A 40-metre lead uncoiled across the lawn looks like a solved problem: the light comes on, the drill runs, nothing gets warm. The installation can still sit outside what the wiring regulations allow, because the fault it hides is not heat but volts.
An extension lead is not a supply
A cable made for a living room, lying in wet grass, with a joint sitting in whatever the last shower left behind: none of it is weatherproof. A reel left coiled traps its own heat too, so a lead good for 13 A fully unwound can cook its insulation with two metres pulled off. Over about 1,000 W, unwind the lot.
- No protection of its own: the lead borrows the kitchen socket's breaker, never sized for a run down the garden.
- It gets unplugged: a shed freezer on a lead borrowed for the hedge trimmer defrosts while you are out.
The longer the run, the thicker the copper has to be
The regulations do not cap how far you can go. They cap how much voltage you may lose on the way: BS 7671 allows 3% for lighting and 5% for everything else, measured from the origin of the installation, which at 230 V is 6.9 V and 11.5 V. That budget is shared with the house, so the garden run adds to whatever the wiring indoors has already spent.
A cable can be entirely safe on current and still be out of specification on voltage: the two are limited by different things. Nothing trips, nothing smells hot. You get undervoltage at the far end instead, a motor that labours and stalls, lights that dip when the compressor kicks in, tools that sound wrong. Load and length are fixed by what goes out there, so the only variable left is the copper's cross-section.
How far each cable size actually reaches
These are the BS 7671 voltage drop figures for two-core armoured cable, worked back to the longest run inside the 5% allowance at 230 V.
Read the 3 kW column against a garden lead: 40 m of 1.5 mm² flex is out of voltage at 30 m. For a workshop drawing 3 kW at 40 m, 2.5 mm² scrapes in and 6 mm² puts the margin back. On a lighting circuit, held to 3%, take 40% off every figure.
What the rules require between the consumer unit and the shed door
A supply to an outbuilding is a new circuit, and a new circuit is notifiable under Approved Document P in England and Wales: a building control application before you start, or an installer on a competent person scheme who certifies and notifies it. Scotland has its own building standards. Swapping a fitting on an existing circuit is not notifiable.
You are not installing a second meter: the circuit leaves the house consumer unit, so the shed lands on the household bill at the same unit rate, with no standing charge beyond the 54.83p a day the price cap sets from 1 October.
- Steel wire armoured cable, buried direct: the armour is the mechanical protection, so no conduit. Most installers work to 450 mm under a lawn, 600 mm under a drive.
- Warning tape above the cable, not beside it, so the next spade finds the tape first.
- 30 mA RCD protection, and a breaker sized for the cable rather than the load you fancy adding later.
- Earthing settled deliberately: the protective conductor travels with the cable, or a local electrode goes in at the shed.
- Fittings rated for the weather, IP66 for an external socket, the lid closing over a plug in use.
What the shed costs once the power is on
Less than people fear, until something cold moves in. At 26.32p per kWh, the price cap rate for direct debit from 1 October, an 18 W LED batten on an hour a day costs £1.73 a year, a 60 W tool charger run an hour twice a week £1.64, and a 258-litre G-rated upright freezer, at the 329 kWh on its label, £86.59: one appliance, 96% of the budget.
The label is optimistic in a shed. An unheated outbuilding pushes a freezer outside the climate class it was built for, so through a cold January it runs closer to continuously than the test cycle assumed. Check what a unit of electricity costs today, and where the rest of the house wastes it, before deciding how full to keep it.
A breaker that drops out when the table saw starts is about inrush, not garden length. The main fuse into the house, usually 60 A, 80 A or 100 A, is a separate ceiling.
So, before anyone digs: list what goes in the shed, summer things included; add up in watts what could run at once, heater and all; then size the cable from those two numbers, one up if it lands near the limit. A cable one size light, or 150 mm too shallow, disappears the moment the trench is filled in.