What this calculator answers
Voltage drop is the amount of voltage lost as current travels through a conductor's own resistance. You enter the source voltage, the load current, the one-way length of the run, the conductor material and size, and the calculator returns the volts lost, the drop as a percentage, the voltage that actually arrives at the load, and the smallest conductor size that meets your chosen limit.
It also works backwards: it reports the longest run the selected conductor can carry before the drop exceeds the limit. That is often the more useful number, because cable size is usually fixed by the load and the length is what you are still deciding.
The formula behind the answer
For DC and single-phase AC circuits the standard approximation is:
and for a balanced three-phase circuit the factor of 2 becomes the square root of 3:
Where I is the load current in amperes, L is the one-way length in feet, CM is the conductor's cross-sectional area in circular mils, and K is the resistivity constant of the conductor material. The factor of 2 is there because current has to travel out to the load and back again, so the effective conductor length in a single-phase circuit is twice the physical distance.
The percentage drop is then simply Vd ÷ V × 100.
Why copper and aluminium give different answers
The K constant is the resistance of a one-foot length of conductor with a cross-section of one circular mil, and it changes with both material and temperature. At the 75°C conductor temperature that most modern terminations are rated for:
| Material | K (ohm-cmil/ft) | Practical effect |
|---|---|---|
| Copper | 12.9 | Reference performance; smaller and more flexible |
| Aluminium | 21.2 | About 64% more resistance, so roughly two trade sizes larger |
Aluminium is not a poor conductor — it is a lighter and cheaper one. For the same ampacity an aluminium conductor needs to be about two trade sizes larger than copper, and it needs terminations listed for aluminium. Where weight matters, such as a long service drop, aluminium frequently wins. Where space matters, copper usually does.
What counts as acceptable drop
| Circuit | Recommended maximum | Source |
|---|---|---|
| Branch circuit | 3% | NEC 210.19(A) informational note |
| Feeder plus branch, combined | 5% | NEC 215.2(A) informational note |
| Voltage drop across a motor running | Do not exceed 5% at the motor terminals | Industry practice; NEMA guidance |
| Sensitive electronics, medical, data | Often specified at 2% or less | Equipment manufacturer requirements |
Two practical notes. First, a drop that is legal is not automatically a good idea: a compressor that starts at 90% of nameplate voltage draws more current and runs hotter, which shortens its life even though nothing has tripped. Second, long runs feeding modern LED drivers and switch-mode power supplies can behave worse than the simple calculation suggests, because those loads are non-linear. When in doubt, size up one trade size.
Three worked examples
1. A 120 V receptacle 100 ft from the panel
A 20 A continuous load on a 120 V single-phase circuit, 100 ft of one-way run, copper conductor. With 12 AWG the drop works out at about 7.9 V, or 6.6% — well outside the 3% target, even though 12 AWG is the correct size for a 20 A circuit on ampacity grounds alone. Moving to 8 AWG brings the drop to roughly 2.0 V, or 1.7%. This is the single most common surprise in residential work: ampacity says 12 AWG, distance says something much larger.
2. A 480 V three-phase feeder
A 100 A load on a 480 V three-phase circuit running 250 ft. Because the voltage is four times higher, the same absolute drop is a quarter of the percentage. A 1/0 copper conductor gives about 5.6 V, or 1.2%. At 480 V you can run a long way before voltage drop becomes the deciding factor — which is exactly why industrial installations use higher distribution voltages.
3. A 24 V DC circuit for a pump
A 10 A load on a 24 V DC circuit 50 ft away. DC uses the same factor of 2 as single-phase. With 10 AWG the drop is about 1.25 V, or 5.2%. That is already past the usual 3% target, so 8 AWG is the practical choice at about 3.1%. Low-voltage DC systems are unforgiving: the lower the voltage, the more the same absolute loss matters as a percentage.
Four mistakes this calculation catches
- Sizing on ampacity alone. Ampacity tables tell you what will not overheat the conductor. They say nothing about whether the equipment at the far end will still work. Both checks are required.
- Using the one-way length twice. Enter the one-way distance. The calculator applies the return-path factor itself. Doubling it manually is a common and expensive error.
- Assuming three-phase uses the same factor. Three-phase uses √3 rather than 2, which makes the drop about 13% lower for the same conductor and current. Using the wrong factor overstates the drop and leads to needlessly large cable.
- Forgetting that K depends on temperature. A conductor operating at 90°C has higher resistance than one at 75°C. The calculator uses 75°C values, which is the usual termination rating in modern equipment.
How this calculator is verified
The resistance constant K and the circular-mil areas follow the values published in the NEC Chapter 9 tables and standard conductor tables. The simplified K method is the one used in practice for feeder and branch-circuit sizing; it neglects AC reactance, power factor, conduit effects and temperature correction, which matters for very large conductors and for circuits with a low power factor.
- NFPA 70, National Electrical Code — Article 210.19(A), Article 215.2(A), Chapter 9 Table 8 and Table 9.
- NEMA — motor terminal voltage tolerance guidance (motors are normally rated for a 10% voltage variation).
- OSHA electrical standards — workplace requirements that reference the NEC for installation methods.
Conductor tables, constants and worked examples last verified: 19 September 2026.