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There is a difference between my calculations and what electricians say. Am I missing something?

If I need to install a 3 horsepower air conditioner. Electricians always choose wires that are more than 4 sq mm. They say that air conditioners consume high current.

My calculations tell me that I can use 1.5 sq mm only. Am I right?

3 horsepower = 2237.1 watt

I = Power / voltage = 2237.1 / 220 = 10.16 Amps

According to the following table, I will choose 1.5 sq mm. http://www.elsewedyelectric.com/BE/ImageFiles/SubProducts/Pdf/Single%20Core%20Cables%20with%20Solid%20or%20Stranded%20Copper%20Conductors%20&%20PVC%20Insulated%20450-750%20V.pdf

So, Why does electricians choose that thick cables?

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    \$\begingroup\$ Wire size is based on fuse or circuit breaker size. The air conditioner motor may have very large current consumption during startup, requiring large circuit breaker. Therefore, large wire is required. Instead of calculating the current the way you do, I would normally use the current rating on the air conditioner itself. Do you know it? \$\endgroup\$
    – user57037
    Dec 10, 2015 at 7:57
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    \$\begingroup\$ Also, 3hp is the mechanical output power. But the motor is not 100% efficient. So the input electrical power will be higher. \$\endgroup\$
    – user57037
    Dec 10, 2015 at 8:00
  • \$\begingroup\$ @mkeith I don't know the current rating because I did not buy anything yet. Thank you for your comments because I did not know that the 3hp is the mechanical output power and I should use the current rating. \$\endgroup\$
    – Michael George
    Dec 10, 2015 at 8:43
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    \$\begingroup\$ If your in US then they will use the NEC "National Electrical Code" which describes the methods to calculate current and cable sizes to be used. \$\endgroup\$
    – Spoon
    Dec 10, 2015 at 10:58
  • \$\begingroup\$ Resistance losses are also a factor, if you size the wire to the minimum \$\endgroup\$
    – Voltage Spike ♦
    Mar 12, 2017 at 4:14

2 Answers 2

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You do not indicate so I wonder if you have considered the cable run length.

Why would you like a thinner cable, if it is to save cost then the length is likely subtantial and therefore more of a reason to use thicker conductors.

Also the electricians are aware that compressor devices have very large inrush currents that are supported by thicker cables.

EDIT:
I found this mentioned in an Off grid guide by JayCar.
Australian Standard AS4509.2 states a surge factor of 7x the continuous power should be used for electric motors, water pumps, fridges, air conditioner, and washing machines. A surge factor of 3x should be used for kitchen appliances and other electronics equipment, and surge factor of 1x for resistive loads.

Another more interesting document on Cold Load Pickup Issues also shows similar inrush current figures.
Similar tests were conducted with on a ¼ HP refrigerator motor. The test results showed an average inrush current of 15 amperes and a maximum of 18 amperes. The average inrush was 7.5 times the steady state run current of 2 amperes. It took 483 milliseconds to reach the steady state condition. The test after a 25 cycle interruption produced lower inrush current and shorter times to reach steady state, 6 – 17 amperes and with 217 milliseconds to reach steady state.

Also tested was a 3 ton air conditioner. The momentary interruption test and the cold load pickup test showed an average inrush current of 90 amperes which lasted for 117 milliseconds. The inrush current was 7.26 times the steady state run current of 12.4 amperes. The steady state value was reached in 267 milliseconds.

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I understand that the newer Inverter type aircons don't have the same 'inrush' issue. So the wattage stated on the AC is not just the operating wattage but also the startup wattage. Some ACs give two ratings - one for startup and one for general operation.

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