How far can an electrical installation run before the voltage drop becomes too large? This question comes up with every new outdoor installation, garage connection or outbuilding. The maximum cable length depends on three factors: the permitted voltage drop (3% under DIN VDE), the cable cross-section and the current flowing through it. The maximum cable length calculator reverses the standard voltage drop formula and gives you the maximum run length directly – no need to solve the equation by hand.
Step by Step: How to Use the Maximum Cable Length Calculator
- Enter system voltage: Typically 230 V for single-phase systems or 400 V for three-phase systems.
- Set the maximum voltage drop: Enter the permitted percentage. DIN VDE 0100 specifies 3% as standard. For critical applications (e.g. control systems), choose 1–2%.
- Enter operating current: Enter the maximum operating current of the load – e.g. 16 A for a fused socket circuit.
- Choose cable cross-section: Select the planned or existing cross-section – e.g. 2.5 mm² copper.
- Read the result: The calculator outputs the maximum run length in meters. If your planned route is longer, you need to step up to a larger cross-section.
Practical Examples
Example 1 – 16 A socket circuit: 230 V, 3% max. drop, 16 A, 2.5 mm² copper. L_max = (0.03 × 230 × 2.5) / (2 × 16 × 0.0178) = 17.25 / 0.5696 = 30.3 m. Any run up to 30 m is safely achievable with 2.5 mm².
Example 2 – Garage 40 m away: Same parameters but 40 m run needed. 2.5 mm² is not enough. With 4 mm²: L_max = (0.03 × 230 × 4) / (2 × 16 × 0.0178) = 27.6 / 0.5696 = 48.5 m. Sufficient for 40 m.
Example 3 – 11 kW wall box supply cable: 400 V three-phase, 16 A, 4% drop (3% house installation + 1% supply), 6 mm² copper. L_max = (0.04 × 400 × 6) / (2 × 16 × 0.0178) = 96 / 0.5696 = 168.5 m. More than enough for typical household distances (under 50 m).
Maximum Cable Length Calculation
Formula: L_max = (ΔU% × U × A) / (2 × I × ρ × 100). At 230 V, 3% max. drop, 16 A, 2.5 mm² copper = approx. 23 m max. length.
Frequently Asked Questions (FAQ)
- Does the maximum cable length apply to the whole run or per socket?
- The maximum length applies to the entire run from the circuit breaker (consumer unit) to the furthest load. In a daisy-chained socket circuit, the distances add up. Always calculate the distance to the most remote device.
- What changes with three-phase (400 V) compared to single-phase (230 V)?
- With a symmetrically loaded three-phase system (all three phases equally loaded), the neutral conductor carries no return current. This effectively doubles the permissible cable run compared to a single-phase system with the same cross-section – or allows a significantly smaller cross-section.
- What do I do if the calculated length isn't enough?
- Three options: (1) Use a larger cable cross-section – the permissible length increases in proportion to the cross-section. (2) Move the distribution board closer to the load (sub-board). (3) For runs over 100 m, consider whether a 400 V feed with a transformer is more practical.
