|NEC Article 820, CATV and Radio Distribution Systems
|Mike Holt for
Be careful with limited-energy coaxial
It's easy to misinterpret 820.1 as limiting Article 820 to
television or cable TV applications. But, it applies to any work you do
installing coaxial cables to distribute limited-energy high- frequency signals.
For example, if you're installing coax for a closed-circuit television in a
security system, Article 820 applies (see Figure 820-1 un820-01 820-01 01.cdr).
If you use coax to connect antennas to equipment [810.3], or for local area
networks, you must follow Article 820 (see Figure 820-2 un820-02 820-01
Perhaps the most important definition in Article 820 is "Point
of Entrance." Knowing this is critical to meeting various grounding
requirements, such as those in 820.33 and 820.40. The point of entrance is where
the cable emerges from an external wall or a concrete floor slab, or from a
rigid metal conduit or intermediate metal conduit grounded to an electrode per
820.40(B). See Figure 820-3 un820-03 820-02.cdr. You must also know the point of
entrance to determine the length of unlisted cable inside a building [820.50,
Exception No. 3].
When installing coax through fire-resistant
rated walls, partitions, floors or ceilings, use approved firestop methods and
materials to maintain the fire-resistance rating. Remove the accessible portion
of abandoned cables, to limit the spread of fire or products of combustion
within a building.
Install only plenum-rated cables in plenums, unless
you run the cables in metal raceways (see 820.61 and 820.53(A)) or if the
plenums are in habitable rooms or areas of a building where the primary purpose
is not air handling [300.22(C)]. Coax installed beneath a raised floor doesn't
need to be Type DP or plenum rated (see 300.22(D) and
Install cables in a neat and
workmanlike manner. Where they run exposed, support them with straps, staples,
hangers, or similar fittings designed and installed so as not to damage the
Don't attach coaxial cable to, or support it with, raceway
[300.1]. Support via "cable tie to conduit" is not an acceptable method (see
Figure 820-19 un820-19 820-52D.cdr). There is one exception: You can support
overhead (aerial) coax to a raceway mast intended for the attachment and support
of communications cables (see Figure 820-20 un820-20 820-52Dx 820-10c
230-28.cdr). Don't support coax by, or attach it to, the power service
Route cables so suspended-ceiling panels don't interfere with
access to electrical equipment (see Figure 800-4 un800-04 800-05.cdr). If you
install cables near framing members, protect them against physical damage from
penetration by screws or nails by 1 1/4 in. separation from the face of the
framing member, or by a suitable metal plate per 300.4(D). See Figure 820-5
un820-05 802-06 02 300-04D.cdr.
If you install cables in hazardous
(classified) locations, follow Chapter 5 requirements. Where practicable, leave
a separation of at least 6 ft between communications wires and cables on
buildings and lightning conductors. CATV coax must be no less than 10 ft over
swimming and wading pools, diving structures and observation stands, towers or
platforms [680.8(B)]. See Figure 820-6 un820-06 820-10F3
[820.33] Ground the metallic sheath of coax
to the earth (electrode) as close as practicable to the point of entrance to the
building or structure (see Figure 820-7 un820-07 820-33.cdr). This doesn't mean
you should drive a separate electrode. The practice of driving a ground rod at a
convenient location and not bonding that ground rod to the power grounding
electrode system is not permitted.
As the 820.33 FPN explains, one
purpose of 820.33 is to limit the potential differences between CATV and other
metallic systems (see Figure 820-14 un820-14 820-40D.cdr). So, connect the coax
shield to the main grounding system. If there's a separate grounding electrode
for the radio and television equipment, bond it to the power grounding electrode
system with a conductor not smaller than 6 AWG (see Figure 820-13 un820-13
Not bonding the electrode to the power grounding
electrode system creates differences in potential between the CATV and other
systems, such as power and telephone-resulting in current flow from lightning
strikes and high-voltage surges. This shock and fire hazard can easily destroy
equipment connected to multiple systems (i.e., the cable tuner is common to
power, CATV and phone).
[820.40] When grounding the coax
Use an insulated grounding conductor listed for the purpose.
Use a grounding conductor made from a corrosion-resistant conductive
material. If copper, it must not be smaller than 14 AWG.
Make the primary
protector-grounding conductor as short as practicable. In one- and two-family
dwellings, it must not exceed 20 ft in length (see Figure 800-8 un820-08
820-40A4 01.cdr). If that length isn't practicable, you can connect it to a
driven rod if you bond the rod to the power grounding electrode system with a
conductor not smaller than 6 AWG (see Figure 820-9 un820-09 820-40A4x.cdr).
Run the grounding conductor to the grounding electrode in as straight a line
Guard the grounding conductor from physical damage, as
Where the grounding conductor runs in a metal raceway, bond each
end of the raceway to the grounding conductor, or the same terminal or electrode
to which the grounding conductor is connected (see Figure 820-10 un820-10
If the building or structure has no grounding means,
terminate the grounding conductor to any of the individual grounding electrodes
described in 250.52. Otherwise, connect the grounding conductor to the nearest
accessible location of one of the following (see Figure 820-11 un820-11
The building or structure grounding electrode system as
covered in 250.50.
The grounded interior metal water-piping system, within 5
ft from its point of entrance to the building [250.52(A)(1)].
bonding means, such as 6 in. of 6 AWG copper conductor connected to the service
equipment or raceway [250.94].
The metallic service raceway
The grounding electrode conductor or the grounding
electrode conductor metal enclosures.
Listing and Markings
When installing coax in a building, use cables listed for the purpose and marked
per Table 820.50, unless:
The cable enters the building from the outside
and runs in rigid metal conduit or intermediate metal conduit, and the raceway
is grounded to an electrode per 820.40(B).
The length of the cable within
the building, measured from its point of entrance, does not exceed 50 ft, and
the cable enters the building from the outside and terminates at a grounding
Many electrical components and materials give off poisonous toxins
when burned. In 300.22 (B) and (C), we find restrictions on the wiring methods
and material you can use in areas of a building used for handling environmental
air. These restrictions help reduce hazards that arise from the burning of
components of an electrical system. Thus, you must pay careful attention to the
listings and markings of the materials you use in a given application in a given
location. Type CATV coax is suitable for general-purpose use, only-refer to
Table 820.50 for other applications, such as risers and
Coax can be in the same raceway or enclosure
with cables of any of the following (see Figure 820-16 un820-16
Class 2 and Class 3-Article 725.
alarm systems-Article 760.
Nonconductive and conductive optical fiber
Communication circuits-Article 800.
network-powered broadband communications circuits-Article 830.
cannot be in any raceway or enclosure with conductors of electric light, power,
or Class 1 circuits, unless:
The coaxial cable is separated from the
power or Class 1 conductors by a barrier (see Figure 820-17 un820-17
If the power circuit conductors are introduced solely for
power supply to the coaxial system distribution equipment. The power circuit
conductors must have a minimum of 0.25 in. separation from the coax.
other applications, you must separate coax by at least 2 in. from any electric
light, power or Class 1 circuit conductors-unless you install those electric
light, power or Class 1 circuit conductors per a Chapter 3 wiring method
(raceway, metallic or nonmetallic sheath, or UF cable). See Figure 820-18
To succeed with Article 820 installations, keep
three primary concepts in mind. First, these are low voltage (under 60V)
applications. Keep the proper separation between coaxial cabling and other
systems. Second, don't violate the fire integrity of a structure. Use approved
firestop methods and materials, and take the necessary steps to minimize any
loss of fire-resistance. Third, there is no special set of grounding physics for
CATV or other coax applications. The basic engineering principles that apply to
power installations also apply to limited-energy high- frequency coax
installations, such as CATV. So, be sure you eliminate any potential differences
between your coax installation and other metallic items. Keeping these three
concepts in mind will help you eliminate any potential violations of Article