Wire Voltage Drop
Calculate power loss in cables.
Watch: How to size a wire for voltage drop
[VIDEO PLACEHOLDER — embed a short walkthrough here]
Reading a resistance-per-meter chart is one thing. Watching a thin gauge wire actually warm up under load, and seeing the voltage sag at the far end on a meter, makes the risk concrete in a way the table doesn't.
Suggested video topics to film or source for this page:
- Measuring voltage drop live on two different AWG wires carrying the same current
- Thermal camera check on an undersized wire under sustained load
- Sizing a wire run for a 12V solar panel to inverter installation
- Comparing copper vs. aluminum wire of the same gauge for drop and heat
Understanding voltage drop
Every wire has some internal resistance. When current flows through it, part of the voltage gets lost as heat along the way instead of reaching the load. That loss is voltage drop. Too much of it, and a device might not get enough power to run correctly, or the wire could overheat enough to become a fire risk.
This calculator works out the voltage loss from wire gauge (AWG), length, and current, so you can pick a wire size that's actually safe for the load you're running.
The math behind the drop (Ohm's law)
The calculation comes straight from Ohm's law, V = IR:
Vdrop = I × Rtotal
Rtotal = 2 × Length × (Resistivity / Area)
Why is length doubled?
Current has to travel from the source to the load and then back again to complete the circuit. A 10-meter cable run actually has 20 meters of wire carrying current, and both directions add resistance.
Standard copper resistance (approx. @ 20°C)
| AWG | Resistance per meter |
|---|---|
| 10 AWG | ~3.28 mΩ/m |
| 14 AWG | ~8.29 mΩ/m |
| 24 AWG | ~84.2 mΩ/m |
Thinner wire (a higher AWG number) has more resistance per meter, which is why long runs at low gauge lose so much more voltage than the same run in a thicker cable.
Practical applications
| Use case | Why voltage drop matters |
|---|---|
| Solar panels | Long runs from roof to a basement inverter waste free solar energy if drop is high |
| Automotive | 12V systems (amps, off-road lights) are very sensitive to drop. A 1V loss is roughly 8% of the system's power |
| Power over Ethernet (PoE) | Security cameras at the end of long CAT6 runs need enough voltage left to operate reliably |
Frequently asked questions
What is an acceptable voltage drop?
NEC guidance suggests a maximum of 3% for critical loads like lighting and electronics, and 5% for non-critical loads. For 12V systems specifically, aim to stay under 3%, since the margin for loss is much smaller than on higher-voltage circuits.
Copper vs. aluminum wire, which loses more voltage?
Aluminum has roughly 1.6 times the resistance of copper for the same gauge. If you're substituting aluminum wire, you typically need to go up two gauge sizes to match copper's performance, for example 6 AWG aluminum in place of 8 AWG copper.
Does AC vs. DC matter for voltage drop?
For resistive voltage drop, no. Resistance is resistance regardless of current type. At very high AC frequencies or with large gauge conductors, skin effect and inductance can raise the effective impedance, but for standard 50/60Hz house wiring, the DC resistance formula is accurate enough to use directly.
How much wire length is too much for a given gauge?
There's no fixed cutoff since it depends on current and acceptable drop, but as a rough guide: doubling the length of a run doubles the voltage drop for the same gauge and current. If a run is already close to the 3% threshold, extending it further usually means stepping up to a thicker gauge rather than accepting the extra loss.
Can voltage drop damage my equipment?
Low voltage at the load can cause motors to run hotter and less efficiently, electronics to brown out or reset unexpectedly, and LEDs to dim or flicker. The wire itself is also a risk: excessive current for its gauge causes it to heat up, which can degrade insulation over time or, in severe cases, start a fire.