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Most inspectors know where GFCI protection is required, but fewer understand what “Class A” means or where that requirement comes from. The NEC mandates GFCI protection in dozens of locations, but it does not define the performance characteristics. That’s because the NEC isn’t a product standard; it’s an installation code. To understand what is expected of a GFCI device, including trip levels and response times, you need to look to UL 943.
A Class A GFCI, per UL 943, is specifically designed for personnel protection. It must trip when the ground-fault current reaches 6 milliamperes (mA), and it must not trip at less than 4 mA. That 4–6 mA range is intentional. It reflects the threshold where current can interfere with human muscle control and create a risk of electrocution. These GFCIs must also trip within 25 milliseconds, minimizing the duration of shock exposure. That speed is essential so that it’s fast enough to prevent cardiac fibrillation and other serious injuries.
The NEC does not define Class A in Article 100. Instead, it references listed GFCIs, which are certified to meet UL 943 Class A standards. The specific reference is in 110.3(B), which requires all listed or labeled equipment to be installed and used in accordance with its listing and labeling. If a device is listed as a Class A GFCI, then that is the standard it must meet, and it becomes enforceable under the NEC. That’s where inspectors come in.
When the NEC says “GFCI protection shall be provided,” as it does in Articles 210.8, 210.63, 422.5, 511.12, 680.32, and many others, it means Class A protection unless otherwise stated. The product being installed must trip at 4–6 mA and within 25 milliseconds. Anything else, even if it’s called a GFCI, doesn’t meet the NEC requirement for personal protection.
You’ll typically find the “Class A” designation:
Here’s why it matters: If a non-Class A GFCI is used, for example, an industrial GFCI that trips at 20 mA, it may not protect people. It may prevent equipment damage or reduce fire risk, but it won’t stop electrocution. That’s not code-compliant for branch circuits serving receptacles in occupiable spaces.
The takeaway is simple. The NEC requires GFCI protection for personnel, and that means Class A GFCIs listed to UL 943. Know where to find the listing, verify the labeling in the field, and never assume “GFCI” means “compliant.” The margin between a safe device and a deadly one is just a couple of milliamps and a few milliseconds.
A Class A GFCI, per UL 943, is specifically designed for personnel protection. It must trip when the ground-fault current reaches 6 milliamperes (mA), and it must not trip at less than 4 mA. That 4–6 mA range is intentional. It reflects the threshold where current can interfere with human muscle control and create a risk of electrocution. These GFCIs must also trip within 25 milliseconds, minimizing the duration of shock exposure. That speed is essential so that it’s fast enough to prevent cardiac fibrillation and other serious injuries.
The NEC does not define Class A in Article 100. Instead, it references listed GFCIs, which are certified to meet UL 943 Class A standards. The specific reference is in 110.3(B), which requires all listed or labeled equipment to be installed and used in accordance with its listing and labeling. If a device is listed as a Class A GFCI, then that is the standard it must meet, and it becomes enforceable under the NEC. That’s where inspectors come in.
When the NEC says “GFCI protection shall be provided,” as it does in Articles 210.8, 210.63, 422.5, 511.12, 680.32, and many others, it means Class A protection unless otherwise stated. The product being installed must trip at 4–6 mA and within 25 milliseconds. Anything else, even if it’s called a GFCI, doesn’t meet the NEC requirement for personal protection.
You’ll typically find the “Class A” designation:
- Stamped or molded into the face of the device (for receptacle-type GFCIs)
- On the label of a GFCI breaker
- In the device's UL file or spec sheet
Here’s why it matters: If a non-Class A GFCI is used, for example, an industrial GFCI that trips at 20 mA, it may not protect people. It may prevent equipment damage or reduce fire risk, but it won’t stop electrocution. That’s not code-compliant for branch circuits serving receptacles in occupiable spaces.
The takeaway is simple. The NEC requires GFCI protection for personnel, and that means Class A GFCIs listed to UL 943. Know where to find the listing, verify the labeling in the field, and never assume “GFCI” means “compliant.” The margin between a safe device and a deadly one is just a couple of milliamps and a few milliseconds.