Clamp Meter
Measuring current without breaking the circuit, and reading amp draw against the data plate.
Tool
Beginner
#tools#electrical#testingSafety
Clamping requires access to energized conductors. Use a correctly rated meter and keep clear of live terminals.
What it is
A clamp meter measures current by sensing the magnetic field around a single conductor. It lets you read amp draw without disconnecting anything, which makes it the fastest way to judge whether a motor or compressor is working harder than it should.
How to use it
- 1Clamp around one conductor only. Clamping two opposing conductors cancels the reading to near zero.
- 2Let the system run and stabilise before recording.
- 3Compare the reading to the rated load amps and locked rotor amps on the data plate.
- 4For very small currents such as a flame sensor circuit, use a meter with the appropriate range rather than a standard clamp.
- 5Use a line splitter carefully when measuring a cord connected appliance.
| Reading | Meaning |
|---|---|
| Near locked rotor amps, then off on overload | Mechanically seized or failing to start, often a capacitor or start component |
| Above rated load amps | Overloaded motor, high head pressure, low voltage or a mechanical problem |
| Well below rated load amps | Undercharged system, low load, or a motor not doing work |
| Zero with the contactor pulled in | Open circuit downstream, open winding or an open internal overload |
Safety
Working near live conductors
Clamping requires access to energized wiring. Use a properly rated meter and insulated jaws, keep your other hand clear, and never force the jaws into a crowded panel. Where practical, use the disconnect to stage the test safely.
Symptoms
Practice & Mastery
- Questions1
- Clamp Meter Troubleshooting Scenarios2
Practice is part of HVAC Pro — $5/month or $50/year.
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