Performing residential electrical sizing – step by step!

Making a residential electrical installation is not something simple, we know that the connections cannot be made in any way, using any conductor gauge, do not perform the distribution of the circuits correctly or do not dimension the circuit breakers. The good dimensioning of the electrical installation directly interfere with its efficiency and safety, therefore we will teach how to dimension the circuit breakers and perform dimensioning of the cables in an installation, according to the NBR 5410 standard and the aid of tables. Come on guys!

We aim to dimension the conductors and circuit breakers of a residential electrical installation, however, it is necessary to survey some data and perform calculations for the correct dimensioning, but rest assured, despite using some calculations they are simple, just know add, multiply and divide! All calculations will be done step by step to facilitate understanding.

Dimensioning of the general circuit breaker:

Before starting this step by step, it is important to note that the tables that will be used to dimension circuit breakers have many variations, which are according to each region, that is, they depend on the responsible concessionaire. In order to make it easier, we will base the design of this circuit breaker according to the tables of our local concessionaire, which is CEMIG.

The dimensioning of this circuit breaker is for a 220V installation, but this step by step can be used to dimension circuit breakers of other values, for that it is enough to replace the table for the desired voltage.

The first step in dimensioning both the circuit breaker and the cables is to survey the total power of all circuits in the installation. To help you understand better we will use as an example a residence that has six circuits, namely:

• 900W Lighting Circuit 1;
• Circuit 2 for 1,100W TUG sockets;
• Circuit 3 another circuit of 1,100W TUG sockets;
• Circuit 4 for 7.500W TUE shower;
• Circuit 5 for the 1200W TUE washing machine;
• Circuit 6 for 1,100W TUE microwave.

So the above circuits are separated into lighting circuits, general purpose outlets (TUG), and special circuits that are special use outlets (TUE), the circuits are separated so that it is possible to apply the demand factor.

It is important to understand that the demand factors are applied on the power value, so we must apply the factor for lighting circuits and TUG circuits. For this to be possible we will add the powers of these circuits which are:

900W of active lighting power of circuit 1, plus 1,100W of power from the TUG sockets of circuit 2 and more 1,100W of power from the TUG sockets of circuit 3, thus obtaining as a result 3,100W

After obtaining the total value of the power, we can consult the demand factor table below, which is used for lighting and TUG sockets, it defines the demand factor according to the installed power ranges. Remember that these tables vary according to your region, so they are available on the dealerships’ websites.

Demand factor table for lighting and outlet points (TUG).

In the previous example the total power was 3,100W, that is, this power range is between 3,001W and up to 4,000W. For this power range, the demand factor to be applied is 0.59. After you have identified, simply multiply 3,100W by the factor of 0.59 and you get the power already set, which is 1,829W.

The next step is to apply the demand factor for the special use circuits which are:
7,500W for the shower, plus 1,200W for the washing machine and to finish the 1100W for the microwave, totaling 9,800W.

This table, which appears below, defines the demand factor according to the number of special use circuits.

Table used to define the demand factor for plug circuits (TUE).

In the example we use, there are three special use circuits, and in this case, the demand factor is 0.84. Now just multiply the power of 9,800W by the factor 0.84 that we will obtain the power already adjusted to 8,232W.

After we have adjusted the power of the circuits, we will now find the total power of the installation adjusted according to the demand. To do this, simply add the lighting power and TUG sockets plus the power of TUE sockets, totaling 10.061W

Table for choosing the correct circuit breaker.

Anyway, we can dimension the meter breaker and for that we will consult a simple table from the dealership. In this case, it belongs to Cemig, where the phase and line voltage is 127 / 220V. The calculated power is in the range between 10.1 and 15KW and for this installation the meter circuit breaker must be a bipolar 60A circuit breaker, as the available connection is 3 wires, in this case two phases and neutral.

Sizing of the QDC circuit breaker:

To perform the dimensioning of the circuit breaker for the QDC we will use Ohm’s law and divide the power by the voltage. So, we divided the power of 10.061W by 220V soon, we will have a result of a current of 45.73A. This is the current to consider the general circuit breaker of the QDC.

It is important to note that in the case of CEMIG there is the possibility of having both voltages 220V and 127V and as in the table the power defines a bipolar circuit breaker, we have to use the line voltage which is the voltage between the phases which is 220V.

Table to choose the exact model of the circuit breaker and its respective curve.

After having found the total circuit current and consulting a circuit breaker table, we found the 50A circuit breaker for the current closest to 45.73A. The circuit breaker curves are defined according to the use, the general circuit breaker always having the longest curve between the circuit breakers! In the case of our example, as there are circuit breakers B and C, the circuit breaker with the largest curve must be used, and the largest curve is C.

We can observe that in the case of CEMIG, there was a difference between the meter breaker which is 60A bipolar, for the QDC breaker 50A bipolar, but this difference is not a problem, because it was calculated and guarantees the selectivity of the circuits. Therefore, we advise you to make the necessary calculations and do not dimension your QDC circuit breaker based on the general circuit breaker, as it is not the correct one, as we have just seen.

Sizing of electrical conductors:

As well as circuit breakers, it is also necessary to survey the loads of each circuit, in order to perform the proper dimensioning of the cables. As we had already done the separation of the circuits according to their power, we will dimension the conductor that leaves the general circuit breaker and goes straight to the QDC, considering that it has a distance between them of 15 meters. And a 220V circuit breaker.

Table for sizing 220V electrical conductors.

As this installation is 127 / 220V we will use the table below, that way it is easier to identify the correct cable, just cross the information. In this situation, we chose the 75A circuit breaker and considered a distance of 20m, because there is no 60A value in the table for the circuit breaker and neither 15m for the distance. Do not use the values ​​below those that have been calculated, as the conductors may experience a voltage drop and the conductors overheat.

If you want to learn how to dimension he gauge of a cable for a single-phase connection of 127V or 220V we have articles that teach you step by step.

We highlight in accordance with the NBR-5410 standard there are minimum sections for conductors according to each circuit of the installation, and for lighting circuits it requires a conductor of at least 1.5 mm, load circuit with conductor of at least 2.5 mm and signaling and control circuits of at least 0.5 mm. It is important to note that these values ​​are minimum requirements, that is, under certain circumstances, cables with gauges lower than these cannot be used.