Why do I need a bus wire?

Track joiners (rail joiners) are unreliable for electrical connection over time. A bus wire running under the layout with regular dropper connections to the track ensures consistent power delivery and eliminates dead spots.

What gauge wire should I use for the bus?

16 AWG (1.3mm²) is recommended for most layouts. It handles the current without significant voltage drop. Smaller layouts may use 18 AWG. Droppers can be thinner (22 AWG) as they carry less current over short distances.

How far apart should droppers be?

The general recommendation is every 60-90cm (2-3 feet) for reliable power. Shorter intervals give better performance but require more wiring. Never rely on track joiners alone for power over more than a few track sections.

How do I size my power supply?

Calculate the maximum number of locos running simultaneously per power district and multiply by current draw per loco (typically 0.5-1.5A for N/OO gauge). Add 20-50% headroom for accessories and future expansion.

ModelCalcs.uk

Model Railway Wiring Calculator — Bus Wire & Droppers

Calculate bus wire length, dropper count, dropper wire and power supply sizing for your model railway layout.

Layout Length

Total length of track run the bus follows under the baseboard, in metres

Power Districts

Independent sections each fed by its own circuit breaker — split only when you exceed ~5A or want short-circuit isolation

Dropper Spacing

Gap between feeder drops to the rails — NMRA advises a drop every ~90cm (3ft); 60cm is a robust default

Bus Wire Gauge14–16 AWG for the main bus keeps voltage drop low; 18–22 AWG is fine for short droppers

How We Calculate This

This calculator estimates wiring requirements based on layout length, dropper spacing and power district count.

The formula

Bus wire = Layout length × 2 wires × (1 + Extra %) + a short link per extra district

Droppers = (Layout length ÷ Spacing) × 2 rails

Power supply = Max locos × Amps per loco × Districts

Power districts subdivide a single layout — they share one track run, so the main bus pair scales with layout length rather than multiplying by district count. Each district beyond the first adds only a short interconnect (≈1 m × 2 wires here) from its own circuit breaker or booster back to the command station. The power-supply figure totals the current of all districts, since each district draws independently from its own booster.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Always verify quantities before purchasing materials.