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
(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
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
(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
(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..
(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:
WARNING - TECHNICAL POWER
(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.
Do not connect to lighting equipment.
For electronic equipment use only.
60/120 V. 1-phase AC
(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.