Siamese Coax Cable: One Pull, Video and Power

Analog CCTV cameras need two connections: video back to the recorder, and power to the camera. Siamese cable exists to make that one pull instead of two, and it is the reason a large part of the installed camera base has a single cable running to each position.

It also introduces a second set of electrical constraints, on the power side, that the video side does not share.

Quick answer

Question	Answer
What is siamese cable?	A coaxial core and a two-conductor power pair under one jacket
What does it connect?	An analog camera to a recorder and a power supply
Why one jacket?	One pull, one route, one cable to fix
Does the power side have a limit?	Yes. Voltage drop over distance, which is separate from video loss
What is the classic symptom of a power problem?	A camera that resets or drops out at night, when its infrared illuminator turns on
Is it used for IP cameras?	No. IP cameras normally take power over the network cable

Why it exists

The analog camera needs video back and power out. Running two separate cables to every camera position doubles the work: two pulls, two routes, two sets of clips, and twice as much to go wrong in a ceiling.

Siamese construction puts both under one jacket. The installer pulls one cable per camera, terminates both at each end, and the recorder end connects the video to the DVR and the power pair to a distribution box or a multi-output supply.

The alternative on many installations is to run power locally at each camera — which requires a mains outlet at every camera position, and is often the reason siamese cable is used instead.

The power pair has its own arithmetic

This is the part that gets skipped, and it is where siamese installations fail.

Video loss is calculated against the coax attenuation curve and the recorder’s tolerance. Power loss is calculated against something else entirely: the resistance of the two power conductors, the current the camera draws, and the voltage the camera will accept.

Voltage drop is a round trip. Current travels out along one conductor and back along the other, so both lengths count:

Drop = 2 × length × current × resistance per unit length

The three inputs are all obtainable. The camera’s current draw is on its specification. The cable’s resistance per unit length is on the cable datasheet. The length is the actual route, not the straight line.

The result has to fit inside the camera’s tolerance. Cameras are usually specified with a voltage range, and the supply’s output voltage is the starting point. If the supply delivers its nominal voltage at the head end and the drop takes the camera below its minimum, the installation is marginal at best.

Why cameras fail at night

The most common power-related fault has a distinctive signature.

A camera with an infrared illuminator draws more current when the illuminator is on than when it is off. During the day the current is low, the voltage drop is small, and the camera works. At night the illuminator switches on, the current rises, the drop increases, and a camera already close to its minimum voltage drops below it and resets.

The installer then tests during the day, finds everything working, and returns to the same fault the following night.

Two fixes, and one non-fix. The right answers are a heavier power conductor in the cable, or a supply with a higher output voltage to compensate for the drop, or moving the supply closer to the camera. The non-fix is to increase the supply’s current rating, which changes nothing about the voltage at the far end.

Terminating siamese cable

Four connections per camera, and two of them are polarity-critical.

The coaxial side terminates with the usual F connector, sized to the cable. The power side terminates with whatever the camera and the distribution box use, and the polarity has to be correct at both ends — a camera powered backwards generally does not work and may not survive.

Label both ends. A siamese cable that has been cut and re-terminated in a ceiling is easy to get backwards, and the fault looks like a dead camera rather than a wiring error.

Where siamese fits now

The IP camera took over most new installations, and it removed the need for a separate power run: power arrives over the same twisted pair as the data. That is one of the strongest practical arguments for choosing Cat6 over coax on new work.

Siamese cable remains the right product for two situations. Adding cameras to an existing analog system, where the recorder and the cameras are analog and the coax is already in place. And retaining a working analog installation, where replacing the cameras is not on the agenda.

Scenarios

Eight analog cameras on a warehouse, all within 40 meters of the head end. Siamese cable per camera, with the power pair sized from the actual route length and the camera current.

A camera 90 meters from the power supply. Work the drop calculation before installing. If it does not close, move the supply or use a heavier power pair rather than accepting a camera that fails at night.

A camera added to an existing analog system. Match the existing cable’s construction so the power distribution stays consistent.

A new installation with no mains at the camera positions. IP cameras on Cat6 with PoE, which avoids the power-drop question entirely.

FAQ

What is siamese cable used for? Analog CCTV cameras, where it carries video and power in one jacket.

Can I use it for an IP camera? No. An IP camera needs a network connection, and normally takes power over that same cable.

How far can siamese cable run? Two separate limits apply: the video loss budget and the power voltage drop. The shorter of the two governs.

Why does my camera only fail at night? The infrared illuminator draws more current, increasing voltage drop. The camera falls below its minimum operating voltage.

What gauge is the power pair? It varies by cable. Check the datasheet, because the power conductor size decides the drop.

Can I increase the power supply’s amperage to fix a drop problem? No. Current rating does not change the voltage at the far end of the cable.