GFCI Class A, B, C, D, E Meaning (With Its Trip Level)

gfci class a, b , c, d, e

It is not that difficult to understand why the NEC encourages consumers to use GFCIs. According to the Home Electrical Fires, electrical issues cause over 50,000 fires annually, killing hundreds of people and injuring thousands. Therefore, you already understand the important role GFCIs play. But what about the class? Does the GFCI class matter?

GFCI Class Meaning

According to NEC 210.8(B), class C, D, and E GFCIs respond to currents that exceed 20mA while their class A counterparts react to current leaks of 5mA or more.

This tells you everything you need to know about GFCI classes.

The class reveals the amount of electricity that has to leak for a particular GFCI device to respond.

An experienced engineer will identify the type of GFCI you need if you tell them the class. If you don’t know the class, they will scrutinize the needs of your applications and projects to figure out the appropriate GFCI class for your situation.

As you can see, the GFCI class is important. People don’t consider it because local retailers rarely mention it. They typically sell the same types of GFCIs to homeowners. This is because most residential areas use a similar GFCI receptacle.

As such, homeowners and their retailers do not have to consider the class of a GFCI. But if you have a complex project in an industrial setting, your contractor will introduce the GFCI class to the discussion.

Advantage Of Using Different Class GFCI

  • Class A responds to a ground fault of 6mA or more
  • Class B responds to a ground fault of 20mA or more
  • Class C is found in systems where the conductors are less than 300VAC
  • Class D is found in systems where at least one conductor is 300VAC to ground or more.
  • Class E GFCIs are similar to class D. The protected equipment has conventional equipment grounding.

Even with those descriptions, the advantage of knowing these classes won’t make sense to you if you don’t understand how GFCIs work.

These devices prevent electrocution by detecting the difference between the ingoing and outgoing current.

Electricity leaves the panel through the hot conductor and returns via the neutral line. The current between these two wires should balance. But if a malfunction occurs and the electricity escapes the wires to flow through water, the human body, or any other conductor, the GFCI will detect this development because the volume of current returning via the neutral line will change.

It will reduce, which is a bad thing. The GFCI will cut the power immediately to protect the user and their equipment. But that raises a question. How big can the difference be between the incoming and outgoing current for the GFCI to trip?

It only takes a current difference of 5mA for the GFCI to take action. On the surface, this is a good thing. The GFCI will shut the power in the outlet off before the current leak can kill you.

But that is only acceptable where ordinary household items are concerned. What if you have powerful devices in commercial settings that use more significant volumes of current?

Machines in factories produce larger current leaks even though a ground fault hasn’t occurred. If you have a conventional GFCI that trips in response to a 6mA difference, it will become a nuisance because factory machines will trigger the device incessantly.

People started looking for GFCIs that could accommodate high voltage applications decades ago. UL introduced tests to determine the viability of higher voltage GFCIs, but none of the available models could meet the organization’s requirements.

That is no longer the case. The market has plenty of class C, D, and E GFCIs that consumers can apply to 600V applications. They trip at a threshold of 20mA, which is low enough to protect workers from fatal shocks but not so low that it attracts nuisance tripping.

You don’t have to limit your selection to GFCIs that trip at 20mA. You can buy models with adjustable thresholds, especially if your objective is to protect devices instead of people. However, organizations like Underwriters Laboratories will not call those devices GFCIs.

Instead, they will use terms like ‘Equipment Ground Fault Protection Devices.’ What does this mean? The class allows you to identify the right GFCI for every application.

The NEC is quite clear on this issue. Homeowners do not realize that the NEC expects them to use class A GFCIs in residential areas. People ignore the class because they don’t know its importance, which makes sense.

If you own a home, class A GFCIs are all that matters to you. You don’t have to concern yourself with class B GFCIs unless you want to apply the technology to swimming pool light fixtures.

But if you want to introduce GFCI technology to higher voltage applications, you must learn everything there is to know about GFCI classes. This is the only way to protect yourself and your equipment with the appropriate GFCI.

Different Classes of GFCI and Its Features

Class A GFCI

Class A GFCIs respond when the current leak exceeds 5mA. But They don’t respond if the current leak is 4mA or less. This prevents nuisance tripping.

Class A GFCIs prevent electrical shocks resulting from malfunctions and defective circuits. The manufacturer will label the GFCI accordingly.

How Does GFCI Of Class A Work?

The GFCI trips when it detects a current leak ranging from 4mA to 6mA. You will find them in outlets in kitchens, bathrooms, and other places that expose electrical equipment to water.

Class A GFCI Trip Level

4mA to 6mA

NEC Code

The NEC puts GFCIs in the class A category if they trip when the current leakage is 4mA to 6mA. They expect consumers to add class A GFCIs to 15 and 20A receptacles in unfinished basements, kitchens, bathrooms, garages, and exterior locations.

Class B GFCI

Class B GFCI trips when the electrical imbalance exceeds 20mA.

You cannot categorize every device with a trip current of over 20mA under the class B banner. Schneider Electric has mentioned that they have devices that trip at 30mA.

And yet, those devices do not meet class B requirements. This should encourage you to apply caution while buying GFCIs. If you cannot find the documentation explaining the GFCI’s class, don’t apply that GFCI until you consult an expert.

How Does GFCI of Class B Work?

A class B GFCI works just like any other GFCI. It protects equipment by tripping after detecting an imbalance in the circuit. But unlike class A GFCIs that trip when the ground fault current is between 4 and 6mA, class B GFCIs will only respond to an imbalance of more than 20mA.

Class B GFCI Trip level

Class B GFCIs trip when the ground fault current exceeds 20mA.

NEC Code

You were more likely to see class B GFCIs before the 1965 NEC was adopted. Consumers added them to underwater light fixtures in swimming pools. Class A GFCIs were not a viable option because of their sensitivity. They would trip incessantly.

Class A VS Class B GFCI

  • You find class A GFCIs in residential settings because they have a trip current of 4 to 6mA. They accommodate home appliances and devices with reasonable watt ratings.
  • You find class B GFCIs in settings with ground fault currents that exceed 20mA. A class A GFCI would repeatedly trip if you added the device to an application that needs class B GFCIs.

Class C GFCI

Class C trip when detecting an imbalance of 15 to 20mA. This is not the only requirement for a class C GFCI. Additionally, the circuit in question shouldn’t have any conductors that exceed 300V to the ground. The line-to-line voltage should be no more than 480V. Make sure the equipment in question has reliable grounding.

That 15 – 20mA range is above the ‘let go threshold.’ Therefore, decent grounding is vital in case an unexpected malfunction occurs.

Many people place class B, C, D, and E GFCIs in the same category because they all have higher trip thresholds than class A GFCIs.

But you have to remember that these GFCIs have significant differences that affect how they are deployed.

With class D GFCIs, you can have at least one conductor on a circuit with more than 300V to the ground. You don’t want to use class C GFCIs on 600V systems.

Class D technology is the best option. Although, you also require reliable grounding.

Class E GFCIs offer highspeed tripping. You don’t need the same oversized ground people associate with class D GFCIs.

The importance of class B, C, D, and E GFCIs cannot be overstated. Class A GFCI trip currents are not fatal. You may not even notice them. But currents as high as 10mA can affect the muscles, causing involuntary contractions.

Higher currents will affect the victim’s breathing. Don’t be surprised if ventricular fibrillation occurs, stopping the heart and killing consumers that cannot be defibrillated quickly enough. The last thing you want is to match a GFCI class to the wrong application.

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One thought on “GFCI Class A, B, C, D, E Meaning (With Its Trip Level)

  1. Hi
    Seems you have been working intensively with GFCI. However the standards or the way to measure is in low frequency range (50 -300Hz – as i recall it). What about harmonics e.g from inverters (speed controlled compressors or pumps) where typically you see harmonics related to the switching frequency. Many GFCI models in the market today seems not to take care and will trip e.g at 3Khz by low current while characteristic of tripping curve is very non linear

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