2002 National 
Electrical Code

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

647.3  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 sensitive electronic equipment locations provided that the following conditions apply.
(1)  The system is installed only in commercial or industrial occupancies.
(2)  The system's use is restricted to areas under close supervision by qualified personnel.
(3)  All of the requirements in 647.4 through 647.8 are met.

647.4  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.
(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 exceed 2.5 percent.
(2)  Cord-Connected Equipment.  The voltage drop on branch circuits supplying receptacle outlets 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.

(FPN):  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.
647.5  3-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 main disconnects.

647.6  Grounding.

(A)  General.  The system shall be grounded as provided in Section 250.30 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 250.122 and run with the feeder conductors.  The technical 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.

FPN No. 1:  See Section 250.122 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..
647.7   Receptacles.
(A)  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 protected.
(2)  All outlet strips, adapters, receptacle covers and faceplates shall be marked with the following words or equivalent:
Do not connect to lighting equipment.
For electronic equipment use only.
60/120 V. 1-phase 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 within 1.8 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 identified for use with this class of system.  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 250.146(D), however, the branch circuit equipment grounding conductor shall be terminated as required in Section 647.6(B).
647.8  Lighting Equipment.   Lighting equipment installed under this article for the purpose of reducing electrical noise originating from lighting equipment shall meet the following conditions (A) through (C).
(A)  Disconnecting Means.  All lighting equipment, luminaires and associated control equipment if provided shall have a disconnecting means that simultaneously opens all ungrounded conductors that shall be located within sight of the luminaire or be capable of being locked in the open position.
(B)  Luminaires.  All luminaires shall be permanently installed, listed and ballast operated.
(C)  Screw-shell.  Lighting fixtures installed under this section shall not have an exposed lamp screw-shell.

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