5 Formulas Electricians Should Have Memorized!

Summary

This video covers five essential formulas every electrician should know, starting with Ohm's Law, which explains the relationship between voltage, current, and resistance. It then explores Joule's Law, focusing on power transfer, and provides examples for calculating wattage and current. Voltage drop formulas for both single-phase and three-phase circuits are discussed, followed by methods for calculating resistance in series and parallel circuits. Lastly, the video covers horsepower calculations for motors, highlighting how efficiency and power factor impact motor output in both single-phase and three-phase systems.

Takeaways

• ⚡ Ohm's Law explains the relationship between voltage, amperage, and resistance. It's often written as E = I * R.
• 🔌 To solve for a specific variable in Ohm's Law, cover the one you're solving for and use the appropriate equation, like E = I * R for voltage.
• 💡 Joule's Law relates to the amount of energy transferred between systems, commonly expressed as P = I * E.
• 🔋 Voltage drop can be calculated using formulas based on Ohm's Law, with additional factors like conductor length and material affecting accuracy.
• 🔗 In three-phase circuits, voltage drop calculations introduce the square root of 3 (1.732) for more accuracy.
• 🧮 For resistances in series, simply add them together. In parallel circuits, use the sum of reciprocals for total resistance.
• 📏 The product-over-sum method is an easier alternative to calculate parallel resistance compared to the sum of reciprocals method.
• ⚙️ One horsepower equals 746 Watts. However, motor efficiency and power factor influence the actual horsepower output.
• 🏎️ Three-phase motors generally produce more horsepower than single-phase motors due to more efficient power transfer.
• 📚 Advanced topics like capacitance, inductive reactance, and RLC circuits require additional study but are important for more complex electrical systems.

Q & A

What is Ohm's Law, and how does it relate voltage, amperage, and resistance?

Ohm's Law describes the relationship between voltage (E), amperage (I), and resistance (R). It states that E = I × R. If you want to solve for one variable, you can rearrange the equation accordingly, such as I = E / R or R = E / I.

How can Ohm’s Law be used to calculate the amperage in a circuit?

To calculate amperage using Ohm’s Law, divide the voltage by the resistance. For example, if you have 120 volts and a 6-ohm resistor, the amperage would be 120 / 6 = 20 amps.

What is Joule's Law, and how does it differ from Ohm’s Law?

Joule's Law relates to the amount of energy transferred, particularly from electrical to another form, like light or heat. It is represented by P = I × E, where P is power (watts), I is current, and E is voltage. Unlike Ohm’s Law, which focuses on voltage, amperage, and resistance, Joule’s Law introduces power.

How can you calculate the total wattage of a circuit using Joule's Law?

Using Joule's Law (P = I × E), if you know the voltage and the amperage, you can calculate total power. For instance, in a 120-volt circuit drawing 20 amps, the total power would be 120 × 20 = 2400 watts.

What is voltage drop, and how is it calculated in single-phase circuits?

Voltage drop is the reduction in voltage in a circuit between the power source and the load. In single-phase circuits, it can be calculated using the formula VD = 2 × K × I × L / CM, where K is a material constant (12.9 for copper and 21.2 for aluminum), I is the current, L is the length of the conductors, and CM is the circular mils of the conductor.

Why is 1.732 used in three-phase voltage drop calculations?

The value 1.732 is the square root of 3 and is used in three-phase voltage drop calculations to account for the three-phase system’s different power transfer characteristics compared to single-phase systems.

What is the difference in voltage drop between copper and aluminum conductors?

Copper has a lower resistance than aluminum, leading to less voltage drop. The constant for copper (K = 12.9) is smaller than for aluminum (K = 21.2), meaning aluminum conductors experience more voltage drop for the same current and distance.

How do you calculate resistance in a series circuit?

In a series circuit, the total resistance is the sum of all individual resistances. For example, if you have three resistors with values of 2 ohms, 3 ohms, and 4 ohms, the total resistance would be 2 + 3 + 4 = 9 ohms.

How do you calculate resistance in a parallel circuit?

In a parallel circuit, resistance is calculated using the reciprocal method: 1 / RT = 1 / R1 + 1 / R2 + 1 / R3. For example, if you have resistors of 2 ohms, 3 ohms, and 4 ohms, you would calculate 1 / RT = 1 / 2 + 1 / 3 + 1 / 4. The total resistance would be the reciprocal of that sum.

What is the relationship between watts and horsepower?

One horsepower is approximately equal to 746 watts. This means that to convert watts to horsepower, you divide the total watts by 746. For example, a motor producing 2400 watts would have a horsepower of 2400 / 746 ≈ 3.2 horsepower.