Air Conditioner Wire Sizing Guide: Chart & Calculation

Apart from the electric current, wire sizes are also depending on other conditions such as ambient temperature and voltage drop. If you are dealing with air conditioners, it is inevitable that you’ll come across wire sizing work. So, I’ve made a wire size chart as well as a guide on how to calculate wire size.

To calculate wire size for air conditioners, divide the maximum current by 80% to find the circuit breaker size. Next, use the wire length and a voltage drop of 2.5% to get the allowable voltage drop. Finally, choose the suitable wire size based on the circuit breaker ampere and the allowable voltage drop.

Sizing wire has been a difficult task for many people even engineers. For most people, using the following wire size chart should be sufficient. For engineers, I’ll go through step by step on how to calculate wire size.

Wire Size Chart by Air Conditioner Capacity

• Ambient temperature = 86°F (30°C)
• Wire length = 50 ft (15 m)
• Wire material = Copper
• Voltage drop = 2.5%

Single-phase (230 volts) air conditioners use 3 wires only (hot wire, neutral wire & ground wire). Hence, there are more current in the wire compared to three-phase (480 volts).

Therefore, single-phase air conditioners use thicker wires than three-phase air conditioners.

The above air conditioner wire sizing chart shows the recommended wire size based on the air conditioner capacity as well as the maximum current of various single-phase air conditioners.

For instance, a 1.5-ton air conditioner needs 2.5mm2 or 14-gauge wires to safely operate given that its maximum current can go up to 10 amps. Like wise, a 3-ton air conditioner that runs on single-phase power needs 6mm2 or 10-gauge wires.

Three-phase air conditioners draw less electrical current than single-phase air conditioners. Hence, their wire size is smaller than single-phase air conditioners.

The above 3 phase wire size chart for air conditioners shows the recommended wire size based on the air conditioner capacity as well as the maximum current of various three-phase air conditioners.

For instance, a 5 ton ac needs 2.5mm2 or 14-gauge wire to safely operate given that its maximum current can go up to 15 amps.

Wire Size Chart by Amperage

• Ambient temperature = 86°F (30°C)
• Wire length = 50 ft (15 m)
• Wire material = Copper
• Voltage drop = 2.5%

Apart from using the air conditioner capacity to size wires, you also can use the amperage to size wires. In fact, it is better this way.

From the above single-phase table, the minimum wire size needed for 20 amps is 4mm2 or 12-gauge wire.

How to Calculate Wire Size for Air Conditioner?

Needless to say, it is critical to run air conditioners with correct wire sizes. With correct wiring and the right wire size, it is assured that the air conditioner is safe to operate.

Calculating the wire size for air conditioners is slightly different from other applications because the running ampere of air conditioners vary based on operating conditions. With that said, here is how you calculate wire size for air conditioners.

1. Identify the Load (Ampere)

The first step to calculating wire size is to identify the load in terms of electric current. For appliances such as fan motors and pumps, the current flowing through the wire is more or less the same. But, it is significantly different for air conditioners.

The amount of ampere drawn by air conditioners is mainly affected by the cooling demand and the outside air temperature. The greater the cooling demand, the higher the ampere. The higher the outside temperature, the higher the ampere.

Many engineers use rated power or rated current to calculate wire size for air conditioners which is inappropriate because the ampere of the air conditioners can be doubled when running at full load, especially during hot days.

For instance, a 415V three phase air conditioner with 67kW of capacity may have a rated power and a rated current of 19kW and 31A respectively. However, its maximum current is stated at 55A. So, if you use 31A to size its wire, it may not be enough.

Hence, the proper way to calculate wire size for air conditioners is to find out the maximum current of the air conditioner and use it to calculate the wire size needed.

2. Determine the Circuit Breaker Size

Next, you need to determine the circuit breaker size. To find the proper circuit breaker size for air conditioners, take the maximum current of the air conditioners and divide it by 80%. Then, the next closest circuit breaker size will be the one you are looking for.

Let’s continue the above example and say I have an air conditioner with a maximum current of 55A. So, my circuit breaker current is 55A ÷ 80% = 68.75A. The next closest circuit breaker size is 70A.

Here is a list of common circuit breaker sizes:

3. Match the Wire Size by Current-Carrying Capacity

Now, use the circuit breaker ampere to find the matching wire size by current-carrying capacity. You may use Mega Kabel wire current-carrying capacity table or any other wire current-carrying capacity table based on your preference and practicing standards.

In Malaysia, we use MS 2112-3 standards for wires and cables. Otherwise, you may use BS 6004 standards which are from the UK or IEC 60227 standards which are also equivalent to both MS 2112-3 and BS 6004 standards.

From the above table, I match 70A, which is the circuit breaker ampere, with a 25mm2 wire size. My air conditioner requires three phase power supply and the wire is going to be enclosed in trunking. So, I use the table to match the wire size accordingly.

Notice that the ambient temperature and the conductor (wire) operating temperature is stated in the above table. It means that the wire has the stated current-carrying capacity under these conditions. If the ambient temperature increases, the wire current-carrying capacity will drop which may cause you to use a bigger wire size.

You may ask why did I use 25mm2 instead of 16mm2 since the maximum current is only 55A.

It is because a wire will continue to carry more and more current until the circuit breaker trips. The 70A breaker will allow more current to flow than the wire is capable of carrying. If a 16mm2 is connected to a 70A breaker, there is a risk of wire failure.

That’s why we practice using the circuit breaker ampere to calculate wire size.

Take note that the above table is meant to copper wires. Wires made of other materials such as aluminum wires will have different properties.

4. Check the Percentage of Voltage Drop

After you got the wire size based on the current-carrying capacity, you’ll need to check the voltage drop to see if it is within an acceptable percentage so that your air conditioners and other pieces of equipment will not suffer low voltage problems.

Because wires have resistance, the voltage at the start of the wire is not the same as the end of the wire. In other words, a 415V power supply may be left with only 400V when reaches the air conditioner because there is a voltage drop across the wire.

NEC (National Electric Code) recommended that voltage drop should not exceed 5%. For me, I practice limiting voltage drop at 2.5% for optimal performance.

Voltage drop is affected by the wire length. The longer the wire, the greater the voltage drop. Hence, to check the voltage drop, you’ll need to estimate the wire length first.

To check the percentage of voltage drop, find the voltage drop value using the wire voltage drop table. Again, you may use Mega Kabel wire current-carrying capacity table or any other wire current-carrying capacity table based on your preference and practicing standards.

Continuing my example, 25mm2 wire has a voltage drop of 1.55 MV/A/m. Assume that the wire length is 30 meters, the percentage of voltage drop is calculated as follow:

Total voltage drop = load x voltage drop x wire length ÷ 1000
Total voltage drop = 70A x 1.55 MV/A/m x 30 meters ÷ 1000
Total voltage drop = 3.255V

Voltage drop % = 3.255V ÷ 415V x 100
Voltage drop % = 0.78%

The result 0.78% of voltage drop is well under both my 2.5% and NEC recommended 5% requirements. Hence, 25mm2 wire is suitable for the 55A air conditioner.

If the resulted percentage of voltage drop exceeds 5%, you’ll need to keep trying the next wire size until the voltage drop satisfies the requirement, regardless of the wire current-carrying capacity.

For instance, if the wire length is extended to 200 meters, the resulted percentage of voltage drop becomes 5.2%. In this case, 35mm2 wire is needed so that the voltage drop is reduced to 3.7%.

Some standards allow their wire/conductor to operate at 90°C instead of 70°C. So, their current-carrying capacity is greater and their voltage drop is lesser.

5. Check the Correction Factor

If you are sizing wires that eventually will be installed in places where the ambient air temperature will exceed or fall below 30°C, you need to apply a correction factor as follow:

To apply the correction factor, multiply the current-carrying capacity by the corrector factor. For example, the 25mm2 wire has a current-carrying capacity of 89A at 30°C. If the ambient air temperature rises to 40°C, the current-carrying capacity of the same 25mm2 wire becomes 77.43A. See Mega Kabel correction factor here.

So, this is how we determine what size wire we need for various air conditioners.

It is quite lengthy if you calculate it manually. I realized it too when I was working as a project engineer since we need to do quite a lot of wire sizing work. Hence, I created an excel sheet that you can purchase and download to speed up the process. Check out the Design Engineer Starter Pack.

Factors that Affect Wire Size

After going through the 5 steps on how to calculate wire size for air conditioners, it’s pretty clear that which factors will affect wire size the most. However, let’s summarize them.

• Load – Wire size depends on electric current or load.
• Wire length – Long wire length can cause a bigger wire size.
• Ambient air temperature – High temperature lead to a bigger wire size.
• Wire operating temperature – High temperature allow a smaller wire size.
• Wire material – Copper is a better conductor than aluminium.
• Wire type – PVC, XLPE or armour wire have different specifications.
• Installation – Wires run on perforated cable tray perform better than enclosed in trunking.

Conclusion

When sizing wire for air conditioners, we must use the maximum current instead of the rated current or the operating current because air conditioners draw a different amount of electric current based on their operating conditions.

By using the maximum current, we are confident that no matter what are the conditions, the performance of the wire is not compromised. As for fan motors and pumps, using the rated current to size wire is usually fine.

This article was originally published on aircondlounge.com. Actions will be taken for unauthorised republication of this article.