Why the table value is never the answer
A cable's current rating is a thermal limit: the current at which the conductor reaches its insulation's maximum temperature. Published tables state that current under reference conditions — 30 °C ambient air, one circuit, a defined installation geometry. Change any of those and the cable runs hotter at the same current, so the rating must be corrected:
where Ib is design current, In the protective device rating and Iz the derated capacity. Both corrections bite hard together: a 0.87 ambient factor with a 0.70 grouping factor leaves only 61% of the tabulated rating.
Worked example
6 mm² copper, PVC, clipped direct, 40 °C ambient, four circuits bunched, 15 A load:
- Tabulated (method C, 2 loaded conductors) = 32 A
- Ambient factor at 40 °C for PVC = 0.87
- Grouping factor for 4 circuits = 0.65
- Iz = 32 × 0.87 × 0.65 = 18.1 A — and a 16 A device fits between 15 A and 18.1 A
Without derating you might have fitted a 32 A breaker on a cable that can only carry 18 A in its actual installation. That's the failure mode this calculation exists to prevent.
Field notes
- Ampacity is one of three checks, not the whole design. Voltage drop often governs on long runs — check it with the cable voltage drop calculator — and fault-loop impedance governs disconnection time.
- Motor circuits carry starting current many times the running value; size from the running full load current but verify the protection tolerates the inrush profile.
- Grouping only counts loaded circuits that are bunched and running simultaneously. Spare cores and idle circuits don't heat the bundle.
- Buried and duct installations use soil thermal resistivity and depth factors not covered here — those need the full standard tables.
- Instrument cable is rarely ampacity-limited — a 4-20 mA loop carries milliamps. There, the constraints are voltage drop and, for RTDs, lead resistance error.
Frequently asked questions
What is cable derating?
Tabulated cable ratings assume reference conditions — typically 30 °C ambient air and a single circuit. Real installations are hotter or more crowded, so the tabulated value is multiplied by correction factors for ambient temperature and grouping. The result is the current the cable may actually carry.
How much current can 2.5 mm² cable carry?
Around 24 A for two loaded copper conductors clipped direct (method C) at 30 °C in PVC. At 45 °C ambient with four circuits bunched, the same cable derates to roughly 13 A — which is why the tabulated figure alone is never the answer.
Why does XLPE carry more current than PVC?
XLPE insulation is rated to a 90 °C conductor temperature versus 70 °C for PVC, so the same conductor can dissipate more heat before reaching its limit — typically 20 to 30 percent more current for the same cross-section.
Does the protective device rating matter?
Yes. The coordination rule is design current ≤ protective device rating ≤ derated cable capacity. A breaker larger than the derated capacity means the cable can overheat without the protection ever tripping.
Representative values based on IEC 60364-5-52 for two loaded copper conductors, provided for reference and education. Installation design must use the full tables of the standard or wiring code that applies in your jurisdiction. See our disclaimer.