DC covered as follows:
1999 NEC Part H. Direct-Current Systems
250-160. General
Direct-current systems shall comply with Part H and other sections of Article 250 not
specifically intended for ac systems.
250-162. Direct-Current Circuits and Systems to Be Grounded
Direct-current circuits and systems shall be grounded as provided for in (a) and (b).
(a)
Two-Wire, Direct-Current Systems.
A two-wire, dc system supplying premises wiring and
operating at greater than 50 volts but not greater than 300 volts, shall be grounded.
Exception No. 1: A system equipped with a ground detector and supplying only industrial
equipment in limited areas shall not be required to be grounded.
Exception No. 2: A rectifier-derived dc system supplied from an ac system complying with
Section 250-20 shall not be required to be grounded.
Exception No. 3: Direct-current fire alarm circuits having a maximum current of 0.030
amperes as specified in Article 760, Part C, shall not be required to be grounded.
(b)
Three-Wire, Direct-Current Systems.
The neutral conductor of all 3-wire, dc systems supplying
premises wiring shall be grounded.
250-164. Point of Connection for Direct-Current Systems
(a)
Off-Premises Source.
Direct-current systems to be grounded and supplied from an off-premises
source shall have the grounding connection made at one or more supply stations. A grounding
connection shall not be made at individual services or at any point on the premises wiring.
(b)
On-Premises Source.
Where the dc system source is located on the premises, a grounding
connection shall be made at one of the following:
1. The source
2. The first system disconnection means or overcurrent device
3. By other means that accomplish equivalent system protection and that utilize equipment
listed and identified for the use
250-166. Size of Direct-Current Grounding Electrode Conductor
The size of the grounding electrode conductor for a dc system shall be as specified in (a)
through (e).
(a)
Not Smaller than the Neutral Conductor.
Where the dc system consists of a 3-wire balancer set or
a balancer winding with overcurrent protection as provided in Section 445-4(d), the
grounding electrode conductor shall not be smaller than the neutral conductor, and not smaller
than No. 8 copper or No. 6 aluminum.
(b)
Not Smaller than the Largest Conductor.
Where the dc system is other than as in (a), the grounding
electrode conductor shall not be smaller than the largest conductor supplied by the system,
and not smaller than No. 8 copper or No. 6 aluminum.
(c)
Connected to Made Electrodes.
Where connected to made electrodes as in Section 250-52(c) or
(d), that portion of the grounding electrode conductor that is the sole connection to the
grounding electrode shall not be required to be larger than No. 6 copper wire or No. 4
aluminum wire.
(d)
Connected to a Concrete-Encased Electrode.
Where connected to a concrete-encased electrode as
in Section 250-50(c), that portion of the grounding electrode conductor that is the sole
connection to the grounding electrode shall not be required to be larger than No. 4 copper
wire.
(e)
Connected to a Ground Ring.
Where connected to a ground ring as in Section 250-50(d), that
portion of the grounding electrode conductor that is the sole connection to the grounding
electrode shall not be required to be larger than the conductor used for the ground ring.
250-168. Direct-Current Bonding Jumper
For dc systems, the size of the bonding jumper shall not be smaller than the system grounding
conductor specified in Section 250-166.
250-169. Ungrounded Direct-Current Separately Derived Systems
Except as otherwise permitted in Section 250-34 for portable and vehicle mounted generators,
an ungrounded dc separately derived system supplied from a stand-alone power source (such
as an engine–generator set) shall have a grounding electrode conductor connected to an
electrode that complies with Part C to provide for grounding of metal enclosures, raceways,
cables, and exposed noncurrent-carrying metal parts of equipment. The grounding electrode
conductor connection shall be to the metal enclosure at any point on the separately derived
system from the source to the first system disconnecting means or overcurrent device, or it
shall be made at the source of a separately derived system that has no disconnecting means or
overcurrent devices.
The size of the grounding electrode conductor shall be in accordance with Section 250-166.
