Journeyman Electrician Exam (NEC)General Electrical Knowledge and Plan ReadingMedium
A residential client reports that some lights in their home flicker intermittently, and certain appliances seem to operate at reduced power. Upon inspection, the electrician measures the voltage at the service entrance and finds it to be 115V AC under load, but when checking a specific branch circuit, the voltage drops to 108V AC when a hair dryer is turned on. What is the approximate voltage drop percentage for this branch circuit?
- A6.1%
- B7.0%
- C3.5%
- D8.2%
Show answer & explanationAnswer & explanation
Correct answer: A. 6.1%
Voltage drop percentage is calculated by dividing the voltage drop by the source voltage and multiplying by 100. The voltage drop is 115V - 108V = 7V. So, (7V / 115V) * 100% = 6.08%, which rounds to 6.1%.
Why the other options are wrong
- B. Incorrect calculation; perhaps used the voltage drop as a percentage of the load voltage.
- C. Incorrect calculation; perhaps used 108V as the denominator.
- D. Incorrect calculation; likely a miscalculation of the voltage drop or denominator.
Voltage Drop Percentage
The ratio of the voltage drop across a circuit or conductor to the source voltage, expressed as a percentage. It indicates how much voltage is lost due to resistance.
- Calculated as (Voltage Drop / Source Voltage) * 100%.
- Excessive voltage drop can cause poor appliance performance and increased current.
- NEC recommends a maximum of 3% voltage drop for feeders and branch circuits.
Memory trick: Drop over Source, then multiply to Percent.