Distribution

The last mile to customers. Learn radial, ring and meshed networks, feeders and distributors, LV earthing systems, voltage-drop calculations, diversity and demand factors, and how distribution transformers are sized.

Distribution networks take power from substations at 11–33 kV and deliver it to homes, shops and factories at 400/230 V. There are far more kilometres of distribution cable than transmission line, and most customer outages originate here.

Designing distribution is about voltage drop, cable sizing, transformer sizing using realistic (diversified) demand, earthing for safety, and network configuration for reliability.

An analogy

If transmission is the motorway, distribution is the network of local roads and driveways — slower, far more numerous, and where most of the everyday traffic problems happen.

Network structures

How feeders are arranged decides reliability.

• Radial: one path from source to each customer — cheap, but a fault disconnects everyone downstream.
• Ring (loop): fed from both ends, usually operated with a normally-open point; after a fault, the section is isolated and supply restored from the other side.
• Meshed: multiple paths, used in dense city centres for the highest reliability.

Voltage levels and limits

Customers must receive voltage within a band (e.g. 230 V +10 %/−6 % in the UK, ±10 % in many countries). Designers budget voltage drop between the MV network, the transformer, the LV feeder and the customer's own wiring.

Diversity and demand factors

Not every appliance runs at once, and not every house peaks at the same time. Demand factor converts connected load to one customer's maximum demand; diversity factor accounts for peaks not coinciding across customers. Ignoring diversity massively oversizes transformers and cables.

LV earthing systems

The earthing arrangement decides how fault current returns and how people are protected.

• TN-S: separate neutral and protective earth from the transformer.
• TN-C-S (PME): combined PEN conductor in the supply, separated at the customer.
• TT: customer's own earth electrode; requires RCDs because earth-fault current is low.
• IT: isolated or high-impedance neutral; used where continuity is critical (hospital theatres).

Automation and DER

Smart distribution uses reclosers, sectionalisers and automated switches for FLISR (fault location, isolation and service restoration) within seconds. Rooftop solar causes reverse power flow and voltage rise on LV feeders, so utilities use on-load tap changers, smart-inverter volt-var control and hosting-capacity studies.

Formulas

• Voltage drop (tabulated): ΔV = (mV/A/m) × I × L / 1000
• Demand: Max demand = connected load × DF
• Group demand: Group max = Σ individual max demands / DivF
• Transformer rating: S = group max demand (kW) / pf

Worked example: Voltage drop on an LV distributor

Given: 3-phase, 400 V; I = 100 A; L = 150 m; 95 mm² Al cable: 0.44 mV/A/m

Find: Voltage drop and percentage.

• ΔV = 0.44 × 100 × 150 / 1000 = 6.6 V
• % = 6.6 / 400 × 100 = 1.65 %

Answer: 6.6 V (1.65 %)

Worked example: Transformer for a housing estate

Given: 50 houses, 10 kW connected each; Demand factor 0.4; Diversity factor 2.5; pf 0.9

Find: Group maximum demand and transformer rating.

• Per-house max demand = 10 × 0.4 = 4 kW
• Group max = 50 × 4 / 2.5 = 80 kW
• S = 80 / 0.9 = 88.9 kVA → choose 100 kVA

Answer: 80 kW → 100 kVA transformer

Common mistakes

• Wrong: Sizing a transformer from the sum of connected loads. — Right: Apply demand and diversity factors to find the realistic maximum demand.
• Wrong: Checking only ampacity when sizing long cables. — Right: Long runs are usually limited by voltage drop, not heating.

Applications

• EV charging: Adding home chargers (7 kW each, low diversity in the evening) is forcing utilities to reinforce LV networks or use smart charging.
• Building services: Electrical engineers apply the same voltage-drop and diversity methods to size risers and sub-mains in large buildings.
• Reliability indices: Utilities report SAIDI and SAIFI (average outage duration and frequency per customer), and are rewarded for improving them.

Key takeaways

• Radial is cheap; ring and mesh improve reliability.
• Size equipment from diversified maximum demand, not connected load.
• Voltage drop, not heating, limits long LV runs.
• Earthing system (TN/TT/IT) decides the protection strategy.

Going further

• Volt-VAR optimisation: Coordinating tap changers, capacitor banks and smart-inverter reactive power flattens feeder voltage, allowing conservation voltage reduction that cuts energy use by a few percent.
• Hosting capacity: Utilities compute how much solar or EV load each feeder can take before voltage, thermal or protection limits are violated, and publish maps to guide connections.
• Microgrids: A microgrid can island itself from the main grid during outages, using local generation and storage with grid-forming control and protection that adapts to low fault currents.