In a commercial building the low-voltage board is where the electrical design becomes visible to everyone else. Landlord supplies, tenant distribution, lifts, HVAC, kitchen equipment, IT and EV charging all leave from the same room, and every one of them needs its own way of being isolated, measured and protected. The equipment itself is not exotic. What takes time is deciding the arrangement early enough that the room still fits and the board can be extended later. This note runs through the decisions that matter, and it is written for the consultant or contractor specifying the LV switchgear and distribution panels for a commercial building.
What the LV Board Has to Do
The board takes its supply from the building transformer – typically an 11 kV or 20 kV primary with a 400 V secondary, and usually a dry-type or cast resin unit because the substation sits inside the building – and distributes it to the landlord and tenant loads. What goes out determines what goes in.
Mechanical loads bring large starting currents. Lighting and small power are numerous and spread over many ways. Life-safety and essential supplies have to stay up when the rest does not. Tenant sections usually need separate billing. Each of those puts a different requirement on the board, which is why the load list matters more than the total kVA. The transformer feeding it is part of the same conversation; our dry-type and cast resin range is what normally goes into an indoor substation of this kind.
Incomer Arrangement and Board Layout
Most commercial boards run a single incomer. Larger buildings, and anything with an essential supply, run two, with a bus section or an automatic changeover between them. The busbar rating is set from the connected load with diversity applied, and it is worth agreeing that diversity figure in writing before the board is built.
The part that gets decided too late is the outgoing ways. A board that is full on day one is a board that gets replaced early, so we leave spare ways and reserve capacity, and we settle how many with you at design stage rather than assuming a number. Landlord and tenant sections are kept physically separate, so that one can be worked on without isolating the other.
Protection, Metering and Monitoring
Protection is chosen so the device nearest a fault clears it and nothing upstream does. That is a discrimination study, and it is far cheaper to do before the board is built than to discover a problem during commissioning. Air circuit breakers at the incomer, moulded-case breakers on the larger outgoing ways, and residual-current protection where the local code requires it.
Metering is usually split by function: landlord, tenant billing, and sub-metering for major plant. If the building runs a BMS or an energy platform, the board is fitted with the communication interface and the metering the platform specifies. That side of the work is described on our switchgear control page.
Fixed or Withdrawable
Withdrawable cassettes let a device be isolated or swapped without disturbing the rest of the board, which matters on an incomer or a critical feeder. For final distribution, fixed devices cost less, take less depth and are perfectly serviceable.
Most buildings end up with a mixture: withdrawable at the top of the board where downtime is expensive, fixed further down where space and budget matter more.
Space, Ventilation and Access
Boards produce heat, and so does the transformer feeding them. The room needs ventilation to remove it, space for cables to enter and bend without being forced, and clearance at the front and rear for someone to work safely. Cable entry from above or below changes how the board is built.
So does the route into the room. A board that cannot be brought through the door is a problem nobody wants to solve on site, and it is entirely avoidable if the access is known before the board is drawn. These details are fixed by the room dimensions you give us.
Standards, Testing and Documentation
Assemblies are built and verified to the standard the project names, with the routine verification done before dispatch and the records issued with the board. Alongside the equipment you get the general arrangement drawings, the schematics, the test reports and the manuals.
Where a client wants to witness testing, that is arranged in advance. Our testing page sets out what a board goes through before it leaves the workshop.
Frequently Asked Questions
How many spare ways should we allow?
Enough that the board is not full on day one. The number follows from the load list and the occupancy plan rather than a rule of thumb, so we settle it with you at design stage and build the reserve into the board.
Fixed or withdrawable: which do we need?
Withdrawable where a shutdown is expensive or the device may need to be swapped – incomers and critical feeders. Fixed for final distribution, where space and cost matter more. Most commercial boards use both.
Can tenant metering go in the same board?
Yes. Tenant metering is usually built as a separate section or an adjacent board with its own isolation, so the meters can be read and worked on without taking the landlord supply down.
What standard do you build the assemblies to?
The one the project specifies. We build and verify to the named standard and issue the verification records with the board, rather than claiming a certification the project has not asked for.
How should we allow for EV charging?
By leaving capacity and spare ways in the board, and by agreeing the charger loads before the board is built. Retrofitting capacity into a full board is the expensive version of this decision.
Send the single-line diagram, the load list with diversity, the transformer rating and impedance, the room dimensions and access, and how the metering needs to be split. We come back with a proposed arrangement, the standard set we would work to, and a quotation. Email info@xsdfftransformer.com or call +8615867897761.