Last updated: June 2026
A CCTV cable that is taut and properly tensioned during a summer installation will sag noticeably during winter, potentially forming water traps, creating strain on connectors, and catching wind that transmits vibration to the camera. The coefficient of thermal expansion of copper is 16.5 parts per million per degree Celsius. A 30-metre cable experiencing a 30-degree temperature change from summer to winter contracts by approximately 15 mm. On a horizontal run, this contraction increases tension. On a vertical drop, it causes the cable to pull upward at the connector.
How Temperature Affects Cable Length and Tension
Copper conductors expand and contract with temperature at 0.0165 mm per metre per degree Celsius. A 50-metre cable installed at 20 degrees Celsius in summer and then exposed to -5 degrees Celsius in winter contracts by 20.6 mm. For a horizontal cable clipped at both ends, this contraction increases tension by approximately 50–100 N, depending on the cable construction and jacket material. For a vertical cable, the contraction pulls the cable upward at the connection point, potentially pulling the RJ45 latch or BNC connector out of its socket or causing the cable to pull out of its gland seal at the camera end.
The Water Trap Problem from Sagging Cables
When a horizontal cable run sags due to thermal expansion, the low point of the sag becomes a water trap. Water runs along the cable sheath and collects at the low point. Over time, if the cable sheath has any pinhole or cut, water enters the cable at the sag point and runs by capillary action to the connector at either end. The connector at the lower end fills with water, causing corrosion that eventually destroys the termination. A properly installed cable with a slight continuous slope (1:100 gradient minimum) and drip loops before each connection prevents this capillary water migration.
Winter-Specific Cable Strain Considerations
The combination of thermal contraction and wind loading creates peak strain on cables during UK winter storms. A cable that was installed with adequate slack for summer conditions may be pulled taut during a -5 degree C night with 50 mph wind, transmitting the wind load directly to the camera connector. The repeated cyclical loading at the connector’s strain relief causes fatigue failure of the cable jacket and conductor wires at the termination point over 2–3 winters. Installers should provide 50–100 mm of service loop at each cable end to accommodate temperature-driven length changes.
Cable Support Intervals for UK Temperature Range
BS 7671 and the relevant communications cabling standards specify cable support intervals based on the cable type and environment. For horizontal runs of Cat6 or coaxial cable, supports should be at 300 mm intervals for vertical runs and 400 mm intervals for horizontal runs. These intervals assume the cable is fixed with appropriate clips or cable ties that allow slight movement (not rigidly clamped). Rigidly clamping the cable at every support prevents thermal expansion from distributing along the cable and concentrates all movement at the termination point. Use cable ties with a torque-limited tool to avoid crushing the cable jacket.
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Frequently Asked Questions
1. Why do my CCTV cables sag more in winter?
Answer: Cables contract in cold weather due to thermal expansion of copper. A 50-metre cable experiences approximately 20 mm of contraction between summer and winter. This changes cable tension and sag. For more detail, see How to maintain Gyms and Fitness Centres CCTV systems - UK guide 2026. Also read our related guide: Ladder Positioning for Safe CCTV Camera Installation. Browse our security technology hub at Uni Blog Security Hub. Official UK guidance on this topic: BSI.
2. How much slack should I leave in CCTV cables?
Answer: Leave 50–100 mm service loop at each cable end to accommodate thermal expansion and allow for re-termination. The loop should be formed into a gentle U-shape, not a tight coil. For more detail, see Does Offices and Commercial Buildings CCTV reduce insurance premiums in 2026? UK guide. Also read our related guide: Right to Light vs CCTV Positioning Conflicts.
3. Can cable sag damage my CCTV connectors?
Answer: Excessive sag creates strain on the connector’s strain relief and can pull the connector out of alignment. A drip loop before the connector also prevents water tracking along the cable into the connection. For more detail, see Self Storage Facilities CCTV - UK legal requirements and GDPR compliance 2026. Also read our related guide: Pre-Action Protocol for Neighbour CCTV Disputes.
4. What is a drip loop for CCTV cables?
Answer: A drip loop is a U-shaped curve in the cable immediately before the connection point. Water runs to the bottom of the loop and drips off rather than tracking along the cable into the connector. For more detail, see Does Warehouses and Logistics CCTV reduce insurance premiums in 2026? UK guide. Also read our related guide: Nuisance Claims Under Common Law for Neighbour CCTV.
5. How often should cable supports be placed?
Answer: Horizontal runs: supports at 400 mm intervals. Vertical runs: 300 mm intervals. Use cable ties that allow slight movement rather than rigid clamps that concentrate thermal movement at the connector. Also read our related guide: Insurance Conditions Requiring CCTV: When It Backfires.
Conclusion
The difference between a security system that works and one that frustrates is understanding the real-world behaviour of cameras, cables, and the environment they operate in. Manufacturers sell specifications. Installers solve problems. The questions above represent the issues that UK homeowners and businesses actually face — the ones the spec sheets do not mention.
Article by Gary Pearce, qualified security systems engineer. For a free security assessment, visit gary-pearce-home-services.pages.dev. This guide was last updated June 2026. Verify current UK regulations with the ICO.