Power Is Knowledge: How NEC Pertains to Low Voltage

It has been some time (October 2003) since we last discussed understanding and properly applying low-voltage power supplies and cabling. This is an important tech topic in the security industry. It is often driven by familiarity with the National Electrical Code (NEC) and the rules from which most of our livelihoods are derived.

Since power supply technologies are covered elsewhere in this issue (see power supplies feature), I thought it made sense to stick with this theme, but to approach it from a slightly different angle.

How NEC Affects Low-Voltage Work
Back in 1987, the NEC introduced some changes that had a major impact on the wire and cable industry. The code now has listing requirements for communication and power-limited circuit cable. These requirements specified fire resistance levels for cable. The code covers initiation of fire by electronic cable, as well as flame spread and electrical shock characteristics of the cable and circuit.

As an alarm technician, your work on low-voltage security systems may not, at this time, fall under permitting and electrical code inspection requirements. However, more than 42,000 jurisdictions in the United States now have some sort of code enforcement in place, and that number is increasing every day. With new security standards such as NFPA 731 coming online, it is a good idea for all low-voltage contractors to brush up on national standards and codes.

Low Voltage Can Still Pose Hazards
Security system installations have often been described as power-limited, limited-energy, low-voltage and nonpower. This can imply that the wiring of these systems is not a major concern since they indicate a nondangerous, nonfire and nonshock environment. While this is the intent of these codes, those installing these systems should understand how to correctly perform low-voltage installations per the NEC.

This can sometimes be confusing because even within the NEC, the term “low voltage” can mean anything from less than 600V, 50V or less, or less than 30V depending on the section you reference. A better definition is needed for security system remote control and signaling circuits.

Some reasons for inspecting low-voltage systems are:

  • Being shocked by low-voltage systems and cable that is not installed properly in proximity of higher voltage power cabling
  • Audio system voltages can be as high as 70VAC
  • Telephone systems have ringer voltages that can be as high as 90VAC
  • Low-voltage systems in dry environments may have a higher risk if they become wet
  • Proper grounding of systems to reduce the risk of fire or electrical shock from dangerous potential voltages between systems
  • New applications of network-powered broadband communication systems such as power over Ethernet (PoE)
  • Workspace requirements for low-voltage distribution equipment

Defining Different Circuit Classes
One term we see a lot on security system power supplies and installations is “Class 2, Power Limited.” What does this mean, and how does it apply to security systems? To find a definition, we need to reference Chapter 7, Article 725:

“Remote-Control, Signaling and Power-Limited Circuits” of the NEC. It refers to power-limited circuits that are not part of a device or appliance. In other words, the power supplies and wiring we install to control remote security devices.

Class 1 remote control circuits and signaling circuits can operate up to 600V with no ampere limitation. This circuit is not typically found in security systems and would be more likely something such as a motor control circuit.?

Power-Limited Class 1 is either an AC or DC circuit. The circuit is supplied by a power source that is limited to 30V and 1, 000VA. An example might be operating low-voltage damper motors in an environmental airflow system. Class 1 is used when the limitations are exceeded in a Class 2 or Class 3 circuit.

Class 2 circuits are designed for safety from fire ignition and electrical shock. These circuits cover mostly everything we install in the security industry. A Class 2 circuit has either a maximum voltage of 20V, 5A for a total of less than 100VA; 21V to 30V, 3.3A for a total of less than 100VA; or 31V 150V, 0.005A for a total of less than 0.5VA.

The intent of the code is to reduce the threat of fire and dangerous shock. This may not always be the case and should not be an excuse for poor installation techniques (see photo on page 24 of April issue). The cabling should be marked CL2, CM or PLTC and have a voltage insulation rating of 150V.

The installation of this cable falls under rules that include 1/4-inch spacing from power cable in an enclosure, such as a power supply, and separation of a listed barrier in the power supply. This class is the most important and should be studied carefully. UL-Listed Class 2 power supplies will be clearly marked “Class 2.”

(TIP: If you want some good training material on this subject, I recommend going to www.mikeholt.com and checking out the code material. There is even a free manual on low voltage and limited energy for download.)

Class 3 circuits pick up where Class 2 leaves off. If you have a circuit that is more than 30V but exceeds 5ma (0.005A), it would probably be a Class 3. Due to the increased power, Class 3 power supplies will be marked accordingly. Class 3 cabling will be marked CL3 and has a rating of 300V.

(2005 NEC HINT: If you use CL3 cable instead of CL2 cable in a Class 2 circuit, you may be exempt from the 1/4-inch spacing rule in enclosures. As always, check with your local inspector.)

The Dangers of Poor Installations
Looking once again at the photo on page 24 of the April issue, we see an example of a door position input sensor in an access control system that, when installed, was pulled so one of the cables was shorted to ground. The first indication of a problem was when the client noticed smoke coming from the door. How could this happen with a low-energy circuit? The point here is that even though we work in a low-voltage, power-limited world, the danger of fire or shock is still possible; especially with poor quality installations.

How could the chance of this accident have been reduced? In this case, it was a short to ground on a hole in steel framing. Anytime when working around steel studs, conduits or boxes, we should be deburring sharp edges and/or using insulated hole bushing. Always use jacketed cable for extra protection, and lubricate tough wire pulls. Avoid resistance in pulling cable; remember that 22 AWG wire has a maximum pull tension of around 25 pounds.


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