A clamp meter is a must have tool for those who regularly need to test amperage, and especially high values. Their biggest advantage over a multimeter is that you don’t need to break into the circuit or switch anything off. In short, they are non-contact, non-intrusive measuring devices that can quickly troubleshoot live electrical circuits.
Amprobe invented and built the first device way back in the 1940s, hence the company’s name. Although initially designed as a single use tool (for testing current) and also featuring analog architecture, they have today typically morphed into multipurpose digital clamp meters. Some incorporate multimeter functions, often to include voltage, resistance and continuity checks, and they are also more accurate.
Design of an Amp Clamp Meter
They have a pair of jaws that open and close around a conductor to measure the current flowing through it inductively via an in-built current transformer or Hall effect sensor. A trigger opens the transformer jaw and a central dial or series of buttons selects the various functions. Displays are digital and can approach the resolution of entry level multimeters. More on internal design…
AC / DC Models
This is the major difference between clamp meters, whether they are AC compatible, DC compatible, or both. Different engineering is required to test one or the other – an AC model will not be able to measure current in a DC circuit, and vice versa.
A transformer is employed in AC clamp meters, which detects the magnetic fluctuations of the alternating current. DC models use the Hall effect sensor method; the meter including a magnet which in turn exerts a force on the current flowing through the unit, whereby a voltage is generated in the Hall element. Readings are calculated from this voltage.
Leakage clamp meters: These differ from power meters in that they measure lower value current leakage and have higher resolution. They can typically pick up low AC measurements in the micro-amps range.
As with multimeters, there is a vast range of devices to suit this or that environment, a range of amperages, and also levels of accuracy. It is more important to look at the safe operating range and temperatures as it is the features.
An AC digital clamp meter should ideally be a True RMS responsive type to more accurately gauge the amperage, and voltage, in variable AC drive output systems. If working largely on motors, having one with inrush measurement capabilities is also very useful. You’ll pay more for one of these as they’re a bit more specialized.
Accuracy would need to be around 2% or less, while auto ranging of other ranges and a nice backlit display are both important features. One that also measures AC and DC voltage, resistance and continuity would be ideal, negating the need for a separate multimeter for basic tasks. The latter would also come with a set of leads and traditional COM/VOLT jack socket inputs.
While you don’t need all the bells and whistles – bearing in mind that some of the cheaper amp clamps might include little-needed features – there are some very useful data logging units on the market. Data hold is another useful function to look out for, and a wrist strap should be a given.
Category IV meters are industrial rated and typically feature a larger jaw design than normal, as well as a tough case to put up with harsher environments. In higher temperature environments, it is usually best to go with an analog meter that does not require batteries.
Types and Brands
Several multimeter manufacturers also make clamp meters. Fluke and Amprobe are two of the main ones that tend to put out the best devices. The likes of Etekcity, Tekpower and Uni-Trend also produce several series.
The inventor of the amp clamp manufactures dozens of units to fit a variety of applications, from the home to industry. Their AMP-200 / 300 series’ are excellent True RMS devices for HVAC and industrial applications. They also make mini amp clamps and swivel types with 180-degree rotating heads for inconvenient places. Their Navigator Series are super-hardwearing.
One of the most highly rated is the Amprobe ACDC-100 TRMS. This unit can measure AC current accurately to 800A, DC currents up to 1,000A. Good for CAT-III installations, it also measures voltage to 600V, along with capacitance and resistance.
Fluke is often the go-to brand for multimeters and, likewise, they churn out a good range of clamp meters. They needn’t be super-expensive either, with the best selling Fluke 323 around the $100 mark, for instance. It is an especially useful AC meter with True RMS response able to measure to 400A.
Some other examples:
- Fluke 368 Current Leakage (with Fluke Connect)
- Fluke 376 AC/DC with iFlex Loop
- Fluke 902 HVAC Clamp
- Fluke a3000 Wireless AC Module
Using a Clamp Meter
Since there is no connection made with any cables, using one of these couldn’t be easier. Simply select the appropriate range, slip it over a live cable and take the readings. There is no further setting to make after you have selected the correct amperage input – AC or DC. Typically, one core is measured at a time, though there are newer clamps that can measure amperage in multi-cores.
How to Video
Although detailing RV electrical diagnostics, Mike Sokol knows his stuff and is nice to listen to:
Clamp Meter vs Multimeter
An AC/DC clamp meter that can also measure voltage and resistance is certainly a match for a basic multimeter, and you get the added current-measuring capacity. Where they typically lose out is on accuracy and resolution. They have their place, but multimeters are inherently more reliable and accurate.
If you are doing general purpose electrics, this is not so much of an issue, but those tasking delicate electronics or HVAC work might be better off with a combination of amp clamp and multimeter.