Klein Cl 390

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The Klein CL390 is an auto-ranging TRMS clamp meter that reads AC and DC current through its jaws, with a non-contact voltage tester built into the clamp. A klein cl 390 user usually wants to know three things: what the meter measures, how to take a current reading without breaking the circuit, and where its limits are. This guide answers those questions from Klein’s own product page and from two reference articles on clamp meters and multimeters. We did not test the meter ourselves, and we say so where a claim comes only from the manufacturer.

Quick answer

The CL390 measures AC and DC current up to 400 A by clamping around one conductor, and it measures AC and DC voltage, resistance, continuity, frequency, capacitance, duty cycle and diodes through its test leads. It also reads temperature with a K-type thermocouple probe. Its safety rating is CAT III 600V, Class 2, double insulation. Before each use, check the meter on a known source, keep the jaws closed on a single conductor, and use the test leads only with the caps the manual specifies.

How we chose these sources

We used three pages that we opened and read. The first is Klein Tools’ product page for the CL390, which lists the functions, ratings, accessories and specifications. The second is the Wikipedia article on current clamps, which explains the types of clamp sensor and why a clamp meter reads current without contact. The third is the Wikipedia article on multimeters, which explains the general meter functions the CL390 shares with other tools. Klein’s page is the only manufacturer source, so every CL390-specific figure below comes from it. We did not rely on search snippets or retailer summaries.

What a clamp meter does

A current clamp has jaws that open around a conductor. The Wikipedia article on current clamps describes the common sensor types: Hall effect clamps, which can read both AC and DC; current-transformer clamps, which work only with AC or pulse waveforms; and iron-vane types. A clamp meter is a meter with one of these clamps built in. The article notes that clamp meters are typically used to read AC current, and that Hall effect types can also measure DC. The CL390 belongs to the Hall effect group because its specification lists AC/DC current measurement.

The practical benefit is that you do not disconnect the wire to measure it. You open the jaws, place one conductor inside, close the jaws fully, and read the display. The clamp reads the current in that conductor only. If two conductors are inside the jaws, the reading is the net of the two, which is why a clamp is usually placed around a single wire.

What the CL390 measures

Klein’s product page lists the following functions for the CL390:

AC and DC current through the clamp, up to 400 A.
Non-contact voltage (NCVT) through the clamp jaw.
AC and DC voltage through the test leads, up to 600 V.
Resistance up to 40 MΩ, continuity, and diode testing through the test leads.
Frequency from 10 Hz to 60 kHz for 8 V to 220 V AC RMS.
Capacitance up to 4000 µF, and duty cycle.
DC microamps up to 200 µA through the test leads.
Temperature from -40 to 1000 °C (-40 to 1832 °F) with the supplied K-type thermocouple probe.

The meter is automatically ranging and uses true root mean square (TRMS) measurement, which the manufacturer describes as improving accuracy. It has a 4000-count reverse-contrast LCD and includes Max/Min, Rel/Zero and Data Hold functions. It is powered by three AAA batteries and auto-powers off after 10 minutes of inactivity, according to the product page.

How to take a current reading with the CL390

The steps below follow the general method for any clamp meter. Read the CL390 instruction sheet before you use it, because the exact button sequence and warnings are in the manual, not in this article.

Inspect the jaws, the clamp, the test leads and the case for damage. Do not use the meter if a part is cracked or the leads are worn.
Select the AC or DC current function. Confirm the function on the display before you clamp the wire.
Open the jaws and place them around one conductor. Close them fully. A gap in the jaws gives a wrong reading.
Keep your fingers behind the jaw guard, and keep the clamp away from other conductors.
Read the display and note whether the meter shows AC or DC. Open the jaws and remove the clamp before you move the meter.

The CL390 offers a Data Hold function, which freezes the reading on the display. Use it when you cannot see the display while the clamp is in place. Release the hold before you take the next reading, so you do not confuse an old value with a new one.

Voltage, resistance and continuity through the test leads

For voltage, resistance, continuity, diode and microamp readings, the meter uses the test leads rather than the clamp. The test leads have CAT III and CAT IV safety caps, according to Klein’s page. Plug the black lead into the common (COM) input and the red lead into the input for the function you select. Confirm the lead position before you touch the probes to a live circuit.

Resistance and continuity readings should be taken with the circuit de-energized. The Wikipedia article on multimeters notes that meters measure several electrical properties and that they include safety features to protect the operator when they are connected to voltages that exceed their rating. That is the point of the CAT rating: the meter is rated for a specific installation category and voltage. Do not connect the CL390 to a circuit above its rated 600 V.

Non-contact voltage and what it cannot tell you

The CL390 includes a non-contact voltage tester integrated into the clamp jaw. An NCVT senses voltage through insulation without touching the conductor. It is a quick screening tool: it tells you that a field is present, and it is useful for finding a live wire before you touch it. It does not give a measured value, and it can give false positives or negatives near metal, in shielded cable, or in a wire that is not carrying current. Use a test lead reading to confirm a result before you work on a circuit.

Temperature with the thermocouple probe

The CL390 includes a K-type thermocouple probe. A K-type probe uses two dissimilar metal wires to produce a voltage that depends on temperature, and the meter converts that voltage to a reading. The listed range is -40 to 1000 °C (-40 to 1832 °F). Allow the probe to reach the temperature of the surface or fluid before you read it, and keep the probe clear of the clamp jaws, which can give a misleading reading if they touch the same surface.

Limits to know before you rely on a reading

The manufacturer’s specifications include a stated accuracy at 18 to 28 °C, with a temperature coefficient that applies outside that range. The product page lists an operating temperature for the meter, and the readings should be taken within that range. The CL390 is rated for residential, office and light-industrial electromagnetic environments, according to the page. It is not a substitute for a meter with a higher rating in an industrial or heavy-utility setting.

Two more limits matter in practice. First, the clamp reads the current in one conductor, so a reading on a cable with several conductors is not the total load. Second, the battery drains over time, and the low-battery indicator is the cue to replace the three AAA batteries before a reading. A low battery can produce a wrong value without an obvious sign.

Maintenance and care

Keep the jaws clean and free of debris, because dirt on the mating faces can change the reading. Store the meter in its carrying case. Do not expose it to moisture or high temperatures beyond the specified range. Replace the batteries when the low-battery indicator appears, and check the test leads for cracks in the insulation before each use. Klein’s product page notes that the meter can withstand a 2 m drop, but a drop onto a hard surface can still damage the jaws or the display.

Where a clamp meter fits among your tools

A clamp meter is the right tool for a fast current check on a live conductor, and the CL390 covers that job plus the usual voltage and continuity checks. It is not a bench instrument, and it does not replace a meter built for precise lab work. If you are choosing a basic meter for test-lead work, read our guide to what multimeter test leads do so you know which input each lead belongs in before you start measuring.

If your work involves more voltage checks than current checks, a dedicated voltage tester may suit the job better. Our review of the best Klein Tools voltage detector compares a simpler non-contact option with a clamp meter for that task.

What we could not verify

We did not test the CL390 ourselves, so we cannot confirm the accuracy figures or the auto power-off timing in practice.
The Klein product page gives the auto power-off time as 10 minutes. We did not open the instruction manual, so we cannot confirm that figure against the manual.
We did not open the CL390 instruction manual PDF. Several copies exist on third-party sites, but we did not read them, so the step-by-step button sequence above is general method rather than manual text.
Current prices and availability are not covered here.

FAQ

Does the Klein CL390 measure DC current?

Yes. Klein’s product page lists AC and DC current measurement through the clamp, up to 400 A.

Do I need the test leads to read current?

No. Current is read through the clamp. The test leads are used for voltage, resistance, continuity, diode, microamp and most other functions.

Can the CL390 measure temperature?

Yes, with the K-type thermocouple probe included with the meter. The listed range is -40 to 1000 °C.

What is the safety rating?

CAT III 600V, Class 2, double insulation, according to the product page.

Is the non-contact voltage tester a substitute for a measured voltage reading?

No. The NCVT only indicates that a field is present. Confirm with a measured reading through the test leads before working on a circuit.

Sources

Klein Tools, AC/DC Digital Clamp Meter, TRMS Auto-Ranging 400 Amp, CL390 product page: https://www.kleintools.com/catalog/clamp-meters/acdc-digital-clamp-meter-trms-auto-ranging-400-amp (opened 2026-10-29)
Wikipedia, “Current clamp” (the “Clamp meter” title redirects here): https://en.wikipedia.org/wiki/Clamp_meter (opened 2026-10-29)
Wikipedia, “Multimeter”: https://en.wikipedia.org/wiki/Multimeter (opened 2026-10-29)