Cable Tray & TrunkingSize Calculator
Calculate the correct cable tray or trunking size with BS 7671 space factor compliance, cable segregation warnings, and support spacing recommendations.
1. Select Containment Type
2. Installation Options
Standard: 45% (trunking), 40% (tray)
3. Add Cables
Important Disclaimer
This calculator is provided for informational and educational purposes only. While we strive for accuracy and compliance with BS 7671:2018+A2:2022 regulations, calculations should be verified by a qualified electrician.
Professional Verification Required: All electrical work must be designed, installed, tested, and certified by a competent person in accordance with BS 7671 (18th Edition) and Building Regulations Part P.
No Liability: London Electrical Distributors and its contributors accept no responsibility for any loss, damage, or injury arising from the use of this calculator. Results are estimates and may vary based on specific installation conditions, environmental factors, and local regulations.
Use at Your Own Risk: By using this tool, you acknowledge that electrical design and installation carries inherent risks and should only be undertaken by qualified professionals with appropriate insurance and certification.
Professional Features
BS 7671 Space Factor Compliance
Automatic compliance with IET On-Site Guide recommendations: 45% for trunking, 40% for cable tray, ensuring adequate heat dissipation and ease of installation.
Cable Segregation Warnings
Critical warnings for Band I/II mixing and fire alarm segregation per BS 7671 Regulation 528 and BS 5839, with practical solutions for compliance.
Comprehensive Cable Database
Includes Twin & Earth, Single Core PVC, SWA 3/4-core, Data cables (Cat5e/6/6a), and Fire Alarm (FP200) with accurate outer diameters and cross-sectional areas.
Support Spacing Guidance
Automatic support spacing recommendations per BS EN 61537: 3m for ladder tray, 1.5m for perforated/solid tray, 1.2m for trunking (horizontal and vertical).
Alternative Size Options
View next-size-up alternatives for future expansion allowance with spare capacity calculations and reduced fill percentages for easier cable additions.
Installation Location Warnings
Outdoor installation warnings for PVC UV degradation, IP rating requirements (minimum IP54), and weather-resistant containment recommendations.
Understanding Cable Tray & Trunking Sizing
What is Space Factor?
Space factor (also called fill factor) is the ratio of the total cross-sectional area of all cables to the internal cross-sectional area of the containment, expressed as a percentage. The IET On-Site Guide specifies maximum space factors to ensure: adequate heat dissipation from current-carrying cables, ease of cable drawing-in during installation, and allowance for future cable additions.
Why Are Fill Factors Important?
- Heat Dissipation: Cables generate heat when carrying current; air space allows cooling
- Installation Ease: Over-filled containment makes cable pulling extremely difficult and risks insulation damage
- Cable Derating: Tightly packed cables require derating factors, reducing their current-carrying capacity
- Future Expansion: Leaving spare capacity avoids costly containment upgrades later
Containment Type Selection
Trunking (45% fill): Fully enclosed, best for concealed installations, dust/moisture protection, and aesthetic installations. Used indoors in commercial buildings, offices, and retail.
Perforated Tray (40% fill): Balance of support and ventilation, cost-effective for medium loads. Common in commercial ceiling voids and plant rooms.
Ladder Tray (40% fill): Heavy-duty, excellent ventilation, 3m support spacing. Ideal for large SWA cables, outdoor installations, and industrial environments.
Cable Segregation Requirements
BS 7671 Regulation 528.1 requires segregation between Band I (ELV ≤50V) and Band II (LV 50-1000V) circuits. Band I includes data cables, fire alarm, telecommunications, and ELV control. Band II includes all mains power circuits (230V/400V). Mixing can cause electromagnetic interference affecting data transmission and fire alarm reliability.
Fire alarm cables (BS 7671 Reg 528.1.4, BS 5839) MUST be segregated from ALL other cables, typically using dedicated red trunking for identification, or maintaining 100mm minimum separation on separate trays.
Support Spacing Standards
BS EN 61537:2007 specifies deflection limits (L/100 for length, W/20 for width) at safe working load. Support spacing must prevent excessive deflection and ensure structural integrity. Ladder tray's strong side rails allow 3m horizontal spacing. Perforated and solid trays need 1.5m spacing. Trunking requires 1.2-1.5m spacing (steel) or 1.0m (PVC) due to material properties.
Frequently Asked Questions
What is the space factor for cable trunking in the UK?
According to the IET On-Site Guide, the space factor for trunking should not exceed 45% of the internal cross-sectional area. This means cables (including insulation and sheath) should occupy no more than 45% of the trunking's internal area, leaving 55% as air space. This allows for: 1) Heat dissipation from current-carrying cables, 2) Ease of cable installation (drawing-in), 3) Future cable additions. For conduit, the limit is 35%, and for cable tray, 40% is recommended (up to 50% in some applications).
Do Band I and Band II cables need to be segregated?
Yes, BS 7671 Regulation 528.1 requires Band I (extra-low voltage ≤50V AC) and Band II (low voltage >50V AC up to 1000V AC) circuits to be segregated unless: 1) All cables are insulated for the highest voltage present, 2) An earthed metal screen or partition separates them, or 3) They are in separate containment systems. Band I includes data cables, fire alarm, telecommunications. Band II includes mains power (230V/400V). Mixing without proper segregation can cause electromagnetic interference and safety issues.
Why is fire alarm cable segregation critical?
BS 7671 Regulation 528.1.4 and BS 5839 Section 26.1 mandate that fire alarm cables MUST be segregated from ALL other cables, including other Band I circuits. This prevents: 1) Electromagnetic interference from other circuits affecting fire alarm operation, 2) Fire in other circuits compromising life safety systems, 3) Voltage transients causing false alarms or system malfunction. Solutions include dedicated fire alarm trunking (typically red for identification), minimum 100mm separation on separate trays, or earthed metal partitions.
What are the support spacing requirements for cable tray and trunking?
Per BS EN 61537 and manufacturer recommendations: Ladder tray: 3m horizontal, 2m vertical (strong side rails allow wider spacing), Perforated/solid tray: 1.5m horizontal, 1.5m vertical (standard for medium duty), Metal trunking: 1.2-1.5m horizontal, 1.5m vertical (steel can use 1.5m), PVC trunking: 1.0m horizontal, 1.2m vertical (requires closer spacing due to material properties). Always check manufacturer specifications and consider cable weight, installation environment, and local regulations.
How is cable cross-sectional area calculated for space factor?
The space factor calculation uses the overall cable cross-sectional area, not just the conductor size. For round cables: Area = π × (diameter/2)². For flat cables (like Twin & Earth): Use effective diameter from width × height dimensions. Our calculator includes accurate dimensions for: Twin & Earth (6242Y), Single core PVC (6491X), SWA 3-core and 4-core, Data cables (Cat5e/6/6a), Fire alarm cables (FP200). The total cable area is then divided by the fill factor percentage to determine required containment area.
Can I mix data cables and power cables in the same cable tray?
While technically possible if properly segregated per BS 7671 Regulation 528.1, it is not recommended due to electromagnetic interference (EMI). Power cables can induce noise in data cables, affecting transmission quality, especially at higher data rates (Cat6/6a for 10Gbps). Best practices: 1) Use separate containment systems for power and data, 2) If sharing is unavoidable, use shielded data cables (STP) and maintain maximum physical separation, 3) Cross power and data cables at 90° angles only, 4) Consider fibre optic cables for complete immunity to EMI, 5) Install earthed metal partition for segregation.
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