Checking Power-Providing Circuits

Measuring a power wire with the intended load disconnected using a DMM  will only find open circuits (open fuse or wire).

• Recommended practice:  Circuits carrying approximately 200-1000 mA* may be loaded with a 250-350 mA test light. Measure circuit voltage with a DMM  while the test light is connected and illuminated. A reduction in the voltage present during test-light-loading indicates excessive circuit resistance.
• Recommended practice:  Circuits carrying more than one ampere* should be loaded with a device requiring similar current (e.g., a headlamp bulb may be effective). A reduction in the voltage present during loading indicates excessive resistance.
• * Circuit current is matched to wire gauge/size; Examples:
  • Conductor sizes of 24 gauge (.5 mm) or smaller are generally used to carry approximately 1 ampere (1000 mA) or less. Use of the test light to load these circuits is appropriate.
  • Conductor sizes of 20 gauge (.8 mm) or larger are generally used to carry approximately 5 amperes (5000 mA) or more. Match the substitute load (measure substitute load current first as necessary) to this current level.