Master Electrician Exam (NEC)General Electrical KnowledgeHard
A master electrician is performing an insulation resistance test on a newly installed 480V motor feeder circuit. What is the minimum acceptable insulation resistance value for this circuit, according to industry standards and common practice?
- A1.0 megohm
- B0.5 megohm
- C10.0 megohms
- D100.0 megohms
Show answer & explanationAnswer & explanation
Correct answer: A. 1.0 megohm
While NEC does not specify minimum insulation resistance values, industry standards (like NETA, IEEE, and manufacturer recommendations) commonly suggest minimum values. For circuits rated 251V to 500V, a common minimum acceptable insulation resistance value is 1.0 megohm. The test voltage applied is typically 1000V for a 480V system.
Why the other options are wrong
- B. 0.5 megohm is typically for lower voltage circuits (up to 250V).
- C. 10.0 megohms is a good reading, but not the absolute minimum for a 480V circuit.
- D. 100.0 megohms indicates excellent insulation, but is not the minimum acceptable.
Insulation Resistance Test Standards
Insulation resistance testing measures the resistance of a conductor's insulation to ground or between conductors. It helps assess insulation integrity and predict potential failures.
- Test voltage is typically 2x nominal voltage plus 1000V, or specific values from NETA/IEEE.
- Minimum acceptable values vary by voltage class, age of equipment, and industry standards (e.g., 1 megohm for 251-500V circuits).
- Readings below acceptable limits indicate potential insulation degradation or moisture issues.
Memory trick: Insulation Resistance: Voltage Guides the Megohms.