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Cable Ampacity Calculator: What Your Copper Cable Can Really Carry
Enter a conductor size and this tool gives you a first-pass ampacity for three-phase copper cables — based on the IEC 60364-5-52 reference values, with ambient-temperature and grouping corrections applied automatically. Run a voltage-drop check in the same breath.
Cable Ampacity Calculator
Three-phase, copper, clipped direct (Method C) — the most common way LV power cables actually get run. Pick your conditions and the tool does the rest.
number of loaded circuits grouped together
How to read this: estimated ampacity = base value × temperature factor × grouping factor. Base values are IEC 60364-5-52 Method C, three-phase, single circuit, 30 °C air. This is a first-pass estimate for planning — always verify against the cable manufacturer's datasheet before you order. For aluminium, buried runs, sections above 120 mm², or long cable groups, ask for a full IEC 60287 calculation.
Why "just look up the ampacity" never works
I'll be straight with you up front: no responsible engineer sizes a cable from a single number. The current-carrying capacity printed in a datasheet is only ever true under one specific set of conditions. Change the conditions and the rating moves — sometimes a lot.
That's why every ampacity question starts with the same reality check. Before you even reach for a table, you need to pin down the things below, because each one quietly multiplies or divides your usable current.
| Factor | Why it matters | How big the swing can be |
|---|---|---|
| Conductor material | Copper conducts heat better than aluminium, so it runs cooler at the same current. | ~25% lower rating for Al vs Cu at the same section |
| Insulation type | PVC tops out at 70 °C; XLPE at 90 °C. A higher ceiling means more current before the insulation degrades. | XLPE typically rated ~15–25% above PVC |
| Installation method | Air can carry heat away easily; a buried cable can't. Method C (clipped direct) vs. in-ground methods differ a lot. | up to ~40% between methods |
| Ambient temperature | Hotter surroundings shrink the gap between cable temp and insulation limit — so less current. | ≈0.5 factor at 60 °C for PVC |
| Grouping / bunching | Each loaded circuit warms its neighbours. Ten cables in a bundle share the pain. | 0.5 factor at 9 bunched circuits |
| Soil conditions (buried) | Dry sand holds heat; moist clay pulls it away. Thermal resistivity of the ground is a big lever. | up to ~30% depending on soil |
And that's before we even mention voltage drop, which is a completely separate limit. A cable can be perfectly fine thermally and still drop too much voltage over a long run. You have to check both. The tool above does both — that's the whole point.
What this calculator assumes (read this before you trust it)
Let's be real: an estimate is only useful if you know exactly what it's an estimate of. Here's the precise basis, so you're not misled by a number that doesn't apply to your job.
Data basis
- Standard: IEC 60364-5-52 reference values, the tables used across most of the world for LV installation sizing.
- Method C — clipped direct / surface-mounted (against a wall, on a surface). The common, everyday way LV power cables run. Perforated cable tray and other free-air runs are Method E, which rates a few percent higher — not covered here.
- Copper conductors only, standard multicore cable, three-phase (3 loaded conductors), single circuit, 30 °C ambient air.
- PVC (70 °C) and XLPE (90 °C) insulation, sizes 1.5 to 120 mm².
Corrections applied
The tool automatically multiplies the base value by two correction factors from the same standard:
- Ambient temperature (Table B.52.14) — adjusts for air temperatures from 10 °C to 60 °C (PVC) or 80 °C (XLPE).
- Grouping (Table B.52.17) — adjusts for 1 to 20 loaded circuits bunched together.
What this tool does NOT cover
Buried installations (they need soil-temperature and thermal-resistivity factors — see our derating guide). Aluminium conductors. Single-phase circuits. Sections over 120 mm². Harmonic-rich loads. Parallel cables sharing one circuit. Armoured construction: the IEC tables don't separate armoured from unarmoured the way BS 7671 does (Tables 4D2A/4E2A vs 4D4A/4E4A), so for an SWA cable confirm the rating in the relevant armoured table. For any of these, an IEC 60287 calculation — or a manufacturer datasheet — is the right tool, and our engineers run those every day.
How to use the result: Ib ≤ Iz, then check voltage drop
You now have an estimated installed capacity, Iz. The golden rule of circuit design is simple:
Design current Ib ≤ Installed capacity Iz
If your load current is above Iz, the cable will run hotter than its insulation rating — that's not a "probably fine", that's premature ageing today and a fire risk in the long run. Step up a size and re-check.
Two things to remember after that:
- Protective device coordination. The overcurrent device also has to fit: it should clear the fault current before the cable overheats. In practice, specifiers check that the breaker rating sits below the cable's Iz (see IEC 60364-4-43 / BS 7671 chapter 43). If the calculator says "OK" but the breaker is bigger than Iz, you've got more work to do.
- Voltage drop is a separate limit. Enter your load current and length and the tool computes three-phase voltage drop. Under BS 7671, keep the total drop to about 5% for power circuits and 3% for lighting; IEC 60364-5-52 Annex G is more conservative, recommending 4% from origin to load. Over the limit and the load at the far end gets noticeably less than it's asking for — motors stall, electronics brown out.
If you entered a load current, the calculator also tells you the smallest size that fits — handy when you're staring at a motor plate instead of a cable spec.
Quick reference: Method C ampacity, copper, three-phase
Prefer a table you can bookmark? Here's the same base data, un-corrected. Remember: this is at 30 °C air, single circuit. Apply your own temperature and grouping factors for the real number.
| Conductor size (mm²) | PVC (70 °C) | XLPE (90 °C) |
|---|---|---|
| 1.5 | 17.5 | 22 |
| 2.5 | 24 | 30 |
| 4 | 32 | 40 |
| 6 | 41 | 52 |
| 10 | 57 | 71 |
| 16 | 76 | 96 |
| 25 | 96 | 119 |
| 35 | 119 | 147 |
| 50 | 144 | 179 |
| 70 | 184 | 229 |
| 95 | 223 | — |
| 120 | 259 | 322 |
Honesty note on the XLPE 95 mm² cell
You'll notice the XLPE 95 mm² cell is blank. The value does exist in the standard — the single-phase (2 loaded conductors) figure for 95 mm² XLPE, Method C, is around 325 A — but I couldn't trace a source-verified three-phase figure for that exact cell, and I'd rather show a gap than invent a number. For anything you're about to actually order, the complete ampacity chart plus a datasheet check is the honest way to finish the job.
When you need the full IEC 60287 calculation instead
This tool is a fast first pass. It's the right call for a quick sanity check or a rough design. But there are jobs where a lookup table — even a corrected one — genuinely isn't enough. Here's when I'd stop estimating and start calculating properly:
- Long cable runs where voltage drop and heat build-up interact over distance.
- Buried or ducted routes, where soil thermal resistivity and burial depth take over from air temperature.
- More than ~20 circuits grouped, or unusual bundle geometry.
- Aluminium conductors, or sections above 120 mm².
- Parallel cables feeding one load — the sharing maths needs care.
- Harmonic-heavy loads (VFDs, UPS) where skin and proximity effects start to bite.
- Anything going into a tender or a compliance submission, where the numbers get audited.
IEC 60287 is the standard that walks through the full thermal balance for these cases. I've written up how it actually works — and when it's worth paying for the calculation — in our IEC 60287 ampacity calculation guide. Or, if you'd rather hand it to people who run these calcs daily, the engineering desk below will do it properly.
Common questions, answered honestly
Why does the same 6 mm² cable show different ampacities in different tables?
Because every table states its own conditions. IEC 60364-5-52 lists values for a dozen installation methods, plus correction factors for temperature, grouping, and soil. American tables (NEC) use their own 60/75/90 °C columns with different derating logic. Same conductor, different assumptions, different numbers — none of them "wrong", they just answer different questions. Always read the conditions line before the value.
Can I size a cable from the mm² alone, without checking anything else?
Short answer: no. The mm² tells you the conductor cross-section, not what it can carry in your specific installation. A 6 mm² PVC cable in a 45 °C plant room carries a lot less than the same cable on a cool wall. That's precisely why the calculator asks for ambient temperature and grouping — two lines of extra input that stop you from undersizing by accident.
What's the real difference between PVC and XLPE ratings?
It comes down to the maximum continuous conductor temperature: 70 °C for PVC, 90 °C for XLPE. Since XLPE can safely run hotter, it dissipates more power at the same section — which is why XLPE values are consistently higher across the table (for example 6 mm²: 41 A vs 52 A). The trade-off is that XLPE costs more and the higher running temperature can matter for adjacent equipment. Neither is "better" — they suit different budgets and environments.
How much does bunched installation really cut the rating?
More than people expect. A single circuit is 1.00; two circuits bunched together drop to 0.80; four to 0.65; nine to 0.50. So nine circuits in one bundle carry half the current each that they'd carry alone. If your bundle is really tight or has extra thermal insulation around it, the practical factor is even lower — worth checking against our installation-method guide for the full table.
Is voltage drop the same as ampacity?
No, and confusing them is a classic sizing mistake. Ampacity is a thermal limit — how much current the cable can carry without overheating. Voltage drop is an electrical limit — how much the voltage sags along the run. A cable can be well within its ampacity yet drop 8% over a 200 m run, which starves the load. That's why the calculator shows both: thermal sizing picks the size, voltage-drop then verifies it's still OK over the actual distance.
Data sources
- IEC 60364-5-52, Low-voltage electrical installations — Part 5-52: Selection and erection of electrical equipment — Wiring systems. Method C reference values and correction factors (ambient temperature, grouping) used in this calculator. webstore.iec.ch — Ed. 3.1
- IEC 60287, Electric cables — Calculation of the current rating of electric cables — the full thermal calculation standard referenced in this article (current editions: IEC 60287-1-1:2023, IEC 60287-2-1:2023).
- IEC 60228, Conductors of insulated cables — conductor DC resistance values (Ω/km at 20 °C) used for the voltage-drop calculation.
- ECalPro, IEC 60364 Table B.52: Cable Current Ratings — interactive table reproducing the standard's ampacity and correction-factor values, cross-checked against the figures above. ecalpro.com/standards/iec-60364/table-b52-current-ratings
- BS 7671:2018+A2:2022, Requirements for Electrical Installations (IET Wiring Regulations) — voltage-drop guidance used here (≤3% lighting, ≤5% other circuits) as indicative limits.
Need the number for your actual project?
Lookup tables get you 80% of the way. For buried routes, aluminium, big sections, or anything that's going to be audited, our engineers run the full IEC 60287 calculation and send you the sized recommendation — free quote, no obligation.
