LV 400 V drop
11.0 %
Figure 4-0 · The electrical chain. Each box is a section below. (Original schematic.)
Section layout
Role → Design method & formula → Worked numeric example → In the software → Code reference. Core formula throughout: three-phase power P = √3 · V · I · pf. Constants, symbols & conversions used in the worked examples
| Symbol | Value / unit | What it is |
|---|---|---|
| √3 | = 1.732 | The three-phase factor. |
| P / S / pf | kW / kVA / 0–1 | Real power / apparent power / power factor. S = P ÷ pf. |
| V / I | volts / amps | Line voltage / current. |
| %Z | e.g. 6% | Transformer impedance — limits fault current. |
| E / Φ / UF / MF | lux / lumens / 0–1 / 0–1 | Target light level / lamp output / utilisation & maintenance factors (lumen method). |
Key formulas: current I = S ÷ (√3·V) · fault Ifault ≈ Ifull-load ÷ (%Z÷100) · volt-drop Vd = (mV/A/m × I × L) ÷ 1000 · luminaires N = (E·A) ÷ (Φ·UF·MF)
4.1 Load estimate & demand
Role: total the connected load, apply diversity/demand factors, and add spare capacity — this sizes the supply, transformers and generators.
Worked example 4.1 · Building maximum demand
Given: stack of floors 100,000 m²; load density (lighting+power+HVAC+VT) ≈ 100 W/m² connected; diversity 0.8; pf 0.9; +15% spare.
Connected = 100,000 × 100 = 10,000 kW; demand = 10,000 × 0.8 = 8,000 kW.
Apparent power: S = P ÷ pf = 8,000 ÷ 0.9 = 8,889 kVA; +15% spare ≈ 10,200 kVA (~10.2 MVA).
~10 MVA design demand for this stack — sets the substation, transformer count and standby sizing. (A full tower is tens of MVA, taken at HV.)
Code
Demand/diversity: SBC 401, IEC 60364-1 / NEC Art. 220; energy: SBC 601 / ASHRAE 90.1.HV INTAKE→transformers→LV gear→risers→boards→socket
4.2 HV intake & substation
Role: the utility (e.g. SEC) delivers at high/medium voltage (e.g. 13.8/33 kV) into the building's main substation — HV switchgear, metering, protection. Tens of MVA can only enter at HV; bringing it in at LV would need impossibly large cables.
Why HV up the tower
Power = √3·V·I. At higher voltage the current for the same power is far lower → much smaller conductors and far less voltage drop. So MV is distributed up the tower to local transformer rooms — the central idea of tall-building electrical design (mirrors how HVAC/water are zoned). Code
Utility interface: SEC distribution code; HV switchgear: IEC 62271; substation: SBC 401, IEC 61936.4.3 Transformers
Role: step MV down to LV (400/230 V). Distributed as unit substations on plant floors up the tower (dry-type/cast-resin indoors for fire safety). Sized in kVA with N+1.
Worked example 4.3 · Transformer count
Given: 10.2 MVA demand; standard 2,500 kVA dry-type units; loaded to ~80%.
Usable per unit ≈ 2,000 kVA → number = 10,200 ÷ 2,000 = 5.1 → 6 units; add 1 spare per group for N+1.
~6–7 × 2,500 kVA cast-resin transformers in distributed substations up the tower, each feeding its zone's LV switchboard. Provide ventilation/cooling for transformer rooms (a heat load back to HVAC).
In the software
Model the whole network in ETAP (or SKM/DIgSILENT): enter sources, transformers, cables, loads → run load-flow (sizing & voltage), short-circuit (breaker duty) and protection coordination. Transformer impedance feeds the fault study. Code
Transformers: IEC 60076; dry-type IEC 60076-11; losses/efficiency: SBC 601 / SASO; rooms: SBC 401/801.4.4 Main LV switchgear & fault level
Role: the main LV board distributes transformer output to risers, with protective devices (ACBs/MCCBs) whose breaking capacity must exceed the prospective short-circuit current. Type-tested assemblies to IEC 61439.
Worked example 4.4 · Fault level & breaker duty
Given: 2,500 kVA transformer, impedance Z = 6%, LV 400 V.
Full-load current I = S ÷ (√3·V) = 2,500,000 ÷ (1.732 × 400) = 3,608 A.
Symmetrical fault ≈ I ÷ Z = 3,608 ÷ 0.06 = 60.1 kA.
Specify switchgear & breakers with breaking capacity ≥ ~65 kA at the main board. Verify with the ETAP short-circuit study (multiple transformers in parallel raise the fault level).
Code
Assemblies: IEC 61439; breaking capacity & protection: IEC 60947 / 60364; SBC 401.4.5 Risers & busbar trunking (busways)
Role: carry power up the tower from substations to floor boards. Large vertical feeders use busbar trunking (busway) with floor tap-offs — compact, fire-rated, and easy to tap, far better than bundles of cables in a tall riser.
Tall-building detail
Busways need expansion units to absorb building/thermal movement over height, fire barriers at floor crossings, and seismic support (SBC 301). Code
Busway: IEC 61439-6; fire barriers & firestop: SBC 801 / UL systems.4.6 Cable / feeder sizing — current, volt-drop, fault
Role: size every feeder for three checks: current-carrying capacity (with derating), voltage drop, and short-circuit withstand. In a tall building, voltage drop on long risers is the tough one.
Voltage drop up a riser — LV vs MV for the same power
For the same power, voltage drop scales with 1/V², so sending it at medium voltage (11 kV) instead of low voltage (400 V) cuts the drop by a factor of ~750. That is why transformers are pushed up the tower and only short LV runs serve each zone. Drag the riser height to mark the drop on each curve.
Cable run from the supply point to the load up the tower.
Typical limit ~3% (final circuits) to ~5% (total). The LV curve is judged against this.
MV 11 kV drop
0.015 %
LV ÷ MV
756 ×
LV verdict
Approximate volt-drop vs riser length for the same power at LV and MV, with your length marked on both curves. The dashed line is the acceptable-drop limit.
Worked example 4.6 · Feeder current & volt-drop check
Given: a 250 kVA board fed at 400 V over 40 m; cable volt-drop 0.6 mV/A/m.
Current I = 250,000 ÷ (√3 × 400) = 361 A.
Volt-drop = (0.6 mV/A/m × 361 A × 40 m) ÷ 1000 = 8.66 V = 8.66 ÷ 400 = 2.2% (< 4–5% limit).
Cable passes volt-drop; also confirm its derated ampacity ≥ 361 A and short-circuit withstand from the fault study. Limits: total volt-drop typically ≤ 4% (power) / 3% (lighting).
In the software
ETAP/SKM auto-sizes cables checking ampacity (with grouping/temperature derating), voltage drop and short-circuit withstand, and runs protection-coordination (TCC) curves. Containment routed/clashed in Revit + Navisworks. Code
Ampacity/derating & volt-drop: IEC 60364-5-52, SBC 401, NEC Art. 310; volt-drop limits: SBC 401 / CIBSE.4.7 Distribution boards & final circuits
Role: floor/zone distribution boards split power into final circuits (lighting, sockets, mechanical, etc.), each protected by an MCB/RCBO sized to the cable and load, with earth-fault/RCD protection for socket and wet-area circuits.
Code
Final circuits, RCD/earth-fault: IEC 60364-4-41, SBC 401, NEC; boards IEC 61439-3.normal supply+STANDBY: GENERATORS + UPS
4.8 Standby power — generators & UPS
Role: keep life-safety and essential loads running when the grid fails — fire pumps, smoke-control fans, firefighting lifts, emergency lighting, fire alarm, BMS, and selected services. Diesel generators (with fuel storage) provide standby; UPS bridges the gap and protects critical/IT loads with no break.
Worked example 4.8 · Generator sizing for essential load
Given: essential load = 30% of the 8,000 kW demand = 2,400 kW; genset loaded to 0.8; pf 0.8.
kW size = 2,400 ÷ 0.8 = 3,000 kW; kVA = 3,000 ÷ 0.8 = 3,750 kVA.
Use e.g. 3 × 1,500 kVA gensets (N+1), with motor-start (transient) and the fire-pump starting kVA checked. Fuel for the required run-time per code.
~3 × 1,500 kVA standby gensets; size fuel storage for the code duration; UPS (e.g. 10–15 min) for FA/BMS/IT and any no-break load. Verify start transients in ETAP.
Code
Emergency/standby power: NFPA 110 (gensets), NFPA 111 (UPS/stored energy), SBC 401/801; life-safety loads: NFPA 101.boards→LIGHTING (the fitting)
4.9 Lighting — the fitting in the room
Role: deliver the design illuminance (lux) at the right quality (uniformity, glare UGR, colour) and low energy. Plus emergency lighting on escape routes. Designed by the lumen method, then verified by point calculation.
Worked example 4.9 · Lumen method (office)
Given: office 100 m²; target 400 lux; luminaire 4,000 lm; utilisation factor UF 0.6; maintenance factor MF 0.8.
N = (E × A) ÷ (Φ × UF × MF) = (400 × 100) ÷ (4,000 × 0.6 × 0.8) = 40,000 ÷ 1,920 = 20.8 → 21 luminaires.
~21 luminaires on a uniform grid; verify uniformity & UGR in DIALux, and check lighting power density against the energy code.
In the software
Model rooms in DIALux evo (or Relux) with real photometric (IES/LDT) files → it computes lux, uniformity, UGR and the lighting power density to check against ASHRAE 90.1 / SBC 601. Emergency lighting verified for escape-route lux & duration. Code
Illuminance & quality: EN 12464-1 / IES; LPD limits: ASHRAE 90.1 / SBC 601; emergency lighting: NFPA 101 / EN 1838.4.10 Earthing & lightning protection
Role: a building-wide earthing/grounding system (equipotential bonding, earth electrodes, low earth resistance) for safety and equipment, plus a lightning protection system (LPS) — essential on a very tall structure that is a prime strike target.
| System | Design point | Code |
|---|---|---|
| Earthing/grounding | Low earth resistance, full equipotential bonding, dedicated clean earth for IT | IEC 60364-5-54, IEEE 142/1100 |
| Lightning protection | Air terminals/mesh, down-conductors, ring earths; protection level from risk assessment | IEC 62305 / NFPA 780 |
| Surge protection (SPD) | At intake & sub-boards to protect electronics | IEC 62305-4 / IEC 61643 |
Code
Earthing: IEC 60364-5-54, IEEE 142/1100; LPS: IEC 62305 / NFPA 780; SPD: IEC 61643.4.11 Installation, accessories & field tricks
| Item | Field rule / trick |
|---|---|
| Cable containment (tray/ladder/trunking/conduit) | Size for ~40% fill + spare; segregate LV/ELV/fire cabling to avoid interference. |
| Fire-rated & fire-survival cabling | Life-safety circuits (fire pumps, smoke fans, FA) on fire-rated cable maintaining circuit integrity. |
| Firestopping at penetrations | Tested/listed firestop at every rated-barrier crossing (busways & trays). |
| Busway expansion & seismic support | Expansion units for building movement; bracing per SBC 301. |
| Labelling, phase identification, as-builts | Clear circuit schedules & labelling for safe maintenance. |
| Segregation of services in risers | Keep power away from sensitive ELV/data; bond cable armour & trays. |
| Spare ways & capacity | Leave ~20–25% spare ways in boards & containment for future loads. |
Code
Installation: IEC 60364-5 / SBC 401 / NEC; fire-survival cable: BS/IEC standards; firestop: UL / NFPA; seismic: SBC 301.4.12 Testing & commissioning
- Insulation resistance, continuity, earth-loop & RCD tests; phase rotation; polarity.
- Functional tests: generator auto-start & load transfer (ATS), UPS autonomy, protection settings (per the coordination study), busway integrity.
- Integrated tests with fire (generators feeding fire pumps/smoke fans), witnessed for AHJ sign-off.
Code
Verification: IEC 60364-6, SBC 401; generator/ATS: NFPA 110; coordination: IEC 60947.Terms & abbreviations
Plain-English meaning of the electrical terms used in this module.
| Term | What it means (plain English) |
|---|---|
| HV / MV / LV | High / Medium / Low Voltage. Power enters at HV/MV and is stepped down to LV (400/230 V) for use. |
| kW / kVA / pf | kW = real power (does work); kVA = apparent power (what the kit must be rated for); power factor (pf) = kW ÷ kVA. |
| √3 (root-3) | The 1.732 factor in three-phase power: P = √3 · V · I · pf. |
| Demand / diversity factor | Not all loads run at once — diversity reduces the connected total to a realistic maximum demand. |
| Transformer | Device that changes voltage (MV→LV); rated in kVA. |
| Impedance (%Z) | A transformer's internal resistance to fault current; sets how big a short-circuit can be. |
| Fault level / breaking capacity (kA) | The short-circuit current that could flow; breakers must safely interrupt at least this much. |
| Switchgear | The assembly of breakers/protection that distributes and protects circuits. |
| ACB / MCCB / MCB / RCBO / RCD | Types of circuit breaker / earth-fault protection, from large (ACB) to final-circuit (MCB), with RCD/RCBO adding shock protection. |
| Busway / busbar trunking | A compact prefabricated power "highway" up a riser, with tap-off points per floor. |
| Voltage drop | The voltage lost along a cable; kept within limits (~4%) so equipment gets enough voltage. |
| Ampacity / derating | A cable's safe current; reduced ("derated") for heat, grouping and installation conditions. |
| Generator (genset) / ATS | Standby engine-generator; ATS = Automatic Transfer Switch that switches loads to it on power loss. |
| UPS | Uninterruptible Power Supply — battery system giving no-break power to critical loads. |
| Essential / life-safety loads | Loads that must keep running on standby power (fire pumps, smoke fans, firefighting lifts, emergency lighting). |
| Lux / lumen method | Lux = light level on a surface; the lumen method calculates how many luminaires give the target lux. |
| UF / MF / UGR / LPD | Utilisation & Maintenance Factors (lighting efficiency), Unified Glare Rating, Lighting Power Density (energy limit). |
| Earthing / grounding & bonding | Connecting metalwork to earth for safety so faults trip protection and people aren't shocked. |
| LPS / SPD | Lightning Protection System / Surge Protection Device — protect the structure & electronics from strikes/surges. |
| NEC / IEC | The US (NFPA 70) and international (IEC 60364) electrical installation code families; SBC 401 is NEC-based. |
References & software map
| Task | Software | Code |
|---|---|---|
| Load-flow, cable & transformer sizing, fault & protection | ETAP, SKM PowerTools, DIgSILENT | IEC 60364, 60076, 61439, 60947; SBC 401 |
| Lighting design & LPD | DIALux evo, Relux | EN 12464-1; ASHRAE 90.1 / SBC 601 |
| Containment routing & coordination | Revit MEP + Navisworks | IEC 60364-5; SBC 401 |
| Lightning risk & LPS | Risk tools to IEC 62305 / NFPA 780 | IEC 62305 / NFPA 780 |
- SBC 401 — Saudi Electrical Code (NEC-based); SBC 601 — Energy; SBC 801 — Fire.
- IEC 60364 (LV installations), 60076 (transformers), 61439 (assemblies), 60947 (switchgear), 60364-5-52 (cables).
- NFPA 70 (NEC); NFPA 110/111 (standby/UPS); NFPA 101 (life safety); NFPA 780 / IEC 62305 (lightning).
- SEC distribution code; IEEE 142/1100 (earthing/power quality); EN 12464-1 (lighting); SASO efficiency.
Note
Worked numbers teach the method, not stamped design. Confirm code editions/clauses against SBC and the project spec; size real equipment in the network software.