Sounds Good: Audio Signal Processing, Amplification and Sound Reproduction

By Mark C. Ode | Mar 15, 2019
CodeApps 0319 Photo Credit: Shutterstock / mw2st / pathdoc




When I was an electrical contractor, I tried to find a specialty where there was less competition and more profit than the normal building wiring. Many ECs doing electrical wiring for dwellings, commercial and industrial installations will have a spinoff business involving electrical service work and forget about the profitable business of audio system installations. Many of the same electricians doing power wiring can also be trained in high-definition audio. Due to the complexity of this subject, this article covers audio basics, and next month, I will cover more specific requirements for permanent, temporary and portable installations.

Article 640 covers the equipment and wiring for audio signal generation, recording, processing, amplification and reproduction; distribution of sound; public address; speech input systems; temporary audio system installations; and electronic organs or other electronic musical instruments. A thorough understanding of the National Electrical Code (NEC) requirements in Article 640 will help ensure these installations are safely and correctly installed and used.

Article 640’s applications and installation requirements do not, however, cover the installation of fire and burglar alarm systems, even though these systems use much of the same audio, video and signal processing as found in Article 640. Pertinent specialty definitions are located in 640.2. A thorough understanding of these definitions will help throughout the remainder of Article 640, so I encourage you to review these definitions before using Article 640.

The three parts to Article 640 are Part I dealing with the general requirements and specialty definitions for these systems, Part II dealing with permanent audio system installations, and Part III dealing with portable and temporary audio installations.

Section 640.4 requires amplifiers, speakers and other sound equipment to be located or protected from physical damage and to guard against environmental exposure that might result in fire, shock or personal hazard. Access to equipment shall not be denied by an accumulation of wires and cables that prevents removal of suspended ceiling panels as 640.5 requires. Mechanical execution of work is covered in 640.6 such that audio distribution cables installed exposed on the surface of ceilings and sidewalls of buildings must be properly supported in accordance with 300.11. Any accessible portion of abandoned audio distribution cables must be removed (that can be accomplished without damage to the building surfaces, such as damage to drywall or other surfaces).

Cables identified for future use shall be marked with a tag of sufficient durability to withstand the environment and must contain the date the cable was identified for future use, the date of intended use, and information related to the intended use. Grounding and bonding of these systems are covered in 640.7, which includes general requirements in (A), separately derived audio systems with 60 volts (V) to ground in (B) (this involves compliance with Article 647 and helps reduce noise on the audio circuits), and the use of isolated ground (IG) receptacles in (C) (referencing 250.146(D) on the installation requirements for IG receptacles).

Section 640.8 states insulated conductors of different systems grouped or bundled in close physical contact with each other in the same raceway or other enclosure or in portable cables or cords, shall comply with 300.3(C)(1). Section 300.3(C)(1) states conductors of both AC and DC circuits, rated 1,000V or less, are permitted to occupy the same equipment wiring enclosure, cable or raceway if all conductors have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable or raceway. Remember, however, the Informational Note No. 1 in 300.3(C)(1) reminds an installer to follow the requirements for Class 2 and 3 circuits in 725.136(A) that has limitations for any Class 2 or 3 circuit conductors since insulation only is not sufficient separation between Class 2 and 3 circuit conductors and any power conductors.

Three subsections within 640.9 cover wiring methods for audio equipment installations. Section 690.9(A) covers wiring to and between audio equipment with 690.9(A)(1), stating any power wiring must comply with chapters 1 through 4 of the NEC unless modified by Article 640. Furthermore, (A)(2) covers separately derived power system compliance with Article 647 for sensitive electronic equipment, and (A)(3) states all wiring not connected to the premises wiring system or to a wiring separately derived from premises wiring shall comply with Article 725 for Class 2 and 3 systems. Section 640.9(B) covers auxiliary power supply wiring based on Article 725, 640.9(C) covers output wiring and listing of amplifiers using Article 725 for Class 1, 2 and 3 circuits, and 640.9(D) covers audio transformers and autotransformers.

Read part two of this series here.

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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