BNC Connector
A bayonet coaxial connector used for video and CCTV signal runs.
Quick Answer
A BNC connector is a bayonet-locking coaxial connector used on video and radio-frequency cable, including analog CCTV cameras and recorders. Two connectors are needed for each coax run. Estimators count them EA, usually derived from camera or device counts on security drawings rather than shown as symbols.
The Full Picture
BNC stands for bayonet Neill-Concelman, and the connector locks with a quarter turn onto a mating jack. It terminates coaxial cable, which has a center conductor and a shield. BNC connectors are made in different impedances, with 75 ohm types common for video and 50 ohm types common for test and data equipment. Impedance must match the cable and equipment, so the schedule or spec for a security system states which is used. One laboratory cabling guide, for instance, lists BNC-terminated 50 ohm coaxial cables for analog logic signals.
In buildings, BNC connectors show up mostly in analog CCTV, older video distribution and some broadcast and audiovisual wiring. Newer security systems use IP cameras on twisted-pair network cabling, which uses modular jacks and not BNC. For a project with analog cameras, each camera run is coax with a connector at the camera and another at the recorder or switch. The connector is a part of the termination, not an item drawn on the plans.
The documents that drive the count are the security or low-voltage plans, which show cameras, recorders and sometimes coax runs, and the specifications under Division 28, which cover video surveillance, and Division 27 where the cabling is in a telecom scope. The estimator counts devices that use coax and then derives connectors: two per run, one at each end, plus a spare allowance if the bid practice calls for it. Cable is measured in LF separately, and the camera, power supply and mounting are separate lines.
The usual misses are forgetting that coax cameras often need a separate power run, buying the wrong impedance of connector, and counting connectors for IP cameras that need none. An estimator should read the camera schedule to see whether each camera is analog or networked before deriving a connector count.
Real Examples
Common Misconceptions
People assume: Any BNC connector fits any coax.
Actually: Connectors are made for specific cable sizes and for 50 or 75 ohm impedance. The type must match the cable and equipment in the specification.
People assume: All modern cameras use BNC.
Actually: IP cameras generally connect by network cable. BNC applies to analog or HD-over-coax systems, so the camera schedule decides whether any are needed.
Frequently Asked Questions
What does BNC stand for?
It stands for Bayonet Neill-Concelman, describing a bayonet-locking connector for coaxial cable.
Where are BNC connectors used in buildings?
Mostly in analog CCTV and some video distribution. IP-based security systems use network cabling instead.
What unit is a BNC connector taken off in?
EA, usually two per coax run, derived from camera or device counts. The coax cable itself is measured in LF.
Do 50 ohm and 75 ohm BNC connectors matter?
Yes. The connector impedance should match the cable and equipment. Video work commonly specifies 75 ohm, but follow the project specification.