Cable Size Calculator

Calculations

Step 1 — Design Current

The full-load current I_b drawn by the load is calculated from the apparent power. For a single-phase circuit: I = P / (V × pf). For three-phase: I = P / (√3 × V × pf). The phase_factor variable (set by the dropdown: 2 for single-phase, √3 ≈ 1.732 for three-phase) handles both cases in one formula by incorporating the effective voltage in the denominator.

Note: for single-phase, the loop voltage is V_system (line-to-neutral), and the phase_factor = 2 accounts for the go-and-return path length in voltage drop calculations later. For three-phase, phase_factor = √3 captures the line-to-line geometry.

Step 2 — Temperature Correction Factor

The tabulated ampacity (current-carrying capacity) of a cable is given at a reference ambient temperature, typically 30 °C. When the actual ambient temperature differs, IEC 60364-5-52 Annex B provides a correction factor C_t based on the ratio of temperature margins:

C_t = √[ (T_rated − T_amb) / (T_rated − 30) ]

where T_rated is the maximum permitted conductor temperature (70 °C for PVC, 90 °C for XLPE) and T_amb is the actual ambient temperature. A higher ambient reduces C_t below 1.0, requiring a larger cable.

Step 3 — Minimum Area from Current-Carrying Capacity

The maximum permissible current density J_max (A/mm²) is set by the conductor material and installation method (from the dropdown). The derated current capacity of a cable of area A is:

I_cap = J_max × C_t × A

Setting I_cap ≥ I_b and rearranging gives the minimum area required to carry the load current without overheating:

A_current = I_b / (J_max × C_t)

Step 4 — Permissible Voltage Drop

The absolute voltage drop limit V_drop is computed from the percentage limit and the system supply voltage:

V_drop = vd_limit × V_system

For example, a 5% limit on a 400 V supply gives V_drop = 20 V.

Step 5 — Minimum Area from Voltage Drop

The resistive voltage drop along the cable (ignoring reactance, which is negligible for cables below ~50 mm²) is:

ΔV = ρ × phase_factor × L × I_b / A

where ρ is the conductor resistivity (Ω·mm²/m), phase_factor accounts for the current path length (2× for single-phase return loop, √3 for three-phase line), and L is the one-way cable length. Rearranging for area:

A_vdrop = ρ × phase_factor × L × I_b / V_drop

Step 6 — Governing Minimum Area

The required conductor cross-section must satisfy both criteria simultaneously. The governing (larger) value is selected:

A_min = max(A_current, A_vdrop)

This is the theoretical minimum continuous conductor area. In practice, the engineer selects the next larger standard IEC cable size from the preferred series: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300 mm².

Step 7 — Verification at Selected Cable Size

Once the engineer selects a standard cable size, it is good practice to verify the actual voltage drop and current density at that size. Enter the chosen standard size below and the worksheet confirms compliance.

Results

The two minimum areas derived from each criterion are compared below, and the selected cable is verified against both limits.

Interpreting the Results

The worksheet reports two minimum conductor areas — one from thermal loading and one from voltage drop. The larger value governs the selection. For the example inputs (15 kW, pf = 0.85, 80 m run on a 400 V three-phase supply), the voltage drop constraint typically dominates for longer cable runs, while the ampacity constraint governs for short, heavily loaded feeders.

The selected 25 mm² cable should be checked: if I_capacity > I_b, the thermal criterion is satisfied; if vd_actual < vd_limit, the voltage drop criterion is satisfied. If either check fails, select the next standard size up and re-enter it as A_selected.

Always cross-check against manufacturer datasheets, grouping derating factors, and any applicable installation-specific factors not covered here (e.g. solar radiation, proximity to heat sources, protective device coordination).

Assumptions & Limitations

• Resistive voltage drop only — cable reactance is neglected (conservative for cables ≤ 50 mm², non-conservative above).
• Uniform current density is assumed — skin and proximity effects are ignored.
• The temperature correction follows the IEC 60364-5-52 Annex B simplified formula for air-ambient installations.
• A single grouping/bunching derating factor of 1.0 is assumed — add further derating if cables are grouped.
• Harmonic distortion in the load current is not considered.
• DC circuits require a different phase_factor (2 for both single and multi-wire DC) and resistivity at operating temperature.
• Earth fault loop impedance and protective device disconnection time checks are outside the scope of this worksheet.
• Voltage drop at starting (motor inrush) is not evaluated — a separate transient check is recommended for motor loads.

Cable Size Calculator

This worksheet determines the minimum required cable conductor cross-sectional area for a given electrical load, ensuring that both current-carrying capacity (ampacity) and voltage drop criteria are satisfied simultaneously.

The governing principle is Ohm's law applied to the cable impedance, combined with the current density limits set by IEC 60364-5-52 (or equivalent NEC 310 tables). The cable size is selected as the larger of the two independently calculated minimum areas, ensuring both thermal and power-quality constraints are met.

This calculation applies to single-phase and three-phase AC systems with copper or aluminium conductors in common installation methods (clipped direct, in conduit, buried, etc.).

System Configuration

Select the conductor material and installation method from the dropdowns below. These selections define the current-carrying capacity (ampacity) per mm² and the resistivity used in voltage drop calculations.

Inputs

Enter the load power, power factor, cable length (one-way), system voltage, and the maximum permissible voltage drop as a percentage of the supply voltage. Typical voltage drop limits are 3% for lighting circuits and 5% for power/motor circuits per IEC 60364-5-52.