Why do LIGHT BULBS DIMMER when you turn on the SHOWER? My dear, have you ever noticed that in some homes, when you turn on the shower, the light bulbs get dimmer, and in some cases, some appliances even stop working? Well, in today's video I'm going to teach you three things: 1 - What is this voltage drop? 2 - How do you find out which cable is the right one, taking this drop into account? 3 - Two ways to solve the problem of light bulbs dimming when you turn on the shower? Next, I'll tell you right away that there's nothing mysterious about it, what's happening is nothing more or less than one of the consequences of a voltage drop. A voltage drop is an effect caused both by the increase in the distance traveled by the electric current in a circuit, as well as by the incorrect dimensioning of the circuit. Therefore, knowing how to correctly dimension an installation makes all the difference. And although many electricians ignore this problem, we will teach you how to do the right calculations so you know how to solve this problem, because sometimes, simply increasing the cable section by one section will not solve the problem. Item 6.2.7 of NBR 5410 presents the maximum permitted values ​​for voltage drop in detail. In summary, NBR 5410 defines that the maximum voltage drop in terminal circuits of use must not exceed 4% of the nominal voltage, and the total voltage drop along the circuit must not exceed 6%. So, how do you know which cable to use to avoid voltage drops? This is simple my dear, just use the following formula: Sc = I x ( L x 2 ) 58 x ΔV Where: S = Cable section ( mm2 ) I = Circuit Current (A) L = Circuit distance 58 = Copper conductivity ΔV = Voltage drop value according to circuit voltage I note that this form is used for single-phase systems (127V / 220V) To better understand, let's look at a not so common but very didactic practical example: I have a shower with a power of 5500W, installed 30 meters from my QDC in an exclusive duct for it. The voltage to power this shower is 127V. Therefore, applying Ohm's law, the current of this circuit is 5500W / 127V = 43.3A, not considering the distance yet. Most “experienced installers” would adopt a 6mm² cable and a 40A circuit breaker, because according to them it CAN WITHSTAND. However, consulting table 36 of the standard, the 6mm2 cable supports up to 41A in installation method B1. However, let's see what the correct cross-section is to serve this circuit. We already know that the circuit current is 43.3A. We also know that the circuit distance is 30 meters. We also know that the conductivity of copper is a constant value of 58. Now we just have to calculate the voltage drop value. As I said before, the voltage drop value should not exceed 4%. In this case, we have 127Vx4% = 5.08V Now just substitute everything in the formula: Sc = 43.3 x (30 x 2) = 2598 = 8.81 58 x 5.08 294.64 The cable cross-section to serve this circuit for it to have maximum efficiency must be 10mm2, since the “gauge” of 8.8 mm² does not exist. Okay André, great, the shower cable in this case is poorly sized, but why does this affect the entire installation, making the bulbs flicker or become weaker? Unfortunately, the electric shower is still the villain of the electricity bill! Its power is very high, and more and more showers are being launched that heat up more, therefore they use more. Even if you correctly size the shower circuit, in this case use a 10mm² cable, it is no use if the house's power supply is poorly sized! Here in Minas Gerais, there are still many single-phase 40A standards, with 6mm² cables supplying an entire house. In rural areas, it is more common for the standard to be close to the distribution pole (since CEMIG limits it to 30 meters) and very, very far from the house itself. I myself have built rural overhead networks that were 100, 200 meters long and, unfortunately, some of them were unable to change the standard due to customer issues. When you turn on the shower in this type of distant network, the voltage drops suddenly, affecting the operation of the lights, appliances and everything else that is connected!