Main low-voltage switchboard: sizing and multi-trade coordination
Three critical interfaces structure the switchboard layout
A main low-voltage switchboard finalised at the detailed-design stage rather than at the sketch stage generates significant on-site cost overruns and delivery delays on an office floor plate of 500 to 800 m²: NF C 15-100 part 5-51 imposes constraints (1.20 m clearance, ventilation, point load) that are non-negotiable once partitions are fixed. On a 600 m² floor plate hosting 70 workstations, the main low-voltage switchboard concentrates a subscribed power of 50 to 100 kVA, a mass of 300 to 800 kg and heat dissipation that can reach 15 kW at full load according to manufacturers’ technical datasheets. Kytom, an office fit-out specialist since 2006, addresses these trade-offs from the sketch stage onwards through a 4-phase design and build method that fixes the structural, HVAC and electrical interfaces before finalising the definitive plans.
The main low-voltage switchboard crystallises three simultaneous trade-offs in a French office, each involving a distinct trade.
- Structural interface: a switchboard’s mass ranges from 300 kg for 50 kVA up to 800 kg for 250 kVA according to manufacturers’ technical datasheets. This point load requires slab reinforcement that is rarely anticipated on an open floor plate. The load take-down must be validated by the structural engineering office before finalising the layout, failing which structural works will need to be redone.
- Thermal interface: dissipation reaches 15 kW for a 250 kVA switchboard at nominal load according to the manufacturer’s thermal assessment. Dedicated ventilation of the technical room is coordinated with the HVAC trade, failing which hot spots derate the protective devices within the meaning of NF C 15-100.
- Operational interface: the minimum 1.20 m frontal clearance, required by NF C 15-100 part 5-51, governs intervention safety. This constraint, often incompatible with floor area optimisation, must be entered on the plan from the preliminary design stage, never afterwards.
Kytom’s position, at odds with the profession’s prevailing practice: conventional wisdom treats the main low-voltage switchboard as a sub-component of the electrical trade, decided at the detailed-design stage. Our reading, drawn from several audits of third-party projects, is the opposite: above 50 kVA installed, the main low-voltage switchboard is a structural and HVAC matter before it is an electrical one. It is the layout that governs the sizing, not the other way around.
For the CFO and Asset Manager: what a poorly coordinated switchboard really costs
The switchboard trade-off is not a secondary technical matter, it is a direct line item on the works income statement and on the lease-up schedule.
- Direct on-site cost overrun: slab rework for an unanticipated point load, additional drilling for ventilation, partition relocation for regulatory clearance. Rework linked to poor switchboard coordination weighs significantly on the initial electrical trade budget, and handling coordination after the detailed-design stage causes delivery slippages of several weeks.
- Asset value at resale or refinancing: an undersized or uninstrumented switchboard weighs on the building’s environmental rating and on its capacity to host a tenant demanding in terms of GDPR compliance and environmental certification. It is a recurring audit point in due diligence.
The ratio is unfavourable: upstream coordination represents a marginal share of the works budget, whereas downstream rework weighs heavily on the electrical trade.
Honest caveat: these orders of magnitude apply to floor plate restructuring operations involving a change of partitioning. On a simple refresh without switchboard intervention, the issue does not arise.
Four recurring mistakes in a sequential approach
The coordination failures observed on office projects concentrate on four recurring points.
- Undersizing of outgoing cables: calculating solely on installed power without factoring in future extensions generates heavy rework. For a floor plate of 500 to 1,000 m² hosting 70 to 100 workstations, the subscribed power must cover the forecast load with sufficient headroom for event-related uses and a 5-year evolution.
- Layout in a hard-to-reach area: a switchboard relegated to the back of the technical room significantly lengthens preventive maintenance intervention times.
- Late validation with the structural trade: fixing the layout before the load note systematically triggers slab rework.
- Omitting heat dissipation: ignoring the room’s heat balance creates thermal drifts in the hot season and reduces component lifespan according to manufacturers’ data on the thermal endurance of modular circuit breakers.
Limits of this grid: this approach is not suited to a floor plate below 200 m² with fewer than 25 workstations and a subscribed power under 36 kVA. In that case, a distribution board suffices, the switchboard-as-project approach is oversized and the ROI of multi-trade coordination is not justified. The Kytom method integrates these four dimensions from the sketch stage onwards, from the threshold of 50 kVA installed.
4-phase coordinated design and build method
Integrating the switchboard structures four steps within the design and build process, each with an identified deliverable and a lead actor.
- Phase 1, needs audit: analysis of power demand by use (IT, lighting, HVAC, general services) and projection of evolution over 10 years. The flexibility of office spaces requires 25 to 35% oversizing on the main outgoing feeders.
- Phase 2, integrated pre-sizing: simultaneous calculation of switchboard, structure and ventilation to validate feasibility before the definitive plans. This step generates a consolidated set of reservations.
- Phase 3, multi-trade synthesis: a coordination meeting of structure, electrical, HVAC and plumbing, which validates the interfaces and the routing of technical ducts.
- Phase 4, operational validation: simulation of maintenance scenarios, extracting a circuit breaker, replacing a residual-current device, to confirm real accessibility beyond theoretical compliance.
| Phase | Deliverable | Lead actor |
|---|---|---|
| 1 Audit | 10-year power assessment | Electrical engineering office |
| 2 Pre-sizing | Load note + thermal assessment | Structural + HVAC engineering office |
| 3 Synthesis | Consolidated reservations plan | design and build synthesis |
| 4 Operation | Maintenance procedures | Future operator |
When this method does not apply: on a project to simply replace an existing switchboard like-for-like in a compliant dedicated room, phases 2 and 3 are reduced to a documentary check.
Frequently asked questions
From what subscribed power threshold is multi-trade switchboard coordination justified?
From 50 kVA installed, multi-trade switchboard coordination is fully justified. Below this threshold, on a floor plate under 200 m² with fewer than 25 workstations and a subscribed power under 36 kVA, a streamlined documentary check is generally sufficient and a distribution board meets the need.
