How to dimension cables for Air Conditioning?

The installation and maintenance of air conditioners have been increasing considerably and with this, several doubts have been raised on the subject, for example, circuit breakers, cable section, an electrical current of circuit breakers and etc. In this article, we will answer your main questions about sizing air conditioning. So come on!

BTU is equal to WATT, electric power of the Air Conditioner?

The most common mistake is to think that BTU equals the electrical power of the air conditioner, when in fact this thought is wrong because in fact, BTU is a quantity of cooling power, different from WATT which is electrical power.

Understand that in air conditioning there are two different powers, the cooling power, which is the capacity that the air conditioning has to change the temperature with the environment and that electrical power is the amount of electrical energy used by the air conditioning to be able to cool the environment.

Air conditioning circuit – Dimensioning

The sizing is calculations that we also use the ohm laws, in order to know what are the levels of electrical current that travel through a given electrical circuit, so that we can then select the cables and circuit breakers appropriately according to each electrical circuit.

For the correct dimensioning of the cables and circuit breaker of the air conditioning electrical circuit, we need to make calculations using the electric power formula which is:

Formula of electrical power.

We have a split inverter air conditioner of 11,500 BTU, with an electrical power of 1,120W and an electrical voltage of 220V, we need to dimension the cables and the circuit breaker for the electrical circuit. The first step is to perform the calculation to obtain the electric current from the air conditioner, for that it is enough to divide the electric power by the electric voltage, thus finding the electric current value.

Application of the power formula to obtain the electric current of the air conditioner.

After the calculation we obtain the electric current of 5.09A, this is the rated current of the air conditioner. We need to multiply the rated electrical current of the air conditioner by the service factor that is equal to 1.25. The service factor is a safety margin so that the circuit breaker does not trip when the air conditioner is in operation.

Multiplication by Service Factor.

The final electrical current with the applied service factor is 6.36A, this will be our reference electrical current for dimensioning the cables and the circuit breaker for the air conditioning circuit. The first step is to select the appropriate cables for the air conditioning circuit, the cables are responsible for conducting electrical energy. In table 36 of NBR-5410 it informs the capacity of conduction of electric current of each cable. We know that the reference electrical current is 6.36A, just look at the table which is the section of the cable that has the capacity to conduct the electrical current of 6.36A.

Table 36 – Current conduction capacities, of NBR-5410.

We saw that a cable with the section 0.75 mm, already has the capacity to conduct 6.36A. But we cannot use this section because, there is another table in NBR-5410, table 47 that informs the minimum section of the conductors (cables), for the power circuit (electrical).

Table 47 – Minimum section of NBR-5410 conductors.

This table informs that for the power circuit the minimum cable section is 2.5 mm², so we must use 2.5 mm² cables for the air conditioning circuit. The 3-way PP type cable should be used because it has double insulation.

Air conditioning I export to weather conditions.

Now we are going to dimension the circuit breaker for the air conditioning circuit. The circuit breaker is a protection device that is present in all electrical circuits, it has the function of protecting the circuit against short circuit and overload, so that it does not damage the installation cables and equipment. The circuit breaker also has curves such as B, C and D for each type of equipment that will be connected to the circuit breaker.

Different circuit breaker curves for each electrical circuit.

Air conditioning is electrical equipment that requires a C-curve bipolar DIN circuit breaker, as it is an electrical equipment that has peak electrical current when the use of the C-curve circuit breaker is mandatory. To select the circuit breaker, we use a reference current of 6.36A. The ideal circuit breaker is with the current closest to the reference current. The following table shows the DIN type circuit breakers that are available on the market.

Table of electrical currents for DIN type circuit breakers.

We observed that there is no circuit breaker with 6.36A electrical current in the table. In this case we selected a circuit breaker with electric current abover, then it would be a 10A bipolar C curve breaker.