Good Vibrations: Audio Signal Processing, Amplification and Sound Equipment, Part 2

Soundwaves Photo Credit: Shutterstock / MW2ST
Photo Credit: Shutterstock / MW2ST

Last month’s article covered the general requirements in the National Electrical Code for audio signal processing, amplification and sound equipment in Article 640. One of the most common mistakes many electricians and electrical contractors make when dealing with low-voltage installations is the impression that Class 2 and 3 remote-control, signaling and power-limited conductors can be placed in the same raceway, cable assembly or enclosure as power conductors. Section 690.9 requires compliance with Article 725, and very specifically, 725.136 requires physical separation from electric light, power, Class 1, non-power-limited fire-alarm conductors and medium-powered broadband communication cables.

Highlighting this issue is important because the conductor insulation on Class 2 or 3 conductors alone is not considered to be isolation from these power conductors. Mixing power conductors with Class 2 or Class 3 conductors in the same raceway, cable or enclosure could result in failure of the insulation of both power and Class 2 and Class 3 conductors and could result in a fire or electrocution. There are some very limited exceptions that I will cover later.

Physical separation of low-voltage and higher-voltage conductors provides isolation, so the higher voltages do not have capacitive coupled or inductively coupled noise and other distortion from the power sources into the audio system. Physical separation also provides safety from power system hazards. Audio amplifiers and other electronic sound systems that carry audio program signals are permitted to use Class 1, 2 or 3 wiring where the amplifier or audio equipment is listed and marked for use with the specific class of wiring method.

This audio equipment’s listing will ensure the equipment’s energy output is equal to the shock and fire risk of the class as covered in 725.2. A Class 1 circuit is essentially a power circuit and can be either power-limited at 30 volts (V) and a maximum of 1,000 volt-amperes—a maximum of 33.3 amperes—or not power-limited and can have unlimited current with a maximum of 600V. These Class 1 circuits are treated as power circuits and require circuit wiring in accordance with Article 300 and Chapter 3 wiring methods.

A Class 2 circuit has a power limitation that considers safety from a fire initiation standpoint and provides acceptable protection from electric shock. A Class 3 circuit has a power limitation that considers safety from a fire initiation standpoint, but it has a higher level of voltage and current than a Class 2 circuit, so additional safeguards are required to provide protection from electric shock. The conductor termination on the listed electrical audio equipment will be marked to identify the Class of wiring required.

Where electrical audio equipment is installed in close proximity to bodies of water (not an issue for audio systems intended for use on boats, yachts or other forms of land or water transportation used near bodies of water), compliance with 690.10(A) and (B) will detail the requirements. If the audio equipment is supplied by branch circuit power, the equipment cannot be located horizontally within 5 feet of the inside wall of a pool, spa, hot tub or fountain or within 5 feet of the prevailing or tidal high water mark of a marina or similar application. In addition, a ground fault circuit interrupter must protect the branch circuit supplying the audio equipment. Audio equipment powered by a listed Class 2 power supply in accordance with 725.121 or, where the audio equipment is listed and marked for Class 2 wiring, shall be restricted to placement near the water based on the manufacturer’s recommendations. Section 110.3(B) would also apply here since the listing agency and inspector must review the equipment listing, labeling instructions and equipment use.

Class 1 circuit conductors used for audio equipment must comply with the wiring requirements in 725.46 for 600V insulation. Class 2 and 3 circuit conductors on the load side of the power supply required by 725.121 must comply with the requirements in 725.130(B) and must be insulated based on 725.179(G). In 725.179(G), Class 2 conductors must have a voltage rating of not less than 150V, and Class 3 conductors must have a minimum of not less than 300V. Class 2 and 3 cables must also have a temperature rating of not less than 60°C (140°F).

There are many more requirements for audio and sound systems than I cover here; however, with experience, electricians and electrical contractors will become more familiar with these systems. Learning the basics first will help overcome common misconceptions and mistakes as well as increase profitability.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and

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