Sizing a Transformer and Cable

Garden lighting fails at the far end of the garden, and it is almost never the fitting's fault. It is the cable.

This page is the arithmetic. It takes about five minutes and it is the difference between a scheme that works for fifteen years and one where the last three lights are visibly dimmer and slightly the wrong colour.

Step 1 — the transformer

Add up the wattage of every fitting on the run, then add 20% headroom.

Ten 6W spotlights = 60W. Add 20% = 72W, so a 150W transformer, not the 60W. The headroom is not wasted: it stops the transformer running at its limit on a warm night, and it leaves room for the two extra lights you will want next year. Every scheme we sell gets specified this way.

Transformers: 60W, 150W, 300W, 600W and 900W.

Step 2 — the current

Amps = watts ÷ 12.

Those ten 6W spotlights draw 5A. That number is the one that matters, because at 12V you are pushing roughly twenty times the current a 230V circuit would for the same light — and current is what voltage drop is made of.

Step 3 — the voltage drop

Every metre of cable costs you voltage. The standard figures for two-core copper are:

Drop in volts = (mV/A/m × amps × metres) ÷ 1,000.

The worked example that changes people's minds

Ten 6W spotlights (5A), 30 metres from the transformer, on 1.5mm² cable:

29 × 5 × 30 ÷ 1,000 = 4.35 volts

The far fitting receives 7.65V instead of 12V. In practice that is a light which is dim, noticeably warmer in colour than its neighbours, and quite possibly not on at all. The fitting is fine. The cable was wrong.

The same run on 4mm² gives 1.65V — better, still too much.

Step 4 — how much drop is acceptable

Aim to keep every fitting within about 1.5V of the transformer, so nothing sees less than roughly 10.5V. Below that, LED drivers start dropping output and the colour shifts.

That gives you this table — the longest run you can put on one cable, by load:

Note what that table really says: the limit is the load on the cable, not the size of the garden.

Step 5 — the fix, which is usually free

Most people reach for thicker cable. That is the expensive answer and often not the best one.

1. Move the transformer. It only needs a 230V socket and somewhere dry. Halving the distance halves the drop. A transformer in the shed at the bottom of the garden beats 4mm² cable from the house.
2. Split the run. Two runs of five lights instead of one run of ten halves the current in each cable, which halves the drop. A three-way T-connector at the transformer costs a few pounds.
3. Feed the middle, not the end. Bring the cable to the centre of a line of lights and run both ways. Same cable, half the effective length.
4. Use the transformer's higher tap. Many 12V garden transformers have 12V, 13V and 14V outputs to compensate for a known drop. Use it only once you have done the arithmetic — it is compensation, not a substitute.
5. Then go up a cable size.

Connectors: the part that actually fails

A buried cable joint sits in wet soil for fifteen years. Ordinary connector blocks corrode, then the joint goes high-resistance, then that section dims, then it stops. Nearly every "the lights stopped working" call is a joint, not a lamp.

• IP68 gel-filled connectors for anything buried. Not IP65, not tape, not a choc-block in a bag.
• Resin-filled joints for anything that will sit in standing water or under a pond liner.
• Strip back only what the connector needs. Exposed copper inside a gel connector is fine; exposed copper outside one is a future fault.
• Leave a service loop — half a metre of slack coiled at each fitting. It is what lets you move a light two metres in three years' time without digging up the run.

In-ground fittings: drainage, not waterproofing

An in-ground uplighter sits in a hole that fills with water. IP67 or IP68 means the fitting survives being submerged; it does not mean the fitting enjoys living in a pond.

Fit a drainage sleeve with 100–150mm of clean pea shingle underneath, so water leaves the hole rather than standing in it. This single detail is the difference between a ten-year fitting and a two-year one, and it costs a few pounds. Every in-ground set we sell includes the sleeves.

Burying the cable

• Depth: 300mm in a border or lawn, 450mm under anything you might dig or drive. 12V cable is safe to cut, but nobody wants to.
• Warning tape above the cable, and a cable marker post where a run changes direction. Future you will not remember.
• Route along edges — bed edges, path edges, fence lines. Never diagonally across a lawn.
• Photograph the trench before you fill it, with something in shot for scale. It is the most useful five seconds of the whole job.

The 230V end — what needs an electrician

The 12V side is extra-low voltage: you can install it, move it and extend it yourself. The supply feeding the transformer is not.

In England and Wales, adding a new circuit is notifiable under Part P of the Building Regulations, and an outdoor socket or a supply to a garden usually is a new circuit. Plugging a transformer into an existing, suitable outdoor socket is not. Scotland and Northern Ireland run their own systems. If in doubt, ask a registered electrician — it is an hour of their time and it is the part that matters.

This page is general guidance, not a design. It does not replace BS 7671 or the advice of a qualified electrician.

Let us do it for you

Send us a rough sketch with distances and what you want lit, and we will size the transformer, the cable and the runs and tell you what it costs. It is free, it takes one email, and it is how most of our schemes start. support@homartuk.com

Or start from a complete scheme — they arrive with the transformer, the cable and the connectors already sized for the fittings in the box.

Sources

Voltage drop figures (mV per amp per metre for two-core copper) and the calculation method are the standard BS 7671 Appendix 4 values. The 1.5V working limit, the run-length table derived from it, and everything about drainage, service loops and trench depth are our own installation practice rather than a published standard.