Copper against aluminium
Copper gives 7.46 V. Aluminium gives 12.25 V — a difference of 4.80 V more (+64.3%).
Same size, same run. Aluminium has about 64% of copper’s conductivity, and the drop rises in the same proportion.
| Input | Copper | Aluminium |
|---|---|---|
| Supply voltage(V) | 240 | 240 |
| Load current(A) | 20 | 20 |
| One-way run length(ft) | 150 | 150 |
| Conductor | Copper | Aluminium |
| Wire size (AWG or kcmil) | 10 AWG | 10 AWG |
| Circuit | Single-phase | Single-phase |
| Voltage dropped | 7.46 V | 12.25 V |
Every figure, both ways
| Step | Copper | Aluminium | Difference |
|---|---|---|---|
| Circular mils | 10,380 | 10,380 | 0 |
| Resistivity constant K | 12.9 | 21.2 | +8.3 |
| Voltage dropped in the run(V) | 7.46 | 12.25 | +4.8 |
| Drop as a percentage(%) | 3.11 | 5.11 | +2 |
| Voltage reaching the load(V) | 232.54 | 227.75 | -4.8 |
| Power burned in the wire(W) | 149.13 | 245.09 | +95.95 |
| Longest run that stays under 3%(ft) | 144.84 | 88.13 | -56.71 |
| Percentage points under the 3% guideline(%) | -0.11 | -2.11 | -2 |
Questions people actually ask
- Does lowering conductor change the answer in proportion?
- The two sides differ by 64.3% in the answer. Read the step table below to see which line moves and which does not — that is where a formula stops being a black box.
- What is held constant between the two columns?
- Everything except conductor: supply voltage, load current, one-way run length, wire size (awg or kcmil), circuit. That is what makes the difference attributable — change two things at once and neither column tells you which one moved the answer.
- Can I compare my own two numbers?
- Yes, from the address bar: add ?a.material=… and &b.material=… to set each side, and a parameter without a prefix sets both at once. The page is computed per request, so any pair works — these two are only the ones worth a link.
Change either side by adding query parameters — ?a.material=… and b.material=…. A parameter without a prefix sets both sides at once. Source: NFPA 70 — National Electrical Code (free online access), Copper Development Association — ampacity adjustments for wire and cable