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Class 4 Fault-Managed Power Systems: An overview of this new classification in the 2023 NEC

By Mark C. Ode | Apr 14, 2023
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A Class 4 classification system with Class 4 jacketed cables, dealing with fault-managed power and cabling, has been accepted into the 2023 National Electrical Code as new Article 726.

A Class 4 classification system with Class 4 jacketed cables, dealing with fault-managed power and cabling, has been accepted into the 2023 National Electrical Code as new Article 726.

This new Class 4 system permits safe transfer of higher-voltage power data with load circuits up to 450V peak AC or DC. Either can be used over much longer distances than before.

What’s new in Class 4?

This new system can provide power distribution to power over ethernet, internet of things, smart building systems, monitoring and control of electronics and appliances. It can also be used for control of security systems and electronic components for large areas of a building, stadium or campus.

New Class 4 systems permit larger and more sensitive cameras, more data transfer for cloud backup, and power and data supply for large appliances. These circuits have the capability of taking substantial power, such as 2,000W, over much longer distances. 

Class 4 systems appear to be much more usable due to lower voltage drop than older Class 1, Class 2 and Class 3 circuits. These new Class 4 systems, despite the higher voltage and current, are allegedly as safe as the Class 2 and Class 3 circuits.

The active components of a Class 4 circuit must be listed devices, based on 726.170. The listing information includes compatible devices, since a listed Class 4 one depends on specific system devices for interoperability, monitoring or control. 

Informational notes

There are two informational notes in 726.170. Informational Note 1 has a reference to UL 1400-1, the Outline for Fault-Managed Power Systems, which provides information on applicable requirements for the listing of Class 4 power systems. Informational Note 2 provides an example of a dependent active device in a Class 4 system as a transmitter that relies on a particular receiver or receivers as part of the monitoring and control system.

Class 4 transmitters and receivers must be manufactured by the same company, listed together as a system and durably marked with the maximum voltage and current output where plainly visible.

In addition, new 726.1, Informational Note No. 1 states that Class 4 fault-managed power systems consist of a Class 4 power transmitter and a Class 4 receiver connected by Class 4 cabling. These systems (transmitter and receiver) monitor the circuit for faults and control the source current to ensure the energy delivered into any fault is limited. Class 4 systems differ from Class 1, Class 2 and Class 3 because Class 4 circuits are not limited for power delivered to an appropriate load. These systems are current-limited for faults between the Class 4 transmitter and receiver, and will shut off within milliseconds if a fault is detected to ensure people and property are safe.

Class 4 transmitters and receivers must be manufactured by the same company, listed together as a system and durably marked with the maximum voltage and current output where plainly visible.

A Class 4 transmitter must interrupt an energized circuit when any of the following six conditions occur between the transmitter and the receiver: a short circuit; line-to-line fault that presents an unacceptable risk of fire or electric shock; ground-fault condition that presents an unacceptable risk of fire or electric shock; overcurrent condition (of any kind); malfunction of the monitoring or control system that presents an unacceptable risk of fire or electric shock; and any other condition that presents an unacceptable risk of fire or electric shock. Testing and listing the systems should alleviate any concern about them becoming a risk of fire or electric shock.

The outputs of a Class 4 receiver and power outputs of Class 4 utilization equipment are considered a separately derived system if the outputs are used as a supply for a feeder or branch circuit. Article 726 does not reference 250.30 for grounding and bonding requirements for separately derived systems, so assume grounding and bonding based on 250.30 would apply.

Class 4 systems are not permitted to be used for dwelling units, especially due to the required voltage limitations in 210.6 of the NEC. Class 4 cables are special cables covered in new Article 722. Connecting hardware must be listed for Class 4 distribution and designed so the connectors are interchangeable with other nonpower-­limited sources located on the same premises. 

Any junction and mating connectors must be constructed and installed to guard against people having inadvertent contact with live parts. These circuits must not be installed with any other power circuits and must follow the requirements in 726.136. The ampacity for these systems is covered in 726.144 based on the cable’s size and temperature rating.

There is much more to learn about these systems. Stay tuned for more coverage in this magazine as we start to see more of these systems being used in commercial and industrial installations.

About The Author

ODE is a retired lead engineering instructor at Underwriters Laboratories and is owner of Southwest Electrical Training and Consulting. Contact him at 919.949.2576 and [email protected]

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