How far can you run a cable to the garden shed?
Every metre of cable costs a little voltage. Indoors nobody notices. On a long run out to the garden you start to: with 30 m of 1.5 mm² carrying 16 A, the shed only gets 218.6 V. That's 5.0% less than at the meter, and the German standards recommend no more than 3%.
Why less voltage arrives at the far end
Copper is a good conductor, not a perfect one. Every conductor has some resistance, which grows with length and shrinks with cross-section:
(kappa) is copper's conductivity, about 56 m/(Ω·mm²). One metre of 1.5 mm² conductor comes out at roughly 0.012 Ω. Tiny, but put a current through it and Ohm's law says some voltage drops across it:
That's the voltage missing at the far end. On a single-phase circuit the current flows out on the line conductor (L) and back on the neutral (N), so the run counts twice:
Think of a garden hose: the longer and thinner the hose, and the more water you push through it, the less pressure is left at the nozzle. Twice the length means twice the drop. Twice the cross-section halves it.
On three-phase the load is shared across three line conductors. The 2 becomes , and the drop is measured against 400 V instead of 230 V:
To compare it with the standards, express the drop as a percentage of the nominal voltage:
The lost voltage turns into heat in the cable:
For 30 m of 1.5 mm² at 16 A that's 183 W warming up the soil for as long as the load runs. Spread over 30 m, the cable itself doesn't mind. How hot a cable gets under a real overload is what the cable heat simulator is for.
How much drop is allowed
The chart below shows two reference lines from German standards. DIN 18015-1, which covers residential buildings, recommends at most 3% between the meter and the appliance. DIN VDE 0100-520 recommends at most 4%, counted from the service connection. Both are recommendations rather than hard limits. Electricians planning a house usually work to the 3%.
That 3% covers the whole path from the meter, not just the bit across the lawn. If there are already 10 m of 2.5 mm² inside the house before the cable leaves the wall, 1.0% is gone at 16 A before the buried section even starts.
The calculator
- Chosen cross-section
- Other cross-sections
- Your run
Each line is one cross-section. Where it crosses the dashed 3% line is the longest run that cross-section can manage at the current you've set. The dot is your run.
Use the rating of the circuit breaker, not whatever you plan to plug in today. If the shed is on a B16, the circuit is allowed to carry 16 A at any time, even if all that's connected right now is a lamp and a radio. A 2 kW fan heater alone draws 8.7 A.
Worked example: 30 m to the garden shed
On 1.5 mm² the drop at 16 A is 5.0%, well over both recommendations. At that current, 1.5 mm² stays under 3% only up to 18 m.
On 2.5 mm² it's 2.98%. That only works if almost nothing is lost inside the house first. Add the 1.0% from the indoor run and you're at 4.0%.
On 4 mm² the garden run stays at 1.9%, or 2.9% including the indoor cable. For 30 m on a B16, 4 mm² is the size with headroom. If the shed only needs lights and a few sockets on a B10, 2.5 mm² comes in at 1.9%.
What too much voltage drop does
A couple of percent too much won't be obvious straight away. A fan heater gets weaker, since its output falls with the square of the voltage: at 218.6 V instead of 230 V it puts out about 10% less heat. Motors in a lawnmower or a circular saw draw several times their rated current while starting up. The voltage dips much further for a moment, the lights in the shed flicker and the motor struggles to get up to speed. You also pay for the heat in the cable on your electricity bill.
Outdoors, use proper buried cable
In Germany the usual cable for burying is NYY-J, for example NYY-J 3×2.5 mm² for single phase or NYY-J 5×2.5 mm² for three phase. For 30 m on a B16, the example above makes 4 mm² the better choice. NYM is not rated for burial.
Better still, run the cable through a conduit, for example a UV-resistant M32 with a 23.5 mm bore. Then it can be swapped later, say for a larger cross-section, without digging up the garden again. It still has to be burial cable inside the conduit, because water collects in there.
The cable or conduit goes in a sand bed about 60 cm down, deeper under driveways. The yellow warning tape goes into the trench some 20 to 30 cm above it, so whoever digs there years later hits the tape first.
The circuit to the shed needs a 30 mA residual current device (RCD). For outdoor sockets it's mandatory.
What the calculator simplifies
It assumes copper at 20 °C. Under full load the conductor warms up and its resistance rises, by about a fifth at 70 °C, and the real drop rises with it. That's why it isn't worth planning right up to the second decimal place of 3%.
It also assumes a purely resistive load (cos φ = 1), which fits heaters and lamps. Motors have a cos φ below 1 and see a slightly smaller drop, so the formula errs on the safe side as long as the cable is almost purely resistive, as it is at these cross-sections.
Length has a second limit: disconnection time. If there's a short circuit at the far end, the breaker has to trip fast enough, which caps the resistance of the fault loop. With a B16 and the lengths here, voltage drop is the tighter limit. An electrician still has to measure the loop impedance.
Simplified model for orientation, not a basis for planning a real installation. Connecting, protecting and testing a supply to a garden shed is a job for a qualified electrician.
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