What is the proper method to measure current in a portion of a circuit?

• A. With the circuit energized, place the meter probes of an ammeter across the component or section of the circuit in question and read the value.
• B. Shut off the power to the circuit, open the portion of the circuit in question, wire an ammeter in series, reenergize the circuit and read the value.
• C. Measure the power output of the section of the circuit in question with an ohmmeter and calculate the current using ohms law.
• D. Measure the voltage output of the power supply and calculate the current using the power equation.

Answer: (B)

Measuring current requires placing the instrument in series with the portion of the circuit you’re testing so the same current flows through both the meter and the load. The proper approach is to power down, open the circuit at the point of interest, insert the ammeter in series, re-energize, and read the current. This ensures you measure the actual current in that path and avoids creating a short or altering the circuit.

Placing the meter across a component would short that path and give an incorrect reading while risking damage or blowing a fuse. An ohmmeter measures resistance, not current, so you can’t reliably determine current from an ohmmeter reading alone. Estimating current from power or voltage alone can be misleading in a multi-load circuit, where conditions vary.