How many amps can 2.5 mm² wire carry? Wire and cable ampacity table

People ask “how many amps does 2.5 carry?” expecting one number. In fact a wire carries the current that keeps its insulation below its limit, 70 °C for PVC. The more the wire is enclosed in an insulated wall, bunched with others in one conduit, or sitting in hot air, the lower the permitted current. So the code gives a table per installation method, then correction factors for temperature and grouping.

The tables below are from SBC 401 (2007 edition), which reproduces IEC 60364-5-52. The current SBC 401-CR-2024 keeps the same structure: Annex B.52 for current-carrying capacities and Annex G.52 for voltage drop. Before final approval, check the official code and your project specification.

How to read it: pick the installation method first, then the number of loaded conductors (two for single-phase, three for three-phase; the earth conductor does not count), then read the value. For example 4 mm² in conduit on a wall, single-phase: 32 A at 30 °C.

The table assumes 30 °C ambient air. Even the Saudi Electricity Company’s planning standard rates its overhead lines at 50 °C air. If a cable runs on a roof, in a service corridor or near outdoor units, take the design ambient set by the consultant and multiply by the factor:

Bunched circuits heat one another. Grouping factors for circuits bunched in conduit, trunking or on a surface (Table A.52-17): two 0.80, three 0.70, four 0.65, five 0.60, six 0.57. Multiply them with the temperature factor.

Example: three 2.5 mm² circuits in one conduit at 45 °C: 24 × 0.79 × 0.70 = 13.3 A. A 16 A breaker no longer fits; use 13 or 10 A, split the circuits into separate conduits, or go up to 4 mm² (32 × 0.79 × 0.70 = 17.7 A) to keep 16 A.

SBC 401 (clause 43-3.2) requires the breaker rating to sit between the load current and the corrected cable capacity, and the conventional tripping current I2 to be no more than 1.45 × Iz. Miniature breakers to IEC 60898-1 trip by 1.45 × In, so with them In ≤ Iz is enough.

• 2.5 mm² in conduit at 45 °C (19.0 A): largest standard breaker 16 A.
• 4 mm² (25.3 A): up to 25 A.
• 6 mm² (32.4 A): up to 32 A.
• 10 mm² (45.0 A): up to 40 A.
• 16 mm² (60.0 A): up to 50 A.

These are for one circuit per conduit; with grouping or higher temperatures, recalculate.

XLPE insulation runs at 90 °C conductor temperature, so its ratings are higher than PVC and less sensitive to heat (factor 0.82 at 50 °C against 0.71 for PVC). In method C with two loaded conductors, 2.5 mm² XLPE carries 33 A against 27 A for PVC. Do not mix tables: read the one for the actual insulation, and make sure equipment terminals are rated for that temperature.

Aluminium carries roughly three-quarters of what copper carries at the same size, so it usually needs one size larger for the same breaker, plus terminations tested and approved for aluminium as the code requires (Table 52-5). It is therefore a designer’s option for large mains, while final circuits in buildings are normally copper.

• Reading method C for wires in conduit: C is for cables clipped direct and rates higher than B1; single wires in conduit are read from B1.
• Counting earth and neutral as loaded: a single-phase circuit has two loaded conductors (line and neutral), a balanced three-phase circuit three, and the earth never counts. Where harmonics are high on a three-phase circuit, the designer treats the neutral per the code’s harmonics annex.
• Forgetting grouping: three circuits in one conduit cut the rating to 70 %.
• Mixing PVC and XLPE tables: each insulation has its own table and temperature factor.
• Trusting the number on the coil box: a marketing figure states neither installation method nor temperature; use the code table or the manufacturer’s table with its stated conditions.
• Ignoring hot sections of the route: if the cable crosses an exposed roof or passes a heat source, size on the worst section.