If you have a multimeter for HVAC work, you most likely have a meter with a CAT III rating on it. Some might also be rated for CAT II at higher voltages or CAT IV at lower ones—or both, like this model of the UEi DL599 from our Multimeter 101 for HVAC 3D video.

In any case, it’s a safety rating to prevent arc faults inside the meter, and each category refers to the suitability of the meter for certain types of work. You can usually find it near the lead ports or on or near the amp clamp. 

Most companies that make multimeters specifically for HVAC/R usage will have CAT III and CAT IV meters (or CAT II 1000V and CAT III 600V, which is generally suitable for residential and light commercial HVAC work). However, some general meter manufacturers might also make meters that are not suitable for our work and may put us at risk of electric shock. Some CAT III 600V meters might also not be suitable for certain applications. 

What CAT Ratings Represent

CAT ratings represent safety categories based on the type of equipment a meter or electrical tester is safe enough to use. These ratings are set by the IEC (International Electrotechnical Commission) and are based on transient voltages, not just typical operating voltages. 

There are four categories: I, II, III, and IV. Broadly speaking, we can think of those groupings based on the voltages associated with items at varying distances from the power source:

• CAT I — Electronics and circuit boards
• CAT II — Standard outlets and plug-in appliances
• CAT III — Electrical panel to high-voltage household appliances
• CAT IV — Incoming power from the utility (service drops or laterals)

A circuit board in a laptop is farther away from the power source than an outlet, which is farther away than a permanent appliance directly wired to the breaker (e.g., HVAC or water heater), which takes in power from a source, usually a service drop (from an overhead power line) or a service lateral (underground)

While most of us aren’t going to be working on service drops, several HVAC meters are CAT IV-rated. You can always use higher-category meters for lower-category applications, but you can NEVER do it the other way around. If there’s transient voltage, that would be a very bad day.

How CAT Ratings Are Determined

Before we get into the numbers, let’s give our friend Georg Ohm a quick visit. (No, that’s not a typo. His name is Georg.)

He’s the guy who came up with a little thing called Ohm’s law. Ohm’s law states that amps go up as resistance goes down (and vice versa) at a constant voltage. The formula is as follows:

Volts (V) = Amperage (I) x Resistance (R)

In a case where we have 120 volts and 4 amps, the resistance would be 30 ohms (120 / 4 = 30). If the resistance drops to 20 ohms but the voltage stays the same, the amps would go up to 6 (120 / 20 = 6). The vast majority of resistance in the circuit comes from the load.

An electrical multimeter has built-in impedance (resistance PLUS reactance) for voltage measurements. This design allows the electricity to be measured without significantly impacting the circuit.

CAT I electrical testers have a relatively high impedance of 30 ohms. This impedance decreases as the categories progress. As a result, CAT II devices (12 ohms) can handle more current than a CAT I tester. CAT III and IV multimeters (2 ohms) can handle even more than a CAT II. The lower impedance values of the CAT III and IV meters allow more current to flow through without arcing.

How are CAT III and CAT IV Different?

Both CAT III and CAT IV equipment have the same impedance: 2 ohms. However, the differences lie in the transient voltages they can handle. 

The IEC sets these guidelines for meters used on test equipment meant to measure less than or equal to 1000V as follows:

Notice how the working voltages are the same across all categories, but the higher the category, the higher the transient voltage rating. CAT IV’s transient voltage ratings are higher than CAT III’s. That’s because there is more robust internal protection in CAT IV meters, even if the impedance is the same as that of CAT III. If an electrician is working on service drops and there’s a sudden high voltage, a CAT IV meter will be less likely to arc internally than a CAT III.

Meters with Multiple CAT Ratings

Many multimeters have two CAT ratings. You may have one that says CAT III 1000V / CAT IV 600V on the front, like UEi’s Everyday Professional Clamp Meter (EPC) pictured above. If you look at the chart from earlier, you’ll notice that the transient voltage is the same for both ratings: 8000V. 

The main difference in the CAT ratings is the context in which you can use the meter. You could use the meter in CAT III applications up to 1000V (which includes items like DC bus voltage on VFDs that may exceed 600V). However, it’s not rated to withstand the intensity of transient voltages in 600–1000V CAT IV applications.

But ultimately, is a CAT III 1000V / CAT IV 600V meter suitable for our typical HVAC/R work? Absolutely. It’s probably the best you’ll find readily available from a safety and performance standpoint.

All that’s to say that CAT ratings matter. Make sure you’re working with at least CAT III meters for the voltages you see in your everyday work, and even then, pay attention to the AC voltage in the rating to pick a meter that’s safe for the job. 

You may come across some meters that are CAT II 1000V / CAT III 600V; whether those are suitable will depend on the work you do. Typical 208V and 240V residential and light commercial equipment—yes. When you get into some heavier commercial, industrial, and specialty equipment where voltage measurements could exceed 600V, you’ll need to pay closer attention to those CAT ratings and their voltages.