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LV Network Design

Multi-node network analysis with SLD generation

Source

Transformer

TX1

Switchboard

Main Switchboard

MSB

Switchboard

Lighting DB

DB-L1

Switchboard

Power DB

DB-P1

Load

Lighting L1

L1

Load

Lighting L2

L2

Load

GPO Circuit

P1

Motor

AHU Motor

M1

PASS WARNING FAIL
MMMBBBMTransformerMain SwitchboardLighting DBPower DBLighting L1Lighting L2GPO CircuitMAHU Motor

How LV Network Design Works

The LV network design calculator models a complete low-voltage distribution system from the transformer secondary to the final distribution boards, evaluating voltage drop, fault levels, and cable adequacy across the entire network.

The calculator cascades calculations through the network hierarchy: transformer secondary, main switchboard, sub-distribution boards, and final circuits. At each node, it computes the cumulative voltage drop (ensuring compliance with the total allowable limit) and the prospective fault current (decreasing with distance from the source).

IEC 60364 provides the complete framework for low-voltage installation design. BS 7671 implements these requirements for UK installations. AS/NZS 3000 adapts the principles for Australian and New Zealand conditions. Results include a single-line diagram with voltage profiles, fault levels at each bus, cable utilization percentages, and a compliance summary per distribution board.

Frequently Asked Questions

How do I design an LV distribution network for a commercial building?
LV network design starts with a single-line diagram (SLD) showing the supply point, main switchboard (MSB), and distribution boards (DBs). Calculate maximum demand per DB using BS 7671 Appendix A or AS/NZS 3000 Section 2, then size submain cables per IEC 60364-5-52 for current capacity, voltage drop (cumulative from MSB to final circuit must not exceed 4-5%), and fault withstand. Each DB bus must be rated for the prospective fault current at its location. The design is iterative — changing cable sizes affects voltage drop and fault levels throughout the network. ECalPro automates this multi-point analysis.
What is the maximum allowable voltage drop across an LV network?
The total voltage drop from the supply origin to the most remote load point must comply with the applicable standard. BS 7671 Appendix 12 recommends 3% for lighting and 5% for other uses. IEC 60364-5-52 Clause 525 sets similar limits but allows national variations. AS/NZS 3008.1.1 Clause 4.5 limits total voltage drop to 5%. The drop is cumulative across all segments: for example, 1.5% in the submain from MSB to DB1, plus 2.5% in the final circuit from DB1 to the load, gives 4.0% total. NEC informational notes recommend 3% per branch and 5% total feeder-plus-branch.
How do I verify fault levels at each distribution board?
Fault level at each DB is calculated by adding the cable impedance of each segment to the source impedance using the IEC 60909-0 method. Starting from the supply transformer (Zt = uk% x U2 / Sn), add each cable segment impedance Zcable = (R + jX) x L to get the total impedance at each point. The fault current is If = c x U / (sqrt(3) x |Ztotal|). Every circuit breaker and busbar must have a short circuit rating exceeding the calculated prospective fault current per BS 7671 Regulation 536.4 and IEC 60364-4-43. Fault levels decrease with distance from the transformer as cable impedance increases.
What is load balancing and why is it important in three-phase networks?
Load balancing distributes single-phase loads as evenly as possible across the three phases of a three-phase supply to minimise neutral current, reduce voltage imbalance, and maximise utilisation of the supply capacity. IEC 60364-5-52 and BS 7671 Regulation 525.2 note that voltage drop calculations assume balanced loads; unbalanced loading increases neutral current (up to sqrt(3) x phase current in worst case) and can cause neutral overheating. AS/NZS 3000 Clause 2.5 requires that the maximum current in any phase does not exceed the supply phase current rating. A phase imbalance exceeding 10-15% is generally considered unacceptable.
How do I coordinate protection across multiple distribution levels?
Protection coordination in an LV network requires discrimination between upstream and downstream devices at each level (MSB main breaker, submain breakers, DB main switches, final circuit MCBs). Per IEC 60947-2 and BS 7671 Regulation 536.2, the upstream device must not trip before the downstream device for any fault within the downstream zone. This is verified by comparing time-current characteristics (TCC curves) and ensuring adequate margins. For MCB-to-MCB discrimination, a ratio of at least 1.6:1 in rated current is typically needed. For MCCB upstream of MCB, time-delay settings provide discrimination up to the instantaneous pickup of the MCCB.
What are the standard distribution board ratings for LV networks?
Distribution board ratings per IEC 61439-3 (for non-expert access DBs) and IEC 61439-2 (for power switchgear) include rated current (63A, 100A, 160A, 250A, 400A, 630A, 800A, 1000A busbar ratings), rated short circuit withstand (10kA, 16kA, 25kA, 36kA, 50kA for 1 second), and rated insulation voltage (typically 690V or 1000V). NEC Article 408 covers switchboards and panelboards, requiring a short circuit current rating label per 408.6. AS/NZS 61439.1 and BS EN 61439 series define testing requirements. The DB rating must exceed both the maximum load current and the prospective fault current at its installation point.

Related Calculators

Maximum Demand
AS/NZSBSIEC
Transformer
IECAS/NZSBS
Voltage Drop
AS/NZSBSIEC
Short Circuit
AS/NZSBSIEC
Cable Sizing
AS/NZSBSIEC

Related Guides & Examples

Hurricane Maria: LV Network CollapseWorked Example
Complete Cable Sizing ComparisonStandards Comparison
Maximum Demand Diversity FactorsTechnical Guide

Key Terms

voltage dropshort circuitcable sizing

Related FAQ

cable sizing FAQshort circuit FAQ

Standards Reference

  • IEC 60364 — Low-voltage installations
  • BS 7671:2018+A2 — Complete wiring regulations
  • AS/NZS 3000:2018 — Wiring rules