Clamp Meter
Test Instrument
Measures current by clamping around a conductor — no circuit breaking, no fuse risk, works at hundreds of amps. The electrician's daily instrument, built on the magnetic field laws from the textbook.
What it looks like
Clamp Meter
A typical test instrument — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Current range
ATypically 400 A or 600 A AC; leakage clamps resolve down to 0.1 mA.
AC or AC/DC
—CT-based clamps read AC only. DC (battery, solar, automotive) needs a Hall-effect clamp — the spec to check first.
Jaw opening
mmMust fit the cable or busbar: 25–35 mm common, more for panels.
True RMS
—Essential around VFDs and electronics — distorted currents are the norm now.
Multimeter functions
—Most clamps include V/Ω/continuity via probes, replacing a basic DMM in the tool bag.
Markings
Jaw marks a conductor-centering line and current direction arrow (matters for DC). Dial: A~, A⎓, V, Ω. CAT III/IV rating printed near jacks.
Standard values
UT210E (100 A AC/DC mini — the hobby favourite for its DC ability), UT203/204 (400 A), Fluke 376 FC class professionally; earth-leakage clamps for RCD troubleshooting.
How to choose
1) Need DC (solar, batteries, EVs)? Hall-type only. 2) Match range: measuring a 50 mA leak needs a leakage clamp, not a 600 A jaw. 3) Jaw size for your biggest cable. 4) True RMS + inrush capture for motor work. 5) Remember: clamp ONE conductor — around a whole cable, fields cancel and you read zero (which is exactly how leakage clamps find earth faults: clamp L+N together, any reading is leakage).
Pinout & package
Trigger-opened transformer jaw on a meter body; probe jacks for voltage/resistance. Hall types have a DC-zero button — use it before every DC reading.
Example circuits
- Reading each phase of a motor to spot imbalance (weak phase, failing winding)
- Clamping L+N together on an RCD circuit to measure actual leakage current
- DC clamp on a solar string checking real panel output
- Inrush mode catching the 6–8× starting surge of a compressor
Common failures
Dirty or sprung jaw faces (low readings — jaws must close perfectly), drift in Hall sensors (re-zero often), and dead range switches. Otherwise clamps outlive DMMs — no fuse to blow for current.
How to test
Wrap N turns of wire carrying a known current through the jaw — it should read N × current (a 10-turn loop of 1 A reads 10 A; this trick also multiplies resolution for small currents). Compare against a series DMM at low current.
Substitutes
A DMM in series for small currents; a shunt + millivoltmeter for fixed installs; CT + panel ammeter for permanent metering. Rogowski flexes substitute where rigid jaws can't reach.