Wiring and connectors
Wire gauge, current, and voltage drop
Find the thinnest copper wire that carries your current without overheating and without dropping so much voltage that the far end stops working. The two limits are separate, and the tool says which one is deciding.
Distance to the load. The return path is counted for you.
3 % is the usual target. A regulator with little headroom needs less.
Wiring context
Short run, open air, heat can escape
Use this gauge
12AWG
Voltage drop is what sized this. A thinner wire would carry the current without overheating.
Voltage drop
0.31V
Drop
2.61%
Voltage at the load
11.69V
Round trip resistance
31.3mΩ
Heat in the wire
3.13W
Rated for
41A
Note
Voltage drop is the binding limit
Heat is not what is sizing this wire. 20 AWG carries 10 A without overheating, but it drops too much voltage over 3.0 m. You are buying copper to hold the voltage up, not to stop the wire melting.
Note
20 AWG will not melt, but the device may brown out
20 AWG handles 10 A on its ampacity rating, so the wire stays intact. It drops 2.00 V, which is 16.7 % of the supply. That shortfall lands on the load. Regulators drop out, motors lose torque, and radios reset on transmit.
The gauges around it
One step thicker and one step thinner, so you can see what the next size buys you.
| Gauge | Drop | Drop % | Loss | Rated | Verdict |
|---|---|---|---|---|---|
| 10 AWG | 0.2 V | 1.6 % | 1.97 W | 55 A | Meets both |
| 12 AWGPick | 0.31 V | 2.6 % | 3.13 W | 41 A | Meets both |
| 14 AWG | 0.5 V | 4.1 % | 4.97 W | 32 A | Too much drop |
Why this matters
Chassis and transmission ratings differ by roughly a factor of three for the same wire, and picking the wrong column is the most common sizing error in a build. The chassis figure assumes a short run in open air; using it for a long run bundled in a loom leaves you with wire running far hotter than you planned. The second mistake is sizing on heat alone. For any run longer than about a metre at low voltage, the binding limit is voltage drop, not temperature, and a wire that will never melt can still starve a regulator until it drops out.
Assumptions and sources
- Copper conductors at 20 °C. Resistance rises roughly 0.4 % per °C, so a hot wire in an enclosure drops more than this shows.
- Resistance is counted for the full round trip, out and back. If your return path is a chassis or a ground plane rather than a wire, the real drop is lower.
- Ampacity figures are the widely published chassis and power transmission tables. They are guidance, not a standard: real limits depend on insulation temperature rating, bundling, and ambient temperature.
- The 3 % default drop target is a rule of thumb, not a rule. Lighting tolerates more. A 3.3 V regulator with 200 mV of headroom tolerates far less.