Why Voltage Drops Along a Cable
No conductor is perfect. Every wire has resistance, and current flowing through resistance produces a voltage drop, exactly as Ohm's law predicts. The energy lost becomes heat in the cable.
DC and single phase: Vdrop = 2 × I × L × R
Three phase: Vdrop = √3 × I × L × R
where I is current in amps, L is the one-way distance and R is resistance per unit length. The factor of 2 matters: current travels out along one conductor and back along the other, so it passes through twice the run length. Three-phase systems use √3 ≈ 1.732 because the return currents partly cancel.
The 3% and 5% Rules
The U.S. National Electrical Code does not mandate a limit but recommends, in informational notes, no more than 3% drop on a branch circuit and 5% total from the service to the furthest outlet. Most other national codes set similar figures.
| Drop | Effect |
|---|---|
| Under 3% | Negligible; equipment operates as designed |
| 3–5% | Noticeable dimming, motors run slightly hot |
| 5–10% | Motors lose torque and overheat, electronics may misbehave |
| Over 10% | Equipment damage likely; wasted energy is significant |
Motors suffer most. Torque falls with the square of voltage, so a 10% drop costs about 19% of torque, and the motor draws more current to compensate — which increases the drop further.
Copper Wire Resistance
| AWG | Copper Ω/1,000 ft | Aluminium Ω/1,000 ft | Typical ampacity (75°C) |
|---|---|---|---|
| 18 | 7.77 | 12.8 | — |
| 16 | 4.89 | 8.05 | — |
| 14 | 3.07 | 5.06 | 15 A |
| 12 | 1.93 | 3.18 | 20 A |
| 10 | 1.21 | 2.00 | 30 A |
| 8 | 0.764 | 1.26 | 50 A |
| 6 | 0.491 | 0.808 | 65 A |
| 4 | 0.308 | 0.508 | 85 A |
| 2 | 0.194 | 0.319 | 115 A |
| 1/0 | 0.122 | 0.201 | 150 A |
| 4/0 | 0.0608 | 0.100 | 230 A |
Aluminium has roughly 60% of copper's conductivity, so an aluminium conductor needs to be about two gauge sizes larger to match a copper one.
Understanding AWG
American Wire Gauge runs backwards: smaller numbers mean thicker wire. The scale is logarithmic, and two rules make it easy to work with:
- Every 3 gauge steps roughly doubles or halves the cross-sectional area.
- Every 6 gauge steps roughly doubles or halves the diameter.
- Going up one gauge size (e.g. 12 to 10 AWG) cuts resistance by about 37%.
Above 1 AWG the numbering continues as 1/0 ("one aught"), 2/0, 3/0 and 4/0, each thicker than the last.
Ampacity Is a Separate Limit
Two independent constraints govern wire size, and both must be satisfied:
- Ampacity — how much current the wire can carry without overheating. This is a safety limit set by code and depends on insulation type, ambient temperature and how many conductors share a conduit.
- Voltage drop — how much voltage is lost along the run. This is a performance concern.
On short runs ampacity decides the size. On long runs voltage drop takes over, and it is common to need a wire two or three sizes larger than ampacity alone would require. Never size below the ampacity requirement to save money on a long run.
Reducing Voltage Drop
- Use thicker wire. The most direct fix; each gauge step cuts drop by about a third.
- Shorten the run. Drop is directly proportional to distance.
- Raise the voltage. The same power at 240 V draws half the current of 120 V, which quarters the power lost in the cable.
- Split the load across two circuits.
- Use copper instead of aluminium where the cost difference allows.
Frequently Asked Questions
Do I enter the one-way distance or the round trip?
One way. The formula already doubles it for the return conductor.
How much drop is acceptable?
Aim for 3% on any single circuit and 5% overall. Sensitive electronics and long motor runs justify tighter targets.
Why does 240 V perform so much better on long runs?
Because the same power needs half the current, and drop is proportional to current. Halving the current halves the voltage drop and quarters the power wasted as heat.
Does temperature matter?
Yes. Copper's resistance rises about 0.4% per °C. The values here are at 75°C, the usual design assumption for building wiring; a cable running cooler will perform slightly better.