A Siamese CCTV cable is sold as one part number and installed as two circuits. One carries baseband video or radio frequency at 75 ohms. The other carries camera power, usually 12 volts DC. They fail for different reasons, they are limited by different published figures, and under the National Electrical Code they belong to different listing families. Buying on the description "RG59 with power" tells you nothing about either half.
The Two Circuits Have Two Sets of Rules
| Coax side | Power side |
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Carries | Baseband video or RF | Camera supply |
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Defined by | 75 ohm impedance | Voltage and current |
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Listing family | CATVP / CATVR / CATV / CATVX | CL2 and CL3 (Class 2 and 3) |
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Primary code article | Article 820, community antenna television and radio distribution systems | Article 725, Class 2 and Class 3 power-limited circuits |
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Limits the run | Attenuation against frequency, and the connector geometry | DC resistance and voltage drop |
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Published where | Attenuation in dB per 100 m, return loss in dB, velocity of propagation | Conductor resistance in ohms per kilometer |
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One jacket, two circuits
The listing requirements for both have been moving. The 2020 edition of the code added Article 800 as a general article for communications systems and renumbered the old Article 800 as Article 805. The 2023 edition then gathered the listing requirements for plenum, riser, general-purpose and limited-use cables into Article 800, including CATV cables alongside communications cables, and the listing requirements for Class 2 and Class 3 cables into new Article 722. The cable did not change. The paperwork did, enough that a specification written three years ago may cite article numbers that no longer exist.
Two Ladders, and Only One of Them Is Communications
The listing ladders climb in the same direction and they are not the same list.
The communications ladder runs CMX, CM, CMR, CMP. The coaxial ladder that matters for CCTV runs CATVX, CATV, CATVR, CATVP. Plenum, riser, general purpose and limited use appear in both, in the same order, which is why they are so often treated as interchangeable. A CATVP is a plenum-listed coaxial cable. It is not a CMP, and a specification that asks for "plenum coaxial" should say which ladder it means.
There is a second rule that explains why a Siamese construction is allowed to exist at all. The code separates cables into two groups by hazard. The low-hazard group — power-limited Class 2, Class 3 and fire alarm cables, communications and CATV cables, and optical fiber — may share a raceway, a tray or a routing assembly. The same low-hazard cables must be separated from electric light and power, Class 1 circuits, non-power-limited fire alarm, and medium-power network-powered broadband. A Siamese cable is legal because both of its halves are low hazard, not because the combination has its own approval.
The 75 Ohm System Is a Geometry
Seventy-five ohms is not a resistor value. It is the relationship between the electric and magnetic fields traveling along the cable, set by the diameter of the center conductor, the inside diameter of the shield, and the dielectric constant of the material between them. Change any one of the three and the local impedance is no longer 75, and a local change reflects part of the signal back down the line.
That is why a kink, a crushed point, a badly seated connector or a short length of a different cable matters more than the meter on either side of it. It also explains why return loss is published as a separate figure from attenuation: attenuation is how much signal the cable loses, return loss is how much it reflects. The foam-dielectric RG6 in the table below publishes return loss better than 20 dB across both VHF and UHF.
Velocity of Propagation Tells You What the Dielectric Is
Velocity of propagation is the speed at which a signal travels along the cable, expressed as a fraction of the speed of light, and it is set by the dielectric.
Compact polyethylene, the solid plastic used in classic RG59, gives about 66 percent. Physically foamed polyethylene, where gas displaces part of the plastic, gives 85 percent. That is a large difference in a number that sounds academic. It is worth reading because velocity of propagation tells you which dielectric you were sold, without cutting a cable open. A datasheet advertising RG6 with a velocity of 66 percent is describing a compact-dielectric construction, and the attenuation that goes with it.
Two Constructions, Compared as Published
| RG59, compact PE | RG6T, physical foam PE |
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Impedance | 75 ohm | 75 ohm |
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Center conductor | Copperweld, 1 x 0.58 mm, nominal 0.26 mm2 | Solid copper, 1.02 mm |
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Dielectric | Compact transparent PE, 3.70 mm | Physical foam PE, 4.57 mm |
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Shield | OFC copper braid, 95% coverage | Bonded aluminum foil + aluminum braid + unbonded aluminum foil |
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Overall diameter | 6.20 mm | 7.06 mm |
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Capacitance | 67 pF/m | 53 pF/m |
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Velocity of propagation | 66% | 85% |
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Inner conductor DC resistance | 154 ohm/km | 21 ohm/km |
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Outer conductor DC resistance | Not published on this datasheet | 30 ohm/km |
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Loop conductor DC resistance | Not published on this datasheet | 51 ohm/km |
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Shielding effectiveness | Not published on this datasheet | Better than 70 dB |
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Return loss | Not published on this datasheet | Better than 20 dB, VHF and UHF |
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Minimum bend radius | Not published on this datasheet | 35 mm |
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Operating temperature | -20 to +70 C | -20 to +80 C |
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Both entries are 75 ohm coaxial cables, both are legitimate, and their published attenuation at 1000 MHz differs by close to a factor of two. The dielectric is most of the reason. The center conductor material is the rest of it.
Attenuation Is a Frequency Question
Frequency | RG59, dB per 100 m | RG6T, dB per 100 m |
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5 MHz | Not published on this datasheet | 1.90 |
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55 MHz | Not published on this datasheet | 5.25 |
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100 MHz | 10.70 | Not published on this datasheet |
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211 MHz | Not published on this datasheet | 10.00 |
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500 MHz | 25.70 | Not published on this datasheet |
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800 MHz | 33.60 | Not published on this datasheet |
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1000 MHz | 38.00 | 21.49 |
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1200 MHz | Not published on this datasheet | 23.59 |
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1500 MHz | 48.50 | Not published on this datasheet |
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2150 MHz | Not published on this datasheet | 31.82 |
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2500 MHz | 66.80 | Not published on this datasheet |
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3000 MHz | 72.70 | Not published on this datasheet |
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Attenuation against frequency, from published points
The table is built from two datasheets with different published frequency points, which is why the columns do not line up. The one frequency both publish is 1000 MHz, and there the gap is 38.00 against 21.49 dB per 100 m.
Two things follow. First, the cable that carries a 5 MHz baseband camera picture is the wrong cable for a satellite intermediate frequency or a coaxial home network, and the difference is on the sheet. Second, attenuation is quoted per 100 m, so run length multiplies it. Fifty meters of that RG59 is roughly 19 dB at 1000 MHz before a single connector is fitted. On a 5 MHz camera signal the same 50 m costs well under 1 dB.
Why the Center Conductor Is Sometimes Steel
The RG59 datasheet in the table publishes a copperweld center conductor, which is copper over a steel core, and the RG6 datasheet publishes solid copper. Neither choice is a defect. A steel core adds tensile strength, which matters on a pulled run and on a camera mounted where the cable moves. The published price is resistance: 154 ohms per kilometer on that RG59 against 21 on the RG6.
The center conductor carries the signal rather than the power, so its resistance matters most in three situations: long runs, tight bends, and any construction where DC is sent down the coax to feed the camera. In the last case the resistance is not a signal detail at all, it is the supply.
The Shield, and What the Percentage Means
The RG59 datasheet publishes an oxygen-free copper braid at 95 percent coverage. The RG6 datasheet publishes a bonded foil, a braid and an unbonded foil, with shielding effectiveness better than 70 dB.
Those are two different measurements of two different properties. Coverage is how much of the surface the braid covers. Shielding effectiveness is how much interference the assembly rejects, measured across a frequency range. A percentage on its own says nothing about rejection at any particular frequency, and a rejection figure without a frequency range says nothing about whether the construction suits the environment. When a supplier quotes one and not the other, ask which question is being answered.
Loop Resistance and the Return Path
The RG6 datasheet publishes three resistances: 21 ohms per kilometer on the inner conductor, 30 on the outer, and 51 as a loop.
The loop figure is the one to use whenever current has to come back — which is every DC-powered camera and every long earthed run. Fifty-one ohms per kilometer over a 200 meter run is a little over 10 ohms in the loop, and at 300 milliamps that is about 3 volts lost before the camera sees the supply. That is arithmetic on a published figure rather than a measured result, and it is the calculation worth doing before assuming a coaxial cable can also carry power.
Grounding the Shield Where It Enters
The shield is a conductor, and the code treats it as one. Article 820.93 addresses grounding of the outer conductive shield of coaxial cables: the shield is grounded as close as practical to the point where the cable enters the building, and a primary protector is installed on each CATV cable outside the premises near the entrance.
Where the shield is bonded at the entry
On a CCTV system the consequence is that a coaxial run coming in from an outdoor camera route is not only a signal cable. It is part of the grounding and protection arrangement for the building, and the entry point is where that is either done properly or skipped. On an internal-only system the same article matters less, which is why it is overlooked on mixed indoor and outdoor jobs.
The code text cited here was read from a code-hosting service whose page summary is machine-generated, so treat it as a pointer to the article rather than a substitute for it.
The Connector Is Where the Geometry Changes
Nothing in either table survives a connector that is not the same impedance as the cable, and the interface matters as much as the body.
An F-type connector uses the center conductor itself as the pin, which is why a stranded or undersized center conductor gives an unreliable contact. A BNC or an N-type has its own defined geometry and its own published limits. The construction details in the first table are the practical warning: a 0.58 mm solid copperweld core and a 1.02 mm solid copper core do not take the same connector, and a connector specified for one is not automatically correct for the other.
One design point follows from the impedance argument. A twist-on connector does not hold the shield concentric with the center conductor. Concentricity is the geometry that makes the cable 75 ohms in the first place, so the connector that is easiest to fit is the one most likely to change the number.
What to Ask Before You Order
- The listing printed on the coax – CATVP, CATVR, CATV or CATVX – and the space it is going into.
- Whether the power pair is a Class 2 or Class 3 circuit, and its conductor gauge and published resistance.
- Attenuation at the highest frequency the system will actually carry, not the lowest.
- Velocity of propagation, which is the fastest way to tell compact dielectric from foam.
- Inner, outer and loop conductor resistance, not just the inner figure.
- Shielding described in dB across a stated frequency range, not as a percentage alone.
- The connector type the center conductor is specified for.
The Short Version
A Siamese CCTV cable is two circuits with two published sets of limits and two code rulebooks. The coaxial side is a 75 ohm geometry whose losses rise with frequency and whose performance is published as attenuation, return loss and velocity of propagation. The power side is a Class 2 or Class 3 circuit whose limit is DC resistance and voltage drop. The phrase "RG59 with power" answers neither question, and both answers are already printed on the datasheet.
We build coaxial and composite constructions and test them against the figures on the sheet. Tell us the run length, the highest frequency the system carries, the camera supply and the space the cable is going into, and we will tell you which construction and which connector the run actually needs.
Sources
- Tasker, RG 59 CPR 75 ohm coaxial cable technical datasheet (Milan S.r.l.) — that the cable is a 75 ohm coaxial type for standard definition video applications and cable assembly; a black soft flame retardant PVC sheath with an outer diameter of 6.20 mm plus or minus 0.20 mm; shielding of oxygen free copper braid covering 95 percent; a core insulation of transparent compact PE with a diameter of 3.70 mm plus or minus 0.20 mm; a stiff wire formation center conductor in Copperweld of 1 x 0.58 mm with a nominal cross section of 0.26 mm2; conductor resistance of 154 ohm per km; capacitance core to shield of 67 pF per meter; velocity of propagation of 66 percent; impedance of 75 ohm; spark test at 4500 V; maximum rated voltage of 2000 V; operating temperature of -20 to +70 C; and the published attenuation in dB per 100 m of 10.70 at 100 MHz, 25.70 at 500 MHz, 33.60 at 800 MHz, 38.00 at 1000 MHz, 48.50 at 1500 MHz, 66.80 at 2500 MHz and 72.70 at 3000 MHz. The datasheet also carries the print legend for the construction including the CPR class — https://www.tme.eu/Document/454cedd91718588a0a68356753ee6f27/RG59CPR.pdf
- Intelek, RG6 T CATV coaxial cable datasheet — the construction of a solid copper inner conductor of 1.02 mm, a physical foam polyethylene dielectric of 4.57 mm, an outer conductor of bonded aluminum foil plus aluminum braid plus unbonded aluminum foil at 5.60 mm, and a PVC or polyethylene jacket at 7.06 mm; capacitance of 53 pF per meter; impedance of 75 ohm; velocity of 85 percent; inner conductor DC resistance of 21 ohm per km; outer conductor DC resistance of 30 ohm per km; loop conductor DC resistance of 51 ohm per km; shielding effectiveness better than 70 dB; minimum bend radius of 35 mm; operating temperature of -20 to +80 C; return loss better than 20 dB at VHF and better than 20 dB at UHF; and the published attenuation in dB per 100 m of 1.90 at 5 MHz, 5.25 at 55 MHz, 10.00 at 211 MHz, 10.82 at 250 MHz, 12.63 at 350 MHz, 13.61 at 400 MHz, 16.08 at 550 MHz, 18.54 at 750 MHz, 20.01 at 865 MHz, 21.49 at 1000 MHz, 23.59 at 1200 MHz, 25.92 at 1450 MHz, 28.70 at 1750 MHz and 31.82 at 2150 MHz — https://www.intelek.cz/db/repository.nsf/v/E046D2F16C3BF5C7C12574980026F6C4/$file/Datasheet_coaxial_cable_RG6T-CATV.pdf
- Communications Cable and Connectivity Association, "2023 National Electrical Code: Revisions to Cable Requirements," by Stanley Kaufman of CableSafe Inc. and Ronald Tellas of Belden, both identified as NEC code-making panel participants — that Chapter 8 of the code covers communications systems and that the 2020 edition added Article 800 as a general requirements article and renumbered the former Article 800 as Article 805 while Articles 810, 820, 830 and 840 were retained; that in the 2023 edition the listing requirements for all plenum cables (CMP, CATVP and BLP), riser cables (CMR, CATVR and BLR), general purpose cable (CMG, CM, CATV, BM and BL) and limited use cables (CMX, CATVX and BLX) are placed in Article 800; that the listing requirements for Class 2 and Class 3 cables were moved to a new Article 722 covering cables for power-limited circuits, fault-managed power circuits and optical fiber; that the code separates cables into low hazard and other groups, with power-limited Class 2, Class 3 and fire alarm cables, communications cables, CATV cables, low-power network-powered broadband communications cables and no-voltage optical fiber cables permitted to be installed together while the same low hazard cables must be separated from electric light and power, Class 1, non-power-limited fire alarm and medium-power network-powered broadband cables — https://cccassoc.org/wp-content/uploads/2022/05/2023-National-Electrical-Code-Revisions-to-Cable-Requirements.pdf
- National Electrical Code, Article 820.93, Grounding of the Outer Conductive Shield of Coaxial Cables, as published by a code hosting service for the 2023 edition — that coaxial cables entering or attached to buildings are subject to grounding requirements; that where the outer conductive shield is grounded no additional protective devices are necessary; that grounding should occur as close as possible to the entry point of the coaxial cable; and that a primary protector must be installed on each CATV cable outside the premises near the entrance, with primary protection equipment not located in hazardous locations unless permitted by other provisions. The page presentation of this section is machine-generated and carries its own accuracy disclaimer, so it is cited here as a pointer to the article rather than as the code text itself — https://up.codes/s/grounding-of-the-outer-conductive-shield-of-coaxial-cables
