Understanding NEC Article 647: Sensitive Electronic Equipment

NEC Article 647 provides specialized requirements for electrical systems designed to power sensitive electronic equipment. These systems operate at 120 volts line-to-line and 60 volts to ground, reducing objectionable electrical noise that could interfere with the performance of audio, video, and similar equipment.

Scope of Article 647

This article applies to:

• Separately derived systems operating at 120 volts line-to-line and 60 volts to ground.
• Systems used in commercial and industrial locations with sensitive audio, video, or electronic equipment.

Key Notes:

• These systems must be installed only in areas supervised by qualified personnel.
• They are designed specifically for noise-sensitive environments.

Key Requirements for Sensitive Electronic Equipment Systems

1. Wiring Methods
   • Standard single-phase panelboards are permitted, provided they are clearly marked for 120/60-volt systems.
   • Disconnecting means must simultaneously open all ungrounded conductors.
2. Voltage Drop
   • Voltage drop is strictly limited to ensure performance:
     • 1.5% maximum for branch circuits supplying fixed equipment.
     • 1.0% maximum for cord-connected equipment.
     • Combined feeder and branch-circuit voltage drop must not exceed 2.5%.
3. Grounding
   • Equipment grounding conductors must connect to a grounding bus labeled Technical Equipment Ground in the panelboard.
   • The impedance of the grounding path must meet or exceed requirements to support 60-volt fault conditions.
4. Receptacles
   • All 15- and 20-ampere receptacles must include GFCI protection.
   • Receptacles must be labeled:
     “WARNING — TECHNICAL POWER. DO NOT CONNECT TO LIGHTING EQUIPMENT. FOR ELECTRONIC EQUIPMENT USE ONLY. 60/120 V. 1ΦAC GFCI PROTECTED.”
5. Lighting Equipment
   • Lighting systems must meet reduced noise standards:
     • Disconnection means must open all ungrounded conductors.
     • No luminaires may use exposed screw shells.
     • Lighting must be permanently installed and listed for 60/120-volt systems.
6. Conductor Identification
   • Conductors must be identified at all splices and terminations using color coding, marking, or tags.

Applications

NEC Article 647 is essential for:

• Recording Studios: Audio and video production environments.
• Broadcast Facilities: TV and radio station equipment rooms.
• Industrial Labs: Locations with precision instruments requiring clean power.

Conclusion

NEC Article 647 provides detailed requirements for electrical systems designed for sensitive electronic equipment. These systems ensure optimal performance by reducing noise interference and meeting stringent grounding and voltage drop standards.