Cable trays and trunking: sizing and scalability

Sizing pits two contradictory imperatives against each other: controlling immediate cost and preserving future flexibility. The trade-off rests on four measurable criteria.

• Initial fill ratio: 30 to 40% for low-current systems (data, voice), 50 to 60% for lighting (NF C 15-100 part 5-52 thresholds and UTE C 15-105 guide).
• Space typology: scalable open space, partitioned offices, equipped meeting rooms, technical rooms.
• Reconfiguration frequency: every 18 months in flex offices, every 5 to 7 years in fixed offices.
• Architectural constraints: usable ceiling height, false-ceiling pathways, vertical technical ducts.

On a commercial floor of 500 to 1,000 m² accommodating 70 to 100 workstations, the subscribed power generally ranges between 50 and 100 kVA, with a forecast load representing a significant fraction of this capacity, a parameter to be refined during the initial electrical assessment. flex office spaces require oversizing coefficients of 60 to 80% versus 30 to 50% for traditional offices.

Kytom contrarian position. Industry conventional wisdom recommends a uniform reserve coefficient of 30% on all commercial projects. Our reading differs: a single coefficient under-protects flex offices, which require significantly higher oversizing, and over-costs floors with stable use, where a modest reserve suffices. The right reflex is to calibrate the reserve to the anticipated reconfiguration frequency, not to an industry standard.

When this oversizing logic is NOT relevant. On floors below 200 m² intended for stable use (notary’s office, medical practice, law firm with no planned densification), a reserve coefficient above 30% becomes an extra cost with no payback: the probability of reconfiguration at 7 years is too low to amortise the oversizing. The tipping threshold sits around 4 expected reconfigurations over 10 years.

For the CFO and the Asset Manager: converting oversizing into avoided cash-flow

The « cable trays » topic seems technical. Above all, it is an initial CAPEX vs 10-year operating OPEX trade-off, legible by the finance department and the asset manager.

CFO reading. On a commercial electrical package of 500 to 1,000 m², the additional cost of scalable sizing remains limited compared with the cost of rework in an occupied site, a favourable CAPEX/OPEX trade-off over the lease lifetime. By contrast, reworking cable trays in an occupied site represents a significant share of the package’s initial budget, on top of which comes the indirect cost rarely quantified: partial immobilisation of the floor for 3 to 8 working days.

Asset Manager reading. A floor whose cable trays support significant densification without structural rework retains its rental liquidity: the incoming tenant does not trigger an additional works phase, which reduces the rent-free period negotiated at signing. The asset value factors in this residual flexibility in the DCF, via a lowered structural vacancy rate.

Architect / IRB reading. Usable ceiling height and coordination with HVAC are the two architectural constraints that make rework retroactively costly. Anticipating false-ceiling pathways with a 30% reserve at the design stage preserves the spatial quality delivered to the tenant.

Three recurring mistakes: multi-technical conflicts and construction rework

The analysis of our electrical distribution projects reveals three sizing pitfalls, drawn from our field experience.

1. Sizing for immediate need: ignoring foreseeable changes creates bottlenecks from the very first reconfiguration. The fill ratio can be reached quickly on a densified open space, making any reconfiguration immediately constrained.
2. Coexistence of high-current and low-current systems: NF C 15-100 (part 5-52, §528) and the NF EN 50174-2 standard require a separation between energy conductors and unshielded data cables, whose value depends on the parallel run length and shielding. Underestimating it generates electromagnetic interference and forces tray rework.
3. Lack of multi-fluid coordination: collisions with HVAC, sprinklers and plumbing are discovered during the works phase, triggering variations whose cost can quickly exceed several thousand euros per impacted zone.

Good practice consists of establishing a coordinated reservation plan from the sketch stage, integrating evolution scenarios at 5 and 10 years. Kytom maps flows by use typology and anticipates power needs according to the equipment planned (computer workstations, screens, videoconferencing units, mobile charging). This anticipation avoids most of the rework observed on undersized projects and secures the works schedule milestones.