WiremanWireman · Check phase rotation: troubleshooting for electricians · 2023 NEC ·
Check phase rotation
Use a phase-sequence meter before you land motors or connect a second source, and fix it by swapping two lines.
Before you start
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How it works
Three-phase power reaches its peak on each phase in turn, and the order (A-B-C or C-B-A) is the phase rotation. A three-phase motor spins in the direction the rotation pushes it, so reversing the order reverses the motor. Swapping any two lines reverses the rotation; swapping all three in a roll keeps it the same. A phase-sequence meter shows the order directly, so you don’t have to bump a motor and hope.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What are you doing?
- Landing a motor or equipment on a three-phase supplyTest: Read the phase sequenceExpect: All three phases present and a steady rotation indication.
- All three present, rotation shownDoes the rotation match what the equipment needs?
- MatchesRotation is correct
- OppositeRotation is reversed: swap two lines
- A phase light is outA phase is missing
- All three present, rotation shownDoes the rotation match what the equipment needs?
- Matching a second source (generator, transfer switch, new service)Test: Compare both sourcesExpect: The same rotation on both sources.
- Same on bothBoth sources have the same rotation
- OppositeSources have opposite rotation
- A phase is missing on one sourceA phase is missing
- A motor already runs backwardRotation is reversed: swap two lines
- Landing a motor or equipment on a three-phase supplyTest: Read the phase sequenceExpect: All three phases present and a steady rotation indication.