Article 647

Sensitive Electronic Equipment

boxes and enclosures (including transfer switches, generators, and power panels) for emergency circuits shall be marked to identify them as components of an emergency circuit or system.

Part IV. Workspace

1. General. Space about electrical equipment shall comply with 110.26.(B)
2. Entrance to and Egress from Working Space. For equipment over 1.8 m (6 ft) wide or deep, there shall be one entrance to and egress from the required working space not less than 610 mm (24 in.) wide and 2.0 m (6 1∕2 ft) high at each end of the working space. The door(s) shall open in the direc‐ tion of egress and be equipped with panic bars, pressure plates, or other devices that are normally latched but open under simple pressure. A single entrance to and egress from the required working space shall be permitted where either of the conditions in 646.19(A) or is met.
   1. Unobstructed Egress. Where the location permits a continuous and unobstructed way of egress travel, a single entrance to the working space shall be permitted.
   2. Extra Working Space. Where the depth of the working space is twice that required by 110.26(A)(1) , a single entrance shall be permitted. It shall be located such that the distance from the equipment to the nearest edge of the entrance is not less than the minimum clear distance specified in Table 110.26(A)(1) for equipment operating at that voltage and in that condition.
3. Working Space for ITE.
   1. Low-Voltage Circuits. The working space about ITE where any live parts that may be exposed during routine servicing operate at not greater than 30 volts rms, 42 volts peak, or 60 volts dc shall not be required to comply with the workspace requirements of 646.19.
   2. Other Circuits. Any areas of ITE that require servicing of parts that are greater than 30 volts rms, 42 volts peak, or 60 volts dc shall comply with the workspace requirements of 646.19.
   Informational Note No. 1: For example, field-wiring compart‐ ments for ac mains connections, power distribution units, and so forth.Informational Note No. 2: It is assumed that ITE operates at voltages not exceeding 1000 volts.
4. Work Areas and Working Space About Batteries. Work‐ ing space about a battery system shall comply with 110.26. Working space shall be measured from the edges of the battery racks, cabinets, or trays.
5. Workspace for Routine Service and Maintenance. Workspace shall be provided to facilitate routine servicing and maintenance (those tasks involving operations that can be accomplished by employees and where extensive disassembly of equipment is not required). Routine servicing and mainte‐ nance shall be able to be performed without exposing the worker to a risk of electric shock or personal injury.

Informational Note: An example of such routine maintenance is cleaning or replacing an air filter.

647.1 Scope. This article covers the installation and wiring of separately derived systems operating at 120 volts line-to-line and 60 volts to ground for sensitive electronic equipment.

1. General. Use of a separately derived 120-volt single- phase 3-wire system with 60 volts on each of two ungrounded conductors to an equipment grounding conductor shall be permitted for the purpose of reducing objectionable noise in sensitive electronic equipment locations, provided the follow‐ ing conditions apply:
   1. The system is installed only in commercial or industrial occupancies.
   2. The system’s use is restricted to areas under close supervi‐ sion by qualified personnel.
   3. All of the requirements in 647.4 through 647.8 are met.
2. Wiring Methods.

1. Panelboards and Overcurrent Protection. Use of standard single-phase panelboards and distribution equipment with a higher voltage rating shall be permitted. The system shall be clearly marked on the face of the panel or on the inside of the panel doors. Common trip two-pole circuit breakers or a combination two-pole fused disconnecting means that are iden‐ tified for use at the system voltage shall be provided for both ungrounded conductors in all feeders and branch circuits. Branch circuits and feeders shall be provided with a means to simultaneously disconnect all ungrounded conductors.
2. Junction Boxes. All junction box covers shall be clearly marked to indicate the distribution panel and the system volt‐ age.
3. Conductor Identification. All feeders and branch-circuit conductors installed under this section shall be identified as to system at all splices and terminations by color, marking, tagging, or equally effective means. The means of identification shall be posted at each branch-circuit panelboard and at the disconnecting means for the building.
4. Voltage Drop. The voltage drop on any branch circuit shall not exceed 1.5 percent. The combined voltage drop of feeder and branch-circuit conductors shall not exceed

2.5 percent.

1. Fixed Equipment. The voltage drop on branch circuits supplying equipment connected using wiring methods in Chapter 3 shall not exceed 1.5 percent. The combined voltage drop of feeder and branch-circuit conductors shall not exceed2.5 percent.
2. Cord-Connected Equipment. The voltage drop on branch circuits supplying receptacles shall not exceed 1 percent. For the purposes of making this calculation, the load connected to the receptacle outlet shall be considered to be 50 percent of the branch-circuit rating. The combined voltage drop of feeder and branch-circuit conductors shall not exceed 2.0 percent.

Informational Note: The purpose of this provision is to limit voltage drop to 1.5 percent where portable cords may be used as a means of connecting equipment.

ARTICLE 650 — PIPE ORGANS 650.3

1. Three-Phase Systems. Where 3-phase power is supplied, a separately derived 6-phase “wye” system with 60 volts to ground installed under this article shall be configured as three separately derived 120-volt single-phase systems having a combined total of no more than six disconnects.
2. Grounding.
   1. General. The transformer secondary center tap of the 60/120-volt, 3-wire system shall be grounded as provided in 250.30.
   2. Grounding Conductors Required. Permanently wired utilization equipment and receptacles shall be grounded by means of an equipment grounding conductor run with the circuit conductors to an equipment grounding bus promi‐ nently marked “Technical Equipment Ground” in the originat‐ ing branch-circuit panelboard. The grounding bus shall be connected to the grounded conductor on the line side of the separately derived system’s disconnecting means. The ground‐ ing conductor shall not be smaller than that specified in Table 250.122 and run with the feeder conductors. The techni‐ cal equipment grounding bus need not be bonded to the panelboard enclosure. Other grounding methods authorized elsewhere in this Code shall be permitted where the impedance of the grounding return path does not exceed the impedance of equipment grounding conductors sized and installed in accordance with this article.
   Informational Note No. 1: See 250.122 for equipment ground‐ ing conductor sizing requirements where circuit conductors are adjusted in size to compensate for voltage drop.Informational Note No. 2: These requirements limit the impe‐ dance of the ground fault path where only 60 volts apply to a fault condition instead of the usual 120 volts.
3. Receptacles.
   1. General. Where receptacles are used as a means of connecting equipment, the following conditions shall be met:
      1. All 15- and 20-ampere receptacles shall be GFCI protec‐ ted.
      2. All receptacle outlet strips, adapters, receptacle covers, and faceplates shall be marked with the following words or equivalent:WARNING — TECHNICAL POWERDo not connect to lighting equipment. For electronic equipment use only.60/120 V. 1φacGFCI protectedThe warning sign(s) or label(s) shall comply with 110.21(B).
      3. A 125-volt, single-phase, 15- or 20-ampere-rated recepta‐ cle having one of its current-carrying poles connected to a grounded circuit conductor shall be located within1.8 m (6 ft) of all permanently installed 15- or 20-ampere- rated 60/120-volt technical power-system receptacles.
      4. All 125-volt receptacles used for 60/120-volt technical power shall have a unique configuration and be identi‐ fied for use with this class of system.
      Exception: Receptacles and attachment plugs rated 125-volt, single- phase, 15- or 20-amperes, and that are identified for use with grounded
      circuit conductors, shall be permitted in machine rooms, control rooms, equipment rooms, equipment racks, and other similar locations that are restricted to use by qualified personnel.
   2. Isolated Ground Receptacles. Isolated ground receptacles shall be permitted as described in 250.146(D); however, the branch-circuit equipment grounding conductor shall be termi‐ nated as required in 647.6(B).
4. Lighting Equipment. Lighting equipment installed under this article for the purpose of reducing electrical noise originating from lighting equipment shall meet the conditions of 647.8(A) through (C).

1. Disconnecting Means. All luminaires connected to sepa‐ rately derived systems operating at 60 volts to ground, and asso‐ ciated control equipment if provided, shall have a disconnecting means that simultaneously opens all ungroun‐ ded conductors. The disconnecting means shall be located within sight of the luminaire or be lockable open in accord‐ ance with 110.25.
2. Luminaires. All luminaires shall be permanently installed and listed for connection to a separately derived system at 120 volts line-to-line and 60 volts to ground.
3. Screw Shell. Luminaires installed under this section shall not have an exposed lamp screw shell.