1996/1999 National
Electrical Code
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Technical Support Information

G.  Separately Derived Systems with 60 Volts to Ground

530-70.General. Use of a separately derived 120-volt single-phase 3-wire system with 60 volts on each of two ungrounded conductors to a grounded neutral conductor shall be permitted for the purpose of reducing objectionable noise in audio/video production or other similar sensitive electronic equipment locations provided that its use is restricted to electronic equipment only and that all of the requirements in Sections 530-71 through 530-73 are met.

530-71. Wiring Methods.

(a) 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 that are identified for operation at the system voltage shall be provided for both ungrounded conductors in all feeders and branch circuits.

(b) Junction Boxes.All junction box covers shall be clearly marked to indicate the distribution panel and the system voltage.

(c) Color Coding.  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.

(d) 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.

530-72. Grounding.

(a) General.  The system shall be grounded as provided in Section 250-26 as a separately derived single-phase 3-wire system.

(b) 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 prominently marked "Technical Equipment Ground" in the originating 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 grounding conductor shall not be smaller than that specified in Table 250-95 and run with the feeder conductors. The technical equipment grounding bus need not be bonded to the panelboard enclosure.

Exception: 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 Part G of this article.

(FPN No. 1): See Section 250-95 for equipment grounding conductor sizing requirements where circuit conductors are adjusted in size to compensate for voltage drop.

(FPN No. 2): These requirements limit the impedance of the ground fault path where only 60 volts applies to a fault condition instead of the usual 120 volts.

530-73. Receptacles.

(a) General.  Where receptacles are used as a means of connecting equipment, the following conditions shall be met:

(1)  All 15- and 20-amp receptacles shall be GFCI protected.

(2)  All outlet strips, adapters, receptacle covers and faceplates shall be marked as follows:

Do not connect to lighting equipment.
For electronic equipment use only.
60/120 V. 1ø AC
GFCI protected
(3)  A 125-volt, single-phase, 15- or 20-ampere-rated receptacle outlet having one of its current carrying poles connected to a grounded circuit conductor shall be located witnin 6 feet 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 identified for use with this class of system.

Exception: 125-Volt, single phase, 15- or 20-ampere-rated receptacle outlets and attachment plugs 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.

(b) Isolated ground receptacles.  Isolated ground receptacles shall be permitted as described in Section 250-74 Exception No. 4, however, the branch circuit equipment grounding conductor shall be terminated as required in Section 530-72(b).

Go to NFPA home page for more information on obtaining a complete copy of NFPA 70 (National Electrical Code).
You may purchase a copy of the NEC from a national construction industry bookstore.

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