# Electrical Distribution in Buildings Electrical systems are the nervous system of any building — powering lighting, HVAC, vertical transportation, communications, security, fire detection, and the increasingly digital infrastructure of modern occupancy. For the practicing architect, electrical design affects core planning (riser locations and sizes), floor plate efficiency (distribution routes), ceiling and floor zone depths, facade coordination (lighting and power outlets), and the building's capacity to adapt to future technological change. The architect does not design the electrical system — that is the electrical engineer's domain — but must understand the spatial, structural, and coordination implications to produce a building that works. --- ## Table of Contents - [Power Supply and Intake](#power-supply-and-intake) - [Distribution Hierarchy](#distribution-hierarchy) - [Electrical Load Estimation](#electrical-load-estimation) - [Cable Distribution Routes](#cable-distribution-routes) - [Lighting Systems](#lighting-systems) - [Emergency and Standby Power](#emergency-and-standby-power) - [Lightning Protection](#lightning-protection) - [Earthing and Bonding](#earthing-and-bonding) - [Low Voltage and Extra-Low Voltage Systems](#low-voltage-and-extra-low-voltage-systems) - [Photovoltaic Systems](#photovoltaic-systems) - [Spatial Requirements](#spatial-requirements) - [See Also](#see-also) --- ## Power Supply and Intake Buildings receive electrical power from the local distribution network operator (DNO) at medium voltage (MV, typically 11kV in the UK) or low voltage (LV, 400/230V) depending on the building's maximum demand. | Building Type | Typical Max Demand | Supply Type | |--------------|-------------------|------------| | Residential (house) | 12-15 kVA | Single-phase LV (230V) | | Residential (apartment block, 50 units) | 200-400 kVA | Three-phase LV (400V) | | Office (5,000 m²) | 300-600 kVA | Three-phase LV or HV with on-site transformer | | Office (20,000 m²) | 1,000-2,500 kVA | HV (11kV) intake; on-site substations | | Hospital (major) | 3,000-10,000 kVA | HV ring main; multiple substations | | Shopping centre | 2,000-8,000 kVA | HV intake; distributed substations | **HV intake room**: For buildings with demand exceeding ~1,000 kVA, an on-site substation is required. This comprises an HV switchroom, transformer(s), and LV switchroom. These rooms must be at ground floor or basement level with direct external access for DNO maintenance and equipment replacement. Minimum room dimensions are typically 6m × 4m per transformer (varies by DNO requirements). --- ## Distribution Hierarchy Electrical power flows through a hierarchy of distribution equipment: ``` DNO supply → HV switchgear → Transformer(s) → LV main switchboard (MSB) → Submain distribution boards (SDBs) per floor/zone → Final distribution boards (DBs) per area → Final circuits → Socket outlets, lighting, equipment ``` | Level | Equipment | Location | |-------|-----------|----------| | **HV intake** | Ring main unit (RMU); HV switchgear | Ground/basement; external access | | **Transformer** | 11kV/400V; oil-filled or dry-type | Adjacent to HV room; ventilation required | | **Main switchboard (MSB)** | LV switchgear; circuit breakers; meters | LV switchroom adjacent to transformer | | **Busbar riser** | Vertical distribution through building | Electrical riser cupboard (each floor) | | **Submain DB** | Floor-level distribution boards | Electrical cupboard per floor | | **Final DB** | Local circuit breakers; RCDs | Within tenant/zone areas | --- ## Electrical Load Estimation **Rules of thumb for preliminary design** (W/m² of gross floor area): | Building Type | Small Power | Lighting | HVAC (electrical) | Total (approximate) | |--------------|------------|---------|-------------------|-------------------| | **Office — standard** | 25-30 | 10-14 | 30-60 | 65-100 | | **Office — high-spec** | 30-40 | 12-16 | 50-80 | 90-130 | | **Retail** | 15-25 | 15-25 | 30-50 | 60-100 | | **Hotel** | 10-15 | 8-12 | 25-45 | 45-70 | | **Residential** | 15-25 | 5-8 | 10-30 | 30-60 | | **Hospital** | 20-30 | 12-18 | 40-70 | 75-120 | | **Data centre** | 500-2,000 | 5-8 | 200-1,000 | 700-3,000 | | **School** | 15-20 | 8-12 | 20-40 | 45-70 | **Diversity factors**: Not all loads operate simultaneously. Apply diversity factors (0.4-0.8 depending on load type) to arrive at the maximum demand from the sum of connected loads. --- ## Cable Distribution Routes ### Vertical Distribution Electrical risers carry power, data, and communications cabling vertically through the building: | Building Scale | Riser Size (typical) | Location | |---------------|---------------------|----------| | Small (1,000-3,000 m² GFA) | 1.0 × 0.6m cupboard per floor | Core area | | Medium (3,000-10,000 m² GFA) | 1.5 × 1.0m cupboard per floor | Core; one per 1,500 m² floor plate | | Large (10,000+ m² GFA) | 2.0 × 1.5m+ per floor; multiple risers | Distributed through floor plate | Risers must be fire-stopped at every floor penetration (minimum 60-minute fire resistance). Riser cupboards require lockable access doors, adequate ventilation, and structural support for busbar/cable weight. ### Horizontal Distribution | Method | Description | Typical Application | |--------|------------|-------------------| | **Ceiling void (cable tray/basket)** | Cables on perforated trays in ceiling void | Standard commercial; concealed | | **Raised access floor** | Cables in 100-300mm floor void | Grade A offices; data centres; maximum flexibility | | **Floor boxes** | Recessed boxes in screed with power/data outlets | Offices without raised floor; open-plan | | **Skirting trunking** | Perimeter cable trunking at skirting level | Refurbishment; education | | **Surface trunking** | Surface-mounted PVC or metal trunking | Refurbishment; industrial | | **Underfloor busbars** | Busbar systems below raised floor | Trading floors; high-density flexible power | --- ## Lighting Systems ### Lamp Technologies | Technology | Efficacy (lm/W) | Life (hours) | CRI | Status | |-----------|-----------------|-------------|-----|--------| | **Incandescent** | 10-15 | 1,000 | 100 | Banned (EU/UK) | | **Halogen** | 15-25 | 2,000-4,000 | 100 | Phased out | | **Compact fluorescent (CFL)** | 50-70 | 8,000-15,000 | 80-90 | Legacy; declining | | **Linear fluorescent (T5/T8)** | 80-105 | 20,000-40,000 | 80-90 | Still common; LED replacing | | **LED** | 100-180 | 50,000-100,000 | 80-97+ | Current standard; dominant | **LED dominance**: LED is now the default specification for all new building lighting. Key selection parameters: colour temperature (2,700K warm to 6,500K cool daylight), CRI (minimum 80; 90+ for retail/healthcare/hospitality), luminaire efficacy (lm/W of complete fitting), driver lifetime, and dimmability. ### Lighting Design Criteria | Space | Maintained Illuminance (lux) | UGR Limit | Uniformity (U₀) | |-------|----------------------------|-----------|-----------------| | Office — general | 300-500 | ≤19 | ≥0.60 | | Office — task area | 500 | ≤19 | ≥0.70 | | Classroom | 300-500 | ≤19 | ≥0.60 | | Corridor | 100 | ≤25 | ≥0.40 | | Retail — general | 300 | ≤22 | ≥0.40 | | Hospital — ward | 100 (general); 300 (examination) | ≤19 | ≥0.40 | | Residential — living | 150-300 | — | — | Standards: EN 12464-1 (interior workplaces); CIBSE SLL Lighting Guide; IESNA Lighting Handbook. See [[Illumination Level Standards]] and [[Daylighting Design Principles]]. ### Lighting Controls | Control Type | Energy Saving | Application | |-------------|--------------|------------| | **Occupancy/absence detection** | 20-40% | WCs; meeting rooms; corridors | | **Daylight dimming** | 20-60% | Perimeter zones near windows | | **Task/ambient lighting** | 15-30% | Open offices with personal task lights | | **Scene setting** | Variable | Hospitality; conference; residential | | **Centralised time scheduling** | 10-20% | All spaces; via BMS | | **DALI digital dimming** | Enables all above | Standard protocol for digital lighting control | --- ## Emergency and Standby Power ### Emergency Lighting Required by building regulations to illuminate escape routes and open areas during power failure: | Type | Duration | Illuminance | Application | |------|----------|------------|------------| | **Maintained** | 3 hours (minimum) | 1 lux (escape routes); 0.5 lux (open areas) | Cinemas; places of entertainment | | **Non-maintained** | 3 hours | 1 lux (escape routes) | Offices; most buildings | | **Standby** | Generator-backed | Near-normal levels | Hospitals; data centres | Standard: BS 5266 (UK); EN 1838 (Europe). ### Standby Generation | Application | Generator Requirement | Transfer Time | |------------|----------------------|---------------| | **Life safety** (fire systems, emergency lighting, lifts) | Essential; statutory | 5-15 seconds (automatic) | | **Hospital** (Category 1) | Full essential services | <15 seconds | | **Data centre (Tier III+)** | Full building load | <10 seconds; UPS bridges gap | | **Commercial office** | Typically life safety only | 10-15 seconds | | **Residential high-rise** | Life safety; firefighter's lift | 10-15 seconds | **Uninterruptible Power Supply (UPS)**: Battery-backed systems providing continuous power during the seconds between mains failure and generator start-up. Essential for data centres, operating theatres, and critical IT equipment. --- ## Lightning Protection Buildings require lightning protection risk assessment per BS EN 62305. Protection comprises: - **Air termination**: Roof-level conductors or rods to intercept lightning strikes - **Down conductors**: Copper or aluminium conductors from roof to ground (minimum 2; more for larger buildings) - **Earth termination**: Buried earth electrodes or foundation earthing - **Surge protection**: SPDs (surge protection devices) at main intake and distribution boards **Protection levels** (BS EN 62305-1): | Level | Rolling Sphere Radius | Typical Application | |-------|----------------------|-------------------| | I | 20m | Hospitals; explosive stores | | II | 30m | Museums; important buildings | | III | 45m | General commercial; industrial | | IV | 60m | Low-risk buildings | **Architectural impact**: Down conductors require discrete routing on the facade — coordinate with facade design early. Flat roofs need air termination networks. Tall buildings attract more strikes and require higher protection levels. --- ## Earthing and Bonding Earthing protects against electric shock and enables protective devices to operate correctly: - **TN-S system**: Separate neutral and earth from DNO supply — preferred for new commercial buildings - **TN-C-S system**: Combined neutral-earth from DNO; separate within the building — most common UK supply - **TT system**: Earth rod at the building; no DNO earth — rural installations All extraneous metalwork (structural steel, pipework, cladding supports) must be bonded to the main earthing terminal. Supplementary bonding in bathrooms and swimming pools per BS 7671. --- ## Low Voltage and Extra-Low Voltage Systems Beyond power and lighting, modern buildings require extensive low-voltage (LV) and extra-low-voltage (ELV) systems: | System | Category | Typical Infrastructure | |--------|----------|----------------------| | **Structured cabling (data/voice)** | ELV | Cat 6A/fibre; floor outlets; comms rooms | | **Fire detection and alarm** | ELV | Detectors; sounders; control panel; cause-and-effect | | **Security (CCTV, access control)** | ELV | Cameras; readers; controllers; recording | | **Intruder alarm** | ELV | Sensors; keypads; monitoring | | **Public address / voice alarm** | ELV | Speakers; amplifiers; fire voice evacuation | | **Disabled refuge** | ELV | Two-way communication at refuges | | **Nurse call** | ELV | Patient call system; healthcare specific | | **AV systems** | ELV | Displays; projectors; conferencing | | **BMS** | ELV | Sensors; actuators; controllers; head-end | **Communications rooms**: Typically one per floor, minimum 3m × 3m, air-conditioned, with dedicated power supply and cable routes to all floor outlets. Main equipment room (MER) at ground/basement level for incoming telecoms, server racks, and main patching. --- ## Photovoltaic Systems | Parameter | Typical Value | |-----------|--------------| | Module efficiency | 18-22% (monocrystalline silicon) | | System yield (UK) | 800-1,000 kWh/kWp/year | | System yield (Southern Europe/India) | 1,200-1,800 kWh/kWp/year | | Area per kWp | 5-7 m² (roof-mounted) | | Weight | 10-15 kg/m² (with mounting) | | Inclination (UK optimum) | 30-40° south-facing | | Design life | 25-30 years (80% output guarantee) | **Integration options**: Roof-mounted (ballasted or fixed); building-integrated PV (BIPV) as cladding, shading, or glazing; car park canopies. Coordinate with structural engineer for roof loading and with facade design for BIPV integration. See [[Net Zero Carbon Buildings]] and [[BREEAM Assessment Framework]] for renewable energy requirements. --- ## Spatial Requirements **Summary of key electrical space allowances**: | Space | Minimum Size | Access Requirements | |-------|-------------|-------------------| | HV switchroom | 6 × 4m (per transformer) | External access; DNO approval | | LV switchroom | 4 × 3m (typical) | Adjacent to transformer | | Electrical riser | 1.0 × 0.6m to 2.0 × 1.5m per floor | Lockable; fire-stopped | | Generator room | 15-40 m² per 500 kVA | Exhaust route; fuel storage; vibration isolation | | UPS room | 10-25 m² | Air-conditioned; battery ventilation | | Communications room | 3 × 3m minimum per floor | Air-conditioned; dedicated power | | Main equipment room | 15-40 m² | Ground/basement; telecom entry | --- ## See Also - [[HVAC Fundamentals]] - [[Fire Safety in Building Design]] - [[Office Building Design]] - [[Hospital Design Principles]] - [[Net Zero Carbon Buildings]] - [[BIM Fundamentals and LOD]] - [[Illumination Level Standards]] --- #services #electrical #lighting #power #emergency #pv