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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 '16 at 5:52
  • 3 phase conversion factor is 1.73% , a motor vibration has nothing to do with the phases / poles it has but how well it is balanced. AC units all vibrate because of the compressor not the motor. Motor failures usually start at the bearings I have seen both single phase and 3 phase motors last decades (most HVAC systems will not make it 20 years)! Last question do you have 3 phase available? If not you would need to create it and you loose any gains that might have been compared to single phase. – Ed Beal Jan 23 '18 at 23:49

11 Answers 11

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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

Power

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.

Vibration

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.

Cost

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

  • I have 2 motors on a test stand a single phase and a 3 phase my accelerometer says they are the same! It is the compressor head that creates the vibration not the motor. – Ed Beal Jan 23 '18 at 23:52
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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
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    @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
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    @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
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    3 phase residential power is very common in Phoenix, AZ – user24428 Aug 11 '14 at 20:27
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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
  • 1.73 is the conversion factor – Ed Beal Jan 23 '18 at 23:57
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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.

  • I don't see 3 phase motors lasting longer, however there are no start run capacitors so this could be an advantage because many folks replace fuses until they fry the motor when the cap fails. – Ed Beal Jan 24 '18 at 0:00
  • This. 3 phase saves the manufacturer and the installer money (less copper all around), not you. Also, most anything 3p is going to be up to commercial or industrial standards; not consumer level garbage. Also, unless your property has 3p incoming service, adding a pole and a breaker does nothing ;) - If it really saved you money, nothing on this planet wouldn't be 3p. – Mazura Jan 24 '18 at 1:29
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Honestly I think there is no meaningful power savings to be had merely because a 5 ton condenser uses three phase vs single phase. Ditto on reliability. A three phase 5 ton compressor is only marginally more reliable than a single phase hermetic unit that size. The issue with the single phase units is that compressor run capacitors do go bad every 10 years or so; but they are cheap and easy to replace.

I have maintained 3 phase 3-5 ton condensers that were installed because it was felt they were more reliable than single phase. Issues like correct installation, EER rating (not to be confused with SEER rating), and correct maintenance unrelated to the compressor drowned out any concerns about the type of power supplied.

5 ton units are sold in both single and three phase configurations because in some commercial environments three phase power is easier to come by than single phase power.

  • Finally someone that understands motors+ – Ed Beal Jan 24 '18 at 0:01
  • The correct maintenance unrelated to basically everything everyone on this page is talking about, is paramount to maintaining efficiency. +1 – Mazura Jan 24 '18 at 1:35
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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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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.

  • Check that calculation again. For 3 phase power 25 amps at 220v: VA (total apparent power) = 25 x 220 x 1.73 = 9513, or about 9.5 KVA (roughly the same as single phase). – user39367 Dec 10 '16 at 3:06
  • This is incorrect. You pay for energy use, not amps, and the only difference in energy use is in the efficiency of the motor. For three phase power you have to multiply by the square root of 3 as @user39367 points out. – Mark Apr 29 '17 at 13:51
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I live in a three phase power area here in Central Phoenix. My 3 phase 5 ton unit is 19 years old. My Home is 2500 ft sq. In all these years for an August power bill I have NEVER paid more than $100 in a monthly power bill. My neighbors who were advised by so called...technicians...to replace their 3 phase units with single phase are paying August power bills of $300-450.

It is your wallet; what are you willing to pay year after year after year

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This is a really simple answer:

1) Power draw is amps * voltage/efficiency. A motor needs a certain amount of power no matter what. You pay by power consumed. So, your bill will be virtually identical because power doesn't change. The only reason it isn't perfectly identical is efficiency:

2) Three phase spreads the load over 3 wires instead of two, reducing current (amps) by 50%. Amperage is what causes heat which causes increased resistance which causes more amperage drawn to make up for it (lowering efficiency). So by reducing amperage, you increase efficiency, lowering power draw. Therefore the amp difference isn't quite 50%; it might be 51 or 52%. The decrease in power is very minimal. A couple percentage points that will get overshadowed by environmental effects.

The bigger points to consider are the cost differences of installing 3 phase vs single phase and cost to wire those, and the total amperage your panel is rated for. If you need fewer amps to fit in the panel and the cost to install isn't significantly higher, go with three phase. If you have three phase and the cost is similar, always go three phase. But there's virtually no lower cost to run by doing it.

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    Also the higher cost for a 3 phase breaker and 3 phase disconnect, and very few residential tech's have a clue how to service 3 phase (at least in my area) – Ed Beal Jan 24 '18 at 0:06
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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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Three phase with current=1, current vector sum is 1.5. Single phase motor, I suppose it is two phase driven, with 4 poles and 90 degrees pole pitch, to generate a vector sum of 1.5, the phase current must be 1.5, 2-phase power consumption=1.5*1.5*4 (4 windings) = 9, 3-phase = 1*1*6 (6 windings) = 6, So 2 phase motor consumes 9/6=1.5 times more power than a 3-phase motor with the same torque output.

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