# Thread: AC Units Consuming More and More (did some due diligence)

1. Originally Posted by dan sw fl
No one would use Your rule for current because I think you absolutely fell off the track.
Now I'd like to see you tell us about the power in an inappropriate way.
Since you are convinced my method is wrong, care to share your ideas on how a comparison should be done?

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Originally Posted by 54regcab
Since you are convinced my method is wrong, care to share your ideas on how a comparison should be done?
http://www.engineeringtoolbox.com/el...tor-d_832.html

4. Using the calculator you linked (cool calculator BTW):

5HP on 3 phase is 14A per leg @ 208V 3 x 14A=42A
5HP on single phase 20A per leg @ 240V 2 x 20A = 40A
Left all other factors the same.

Not 100% accurate according to the calculator, but close. Not enough of a difference to lodge a compliant against the landlord. In reality 3 phase motors tend to be slightly more efficient with better power factors, negating the much of the 2A difference. Other factors such as dirty coils, system size, system charge, etc, would have more of an influence on power consumption differences.

But wait, there's MORE: I upped the power to 50HP to give the calculator better resolution. 405A (135 x 3) vs 406A (203 x 2). "Myth confirmed", adding the 3 phases @ 208V is the same as adding 2 phases @ 240V. Give it a try yourself
Last edited by 54regcab; 11-01-2012 at 10:16 PM.

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Originally Posted by 54regcab
Using he calculator you linked:
5HP on 3 phase is 14A per leg @ 208V 3 x 14A=42A
5HP on single phase 20A per leg @ 240V 2 x 20A = 40A
Left all other factors the same.
+ the Square Root of 3 = 1.732

6. The 1.732 is already factored into the calculator
Take the 3 phase amps x 1.732 and you get single phase amps.

I think you're getting lost on the voltage conversion. There is 208v between phases on a 240v supply.
The OP is measuring amps with each leg, if his building has 240V to single phase it will have 208 between phases.
The lower voltage between phases will raise the amps read on his meter for the same HP motor.

Please share your calculations of what the amps should be on a 3 phase 208v motor vs a 240v single phase motor of the same HP.

7. Attached are screenshots from the calculator you linked:

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Originally Posted by 54regcab
Using the calculator you linked (cool calculator BTW):

But wait, there's MORE: I upped the power to 50HP to give the calculator better resolution. 405A (135 x 3) vs 406A (203 x 2). "Myth confirmed", adding the 3 phases @ 208V is the same as adding 2 phases @ 240V. Give it a try yourself
BTW... It appears that You may be LOST FOREVER! \
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/

9. I still don't see your calculations. Looks like you're the one who can't back up your point of view...
How can you call my calculations wrong when you don't have any of your own?
If you really knew what you were talking about you could provide calculations.

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Hey there,
Sorry but this argument on calculations has confused me further.
a) The AC is cooling just fine and keeps the apt cool during the hottest months
b) I just checked the coils (surrounding the fan) and they dont seem to be too clogged up but I cant spray anything on to it a sits on the roof of the apt bldg
c) I had the AC units running at 20*C with an outdoor temp of about 28*C for about 20 minutes before going out to check the tubes as suggested to see if they were hot. They were not hot but the relative temperature was not hot as the weather has cooled off so I'm assuming the AC did not have to work hard
d) Despite a debate over some calculations on this thread, I'm still not sure if the numbers I presented indicate that the units may be consuming more than they should (I know the fan running at 4 amps is too high) and that the units' amp readings were higher than the stated RLA on the panel. All this while the weather is cooling off...so I gather that the last three months when the weather was in the 40-45*C range, the amp readinsg would have been drastically higher - can anyone respond to this directly?

Again, thanks for all your help. You guys are awesome

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Originally Posted by 54regcab
I still don't see your calculations. Looks like you're the one who can't back up your point of view...
How can you call my calculations wrong when you don't have any of your own?
If you really knew what you were talking about you could provide calculations.

1.732 ___ 1.732 .. SQRT 3 for 3-phase power
12 ___ ___ 12 Amps
208 ______ 220 V
4323.1 ___ 4572.48 Volt-Amps

Voltages & Power Factor are not known with certainty.
There is nothing that even needs to be calculated.
The Measured power is stated as 4,400 Watts.

So , if OP wishes one to evaluate the performance as stated in post #1 ( copied below)
the air flow rate, supply air temperature, model numbers of the condenser and air handlers must also be provided.
Room temperature and outdoor temperature are provided.
Relative humidity inside would help, but one can assume it's ~40'F for discussion purposes.

Unit #1:

3-Phase, RLA 7.0, Min. Supply Circuit Ampacity 10
With AC running at 22*C with outdoor temp at 30*C the amp readings were 11-12 amps per phase
and 12 amps all together including fan

Wattage measurement with unit set at 20*C with outdoor temp at 30*C (night) = 4400 watts

Unit #2:

1-Phase, RLA 13.7, MSCA 19 amps
With AC running at 22*C with outdoor temp at 30*C the amp reading was
14 amps and 18 including fan
Wattage measurement with unit set at 20*C with outdoor temp at 30*C (night) = 3000 watts

12. If the OP wants to save a little money, move the a/c setpoint from 20°C to 22°C (68 to 72°F).

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Originally Posted by Shophound
If the OP wants to save a little money, move the a/c setpoint from 20°C to 22°C (68 to 72°F).
OR if he doesn't like to be at the mercer of his utility company ...
Raise it to 78'F / 25.6'C .:. reduce electric cost\$ > _ 40%_

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