Wires get warm or drop voltage on a robot: choose the right wire gauge

Use thick, short wire for the battery and motor paths. Thin jumper wires are only for signals.

What the message means

Wire has resistance. Current through it makes heat and a voltage drop, both larger for thin and long wires.

Fix it in this order

Estimate the largest current in each path, including the stall current of motors.
Use a wire gauge chart from a reliable source and pick a gauge with margin.
Keep battery and motor wires as short as the layout allows.
Use silicone wire, which stays flexible, for moving parts.
Do not use breadboard jumpers for motor current.
Check the connectors are rated for the same current.

Why it happens

A wire is a resistor. By Ohm’s law the voltage lost along it is the current times its resistance, and the heat it gives off is the current squared times that resistance. Resistance goes up as the wire gets thinner and as it gets longer, so a thin, long run loses more voltage and gets warmer.

Motors draw far more current when they stall or start than when they run freely, so a wire that stays cool at idle can heat up the moment the robot pushes against something. Size the wire for the stall current, and read the safe current for your gauge from a chart you trust, since charts differ with insulation and how the wire is bundled.

If you see this, do this

You see	Do this
Wire is warm to touch	Too thin for the current.
Voltage low at the load	Resistance in wire or connectors.

18650 holder drop, battery sag.

Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.

Why this happens

Copper wire has a resistance per metre that depends on its cross-section. Typical values for solid copper are about 13 milliohm per metre for 16 AWG, 21 milliohm per metre for 18 AWG and 53 milliohm per metre for 22 AWG. The wire carries current out and back, so the length in the calculation is the round trip. The voltage lost is I x R and the heat is I squared x R.

Those two figures behave differently as current rises. Doubling the current doubles the voltage drop but quadruples the heat in the wire. Heat is what limits a wire in practice, because insulation softens when it gets too hot. A bundle of wires or a closed sleeve gets rid of heat more slowly than a single wire in air, so charts give lower safe currents for bundles.

How to confirm the cause

Measure the voltage at the battery terminals and at the motor driver input while the robot is working hard. The difference is the loss in the wire and connectors.
Touch the wire and the connectors after a short test, with power off first if you are unsure. A connector warmer than its wire is a poor contact.
Measure the voltage across a single connector or joint while current flows. A few tenths of a volt across one joint is a bad connection.
Check the stall current of the motor from its data, not only the running current.
Compare the current in each path with a gauge chart from a reliable source.

Worked example

A 5 A load, with 0.5 m from the battery to the board, so the round trip is 1.0 m. These are calculated values, not measurements.

22 AWG: R = 0.053 ohm/m x 1.0 m = 0.053 ohm
        drop = 5 A x 0.053 = 0.27 V
        heat = 5 x 5 x 0.053 = 1.3 W
18 AWG: R = 0.021 ohm/m x 1.0 m = 0.021 ohm
        drop = 5 A x 0.021 = 0.10 V
        heat = 5 x 5 x 0.021 = 0.53 W

Common mistakes

Counting only the one-way length and forgetting the return wire.
Using breadboard wires and thin jumper leads for motor current.
Choosing the wire from a chart for free-air use when the wires are bundled together.
Using a good wire with a small connector that cannot carry the current.
Soldering a thick wire to a thin board trace and calling it done, since the trace is now the thin part.

If this did not fix it

Check the internal resistance of the battery. A tired battery sags under load even with perfect wiring.
Check the connectors rated for the current, such as XT60 type for high current, rather than small hobby headers.
Check for a fuse or switch in the path that adds its own resistance.