The constraints that decide cable size
Cable sizing per IEC 60364-5-52 is a multi-constraint problem. The cable must be large enough to carry the design current without overheating its insulation, large enough that the voltage drop across its length stays within the regulated maximum, and — where the fault level is known — large enough to survive a short circuit until the protective device clears it. Most spreadsheet calculators handle one of the three. Picking the largest of the answers is the only correct result, and which constraint bites depends on the run length, the power factor, how aggressively the site has been derated, and how stiff the supply is.
Ampacity by installation method, then derated
IEC 60364-5-52 tabulates current-carrying capacity per reference installation method, because the same conductor sheds heat very differently in a thermally insulated wall (A1 / A2), in surface conduit or trunking (B1 / B2) and clipped direct to a wall or tray (C). A 16 mm² XLPE copper cable is good for 100 A clipped direct but only 73 A buried in an insulated partition. Two corrections then derate the table figure: Ca for ambient temperature (hot sites cut it sharply — at 50 °C, PVC is at 71% of its 30 °C rating) and Cg for grouping (touching cables derate each other — 3 circuits cut ampacity to 70%, 6 circuits to 57%). For aluminium, multiply the copper ampacity by 0.78.
Voltage drop: the second constraint
Voltage drop scales linearly with cable length and current, and inversely with cross-section. For cables under 25 mm², resistance dominates and the simplified formula Vd = k × ρ × L × I × cos φ / S gives the right answer. Above 25 mm² (and above 16 mm² when power factor drops below 0.9), reactance becomes a meaningful share of the total and the formula expands to include the X sin φ term. National codes set the limits: 5% from origin to load is the IEC baseline, 2.5% for DEWA final circuits, 3% is common practice for sub-mains and feeders.
DC circuits
Battery-string and DC-bus cables are the UPS dealer's most common sizing job and the most often done wrong. There is no power factor and no reactance: the current is simply I = P / V, and because a 48 V or 240 V DC bus carries far more current for the same power than a 400 V AC feed, voltage drop bites early — Vd = 2 × ρ × L × I / S across the go and return conductors. Ampacity uses the two-loaded-conductor column. Remember that a DC fault has no current zero to help the fuse clear it; check the string fuse's DC rating and let-through as well as the cable.
Short-circuit withstand (adiabatic check)
During a fault the conductor has no time to shed heat, so all the energy I² t goes into warming the copper. IEC 60364-4-43 §434.5.2 gives the minimum cross-section as S ≥ √(I² t) / k, where k depends on the conductor and the insulation's short-circuit temperature limit: 115 for copper / PVC, 143 for copper / XLPE, 76 and 94 for aluminium. Enter the prospective fault current at the cable and the protective device's disconnection time from its curve; if the thermal floor is above the ampacity answer, the calculator steps the size up and says so. A current-limiting MCCB or fuse with a short clearing time is often cheaper than the larger cable.
Why the regional defaults matter
Indoor ambient in a chilled office in northern Europe is 20–25 °C; outdoor in summer in the Gulf is 45–50 °C. Sizing the same load for both sites gives different answers — and a cable that's ampacity-correct in a temperate site can be one or two size steps short when installed in a hot region. The region defaults in this calculator preset Ca and the voltage- drop limit to the typical values for KSA, UAE, and Egypt, and fall back to a generic IEC baseline (30 °C ambient, 5% drop limit) for everywhere else. Override the defaults whenever your site differs.
Standards behind the math
Ampacity values from IEC 60364-5-52 Ed. 3.1 (2024-11) Tables B.52.4 (PVC) and B.52.5 (XLPE), copper, reference methods A1, A2, B1, B2 and C. Free-air methods E / F / G and buried methods D1 / D2 are not included — their ratings depend on spacing, arrangement and soil resistivity. Aluminium ampacity is the IEC convention of 0.78 × copper. Ca from Table B.52.14; Cg from Table B.52.17 (touching, single layer). Resistivity from IEC 60228 with the standard temperature coefficient applied at the insulation rating (70 °C PVC, 90 °C XLPE). Reactance simplified to 0.08 mΩ/m, the Schneider EIG cross-vendor value for typical LV cable. Adiabatic k from IEC 60364-4-43 Table 43A.