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Summary

Professor Gelasio Boz explains how to select and calculate the appropriate electrical cables for different applications, focusing on current-carrying capacity, minimum section, and voltage drop. Using a practical example of an industrial electric oven, he walks through the process of determining the right cable size based on the NBR 5410 Brazilian technical standard. The lesson emphasizes the importance of considering both electrical current and permissible voltage drop for safe installations. The professor also encourages viewers to like, share, and follow his content for future lessons, including upcoming topics like disjunction calculations.

Takeaways

• 😀 The lesson focuses on selecting cables for electrical installations, specifically calculating current carrying capacity, minimum cross-section, and voltage drop.
• 😀 The content is based on the Brazilian Technical Standards (ABNT), particularly NBR 5410, which governs low voltage electrical installations.
• 😀 The circuit used for the calculations is hypothetical, with a power of 4200 watts, a voltage of 220V, and a project current of 19.09A at a temperature of 50°C.
• 😀 The initial calculation determines that the required cable has a capacity of 6mm², which meets the current carrying needs.
• 😀 For minimum cross-section, the standard size is 2.5mm² for lighting and outlet circuits, regardless of whether the circuit is mixed (lighting and outlets).
• 😀 Voltage drop limits vary: 7% for circuits with transformers, up to 1% for low-voltage networks, and 4% from the terminal panel to the load.
• 😀 The voltage drop formula is provided for both single-phase and three-phase circuits, with distinctions in constants for each.
• 😀 The resistivity of copper (0.017) is used for calculations, and the formula takes into account circuit length, current, and desired voltage drop.
• 😀 The instructor prefers using a formula that allows flexibility in adjusting the voltage drop to meet specific client needs, rather than strictly following norms.
• 😀 After performing the calculations, a 3.3mm² cable is chosen, though it is rounded to 4mm² as standard sizes must be used in practice.
• 😀 To select the correct cable, the highest value from calculations (current carrying capacity, minimum section, and voltage drop) determines the final cable size.

Q & A

What is the main focus of this video lesson by Professor Gelasio Boz?

The lesson focuses on the continuation of electrical cable calculations, specifically covering current-carrying capacity, minimum cross-section, and voltage drop according to Brazilian standards (NBR 5410).

Which Brazilian standard is the lesson based on for electrical installations?

The lesson follows the Associação Brasileira de Normas Técnicas (ABNT), specifically NBR 5410, which regulates low-voltage electrical installations.

What are the three main criteria to consider when selecting an electrical cable?

The three main criteria are: current-carrying capacity, minimum cross-section of the cable, and voltage drop.

How is the minimum cable section determined for different types of circuits?

For lighting circuits, the minimum section is 1.5 mm², for power/plug circuits it is 2.5 mm², and for mixed lighting and plug circuits, the minimum section is 2.5 mm².

What is the maximum voltage drop allowed according to the NBR 5410 standard?

The maximum voltage drop allowed is 7% if a transformer or generator is present. For low-voltage networks without a transformer or generator, the drop from the main panel to the terminal panel is 1%, and from the terminal panel to the load, it is 4%.

How is the voltage drop calculated for single-phase and two-phase circuits?

Voltage drop is calculated using a formula that considers cable resistivity, circuit length, project current, and maximum allowable voltage drop. The difference between single-phase and two-phase calculations is the constant used in the formula, which accounts for the circuit configuration.

What example is used in the lesson for a hypothetical industrial load?

The example used is an industrial electric oven with a power of 4200 W, voltage of 220 V, project current of 19.09 A, ambient temperature of 50°C, and grouped circuits.

How is the final cable size determined when considering all three criteria?

The final cable size is determined by calculating the required section for current-carrying capacity, minimum section, and voltage drop, and then choosing the largest value among the three calculations.

Why does the lesson emphasize using copper wire resistivity in calculations?

Copper wire resistivity (ρ = 0.017) is used because the calculations for voltage drop assume copper conductors. If aluminum wires were used, a different resistivity value would be required.

What is the main teaching tip given by Professor Gelasio Boz for selecting cables?

The key tip is to calculate the three criteria—current-carrying capacity, minimum cross-section, and voltage drop—and always select the cable size that meets the highest requirement to ensure safety and compliance with standards.

What is the purpose of explaining voltage drop variations in practical applications?

It allows electricians to adjust the allowable voltage drop for clients who are at the end of the network or experiencing poor voltage quality, using lower percentages to improve energy efficiency and reliability.

How does the lesson connect theoretical calculations to practical cable selection?

The lesson demonstrates using formulas, resistivity, current, and length to calculate the required cable section, then shows practical steps to round up to commercially available cable sizes and ensure compliance with standards.