase 220VAC infrastructure. \$\endgroup\$ – Tinkerer Aug 28 '15 at 21:31
  • \$\begingroup\$ I'm afraid I actually don't know enough to be more specific. My current real-life situation is similar to this. I'm trying to choose between 2 locations (one has 3-phase with 100 amps and the other has single-phase with 400 amps). The equipment I use in the future may come in either 3- or single-phase. I don't know which is the better location to work with. \$\endgroup\$ – user2323030 Aug 28 '15 at 21:35
  • \$\begingroup\$ The 3-phase facility is probably easier to work with. An electrician can easily hook up 1-phase equipment to a 3-phase system, but going the other way around is often impractical. Also note power companies charge based on energy (kW•h) and power factor in an industrial site, not wattage. \$\endgroup\$ – jbarlow Aug 28 '15 at 21:40
  • \$\begingroup\$ Provide the highest Voltage potential at the 3 phase location and the highest Voltage at the single phase location. You can then calculate which location has the infrastructure to supply the most power. Provide some more info on the equipment you are operating within the location... \$\endgroup\$ – Tinkerer Aug 28 '15 at 21:41
  • \$\begingroup\$ One way of looking at this question might be to ask "How much will it cost me to get three-phase added?" vs. "How much will it cost me to get higher current service to my existing three-phase?" My guess would be that higher current service might be the cheaper upgrade; it may just be a transformer replacement. Getting three-phase run to a single-phase location can cost literally hundreds of thousands of dollars, depending on how remote it is. \$\endgroup\$ – Stephen Collings Aug 29 '15 at 2:26
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If you have a single-phase facility, you can't just plug in a piece of 3-phase equipment. You will need to find a way to adapt it for single-phase power. That will probably not be particularly easy and might not be possible at all. It could be done with no loss in efficiency and performance, but is more likely to be less efficient and not perform as well.

1A. If it can be adapted, that can be done safely.

2A. The efficiency reduction will probably be in the range of 0 to 15%.

3A. It is not a matter of adjustment, it a matter of changing the design in some way.

4A. The current increase will probably be in the range of 0 to 15%.

It is relatively easy to use single-phase equipment in a 3-phase facility. Three-phase facilities usually have some single-phase equipment connected. The only concern is that you should try to keep the overall usage balanced among the phases.

1B. Safety is not an issue.

2B The single phase equipment may be less efficient than equivalent 3-phase equipment by as much as 15%.

3B No adjustments will be required.

4B The current increase will be somewhat comparable to the efficiency decrease.

The basic ground rule and method for determining cost effectiveness is to list all the anticipated equipment required and get prices and specifications from the suppliers. Also get information from the utility that will supply the power and from installers.

Edit: You can not really make a good decision with out learning more specifics. The more specific information you put int your planning, the more likely you can make a good decision.

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Most single-phase equipment will work fine on three-phase, assuming all other necessary specs are met. You just connect two of the three legs. The load doesn't know there's a third leg and two more phases running around the universe somewhere, any more than your 110V blender knows there's a 220V outlet that your dryer is plugged into. (If you're not in the US and everything is 220V where you are, pretend I didn't say that.) For that matter, your entire house is a single-phase load on a three-phase grid!

Now, the effect the load will have on the source is a different question. Unbalanced loads (loading one phase and not the other two) cause triplen harmonics (3, 9, 15, 21...), which add up as currents in the neutral conductor. This can lead to overheating of that conductor, if it's bad enough. The line distortion can also affect other loads, or cause your bill to go up if the utility cares enough.

As to whether you can run three-phase equipment single-phase, the answer is "maybe, but with derating in most cases". Most three-phase equipment I've seen is either a motor or motor-related. If you try to start a single-phase motor, you'll need external equipment to guarantee that it rotates, and in the direction you want. I've never thought about running a three-phase motor single-phase, but I suspect it may not even rotate. Even if it did, I expect the four windings you'd be using would overheat as well, unless you derated the motor; you can't shove the same amount of power through a third less copper and expect everything to work out the same.

You could use a variable-frequency drive to output three-phase to the motor, and run the VFD off your single phase. But what if the VFD itself was expecting three-phase input? Then if you run it at single-phase, the caps will see 3x the ripple currents they were designed for, and the rectifier will be overloaded (four diodes doing the work of six). So you'd need to oversize your drive, typically by at least 2x, sometimes more. Alternately, you could use an external oversized rectifier with extra caps built in, which is a commercially available product for exactly that circumstance.

Obviously my experience does not cover the entirety of extant three-phase equipment. If you have some other kind of three-phase load in mind, I'd be happy to try to expand this answer to cover it.

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  • \$\begingroup\$ I can tell that this is also a great answer – if only I knew more about electricity. Unfortunately, most of it went over my head and I could only make sense of the first sentence. :\ \$\endgroup\$ – user2323030 Aug 29 '15 at 4:20
  • \$\begingroup\$ @user2323030 I think you may have a larger problem, then. Without offense intended, why are you the one selecting a building by its electrical service if you know very little about electricity? Is there someone else in your organization you can leverage? \$\endgroup\$ – Stephen Collings Aug 29 '15 at 12:55
  • \$\begingroup\$ It's a fair question. It's because my electrician just went on vacation and I couldn't find anyone else knowledgeable enough to ask. \$\endgroup\$ – user2323030 Aug 30 '15 at 5:31

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