WiremanWireman · Clamp meter: amps, inrush and balance: troubleshooting for electricians · 2023 NEC ·
Clamp meter: amps, inrush and balance
Read running current, starting current, phase balance and leakage without opening the circuit.
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How it works
Current flowing through a wire makes a magnetic field around it, and a clamp meter measures that field through its jaws. Clamp one conductor and you read its current. Clamp a whole cable and the hot and neutral fields cancel, so a working load reads near zero. That cancelling is also a tool: clamp all the circuit conductors together and anything you read is current leaking out another path.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What do you want to know?
- How much a load draws runningTest: Measure running currentExpect: At or below the nameplate amps (FLA, RLA or “A”).
- About the nameplate or lessNormal reading
- Higher than nameplateLoad drawing more than nameplate
- Near 0 A with the load runningWhat was in the jaws?
- A whole cable or cordThe currents cancelled in the jaws
- One conductorLittle or no current is flowing
- Starting surge (inrush)Test: Capture starting currentExpect: For an across-the-line motor, roughly 5 to 8 times running current, settling in a second or two. Soft starters and drives start much lower.
- In the normal range and settles quicklyNormal reading
- Much higher, or takes seconds to settleHard or long start
- Balance between phases or neutral currentTest: Compare each phase and the neutralExpect: A three-phase motor’s lines within about 10% of each other. On a single-phase panel, the neutral carries about the difference between L1 and L2.
- Balanced within about 10%Normal reading
- One phase much higher or lowerCurrent unbalance
- Three-phase neutral near or above phase currentHarmonic current on the neutral
- Leakage to groundTest: Clamp all the circuit conductors togetherExpect: Near 0 mA. Some electronic equipment normally leaks a few milliamps through its filters.
- Near 0 or a few mANormal reading
- Several mA or moreCurrent leaking out of the circuit
- How much a load draws runningTest: Measure running currentExpect: At or below the nameplate amps (FLA, RLA or “A”).