The formula
Voltage drop = 2 × Length × Current × 0.01724 ÷ Area (mm²)
This resistive calculation is accurate for the small cable sizes used in leads and sub-mains. For large cables and long distances, an electrician will also consider reactance and the drop already present at the supply.
Worked example
A standard 25 m 2.5 mm² campsite hook-up lead at 16 A: 2 × 25 × 16 × 0.01724 ÷ 2.5 = 5.5 V, or 2.4% at 230 V. That's fine. A 50 m reel of 1.5 mm² cable at 13 A would lose 15 V (6.5%), and must be fully unwound to avoid overheating.
Guidance
- UK wiring regulations (BS 7671) give a typical limit of 3% for lighting and 5% for other uses from the origin of the installation.
- Always fully unwind cable reels when drawing high current.
- Outdoor, shed and caravan supplies need RCD protection. Use a qualified electrician.
Frequently asked questions
What is the maximum length for a hook-up cable?
UK guidance limits caravan hook-up leads to 25 m in one length, with 2.5 mm² conductors for 16 A.
How much voltage do I lose on a 50m extension lead?
With 1.5 mm² cable at 10 A, about 11.5 V (5% at 230 V). At 13 A, about 15 V.