Klein Tools’ CL390 measures up to 400 amps AC or DC through its clamp jaw and up to 600 volts through its test leads, and its manual lays out a short routine for each job: pick the function on the dial, press SEL to switch between AC and DC, and press REL/ZERO before a DC current reading. That last step is the one most first-time users skip, and it’s the difference between a clean DC amp reading on a solar cable and a number that’s off by a few tenths of an amp. Here’s how to use every major function, in the order you’ll most likely need them.
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I use the Klein Tools CL390 clamp meter to check my solar panels, and the video below shows me testing a 10-year-old panel’s voltage and current. Steps and specs come from Klein’s CL390 instruction manual and product page. For my results on that panel, see my Klein CL390 solar panel test review.
Know the controls first
| Control | What it does |
|---|---|
| Function selector dial | Turns the meter on and picks the measurement (current, voltage, continuity/resistance/capacitance/diode, frequency, temperature, DC µA) |
| SEL/NCV | Toggles AC/DC and cycles sub-modes; also triggers the non-contact voltage tester |
| REL/ZERO | Zeroes the DC current offset; relative mode on other functions |
| HOLD/Brightness | Freezes the reading; adjusts backlight |
| MAX/MIN | Holds the highest and lowest readings |
| RANGE | Switches from auto-ranging to manual ranges |
| VΩµA jack (red) / COM jack (black) | Test lead inputs for every lead-based measurement |
The meter powers off after about 5 minutes of inactivity per the manual; press any button to wake it.
CL390 specs at a glance
| Function | Range | Accuracy (Klein) |
|---|---|---|
| AC/DC current (clamp) | 40.00A / 400.0A | ±(2.0% + 9 digits) |
| DC voltage | 400mV to 600V | ±(0.8% + 3 digits) on 4–400V ranges |
| AC voltage | 400mV to 600V | ±(1.2% + 5 digits) on 40V and 400V ranges |
| Resistance | 400Ω to 40MΩ | ±(1.2% + 3–5 digits) |
| Capacitance | 40nF to 4mF | ±(4% + 8 digits) mid ranges |
| DC microamps | 200µA | ±(1.0% + 5 digits) |
| Temperature | −40 to 1,832°F (K-type) | ±(1.2% + 6 digits) |
| Frequency | 10Hz to 60kHz | ±(0.1% + 5 digits) |
Other details from Klein: 4,000-count reverse-contrast display, 1.6-inch jaw, CAT III 600V rating, 6.6-foot drop protection, 3 AAA batteries, and a minimum measurable current of 0.3A.
How to measure amps
AC current
- Unplug the test leads. Klein says to disconnect them when measuring with the clamp.
- Turn the dial to the 400A current setting. The meter starts in AC mode.
- Squeeze the trigger, place the jaw around one conductor, and release so the jaw closes fully.
- Center the wire in the jaw, lined up with the arrow markings, and read the display.
- If the reading is under 40A, move the dial to the 40A setting for finer resolution (0.01A).
Clamping a whole cord with hot and neutral together gives a near-zero reading because the currents cancel. Clamp a single wire, or use a line splitter.
DC current (solar, batteries, 12V)
- Select the current setting and press SEL until the DC icon shows.
- With the jaw closed and nothing in it, check the display. If it shows a non-zero value, press REL/ZERO. The ZERO icon appears, and later readings subtract that offset.
- Clamp around a single positive or negative cable and read the amps.
- Press REL/ZERO again to leave zero mode.
Klein warns not to use DC current mode with a magnetic hanger attached to the back of the meter, because the magnet can throw off the reading.
How to measure volts
- Plug the red lead into VΩµA and the black lead into COM.
- Turn the dial to the voltage setting. It defaults to AC; press SEL for DC.
- Touch the leads across the circuit. The meter auto-ranges.
- A “–” sign on DC means the leads are reversed, which is useful for checking polarity on a battery or panel.
Klein notes that a few millivolts can show with the leads open; that’s noise and normal. The hazardous voltage icon lights above 25V AC or 60V DC. For the theory behind the numbers, see my volts, amps and watts explainer.
How to test capacitors
- Leads in VΩµA and COM, dial on the continuity/resistance/capacitance/diode setting.
- Press SEL/NCV repeatedly until the capacitance icon (F) shows.
- Remove power from the circuit. With motor run and start capacitors in HVAC gear, standard practice is to safely discharge the capacitor before touching the terminals.
- Connect the leads across the capacitor terminals and wait for the reading to settle.
Compare the reading to the µF rating printed on the capacitor. On a 45µF run capacitor, Klein’s ±(4% + 8 digits) accuracy on the 400µF range works out to roughly ±2.6µF (estimate), so a reading of 43–47µF is within meter tolerance of the label.
Continuity, resistance and diodes
- Continuity: Same dial position; the meter defaults to continuity. It beeps at 10Ω or less. Power must be off.
- Resistance: Press SEL until Ω appears. An open circuit shows the overload symbol, which is normal.
- Diode: Press SEL until the diode icon shows. Klein says 200–800mV indicates forward bias; about 0mV means shorted.
Temperature and frequency
Plug the K-type thermocouple into the jacks observing polarity, turn to the temperature setting, and press SEL to switch from °F to °C. Klein notes the included probe is only suitable below 446°F (230°C). For frequency, use the Hz% setting; press SEL once for duty cycle.
Non-contact voltage tester
The NCV antenna sits in the clamp jaw. A red glow and beep indicate AC voltage. Klein is clear about its limits: it will not detect DC voltage or shielded wire, and no indication doesn’t guarantee a dead circuit. Verify on a known live outlet before and after use.
Worked example: testing a solar panel
In my video I checked an older panel rated 32.8V open-circuit and 8.05A short-circuit. I measured about 27–29V and 5–6A in California sun. Here’s the routine, with estimates for a typical 100W panel rated around 22V Voc and 5.8A Isc:
- DC voltage across the disconnected panel leads: expect a reading near the rated Voc in strong sun, lower in heat or haze.
- Short the panel through a suitable switch or breaker, then clamp one lead in DC amps (after zeroing): expect a reading near Isc in full sun.
- Rough power potential ≈ Vmp × Imp, not Voc × Isc; on a 100W panel that’s about 18V × 5.5A ≈ 100W (estimate).
My solar panel degradation guide explains how to compare those readings with the label over time, and how to use a multimeter covers the meter basics in more depth.
FAQ
Does the Klein CL390 measure DC amps?
Yes, up to 400A DC through the clamp. Press SEL for DC and zero it with REL/ZERO first.
Why does my CL390 show amps with nothing clamped?
That’s the DC offset. Press REL/ZERO in DC current mode to subtract it.
Can it test capacitors?
Yes, from 40nF to 4mF, on the continuity/resistance/capacitance setting after pressing SEL.
Does the CL390 auto-range?
Yes. Press RANGE for manual ranges if you need them.
What batteries does it use?
Three AAA batteries, included.
Can I measure a power cord’s current?
Only by clamping one conductor. Clamping the whole cord cancels the reading.
Bottom line
The CL390 is simple once the routine sticks: dial, SEL for AC or DC, REL/ZERO before DC amps, one wire in the jaw, leads in VΩµA and COM for everything else. That covers solar panels, 12V systems, household circuits and HVAC capacitors with one meter. For a look at other testers I’ve used, see my Proster smart multimeter review.