Running power to a backyard shed is one of the most useful upgrades a NZ homeowner can make. The good news is that digging the trench itself is work you can do — but connecting the cable to your switchboard and the shed's wiring is restricted electrical work that must be carried out by a registered electrician. Here's a practical guide to getting the groundwork right.
Understand What You Can and Cannot Do
In New Zealand, restricted electrical work is defined under the Electricity Act 1992. This includes connecting cables to your home's switchboard, installing a sub-board in the shed, and terminating any fixed wiring. These tasks must be performed by a registered electrician who will issue you a Certificate of Compliance (CoC).
What you can do as a homeowner is prepare the trench — digging, laying conduit, and backfilling — ready for the electrician to pull the cable through. Always confirm the scope with your electrician before you start. You can find registered electricians through the Electrical Workers Registration Board (EWRB) or on tradedirectory.co.nz.
Check Whether You Need a Building Consent
Adding a new circuit to a shed may or may not require a building consent, depending on the scope of work and your local council's rules. Schedule 1 of the Building Act lists certain exemptions, but electrical work still requires a CoC regardless. Check with your local council before starting — requirements vary around the country.
For general guidance on consents, visit MBIE Building Performance.
Plan Your Trench Route
Before you pick up a spade, plan the cable route carefully:
- Choose the shortest practical path from the house to the shed, avoiding large tree roots, garden features, and areas prone to waterlogging.
- Mark the route with spray paint or string line.
- Call Before You Dig — contact Dial Before You Dig (0800 248 344) or visit their website to identify any underground services (gas, water, telecoms) along your route. This is essential and free.
- Discuss the cable type and conduit size with your electrician before you dig, so the trench dimensions are correct.
How Deep Does the Trench Need to Be?
Cable burial depth in NZ is governed by NZS 3000 (the Wiring Rules) and your electrician will specify the exact requirement. As a general guide:
| Installation method | Typical minimum depth |
|---|---|
| Cable in rigid conduit under a lawn or garden | 500 mm |
| Cable in rigid conduit under a driveway or vehicle crossing | 600 mm or more |
| Direct-buried armoured cable (if specified) | As per electrician's spec |
Always confirm the required depth with your electrician — they are responsible for the installation and will know what depth suits the cable type they intend to use.
What You'll Need
- Spade and/or mattock for hand digging, or a mini excavator (available to hire from most hire centres)
- Rigid PVC conduit — your electrician will specify the diameter; 25 mm or 32 mm is common for a shed circuit
- Conduit fittings and bends
- Draw wire (pull wire) left inside the conduit so the electrician can pull the cable through
- Warning tape (yellow/red underground cable marker tape) to lay above the conduit before backfilling
- Compactable backfill — fine soil or sand bedding around the conduit, then the excavated soil
Conduit and fittings are available from Bunnings, Mitre 10, and PlaceMakers.
Digging the Trench
- Mark out the full route before breaking ground.
- Dig carefully — use a spade by hand near any known services, even if Dial Before You Dig shows the area as clear.
- Keep the trench sides vertical and the base reasonably flat and free of sharp rocks.
- Lay a 50–75 mm bed of fine sand or sifted soil in the base of the trench to protect the conduit.
- Lay the conduit, joining sections with solvent-weld fittings and using sweep bends (not sharp elbows) at any changes of direction to make cable pulling easier.
- Leave a draw wire protruding at both ends.
- Lay warning tape approximately 150–200 mm above the conduit before backfilling.
- Backfill in layers, tamping gently to avoid damaging the conduit. Avoid large rocks or rubble directly against the conduit.
Dealing with NZ Soil and Conditions
NZ soils vary enormously — from Auckland's heavy clay to Canterbury's stony river gravels. In clay soils, the trench walls can collapse if left open too long, especially after rain. In rocky ground, a mini excavator or even a rock saw attachment may be needed. If you're unsure about ground conditions or the trench is longer than about 20 metres, hiring a mini excavator will save significant time and effort.
When to Call a Professional
Always engage a registered electrician for the electrical work — this is a legal requirement in NZ, not just a recommendation. The electrician must:
- Specify the correct cable type and conduit size
- Pull and terminate the cable
- Install the sub-board or outlet in the shed
- Connect to the house switchboard
- Issue a Certificate of Compliance
If your trench route is complex, involves rock breaking, or crosses a driveway, consider hiring a professional excavator operator to ensure the job is done safely and efficiently. You can find qualified local excavators and electricians on tradedirectory.co.nz.
Safety note: Never attempt to connect or modify any electrical wiring yourself. Underground cables carry mains voltage and pose a serious risk of electrocution and fire if incorrectly installed.
Licensed Building Practitioner (LBP)
