How to calculate the right cable cross-section?

What determines the cable cross-section?

The correct cross-section follows from three criteria: current-carrying capacity (protection against overheating), permissible voltage drop (function at the load) and short-circuit protection. The largest of the resulting cross-sections always governs.

Current-carrying capacity defines the continuous current a cable may carry without the insulation getting too hot. It depends strongly on the installation method, ambient temperature and grouping of several cables. In Germany these influences are set out in DIN VDE 0298‑4.

• Current-carrying capacity: protects the insulation from thermal overload.
• Voltage drop: ensures enough voltage reaches the load.
• Short-circuit and overload protection: fuse must match the cross-section.
• Installation method and grouping markedly lower the permissible current.

How do you determine current capacity?

DIN VDE 0298‑4 assigns each installation method a reference method (A1 to E) and states the base current capacity. Embedded in a thermally insulated wall (B2), a cable carries less current than free in air (E), because heat is dissipated more poorly.

The table values apply at 30 °C ambient temperature and a single cable. If several loaded cables are grouped, or it is warmer, the values are multiplied by correction factors - the permissible current capacity falls.

• Ambient temperature above 30 °C: apply a temperature factor below 1.
• Several loaded conductors or cables: apply the grouping factor.
• Thermal insulation or conduit: plan for poorer heat dissipation.

How do you calculate voltage drop?

Even if the current capacity fits, a long cable can lose too much voltage. The load then receives too little voltage, motors run weak, LEDs flicker. Recommended limits are max. 3 % voltage drop for lighting and 5 % for other loads (guide values per DIN VDE 0100‑520).

For direct current or single-phase AC: ΔU = 2 · L · I / (κ · A). Here L is the one-way cable length in metres, I the current in amperes, A the cross-section in mm² and κ (kappa) the conductivity - about 56 for copper and about 35 m/(Ω·mm²) for aluminium. For three-phase the 2 is dropped and a factor √3 is added.

• Voltage drop too high: choose the next larger cross-section.
• Quick rule: double the length means double the voltage drop.
• Aluminium has only about 62 % of the conductivity of copper.

How do you proceed in practice?

In practice you work through both routes - current capacity and voltage drop - and take the larger cross-section. Then you check that the upstream fuse disconnects the cable in time in the event of a short circuit.

• 1. Determine the operating current I and cable length L.
• 2. Define installation method, temperature and grouping.
• 3. Select the cross-section by current capacity from DIN VDE 0298‑4.
• 4. Recalculate the voltage drop, enlarge the cross-section if needed.
• 5. Match the protective device (fuse/MCB) to the cross-section.

Frequently asked questions