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Extech 380942 Review: Low-Current True-RMS AC/DC Clamp Meter

The Extech 380942 is a specialist: a 30 A clamp with 0.1 mA resolution for currents that a 400 A clamp cannot resolve. This review explains what low-current clamp measurement is for, why the resolution matters, and where the 380947 and 380950 fit alongside it.

By the Akermin Editorial Team · 4 min read

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Extech 380942 Review: Low-Current True-RMS AC/DC Clamp Meter

Extech · Extech 380942 True RMS 30A AC/DC Mini Clamp Meter, 0.1mA Resolution

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Price and availability are shown on Amazon and can change at any time.

Verdict

The Extech 380942 is a mini clamp meter whose whole purpose is small currents. It reads AC and DC current to 30 A with True RMS and a resolution of 0.1 mA, which means it can see the current in a 24 V control loop, a sensor supply, a small motor or an LED lighting circuit that a 400 A clamp would show as zero or as a single flickering digit. A word on the listing: at the time of writing, the feature bullets on the Amazon page describe a light meter and do not belong to this product. The listing title, the model number and Extech’s documentation are consistent, and this review is written from those. For an instrumentation, controls or automotive technician, the documentation supports the 380942 as a useful complement to a full-size clamp; it is not a replacement for one.

Who It Is For

  • Controls and instrumentation technicians working on 4 to 20 mA loops and 24 V control circuits.
  • Automotive and marine technicians tracing small DC draws.
  • Lighting and building services engineers measuring LED driver and ballast currents.

Our guide to choosing a clamp meter explains where low-current clamps sit in the range.

Documented Specifications

Item Documentation
Current AC and DC to 30 A, True RMS (listing title and manufacturer)
Resolution 0.1 mA (listing title and manufacturer)
Form Mini clamp, same family as the 380947 and 380950
Listing note Amazon feature bullets currently describe a different product

The voltage range, display count, jaw opening and safety category rating are on the manufacturer’s datasheet, which should be consulted given the state of the listing.

Why a Low-Current Clamp Exists

Clamp meters are specified by their maximum current, and the maximum sets the resolution. A 4000-count display on a 400 A range shows 399.9 at most, so the smallest step is 0.1 A. A 200 mA control circuit reads as 0.2 with the last digit uncertain; a 20 mA loop reads as zero. Reducing the range to 30 A moves the decimal point: the same display now resolves 0.01 A, and on the lower ranges the 380942 reaches 0.1 mA. That is the difference between guessing and measuring on a control panel. The traditional alternative is to break the circuit and insert a multimeter in series, which is slow and, on a live control loop, disruptive. A low-current clamp reads the loop without opening it. For the general technique see measuring current with a clamp meter.

DC Current and True RMS at Low Levels

DC through a clamp relies on a Hall-effect sensor, which drifts with temperature and retains a little magnetism after a large current. On a 30 A clamp the drift is a larger fraction of the reading than on a 400 A clamp, so zeroing before every DC measurement is essential; the meter provides a zero function for this. True RMS matters at low currents for the same reason as at high ones: LED drivers, electronic ballasts and switch-mode supplies draw peaky current that an average-responding meter reads low. Owners who use the 380942 on lighting circuits mention this as the reason they chose it over a cheaper mini clamp.

Practical Notes

Keep the conductor centred in the small jaw; off-centre placement matters more on a small jaw than a large one. For very small currents, several turns of the conductor through the jaw multiply the reading and the display is divided by the number of turns. Because the listing is currently mismatched, check the model number on the product you receive against the 380942 and download the datasheet from the manufacturer before relying on a range figure.

Strengths

  • 0.1 mA resolution reads control and sensor currents without opening the circuit.
  • AC and DC with True RMS.
  • Mini form fits control panels and vehicle harnesses.

Limitations

  • 30 A ceiling; not a general-purpose clamp.
  • Amazon listing bullets are currently wrong for this product.
  • DC readings need careful zeroing.

How It Compares

The Extech 380947 is the 400 A True RMS mini clamp in the same family, with 1 mA resolution and a 23 mm jaw. The Extech 380950 is the 80 A mini clamp with a full multimeter function set. The Extech MA120 is the 200 A mini AC/DC clamp with an NCV detector. Most technicians pair the 380942 with one of these rather than choosing between them.

How We Research

This review is based on the manufacturer’s published documentation, the Amazon listing title and owner reports. Because the listing’s feature bullets currently describe a different product, we have not used them. We did not run laboratory tests.

Extech 380947 400A True RMS AC/DC Mini Clamp Meter

Extech 380947 400A True RMS AC/DC Mini Clamp Meter

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Extech 380950 Mini AC/DC 80A Clamp Meter, 4000 Count, Full DMM

Extech 380950 Mini AC/DC 80A Clamp Meter, 4000 Count, Full

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Extech MA120 200A AC/DC Mini Clamp Meter with Voltage Detector

Extech MA120 200A AC/DC Mini Clamp Meter with Voltage Detector

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Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

Why buy a 30 A clamp when 400 A clamps exist?

Resolution. A 400 A clamp with a 4000-count display resolves 0.1 A at best. The 380942 resolves 0.1 mA, which is what control circuits, sensor loops and leakage checks need.

Does it measure DC current?

Yes. The listing describes it as an AC/DC clamp, so it reads DC control circuits and small DC loads through the jaw.

Are the Amazon listing bullets correct?

At the time of writing the feature bullets on the listing describe a light meter, which is an error. The title, the model number and the manufacturer's documentation describe a 30 A True RMS mini clamp with 0.1 mA resolution.

Can it measure leakage current?

It can read small AC currents in a single conductor. Whether it is suitable for a formal leakage test depends on the resolution and accuracy figures on the datasheet, which should be checked against the test requirement.

Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.