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Extech · Extech 380947 400A True RMS AC/DC Mini Clamp Meter | 1mA resolution, frequency to 10kHz, bargraph, auto zero, min/max, data hold, battery, case
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Verdict
The Extech 380947 is the general-purpose member of Extech’s mini clamp family: 400 A AC and DC with True RMS in a body small enough for a shirt pocket, with a 0.9 inch (23 mm) jaw. Its distinctive functions are non-contact frequency measurement to 10 kHz through the jaws, a fast 40-segment bar graph and one-touch auto zero for DC. The 1 mA AC resolution on the low range gives it useful digits on small loads without giving up the 400 A ceiling. It has no non-contact voltage detector and, as far as the listing states, no inrush capture. For an electrician or HVAC technician who wants a capable clamp that disappears into a pouch, the documentation supports it; for large feeders or crowded panels, the small jaw is the constraint.
Who It Is For
- Electricians who want a small everyday clamp with True RMS and DC.
- HVAC technicians working inside air handlers and condensing units.
- Drive and motor technicians who want frequency without leads.
Our guide to choosing a clamp meter compares mini and full-size clamps.
Documented Specifications
| Item | Documentation |
|---|---|
| Current | AC and DC to 400 A, True RMS |
| Resolution | 1 mA AC on the low range |
| Jaw opening | 0.9 inch (23 mm) |
| Frequency | Non-contact through the jaws to 10 kHz |
| Display | Digital readout with fast 40-segment bar graph |
| DC zero | One-touch auto zero |
The voltage range, display count and safety category rating are on the manufacturer’s datasheet; consult the current revision.
Frequency Through the Jaw
Most clamps that measure frequency do so through the test leads on a voltage signal. The 380947 reads it from the current in the jaw, which means a technician can clamp a motor lead and see both the current and the frequency the drive is delivering without touching a terminal. The 10 kHz limit is far above the fundamental frequencies of mains, generators and drives, and lets the meter register switching-frequency content when it is present. Owners use it to confirm a drive is running at commanded speed and to check a generator’s frequency under load. For the underlying technique see measuring current with a clamp meter.
Bar Graph and Auto Zero
A digital display updates a few times a second and shows a number; a bar graph updates faster and shows movement. The 40-segment bar graph on the 380947 makes a fluctuating current visible as a moving bar, which is how a technician spots a cycling load or an unstable drive before the digits settle. One-touch auto zero is the practical answer to Hall-effect drift: press it with the jaw closed on nothing and the DC offset disappears. Owners note that on a mini clamp, whose sensor is smaller, this matters more than on a full-size meter, and that the single-button implementation makes it a habit rather than a chore.
The Mini Clamp Family
Extech’s 3809xx mini clamps share a body and differ in range and function. The 380942 is the 30 A low-current model with 0.1 mA resolution. The 380947 is the 400 A general-purpose model reviewed here. The 380950 is the 80 A model with a full multimeter function set. A technician who works mainly on control circuits pairs the 380942 with a full-size clamp; one who wants a single small clamp for everything from lighting to compressors chooses the 380947.
Practical Notes
Centre the conductor in the small jaw and close it fully; a partially closed jaw reads low. Use the auto zero before every DC reading and again if the meter has just read a large current, since the sensor retains some magnetism. A two-conductor cable reads near zero, so a line splitter is needed for appliance loads. Owners note that the bar graph is the quickest way to see whether a compressor’s current is stable or cycling, and that the small display benefits from the hold function when the meter is deep inside an enclosure.
Strengths
- 400 A True RMS AC/DC in a pocket-sized body.
- Frequency read through the jaw without leads.
- Fast bar graph for fluctuating loads.
- One-touch auto zero for DC.
Limitations
- 23 mm jaw excludes larger feeders.
- No non-contact voltage detector or inrush capture stated on the listing.
- Small display compared with full-size clamps.
How It Compares
The Extech 380942 is the low-current sibling for control circuits. The Extech 380950 is the 80 A mini clamp with full multimeter functions. The Extech MA445 is the full-size 400 A AC/DC clamp with a 40000-count display, inrush and an NCV detector for those who can accept a larger body.
How We Research
This review is based on the manufacturer’s published specifications, the Amazon product listing and owner reports. We did not run laboratory tests.
Recommended Tools
Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.
Frequently Asked Questions
How can a mini clamp read 400 A?
The jaw size limits conductor diameter, not current. The 23 mm jaw fits the conductors found in residential and light commercial panels, and the sensor is rated to 400 A through it.
What is non-contact frequency measurement?
The meter reads the frequency of the current in the conductor inside the jaw, without test leads, to 10 kHz. It is useful for confirming drive output frequency and for spotting harmonics.
What does one-touch auto zero do?
It cancels the DC offset of the Hall-effect sensor with a single button press, which should be done before each DC current reading.
Is it True RMS?
Yes. The listing specifies True RMS measurements with 1 mA AC resolution.
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