CSLB C-10 Electrical ContractorElectrical Theory and CalculationsEasy
An electrician is installing a 240V, single-phase circuit to power a new resistive oven drawing 30 amps. What is the total impedance of the circuit?
- A12 ohms
- B0 ohms
- C7200 ohms
- D8 ohms
Show answer & explanationAnswer & explanation
Correct answer: D. 8 ohms
For a purely resistive circuit, impedance (Z) is equal to resistance (R). Using Ohm's Law, R = V/I. Therefore, 240V / 30A = 8 ohms. The total impedance is 8 ohms.
Why the other options are wrong
- A. This would be the result if the voltage was 360V, or current was 20A, or a calculation error.
- B. 0 ohms would imply a short circuit with infinite current.
- C. This is the power (P = V*I), not the impedance or resistance.
Impedance in Resistive Circuits
In a purely resistive AC circuit, impedance (Z) is equal to the resistance (R) because there are no reactive components (inductors or capacitors) to cause phase shifts. It represents the total opposition to current flow.
- For purely resistive circuits, Z = R.
- Calculated using Ohm's Law: Z = V/I.
- Measured in Ohms (Ω).
Memory trick: Resistive means Impedance is just Resistance, simple Ohm's Law.