NEC Class 2 DC Circuit
The National Electrical Code (NFPA 70) defines special safety requirements for low voltage circuits. The NEC refers to such circuits as Class 2 circuits – the portion of the wiring system between the load side of a Class 2 power source and the connected equipment. Due to its power limitations, a Class 2 circuit is considered safe from a fire initiation standpoint and provides acceptable protection from electrical shock.
NOTICE: NEC Class 2 circuits are only allowed to be powered from a power source with an output power of lower than 100VA and an output current of lower than 8A.
WHEN TO USE A CLASS 2 POWER SOURCE
Use this power source for any circuit powering any device that is UL certified with a requirement of a Class 2 power source. When there is an application in regions other than the US, the NEC Class 2 requirements do not carry any authority.
However, the application designer must consider the component product design requirements when designing power sources for those components, wherever they are applied.
WHERE NEC CLASS 2 CIRCUITS ARE CERTIFIED FOR USE
Most DC sensors, from any provider, when UL certified, are certified for use in Class 2 circuits (e.g., powered by a Class 2 Power Source). Most Rockwell Automation® DC sensors including IO-Link, Zone Control, and DeviceNet sensors are UL certified for Class 2 circuits. SensaGuard™ products are UL Certified for Class 2 circuits. Safety Relays, the MSR42 for instance, are UL Certified for Class 2 circuits. Many condition sensing devices from Rockwell Automation are UL certified with Class 2 circuits.
Examples of products that require NEC Class 2 power supply:
- SensaGuard Interlock Switches
- TLS-Z Guard Locking Switches
- 440G-LZ and 440G-MZ Guard Locking Switches
- Magnetically Coded Non-Contact Switches
- MSR42 Safety Relays and MSR45 Safety Relay Expansion Modules
HOW CIRCUITS ARE IMPLEMENTED
1. Class 2 power sources can be designed in from the beginning
2. A Class 2 source can be integrated into an existing system
Rockwell Automation® has products suited for either situation. In the case of a new system using Class 2 power sources when required, designing from the start is makes things easier. Due to the relatively limited power available from a Class 2 source it is not uncommon for a new system to include both Class 2 and standard non-Class 2 power sources. While non-UL Certified Class 2 DC powered devices may be included in a Class 2 circuit; generally, the Class 2 power source is sized to handle the power requirements of only the Class 2 devices only.
A standard supply with a higher current capacity is provided to power devices that do not require a Class 2 power source. The Bulletin 1606-XLP series has a wide selection of Class 2 power supplies.
There can be a third way. In many instances the combination of a larger “standard” power supply and the Bulletin 1694 ECP may offer another alternative. See more information below as to the solutions Rockwell Automation offers for NEC Class 2 DC circuits.
WHY THE CIRCUITS ARE USED IN CONTROL PANELS AND WHO IS RESPONSIBLE TO POWER THE DEVICE
1694 Electronic Circuit Protection
These circuits are used to minimize electric shock hazards. The standard in Article 725 requires that low-voltage DC circuits have a maximum voltage limit essentially the same as that defined in IEC 60950-1. Because of the 100VA max capability requirement, power sources meeting Class 2 requirements generally have lower current capability than non-Class 2 power sources.
Whenever the connected component(s) require a class 2 circuit, it is the installer’s responsibility to power that device with a Class 2 power source.
Need More Information?
If you have any questions on this topic or other product solutions from Rockwell Automation, please contact your local Smart Components, Sensor, and Safety Specialist.