Electrical Cable Size Calculator
This electrical cable size calculator sizes a cable by voltage drop only. Enter the load current, the one-way cable length, the supply voltage and the largest drop you will accept, pick copper or aluminium, and it returns the exact minimum cross-section in mm², the next standard cable size up, and the actual voltage drop that size will give you. It does not check ampacity, temperature or grouping: those checks still have to happen separately, and the article below says exactly where.
Quick Answer
For a 20 A load on a 30 m one-way run at 240 V with a 3% drop limit in copper, the exact minimum size is 2.87 mm², so the recommended standard cable is 4 mm². That size drops 5.17 V in practice, which is 2.16%, comfortably inside the 3% limit. The same run in aluminium needs a 6 mm² cable.
Key takeaways
- This tool sizes by voltage drop only. A cable can pass the drop test and still be too small to carry the current safely, so an ampacity check is a separate, mandatory step.
- Length is one way. The formula already counts the return conductor, which is where the factor of 2 comes from.
- Aluminium needs roughly one to two standard sizes more than copper for the same drop, because its conductivity is 35 against copper at 58.
- Low-voltage runs are the demanding ones: at 12 V a 3% limit allows only 0.36 V of drop, so sizes climb fast.
- Quick Answer
- How to use this cable size calculator
- The voltage drop formula behind the result
- Worked example: 20 A over 30 m at 240 V
- What this calculator does not check: ampacity comes second, not never
- Copper or aluminium: the trade-off in one comparison
- What competing calculators miss
- Choosing your drop limit: 3%, 5%, and the low-voltage trap
- Frequently asked questions
- Add This Calculator to Your Website
- References
How to use this cable size calculator
Enter the run you are planning, then click Calculate. Results stay hidden until you do, so you can test several lengths and limits before settling on a size.
- Load current: the amps the load actually draws. If the nameplate gives watts instead, a amp and watt calculator converts between the two for a known voltage.
- Cable length: the one-way distance from the supply to the load, in meters. Do not add the return run: the maths adds it for you.
- Voltage: pick 12, 24, 120, 240 or 480 V.
- Max voltage drop: your limit as a percentage of that voltage. The pre-filled 3% matches the branch-circuit design target most electricians start from.
- Material: copper at a conductivity of 58, or aluminium at 35, in meters per ohm mm².
Read the three outputs together. The minimum required size is the exact maths answer. The recommended size is that answer rounded up to a cable you can actually buy. The actual drop is what the rounded-up cable will really produce, and it is the figure to compare against your limit.
The voltage drop formula behind the result
The calculator uses the standard single-phase and DC voltage-drop sizing formula, the same relationship set out in conductor resistance references such as the Engineering ToolBox table of resistivity and conductivity. First it turns your percentage limit into volts, then it solves for the cross-section that keeps the drop at or under that limit.
Minimum size A (mm²) = (2 × Current × Length) ÷ (Conductivity × Allowed drop)
Actual drop (V) = (2 × Current × Length) ÷ (Conductivity × Chosen standard size)
Three details matter. The factor of 2 exists because current travels out on one conductor and back on another, so the resistive path is twice the one-way length. Conductivity is the inverse of resistivity: copper at 58 m/Ωmm² is the same material property as a resistivity of about 0.0172 Ωmm²/m, and aluminium at 35 is about 0.0286 Ωmm²/m, values consistent with the HyperPhysics resistivity reference. And the formula is resistive only: it ignores cable reactance, power factor and conductor heating, which is reasonable for the short to medium runs and modest sizes this kind of quick sizing is used for, and one reason the result is a starting point rather than a final design.
Worked example: 20 A over 30 m at 240 V
A workshop circuit draws 20 A. The board is 30 m from the load, the supply is 240 V, and the design limit is a 3% drop. Here is the calculation the tool runs, step by step.
Change one input and the picture shifts. The table below uses outputs produced by this calculator, so you can type any row in and reproduce it exactly.
| Scenario | Minimum size | Recommended | Actual drop |
|---|---|---|---|
| 20 A, 30 m, 240 V, 3%, copper | 2.87 mm² | 4 mm² | 5.17 V (2.16%) |
| 20 A, 30 m, 240 V, 3%, aluminium | 4.76 mm² | 6 mm² | 5.71 V (2.38%) |
| 16 A, 50 m, 240 V, 3%, copper | 3.83 mm² | 4 mm² | 6.90 V (2.87%) |
| 32 A, 25 m, 240 V, 3%, aluminium | 6.35 mm² | 10 mm² | 4.57 V (1.90%) |
The 16 A row is the instructive one. A longer run at a lower current lands within a whisker of the limit: the 4 mm² cable drops 2.87% against a 3% ceiling. There is almost no headroom for a hotter day, a longer route than planned, or a load that grows later, which is why many designers step up one size when the actual drop comes out that close.
What this calculator does not check: ampacity comes second, not never
Voltage drop answers an efficiency question: will enough voltage reach the load? Ampacity answers the safety question: can this cable carry the current without overheating its insulation? A complete cable selection has to pass both tests, and this tool performs only the first one.
That gap is not theoretical. On a short, high-current run the drop calculation can happily return 1.5 mm² while the current is far beyond what a 1.5 mm² cable may carry in a wall or conduit. The ampacity limits themselves live in the wiring codes: NFPA 70, the National Electrical Code, sets conductor ampacities in Table 310.16 for US work, while IEC 60364-5-52 does the same job internationally and the IET Wiring Regulations, BS 7671, carry the UK tables. Installation method, ambient temperature, grouping with other loaded cables and insulation rating all move those numbers, which is why no honest drop-only tool folds them into a single silent answer.
Use the result here as the voltage-drop candidate, then verify that same size against the ampacity table for your actual installation conditions, and take the larger of the two sizes. If the circuit is three-phase, the drop maths itself also changes: the factor of 2 is replaced by the square root of 3, and a dedicated 3-phase current calculator is the right companion for the load side of that work. A final selection on a real installation belongs with the code book and, where the work is regulated, a licensed electrician, a point the OSHA electrical safety guidance makes for good reason.
Copper or aluminium: the trade-off in one comparison
Both materials run through the identical formula; only the conductivity changes. That single number explains most of the practical difference.
Copper: conductivity 58
- Smaller cable for the same drop: 4 mm² does the worked example.
- Easier terminations, well understood by every installer.
- Heavier and usually more expensive per meter at larger sizes.
Aluminium: conductivity 35
- Needs a larger size for the same drop: 6 mm² for the same run.
- Much lighter, often cheaper on long feeder runs.
- Terminations need the correct lugs and anti-oxidant practice.
The pattern to remember is that aluminium at 35 has about 60% of copper’s conductivity, so the required cross-section grows by a factor of about 58 ÷ 35, or 1.66. In standard sizes that usually lands one or two steps up, exactly as the worked example shows. Weight and price then decide whether the bigger aluminium cable beats the smaller copper one on your run.
What competing calculators miss
The big manufacturer tools and the general calculator sites each give up something this page keeps.
- AWG only: the best-known voltage drop tools size in AWG and kcmil for NEC work and never show an mm² answer, which leaves metric projects converting by hand. This tool is metric from the first input to the last output.
- Ampacity folded in silently: several leading tools limit the answer by ampacity without saying which test decided the size. Here the scope is stated plainly: drop only, with the ampacity check named and pointed to its tables.
- Fixed assumptions buried in charts: some reference charts quietly assume one voltage, one power factor and 20 °C. This calculator exposes every assumption it uses, voltage, drop limit and conductivity, as an input you can change.
- No exact minimum: tools that jump straight to a standard size hide how close the run is to the edge. Showing the exact 2.87 mm² next to the 4 mm² recommendation tells you whether you have headroom or none.
- No failure signal at the top end: when even 300 mm² cannot hold the drop, most tools just print 300 mm². This one flags it in red, because a capped answer that still breaks your limit is a design problem, not a result.
Choosing your drop limit: 3%, 5%, and the low-voltage trap
Pick the limit before you pick the cable. The NEC treats voltage drop as a design recommendation rather than a hard rule: its informational notes suggest holding a branch circuit to about 3% and the combined feeder plus branch run to about 5%, so equipment still receives close to its nameplate voltage. Lighting and electronics with tight supply tolerances argue for the strict end; a tolerant resistive load on a short run can live with more.
Percentage limits hide an absolute truth: the allowed volts shrink with the supply. Three percent of 240 V is 7.2 V, a comfortable budget. Three percent of 12 V is 0.36 V, which is why 12 V solar, vehicle and boat runs so often end up in surprisingly thick cable. Try the worked example at 12 V in the tool and watch the recommended size jump: nothing about the load changed, only the voltage budget did. If a low-voltage result looks absurd, raising the system voltage, where the equipment allows it, usually beats buying ever larger copper.
One more honest limit: the standard size list in this tool stops at 300 mm². If your exact minimum comes out above that, the tool returns 300 mm² with a red flag and the true drop percentage, and the real design answer is parallel conductors or a rethink of the run, handled in the code tables rather than here.
Frequently asked questions
No. It sizes by voltage drop only. Ampacity, the safe current-carrying limit for your installation method, temperature and grouping, must be checked separately in NEC Table 310.16 or IEC 60364-5-52, and the final cable must satisfy both checks. Take the larger of the two sizes.
Current flows out to the load on one conductor and back on another, so the resistive path is twice the one-way distance. Enter the one-way length only; the calculator applies the factor of 2 for you.
Not directly. The factor of 2 is the single-phase and DC case. Three-phase voltage drop uses the square root of 3 instead of 2, along with line voltage, so a three-phase run sized here would come out larger than it needs to be. Use a three-phase method for those circuits.
3% is the common design target for a single branch circuit, and 5% for a feeder and branch combined, following the NEC informational notes. Use a tighter limit for sensitive electronics and long runs, and remember that at 12 V even 3% allows only 0.36 V, which forces much larger cables.
Aluminium conducts at 35 m per ohm mm² against copper at 58, about 60% as well. For the same current, length and drop limit, the aluminium cross-section must be about 1.66 times larger, which usually means one or two standard sizes up. In the worked example, copper needs 4 mm² where aluminium needs 6 mm².
The standard size list stops at 300 mm². If the exact minimum is above that, the calculator returns 300 mm² with a red warning and shows the real drop, which will be above your limit. The practical answers are parallel conductors, a higher supply voltage, or a shorter run, all decided with the code tables.
Yes in reality, no in this tool. Conductor resistance rises as the cable warms under load, so a hot cable drops slightly more voltage than the 20 °C style conductivity values used here suggest. The difference is small on typical runs, but it is one more reason to step up a size when the actual drop lands very close to your limit.
Add This Calculator to Your Website
Embed this calculator on your own site with the code below. Pick a size to preview it live, then copy the code.
Powered by Calculator Zilo
References
- National Fire Protection Association. NFPA 70, National Electrical Code. Accessed October 21, 2026.
- Institution of Engineering and Technology. BS 7671, Requirements for Electrical Installations (IET Wiring Regulations). Accessed October 21, 2026.
- Engineering ToolBox. Resistivity and Conductivity: Temperature Coefficients for Common Materials. Accessed October 21, 2026.
- Georgia State University, HyperPhysics. Resistance and Resistivity. Accessed October 21, 2026.
- U.S. Occupational Safety and Health Administration. Electrical Safety Standards and Guidance. Accessed October 21, 2026.