WiremanWireman · Test a capacitor: troubleshooting for electricians · 2023 NEC ·
Test a capacitor
Discharge, inspect, measure microfarads, and check it under load.
Before you start
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How it works
Motor capacitors store charge and release it out of step with the line, which shifts the current in a motor’s start winding so the field rotates. Run capacitors stay in the circuit all the time and are rated in microfarads (µF or MFD) with a tolerance. Start capacitors are switched in for a second and have a range printed on them. A capacitor can hold a dangerous charge after power is off, so discharge it before you touch the terminals.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- Test: Discharge and verify 0 VExpect: 0 V across every pair.
- Reads 0 VWhat does the capacitor look like?
- Bulged or domed top, leaking, cracked or burnedCapacitor has failed
- Looks normalIs it a run or a start capacitor?
- Run capacitor (single or dual)Test: Measure the run capacitorExpect: Within the printed tolerance, often ±5% or ±6% of the rated µF.
- Within toleranceTest: Check it under loadExpect: Within the label tolerance.
- Matches the ratingCapacitor tests good
- Lower than the ratingWeak or open capacitor
- Below tolerance, or reads openWeak or open capacitor
- Within toleranceTest: Check it under loadExpect: Within the label tolerance.
- Start capacitorTest: Measure the start capacitorExpect: Within the printed range.
- Within the rangeStart capacitor good: check what switches it
- Below the range or openWeak or open capacitor
- Run capacitor (single or dual)Test: Measure the run capacitorExpect: Within the printed tolerance, often ±5% or ±6% of the rated µF.
- Still reads voltageThe capacitor is still charged or power isn’t off
- Reads 0 VWhat does the capacitor look like?