How to dimension the general circuit breaker

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The circuit breaker is the electromechanical device that has the function of protecting the electrical installation from damage that originates in short circuits and or overloads. When we talk about the general circuit breaker, we are talking about the main circuit breaker of an electrical installation that protects other partial circuit breakers in the circuits of installation, that is, it is the circuit breaker upstream of the partial circuit breakers.

In electrical it is common to use the thermal upstream and downstream to define the position of a component in relation to other components, this way we can say that the general circuit breaker is the circuit breaker upstream of the partial circuit breakers and we can also say that the partial circuit breakers are the circuit breakers. downstream of the general circuit breaker.

  • one component being upstream means that it is before the other in question, closer to the energy source.
  • one component downstream meaning that it is after the other in question, closer to the final load.

When we talk about partial circuit breakers and general circuit breakers in an installation we need to understand the importance of correctly dimensioning circuit breakers, both partial and general, and this correct dimensioning has the function of ensuring what in electrical we call selectivity.

Selectivity

Selectivity is the property that an installation has that in the event of a fault, only open the short circuit protection device, in this case, the circuit breaker that is closest to the fault point, this ensures that the part of the circuit that will be off and inoperative be as small as possible.

Schneider Easy9 circuit breaker line.

In our house, when an outlet is short, the idea is that only the outlet circuit is disconnected and the rest of the installation continues to function, that is, selectivity. In order for selectivity to occur, circuit breakers must be correctly dimensioned.

Knowing the installed load

The first step to dimension the general circuit breaker of the distribution board of installation is to have the powers installed in each circuit and what are the types of loads. It is important that the loads are divided into circuits, and that the loads that have a nominal current greater than 10A are in separate circuits, as required by NBR5410 – Low voltage electrical installations.

See the example of loads for an installation.

Division of installation circuits.

Demand factor

Knowing the circuits and loads now it’s time to understand what the demand factor is.

In a general-purpose socket circuit, for example, that has 1100W of total power and this circuit has 10 sockets, we understand that each socket has 110W to be used if all the sockets are used at the same time at maximum power, but we know that in a residential installation this is unlikely to happen. In these cases we use the appropriate demand factor, to bring the power that is actually used in order to scale the circuit breaker to a current closer to the average usage, thus ensuring better protection and selectivity.

Demand factors are calculated and made available in tables by the concessionaires in their distribution rules.

For residential installations, we will use two demand factors, one that groups the general-purpose socket circuits (TUG) and lighting circuits, and a second demand factor for special use socket circuits (TUE).

Demand factors by type of electrical circuit.

Now it’s time to add the power values ​​of the circuits for each type of demand factor that will be applied. In the first case, the TUG and lighting circuits.

1st Step – Demand factor for TUG circuits and lighting.

In our example we see that the total power is 3100W, this would be the power in case all the sockets and lighting points are being used simultaneously, which we already saw back there that it is very unlikely to happen, so we must apply the factor demand. The demand factor for TUG and lighting circuits is based on the installed power range, in this case, a range between 3001W to 4000W, as we can see in the table below:

2nd step – Demand factor table, TUG circuits, and lighting.

For this range, the power factor indicated by the concessionaire as most appropriate is 0.59, this value has no unit but in percentage form, it represents 59% of the installed power, that is, the concessionaire tells me that of all the installed power on average only 59% will be used. We must multiply the original power value by the indicated factor, which in our example gives us a total of 1829W.

Now we have the power of the appropriate TUG and Lighting circuits and we are going to adjust the power for the TUE circuits.

In this case, we must proceed with the sum of the power of the TUE circuits.

3rd step – Demand factor for TUE special use outlet circuits.

Unlike the demand factor for TUG circuits and lighting, the demand factor for TUE circuits is indicated by the number of circuits and not by power range. It is interesting to realize that due to the nature of the circuits being the equipment with specific power, this demand factor attenuates the power range less than the previous factor.

4th step – Demand factor table for TUE special use outlets.

The demand factor value indicated by the concessionaire for 3 special use circuits is 0.84, that is, of the total installed power for these circuits, on average 84% will actually be used on a daily basis.

With the value of the demand factor, we must multiply by the installed power, in our example, we will obtain a result of 6132W

The power of our installation is already adequate to the demand factors and the next step is to add the power with applied factors.

Final power value of the installation.

Calculating the general breaker

With the adequate installed power with the respective demand factors, we can already calculate the current of the general circuit breaker of our installation, this current will be calculated through Ohm’s Law.

Current calculation for the general circuit breaker.

The general circuit breaker for our example must be at least 63A for a voltage of 127V. The choice of the current range depends on the availability of the manufacturers.

The manufacturer Schneider Electric has the Easy9 line of circuit breakers and other components, the catalog of products in the Easy9 line is complete and rich in information.

Table for Schneider Easy9 line circuit breakers.

General circuit breaker curve

The last information about the general circuit breaker is the broken curve in the case of DIN circuit breakers. Regarding the curve of the general circuit breaker, we have to consider the curves of the partial circuit breakers, the choice of the curve will always be the largest curve between the partial ones.

In the case of our example, we would have the following table for partial circuit breakers:

Table of partial installation circuit breakers.

In this case we have circuit breakers B curve and curve C and following the rule of curves, we have to select a 63A circuit breaker with curve C as the general circuit breaker.

Not all electricity professionals effectively know how to perform the correct dimensioning of the general circuit breaker, some make a simple summation of the partial circuit breakers, which in the case of the example we use for circuits, would give us a general circuit breaker of 100A that would be completely out of that performed in this installation.

In the Partner Portal, an area created by the Schneider company for electricity professionals, it is possible to have access to various materials such as handouts, videos, catalogs and others. This portal is an interesting study and learning tool for the electricity professional.

In the video below you can see this sizing process.

Following the steps described here, the circuit breaker will be dimensioned according to the selectivity criteria, guaranteeing safety for installation and users, and the correct operation in case of failure. It is important to understand that there is no standard installation or common residence and this calculation will always change according to the loads installed in each residence.