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I'm planning to install a 5 ton central AC. I'm deciding between a condensing unit that runs on 220V and a costlier unit that runs on 207Y three-phase.

My understanding is that three-phase motors use less power, and that this is their primary advantage. But no one seems to be able to tell me how much less power. Without knowing that, it's hard for me to make a decision.

I'm sure the answer is some form of "it depends," but can anyone give me a ballpark? Should I figure 10% less power consumption, or more like 50% less. Any guidance is greatly appreciated.

Also, the wikipedia article on three-phase power claims that three-phase motors vibrate less. Is this something I should care about? Will a three-phase motor therefore last longer than a single phase motor?

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Is it an 'inverter' unit? If so, then the motor is already a three-phase unit with a variable-frequency drive built into the unit. The only advantages are going to be wire size, and better load spreading across the phases. – Someone Somewhere Mar 30 at 5:52

If you have the condensers already selected, you should be able to get the two pieces of data you need to make a decision.

1) price increase for 3 phase condenser
2) operating power for single phase and 3 phase units.

I suspect they will quote you that the two units consume the same amount of operating power.

Yes, theory says 3 phase motors run smoother just like V8s run smoother than 4 cylinder engines. But the practical difference in motor life is something you will probably never see.

One possible reason that a 5 ton unit is available in 3 phase is that the amperage is lower and thus you could save some money on the wiring cost. This comes at the cost of an extra circuit breaker.

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Conversely, if three-phase did not provide a significant benefit for a 5 ton system, why would manufacturers bother to make both three-phase and single-phase versions. I mean, they do for 5 ton, but not for 3 ton. I have three phase service already, so there is no additional wiring cost to use three phase. – Andrew Cone Jun 1 '12 at 15:08
@AndrewCone You have three-phase in residential? Really? – Tester101 Jun 1 '12 at 18:09
@Tester101 it's plentiful in older homes in the higher end country clubs in the area I live in... or if your home is 27,000 square feet. – lqlarry Jun 2 '12 at 1:13
@Tester101 The building in question is a 35-unit apartment building with seven retail spaces. The three phase may have ben installed for the washers and dryers, which are the only things currently using it. The AC is for a common area of that building, with lots of people coming and going. – Andrew Cone Jun 2 '12 at 17:09
3 phase residential power is very common in Phoenix, AZ – user24428 Aug 11 '14 at 20:27

Think I'll chime in here and add my two cents to these other good answers. There are two components to the original question:

  1. How much less power does will the 3 phase A/C compressor unit use?
  2. Longevity of three-phase motor versus single phase


Motor Efficiency = Power Output / Power Input

It takes a certain amount of power to run the compressor regardless of single or three phase power... this is the power output. The power input is what you are trying to minimize, thus you want to increase efficiency as much as possible. The trouble is this metric (efficiency) is a hidden combination the components inside the compressor unit (compressor, fans). It is not necessarily true (but it usually is true) that three phase is more efficient than single phase.

I would look at the SEER rating which 'automatically' includes the motor efficiency for a better indicator of "cost to cool".

Longevity and Reliability

Three-phase motors and compressors are generally more reliable than their single-phase couterparts. But like anything else, there is more to it than that one attribute. Using Philps' automotive engine example, a 4 cylinder Honda might be more reliable that that old smallblock V8.


Certainly less in the three-phase motor. And generally less vibration equals greater reliability.

Motor Starting

Three-phase motors have lots of starting torque and don't (typically) require any special circuitry to start (capacitors, centrifugal switches). And of course fewer components means fewer things to go wrong.


Is disappointing that the three-phase equipment is more expensive. Almost always, three-phase motors are actually cheaper than an equivalent single-phase motor.

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Single phase

1hp = 756 watts

5hp = 3730 watts

3730 watts at 230 volts = 17 amps

Three Phase

3730 watts = 3730(watts) / (207(v) x 1.73) or 3730 / 358.11 = 10.4 amps.

Three phase is 40% less amperage.

There are other things to make this accurate, like motor efficiency and power factor. I used 1 for each.

Another savings is that with the reduced amperage you can reduce your conductor size for your feeder. With the price of copper today that could help matters too.

I used 5hp for no special reason, just a nice round number.

I suggest using three phase.

Formula found here.

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Less amperage seems like a good thing, but I'm really concerned about wattage, because that's what I pay for every month. – Andrew Cone Jun 2 '12 at 17:07
Wattage is wattage, can't help you there. However I think three phase will let you do in less horsepower also, and less horsepower is less wattage. Does a 5 ton unit use the same motor on three phase as it does in single phase? – lqlarry Jun 2 '12 at 22:24

The above example of 5hp at 240v = 3730 watts and 5hp at 207v three phase = 3730 watts is correct. You draw less amperage with three phase but with more added voltage. Power is power and you draw 3730 watts with either scenario. Your savings will come in the durability and longer life of a 3 phase motor. You will also reduce your feed supply conductor size but will have to add another pole to your breaker or disconnect switch and one more conductor too! By using a three phase motor you will also balance your load on your service and probably increase your power factor efficiency which your hydro utility probably charges you extra based on a lower one, (check your bill). The nod goes to three phase but only slightly. Pay hundreds more for a 3 phase not thousands.

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Volts x Amperage=Wattage I have a 220 volt device (motor, dryer whatever... it makes little difference for this example). Single phase device draws 43 amps (43 x 220 = ~ 9460 Watts or 9.5 KW) Same unit with 3 phase draws 25 amps (25 x 220 =~ 5500 Watts or 5.5 KW) *** The same amount of work is output with either unit despite the 3 phase using less power because it is more efficient.

I currently pay $0.14 / KW and Run the unit 6 hours a day. Single phase costs me (9.5 x .14 x 6) per day to operate or ($6.65). Three phase costs me (5.5 x .14 x 6) per day to operate or ($4.62) a difference of $2.03/day. Therefore the difference in 1 year (5 days a week) is a yearly savings of $527.80.

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208/120Y is cheaper for supplying condominiums because one neutral can carry the unbalanced loads of three ungrounded conductors. But for the consumer, the voltages difference of 208V vs 240V usually equates to the same overall wattage, with the only negligible concern being it takes longer for heating elements to reach their desired temperature. As for motors, whether or not they're more efficient is not a question, but rather if you'd notice that difference in your monthly power bill. That I would probably say no just because being a residential area.

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AS WE KNOW 1 TR = 3.516 KW

FOR 5TR IT IS= 3.516X5 =17.58 KW

CURRENT FOR 430 V SUPPLY = 17.58X1000/(1.73*430*0.9) ASSUMING pf 0.9 =17580/669.51 =26.2 AMP (APPROX)

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Where did 430V come from? What is the comparison here? – mjohns Jul 20 '15 at 13:27

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