Short answer: cable size comes from two checks that must both pass — ampacity, which asks whether the cable will run too hot, and voltage drop, which asks whether the load at the far end will still see enough volts. Ampacity is not a property of copper alone: the same 4 mm² carries about 32 A clipped to a wall and about 28 A in conduit with three loaded conductors, and less again at 45 °C ambient or in a bunch of six circuits. Add the breaker rule Ib ≤ In ≤ Iz and the trip rule I2 ≤ 1.45 Iz, and the table stops being a lookup and becomes a calculation.
We export breakers, cable accessories and panel components as an independent trading company — not a manufacturer and not an appointed distributor of any brand. This is the method we use when a customer’s drawing shows a breaker size but not a cable size.
The reference table, and its conditions
The numbers below follow the IEC 60364-5-52 approach for copper conductors with 70 °C PVC insulation, at 30 °C ambient, in two installation methods: B1 (conductors in conduit on a wall) and C (cable clipped direct). They are the two methods that cover most export panel and small-building work.
| Section (mm²) | B1, 2 loaded conductors | C, 2 loaded conductors | B1, 3 loaded conductors | C, 3 loaded conductors |
|---|---|---|---|---|
| 1.5 | 17.5 A | 19.5 A | 15.5 A | 17.5 A |
| 2.5 | 24 A | 27 A | 21 A | 24 A |
| 4 | 32 A | 36 A | 28 A | 32 A |
| 6 | 41 A | 46 A | 36 A | 41 A |
| 10 | 57 A | 63 A | 50 A | 57 A |
| 16 | 76 A | 85 A | 68 A | 76 A |
| 25 | 101 A | 112 A | 89 A | 96 A |
| 35 | 125 A | 138 A | 110 A | 119 A |
| 50 | 151 A | 168 A | 134 A | 144 A |
| 70 | 192 A | 213 A | 171 A | 184 A |
| 95 | 232 A | 258 A | 207 A | 223 A |
| 120 | 269 A | 299 A | 239 A | 259 A |
Three conditions are baked into that table and each one is a place where a real installation differs: 30 °C ambient, one circuit, and PVC insulation. Change any of them and the number moves.
Correction factors: the part that gets skipped
Ambient correction is the first. At 40 °C a PVC cable loses about 13 % of its tabulated current, at 45 °C about 21 %, and at 50 °C about 29 %. A panel in Lagos or Dubai designed from a European table without this step is designed for the wrong climate.
Grouping is the second. Cables bunched together cannot each shed heat as if they were alone, so a group of six circuits in one tray is derated by roughly 20–30 % depending on the arrangement. The full factors are in the standard’s tables; the point to remember is that a bundle is not a set of independent cables.
| Condition | Typical effect on tabulated current | When it matters |
|---|---|---|
| Ambient 40 °C instead of 30 °C | ≈ 0.87 | Most tropical and Gulf projects |
| Ambient 45 °C | ≈ 0.79 | Plant rooms, roof spaces, containers |
| Ambient 50 °C | ≈ 0.71 | Solar combiner boxes, engine rooms |
| Six circuits grouped | ≈ 0.72–0.8 | Busy cable trays, duct banks |
| Buried in soil, thermally poor | 0.7–0.9 depending on soil and depth | Underground runs, not panel work |
The two rules that tie cable to breaker
The design current Ib must not exceed the breaker’s rated current In, and In must not exceed the cable’s corrected capacity Iz. That is the first rule, and it is the one that keeps the cable from being overloaded before the breaker notices.
The second rule covers the breaker’s tolerance band: the current that reliably trips it, I2, must not exceed 1.45 times Iz. For a standard MCB to IEC 60898-1, I2 is conventionally taken as 1.45 In, which is why the rule collapses into “In ≤ Iz” for that family — a convenience that stops being true for adjustable or higher-tolerance devices, where the actual trip current has to be checked.
Both rules are necessary and neither is sufficient: they say nothing about voltage drop, which is a separate check with its own limit, usually 3 % for a final circuit and 5 % overall from the origin.
Voltage drop changes the answer at low voltage
Ampacity is about heat; voltage drop is about length. A 2.5 mm² cable that is perfectly adequate for 20 A over 10 m is not adequate over 60 m, because the volts lost in the conductor scale with length and current. That is why long runs are often sized two steps above the ampacity table.
The effect gets sharper as system voltage falls. The same absolute drop is a larger percentage of 127 V than of 230 V, so a 110–127 V installation can hit its 3 % limit at a length where a 230 V installation is still comfortable. For three-phase circuits the drop is lower for the same power because the current is lower — the same reason a 400 V motor needs thinner cable than a 230 V one of equal kW.
Aluminium, temperature and other variations
Aluminium carries roughly 78 % of the current of a copper conductor of the same section, so it needs about one size up for the same duty, plus terminals rated for aluminium and a jointing method that deals with its oxide layer. Flexible multi-strand cable in a moving application is a different rating again. And XLPE insulation (90 °C conductor) carries more than PVC for the same section, which is why the insulation type has to be on the order, not just the size.
What to put on the purchase order
Section and conductor material; number of cores and whether there is a protective conductor; insulation type (PVC or XLPE) and its voltage grade; installation method; ambient and grouping conditions; the breaker rating it must coordinate with; the run length and the permitted voltage drop; whether the cable must be flame-retardant or low-smoke for the destination; and the certification the market requires. Add the destination country’s voltage and frequency, because the volt-drop check and the breaker selection both depend on them.
FAQ
Can I use the ampacity table without correction factors?
Only if the installation really is 30 °C, one circuit and PVC in the stated method. In practice, most export projects need at least the ambient correction, and busy trays need the grouping correction as well.
Why is 4 mm² sometimes rated at 32 A and sometimes at 28 A?
Because of the number of loaded conductors and the installation method. In conduit with two loaded conductors it is about 32 A; with three loaded conductors in the same conduit it is about 28 A. The heat from the third conductor is the difference.
Does a 32 A breaker protect a 4 mm² cable?
Usually yes for a single-phase circuit in method C at moderate ambient, but not automatically. Once ambient correction and grouping are applied, the corrected capacity can fall below 32 A, and then the breaker must be reduced or the cable increased.
Is voltage drop part of the cable standard?
It is part of the installation requirements rather than the cable’s own rating, but it decides the size just as often as ampacity does. Check both, and check the length before choosing the section.
Related: Voltage drop calculation for a cable run · Breaker and cable coordination: Ib, In, Iz · Copper busbar current rating · Planning an enclosure in DIN modules · MCB range overview · Cross-reference an existing device · Request a quotation