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Voltage Drop in High-Rise Buildings: How to Optimize Riser Cable Sizing
In a 30-storey tower, voltage drop along the riser can force you to oversize cables by two or three steps. Here’s how to calculate it, optimise it, and know when to switch to busbar trunking.
★ Building Services — Electrical DesignI’ve sized riser cables for enough high-rise projects to know which constraint bites first. In a low-rise building, cable sizing is driven by load current. In a high-rise, it’s almost always voltage drop — the accumulated loss along a vertical cable run can exceed the 5% limit long before the cable reaches its full ampacity.
The result? A riser cable that’s two or three sizes larger than the load requires, just to keep the voltage at the top floor within tolerance. That extra copper adds cost, fills up riser shafts, and increases installation difficulty.
But there are ways to optimise. Let me walk through the calculation method, the design strategies that work, and when busbar trunking becomes the smarter choice.
Voltage Drop Limits in Codes
Every wiring regulation sets a maximum allowable voltage drop. The good news is the limits are consistent across major standards.
3%BS 7671 limit for lighting circuits
5%BS 7671 limit for all other circuits
20 VMaximum drop at 400 V 3-phase (5%)
6.9 VMaximum drop at 230 V 1-phase (3%, lighting)
Under BS 7671 (IET Wiring Regulations) Appendix 4 and IEC 60364-5-52, the voltage drop from the origin of the installation to the load point must not exceed:
- 3% for lighting circuits
- 5% for all other circuits (power, HVAC, lifts)
These limits apply cumulatively. If a lighting circuit on the 25th floor receives power through a main riser (say 2.5% drop) plus a final sub-circuit (another 1.5% drop), you’re already at 4% — exceeding the lighting limit. This is why in tall buildings, voltage drop often dictates the riser size independently of the ampacity calculation.
The Calculation Method
BS 7671 uses the mV/A/m method: the voltage drop per ampere per metre, tabulated for each cable size and type.
VD (V) = (mV/A/m × Ib × L) / 1000
Where mV/A/m comes from BS 7671 Appendix 4 tables, Ib is the design current (A), and L is the cable length (m). For 3-phase systems, the tabulated value is already for line-to-line voltage.
The hidden factor: For cables above 95 mm², the power factor significantly affects voltage drop. The tabulated mV/A/m values assume a particular power factor (typically 0.85). If the load power factor is lower (common with LED lighting or VFD-driven lifts), the reactive component increases and the actual voltage drop can be higher than the tabulated value suggests. For critical riser designs, use the full formula:
VD = ∝3 × I × (R cosφ + X sinφ) × L / 1000 (3-phase)
Riser Design Strategies
Several techniques can reduce the voltage drop penalty in tall buildings. The right approach depends on the building height, load density, and floor plan.
1. The Barycentric Approach (BS 7671 Appendix 17 / IEC 60364-8-1)
This is the single most effective optimisation. Instead of feeding all floors from one riser at the building’s edge, position the main switchroom and transformer as close to the centre of electrical load as possible — ideally at mid-height rather than at ground level.
For a 30-storey building with the transformer at ground level, the top-floor circuit sees the full 100% of the riser length. Move the transformer to the 10th floor (in a dedicated electrical floor), and the longest vertical run drops to roughly 65%. The voltage drop reduces proportionally, and the riser cable can be one to two sizes smaller.
BS 7671 Appendix 17 (based on IEC 60364-8-1) formalises this as the barycentric method for reducing energy losses in building electrical systems. It’s not a mandatory requirement (Appendix 17 is informative) but the energy savings over 30 years are substantial.
2. Dual Risers vs. Single Oversized Riser
For very tall buildings (40+ storeys), a single riser sized for the cumulative load of all floors becomes impractical. The cable size at the base can reach 400 mm² or more — difficult to bend, support, and terminate.
Splitting the building into two or three vertical zones — each with its own riser fed from a dedicated transformer or sub-distribution board — dramatically reduces the maximum cable size and voltage drop in each zone. A common split: low zone (G–10), mid zone (11–25), high zone (26–40). Each riser sees only one-third of the total height.
3. Busbar Trunking vs. Cable — When to Switch
At a certain current, busbar trunking becomes more economical than cable risers. The crossover point is typically around 400–800 A:
| Factor | Copper Cable Riser | Busbar Trunking |
|---|---|---|
| Initial cost (equipment) | Lower for currents <400 A | Lower for currents >800 A; competitive 400–800 A |
| Installation time (20 floors) | 2–3 weeks (multiple cable pulls) | 2–3 days (bolt-together sections) |
| Voltage drop | Baseline | Up to 50% less (lower impedance per amp; project-specific calculation required) |
| Short-circuit withstand | Limited by individual cable size | 60–70 kA typical (per manufacturer data, design-verified joints) |
| Shaft space | Large — multiple cables on trays | Compact — 1600 A busbar ~185 × 180 mm |
| Fire load | High (cable-density dependent: 36–108+ MJ/m² depending on fill) | Low (~5 kWh/m² — 80–90% less) |
| Flexibility for future changes | Difficult — new cables required | Easy — plug-in tap-offs, even live |
| Skilled labour needed | 3–4 electricians | 2 technicians |
| For high-rise risers above 400 A and runs longer than 30 m, busbar trunking typically offers lower total installed cost and superior electrical performance. | ||
💡 Engineering judgment: I tend to use the following rule of thumb: below 250 A peak demand per floor, a cable riser is almost always the right choice. Above 630 A per floor, busbar trunking wins on cost, voltage drop, and fire safety. In the 250–630 A range, do a full TCO comparison — the answer depends on building height, shaft space availability, and local labour rates.
4. Grouping Derating in Riser Shafts
Here’s one that designers often overlook late in the project. A riser shaft typically contains multiple cables — the main riser, fire alarm cable, emergency lighting supply, data cables, etc. BS 7671 Table 4C1 requires grouping factors to be applied when cables are bunched together. For 6–8 cables in a single shaft, the grouping factor can be as low as 0.52–0.57.
This means a riser cable with a base rating of 400 A might only be good for about 220 A once grouping is applied. The designer then upsizes the cable to compensate, which increases both cost and the physical space required — which can trigger even more derating. It’s a spiral that’s best avoided by keeping dedicated riser compartments with adequate spacing.
⚠ Common mistake: Sizing the riser cable based on load current and voltage drop alone, then discovering at the installation stage that grouping in the shaft requires a further upsizing. The riser shaft space is already fixed, and there’s no room for the next cable size. Always include grouping derating in the initial shaft design. A simple fix: specify a 20% spare capacity in shaft cross-section to accommodate future upsizing or additional circuits.
Economic Cable Sizing Example
Here’s a worked example that brings the theory together.
Scenario: 25-storey commercial tower, 250 A per floor (peak demand), 3-phase 400 V supply. Transformer at ground level. Maximum floor-to-floor height 3.8 m, total riser run approximately 100 m (including horizontal sections at base and top). Cable installed in a dedicated riser with grouping factor 0.75 (4 circuits in shaft). Power factor 0.85.
Step 1 — Design current: Ib = 250 A. Protective device rating In = 315 A. Load requires cable ampacity Iz ≥ 315 A.
Step 2 — Base cable size: From BS 7671 Table 4E4A (XLPE/SWA, Method C, clipped direct), 95 mm² is rated for 289 A — too low for 315 A. Try 120 mm² at 335 A. With grouping factor 0.75: 335 × 0.75 = 251 A. Still needs upsizing. Try 185 mm² at 441 A base: 441 × 0.75 = 331 A ≥ 315 A. Minimum size from ampacity + grouping: 185 mm².
Step 3 — Voltage drop check: 185 mm² has approximately 0.26 mV/A/m (3-phase). Over 100 m at 250 A: VD = 0.26 × 250 × 100 / 1000 = 6.5 V = 1.6%. Well within 5%. The voltage drop is not the limiting factor in this case — grouping derating is.
Step 4 — Energy efficiency optimisation (BS 7671 Appendix 17): The I²R loss over the cable life can justify upsizing to 240 mm². A simple payback: the additional cable cost (~$4,500 for the extra 55 mm² over 100 m) reduces I²R losses by ~25%, saving about $1,200–$1,500 per year in energy. Payback period: 3–4 years. Over a 25-year building life, the upsizing saves approx. $20,000–$30,000 net.
Final selection: 240 mm² 4-core XLPE/SWA/PVC — optimised for both ampacity and lifetime energy efficiency. ✅
Sorivo Large-Section Building Wires
For high-rise riser applications, Sorivo offers a range of large-section cables designed for vertical installation.
| Application | Recommended Cable | Key Feature for Risers |
|---|---|---|
| Main LV riser (0.6/1 kV) | CU/XLPE/SWA/PVC | XLPE insulation for high ampacity, SWA for mechanical protection during vertical installation, sizes up to 400 mm² |
| LSZH riser (fire-sensitive zones) | CU/XLPE/LSZH/SWA/LSZH | Zero halogen sheath, low smoke emission — required for riser shafts in public buildings and high-occupancy towers |
| Emergency circuits / fire alarm | BS 6387 CWZ fire resistant | Maintains circuit integrity during fire, LSZH sheath, SWA for mechanical protection in shaft |
| Sub-mains / floor distribution | CU/XLPE/SWA/PVC 16–95 mm² | Economical for shorter floor runs, same construction as main riser for consistent termination practice |
Sorivo vs. Economy-Grade Power Cables
| Feature | Market Generic / Economy | Sorivo Premium Grade |
|---|---|---|
| Conductor | Bare copper, may contain impurities → higher DC resistance → increased voltage drop | High-purity annealed copper per IEC 60228 Class 2, verified DC resistance within ±2% of specified value |
| XLPE insulation | Variable cross-linking → inconsistent thermal performance | Monitored cross-linking, consistent 90°C rating, high short-circuit capacity (250°C for 5 s) |
| Armour | Under-gauge galvanised wire → corrosion risk in riser shafts | Full-gauge SWA per BS 5467, hot-dipped galvanised, verified tensile strength |
| Fire performance | PVC sheath → toxic smoke, flaming drips, high fire load | LSZH option (Type LTS3 per BS 7655) — low smoke, halogen-free, classified per BS EN 50399 |
| Traceability | None → impossible to verify actual conductor size after installation | Metre-marked sheath, batch traceable, full certification available |
| Warranty / design life | 1–5 years | 25-year design life per IEC 60216 thermal endurance testing |
Frequently Asked Questions
Is voltage drop always the limiting factor for high-rise riser sizing?
Not always, but it often is for buildings over 15 storeys. For lower floors, the cable is usually sized by ampacity. For the upper half of a tall building, voltage drop accumulates and typically becomes the governor. In my experience, about 70% of high-rise riser designs I review are voltage-drop-limited rather than ampacity-limited. The exception is buildings with high floor-to-floor loads (data centres, hospitals) where ampacity is the primary constraint.
Does using aluminium conductors help with voltage drop in tall buildings?
Aluminium has about 61% of copper's conductivity, so for the same current and voltage drop, you need approximately 1.6 times the cross-sectional area of copper. This makes aluminium risers physically larger, which is a problem in tight shafts. However, aluminium is lighter (about 30% of copper's density), so the weight load on cable cleats and supports is lower — a real advantage in vertical installations over 50 m. The economics depend on copper/aluminium price ratio and shaft space availability. For current-carrying capacity above 500 A per phase, aluminium busbar trunking is worth considering.
How do harmonics from LED lighting and VFD lifts affect voltage drop calculation?
Harmonic currents — particularly 3rd-order harmonics in LED lighting and VFD-driven lifts — increase the neutral conductor current in 3-phase systems and add to the total RMS current without contributing useful power. This increases I²R losses and effective voltage drop. For risers supplying mixed loads with significant non-linear equipment, the neutral should be considered as a current-carrying conductor for grouping derating purposes (per BS 7671 Table 4C1 note). BS 7671 Appendix 17 highlights harmonic effects as an energy efficiency consideration.
What is the practical maximum cable size for a riser installation?
Above 240 mm², the practical difficulties increase significantly. A 400 mm² 4-core SWA cable weighs approximately 20–25 kg/m and has a minimum bending radius of about 1.2 m. Installing it in a vertical shaft requires substantial cable cleats at every floor level, careful pulling tension management, and generous bend space at the base. For currents requiring cables above 300 mm², I typically recommend switching to either parallel smaller cables (two runs of 185 mm² instead of one 400 mm²) or busbar trunking.
Should I include spare capacity in the riser for future building modifications?
BS 7671 Appendix 17 recommends considering future load growth in cable sizing for energy efficiency. In practice, I suggest including 20–25% spare capacity in the initial riser design, particularly in commercial buildings where tenant fit-outs change regularly. The incremental cost of upsizing by one cable size during original installation is typically 10–15%, while retrofitting a new riser later can cost 3–4 times as much. If using busbar trunking, the plug-in tap-off system makes future modifications much easier without spare capacity concerns.
Need large-section power cables for your high-rise project?
Sorivo supplies CU/XLPE/SWA/PVC and LSZH armoured power cables up to 400 mm², with full DC resistance and voltage drop data for accurate sizing. Contact our team for project-specific cable schedules.
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