The space factor for trunking, specifically when the type and size of the conductor are not identified in Table E5 and Table E6 as per the provided reference, should not exceed 45% of the internal CSA (Cross-Sectional Area).
Understanding the space factor is crucial in electrical installations, particularly when planning cable routes using containment systems like trunking.
What is Space Factor?
Space factor is essentially a measure of how full a containment system is with cables. It represents the ratio of the total cross-sectional area occupied by the cables to the internal cross-sectional area of the containment (like trunking or conduit).
- Low Space Factor: More free space, easier for pulling cables, better heat dissipation.
- High Space Factor: Less free space, potentially more difficult to install cables, increased risk of overheating due to reduced air circulation.
The 45% Rule for Trunking
According to the provided reference, a key guideline for calculating cable capacity in trunking relates directly to the space factor. The rule states:
It is important to remember that where the type and size of the conductor are not identified in Table E5 and Table E6, the space factor should not exceed 35% for conduit and 45% of the internal CSA for trunking.
This means that, under the conditions mentioned (when specific tables E5 and E6 are not used for reference on conductor type and size), the total cross-sectional area of all the cables installed inside a piece of trunking must not take up more than 45% of the internal area available within the trunking itself.
Why is this limit important?
Limiting the space factor to 45% in this context helps ensure:
- Sufficient space remains for easier installation and future modifications.
- Adequate air circulation around the cables to dissipate heat, preventing potential overheating and damage.
- Compliance with electrical safety standards.
| Containment Type | Space Factor Limit (when not using Tables E5/E6) |
|---|---|
| Conduit | 35% |
| Trunking | 45% of the internal CSA |
It's important to note that specific regulations and standards (like BS 7671 in the UK) provide detailed tables and calculation methods for determining cable capacity based on various factors, including cable type, size, containment type, and installation environment. The 45% rule highlighted in the reference applies under a specific condition related to referencing Tables E5 and E6, suggesting it might be a default or simplified approach when more detailed data from those tables isn't being used.
Understanding and applying the correct space factor is a fundamental part of designing safe and effective electrical installations.
