A device at the end of a long cable does not receive the same voltage as one plugged directly into a socket. This voltage drop is physically unavoidable – but under DIN VDE 0100 it must not exceed 3%, otherwise devices will not function correctly. The problem is especially critical with long garden cable runs, garage connections or sub-distribution boards in outbuildings. The voltage drop calculator shows whether your planned cable cross-section is adequate or whether you need to step up to a larger one.
Step by Step: How to Use the Voltage Drop Calculator
- Enter cable length: Measure the length of the cable from the distribution board to the load – e.g. 35 meters for a garden shed.
- Enter current: Enter the rated current of the load. A 2,000-watt circular saw at 230 V: I = P ÷ U = 2,000 ÷ 230 = 8.7 A.
- Choose cable cross-section: Select the planned cross-section – e.g. 1.5 mm² for standard sockets or 2.5 mm² for higher loads.
- Select material: Copper (resistivity ρ = 0.0178 Ω·mm²/m) is standard. Aluminium (ρ = 0.028) is used less often.
- Check the result: If the calculated voltage drop exceeds 3%, you must increase the cross-section or shorten the cable run.
Practical Examples
Example 1 – Garden shed connection: Cable length 40 m, load 16 A, cross-section 2.5 mm² copper. ΔU = (2 × 40 × 16 × 0.0178) / 2.5 = 9.1 V = 3.96% – too high! With 4 mm² cross-section: ΔU = 5.68 V = 2.47% – within limits.
Example 2 – LED lighting in a basement: 20 m cable run, 3 A total load, 1.5 mm² copper. ΔU = (2 × 20 × 3 × 0.0178) / 1.5 = 1.42 V = 0.62%. Well under the limit – correctly sized.
Example 3 – EV charging station: 50 m cable run, 32 A (11 kW wall box), planned 6 mm² copper. ΔU = (2 × 50 × 32 × 0.0178) / 6 = 9.49 V = 4.12% – critical! With 10 mm²: ΔU = 5.69 V = 2.47% – within limits.
Voltage Drop Calculation: Formula and Practice
Formula: ΔU = (2 × L × I × ρ) / A. DIN VDE 0100 permits a maximum 3% voltage drop. Copper: ρ = 0.0178 Ω·mm²/m.
Frequently Asked Questions (FAQ)
- Why is the cable length doubled (factor 2) in the formula?
- The current has to travel from the distribution board to the load and return via the neutral conductor. The electrical path is therefore twice as long as the physical cable length. That's why L in the formula represents the one-way cable length but is multiplied by 2 to account for the outward and return path.
- What happens if the voltage drop is too large?
- Electric motors (e.g. in circular saws or pumps) run hotter at reduced voltage and can fail prematurely. Lighting may flicker or dim. Electronic devices can produce erroneous readings or shut down. Additionally, with a large voltage drop the circuit breaker may fail to trip reliably in a fault condition.
- Does the 3% limit apply to all installations?
- The 3% limit under DIN VDE 0100-520 applies to domestic and commercial installations at 230/400 V. The same 3% applies to lighting circuits; for other loads, up to 5% may be tolerated in some cases. In low-voltage systems (12 V, 24 V) the limit is often applied more strictly because absolute losses have a proportionally greater effect.
