Wire Gauge & Voltage Drop Calculator

Description

Every foot of bus or feeder wire has a little resistance, and current flows out through one conductor and back through the other — so a run's resistance counts twice, not once. Over a long enough run or a high enough current, that adds up to a real voltage drop at the far end of your layout, which shows up as dimmer lights, flickery sound decoders, or locomotives that stall sooner on a curve or turnout than they should.

The default mode gives you the voltage drop for a wire gauge, run length, and current. The other two modes work backward: enter the most voltage drop you're willing to accept, and either get the longest run that stays within it for a given gauge, or the thinnest (most economical) gauge from this calculator's table that still keeps a given run within it.

Sources

Voltage drop is standard electrical engineering: V = 2 × length × (resistance per unit length) × current, the factor of 2 accounting for the round trip through both conductors — not a modeling-specific formula. AWG wire resistance values (solid bare copper at 25°C) are from Engineering Toolbox's published copper-wire resistance table, a standard reference, not an invented or modeling-specific number.

There's no DCC-specific standard for how much voltage drop is acceptable — that depends on your decoders' and boosters' own tolerance. As a reference point, the National Electrical Code's own informational notes (210.19(A) for branch circuits) suggest keeping household wiring's voltage drop to around 3% at the farthest outlet, or 5% combined with the feeder — informational guidance, not a mandatory limit, and written for household wiring, not model railroads. Use it as a starting point for your own "maximum acceptable drop," not a rule.

Disclaimer: stranded wire and higher operating temperatures have slightly higher resistance than these solid-wire, 25°C table values — treat results as a close estimate, not an exact lab measurement.