Cable Tray Sizing Calculator

Width depends on the number of cables, their overall diameters and whether they are laid touching or spaced. Touching single layer: width = cable OD × number of cables; you must then apply a BS 7671 Table 4C1 grouping factor to the current ratings. Spaced single layer for no derating: the BS 7671 Table 4C1 note gives a grouping factor of 1.0 (no derating) only where the horizontal clearance between adjacent cables exceeds twice the cable overall diameter, so width = (OD × count) + (count − 1) × 2 × OD. That is a much wider tray than a touching layout but preserves the tabulated ratings. Allow margin for future additions and confirm cable ODs from the manufacturer datasheet.

There is no fixed regulatory spacing — the maximum span is set by the tray manufacturer's BS EN 61537 load/span table for your tray and load (the safe working load is reached when mid-span deflection hits span/100). As typical guidance only, lighter loads allow wider spans (around 1.5-2.0m) and heavier loads need closer supports (around 0.9-1.2m), with vertical runs supported more frequently (about 1.0m). Per the BEAMA Best Practice Guide, add a support within roughly 300-600mm of every bend, tee or other fitting on each side. Always size the span from the manufacturer's data, not a rule of thumb.

Yes, where possible. BS 7671 current-carrying capacity tables assume a single layer. If cables touch in a single layer on a perforated tray, BS 7671 Table 4C1 grouping factors apply: about 0.88 for 2 circuits, 0.82 for 3, 0.77 for 4, 0.75 for 5 and 0.73 for 6, falling further with more circuits. No grouping factor is needed where the horizontal clearance between adjacent cables exceeds twice their overall diameter (Table 4C1 gives a factor of 1.0 for that spaced arrangement). Adding a second layer worsens the derating substantially — apply the BS 7671 multi-tier/number-of-trays reduction (Table 4C4 for multicore cables). Single-layer also makes cables easier to trace.

SWA is heavier than non-armoured cable because of the steel wire armour. Approximate manufacturer values (Prysmian BS5467 datasheet) for 4-core XLPE/SWA/PVC are: 2.5mm² ≈ 0.47kg/m, 16mm² ≈ 1.45kg/m, 25mm² ≈ 2.06kg/m, 50mm² ≈ 2.875kg/m, 70mm² ≈ 4.25kg/m. So 10 × 4-core 25mm² SWA is about 20.6kg/m. These vary by maker — always confirm against the actual cable datasheet, and add the tray self-weight (typically 2-5kg/m) to the total loading.

Yes — metal cable tray must be earthed as it is an exposed-conductive-part. Per BS 7671, the tray should be bonded to the main earthing terminal using a separate CPC, or verified as providing a reliable low-impedance earth path throughout its length. Tray sections must be electrically continuous — use bonding links or earth straps at joints. The minimum CPC size depends on the circuit protection but is typically 4mm² minimum for the tray bonding conductor. Use our Bonding Conductor Calculator for correct bonding sizes.