Work out how many metres or feet of electrical cable you need for a job. Add multiple cable runs, pick the circuit type, and get a total length with waste allowance — so you buy the right reel size first time.
Getting the right amount of cable sounds simple — measure the route and buy a reel. But most people forget the return leg, underestimate wall drops, or skip the waste allowance, and end up a few metres short with the power off and the merchant closed.
| Circuit | Typical one-way run | Notes |
|---|---|---|
| Ring main (sockets) | 40–60 m (130–200 ft) | Full loop — cable runs out and returns to the consumer unit |
| Radial (sockets) | 15–25 m (50–80 ft) | One-way — double for there-and-back |
| Lighting circuit | 20–40 m (65–130 ft) | Loop-in at each light — measure the full daisy-chain route |
| Cooker / oven | 5–15 m (16–50 ft) | Dedicated radial — double the one-way distance |
| EV charger | 10–25 m (33–80 ft) | Garage or drive — usually SWA if crossing outside |
| Outdoor / garden | 10–40 m (33–130 ft) | SWA armoured cable for buried runs |
Trace the actual route the cable will follow — along walls, through ceiling voids, down to sockets — and measure that path. Do not measure a straight-line distance; real cable routes are always longer because they follow building surfaces and pass through joists.
For most dedicated circuits (radial sockets, cooker, shower, EV charger), yes — the cable goes from the consumer unit to the point and back. Ring mains are different: measure the full loop distance. Lighting circuits are usually daisy-chained, so measure the total chain route.
A 10% allowance covers most jobs. Add 15–20% if the route has lots of bends, tight loft spaces, or if you are less experienced and might mis-cut a length. Running short mid-job is far worse than having a few metres spare.
In the UK, Twin & Earth and SWA cable typically comes in 10 m, 25 m, 50 m and 100 m reels. In the US, Romex (NM-B) is commonly sold in 25 ft, 50 ft, 100 ft and 250 ft spools. Always round up to the next size — mid-run joints are not allowed under most wiring regulations.
Under BS 7671 (UK) and NEC (US), joints must be accessible and housed in an approved junction box — you cannot bury a joint in a wall or ceiling void. It is always better to buy enough cable in one length and avoid joints altogether.
