Routing Rules
How to plan and run wire routes that are safe, durable, accessible, and free from damage hazards.
Why This Matters
How a wire is routed matters as much as the wire’s capacity. A correctly sized conductor buried in insulation overheats because it cannot dissipate heat. A wire running along a baseboard gets damaged by foot traffic and furniture. A wire that crosses through damp areas corrodes and faults. A wire buried in a wall with no documentation cannot be found for repair — or for the nail driven through it during renovation.
Routing rules are the physical discipline of wiring. They are not arbitrary regulations — each rule exists because someone experienced a fire, shock, or failure that the rule prevents. Learning the reasoning behind each rule lets you apply it intelligently in novel situations.
Core Routing Principles
1. Protect Wires from Physical Damage
Wire in free space can be snagged, cut, bent, and abraded. Any wire exposed to foot traffic, vehicle traffic, stored goods, or outdoor weather must be protected:
| Exposure Type | Protection Method |
|---|---|
| Floor crossing | Steel conduit, minimum 25 mm diameter |
| Wall surface | PVC trunking, metal conduit, or recessed channel |
| Overhead outdoor | Armored cable, or bare conductor on insulators (aerial) |
| Underground | Direct-burial cable (armored) in sand bed, 600 mm depth minimum |
| Through timber | Drill hole, grommet to protect against abrasion |
| Through metal | Rubber grommet mandatory — sharp edges cut insulation |
2. Maintain Separation from Heat Sources
Insulation has a temperature rating — typically 70°C for standard PVC, 90°C for XLPE. Routing wire within 50 mm of hot pipes, flues, or heating elements degrades insulation prematurely.
- Minimum 50 mm separation from hot water pipes
- Minimum 300 mm from flue pipes and chimneys
- If unavoidable, use high-temperature cable (silicone-insulated) or heat-resistant conduit
3. Keep Wire Routes Horizontal and Vertical
Route wires horizontally along ceilings and floors, and vertically between them. Diagonal routes are harder to locate later and more likely to be pierced by fasteners. When anyone drills a hole or drives a nail, they check the vertical and horizontal zones from switches and outlets — diagonal routes are in the “unexpected” zone.
Standard “safe zones” for wiring inside walls:
- Directly above or below a switch or outlet (vertical)
- Horizontally along the top or bottom 150 mm of a wall
4. Radius of Bend
Every wire type has a minimum bend radius. Bending tighter crushes the conductor and cracks insulation:
| Cable Type | Minimum Bend Radius |
|---|---|
| Single-core flexible (≤ 10 mm²) | 3 × outer diameter |
| Multi-core flexible | 4 × outer diameter |
| Armored cable | 6 × outer diameter |
| Rigid conduit-wired | 6 × conduit diameter |
A 10 mm outer diameter cable should not be bent tighter than 30 mm radius.
5. Avoid Running Parallel to Data/Signal Cables
Power cables generate electromagnetic interference (EMI) that corrupts signals in nearby data cables. Keep minimum 150 mm separation between power and data cables for parallel runs. Where they must cross, cross at 90° to minimize the coupling length.
Conduit and Trunking
Why Use Conduit
Conduit is pipe through which wires are pulled. It protects wires from physical damage and allows conductors to be replaced without cutting walls. In any permanent installation, conduit is strongly preferred.
Types:
- Steel conduit: Strongest protection, can be grounded. Used in mechanical and outdoor applications.
- PVC conduit: Lighter, cheaper, cannot be grounded. Adequate for dry indoor.
- Flexible conduit: Short sections where rigid conduit would require tight bends — motor connections, equipment with vibration.
Conduit Fill
Don’t overfill conduit — you must be able to pull conductors through. Standard fill rules limit total conductor cross-section to 40% of conduit internal area for 3+ conductors.
| Conduit ID (mm) | Max Conductors (2.5 mm²) |
|---|---|
| 16 mm | 4 |
| 20 mm | 7 |
| 25 mm | 11 |
| 32 mm | 19 |
Trunking
Surface-mounted plastic or metal channel with a clip-on lid. Faster to install than conduit, easier to add circuits later. Less mechanical protection. Ideal for surface wiring in workshops and utility areas.
Underground Routing
When running cable between buildings:
- Excavate trench — 600 mm deep minimum; 750 mm under roads and driveways.
- Lay sand bed — 75 mm of clean sand at bottom of trench.
- Lay cable — in a shallow S-curve, not straight. This allows for soil settlement without stretching the cable.
- Cover with warning tape — bright red or yellow plastic tape placed 150 mm above the cable. Future excavation work cuts the tape as a warning before hitting the cable.
- Backfill and compact.
- Mark route — on your site plan with measurements from fixed reference points (building corners, fence posts).
Wet and Damp Locations
Water and electricity are incompatible. Any location that may be wet — bathrooms, kitchens, basements, outbuildings, outdoor — requires:
- Minimum IP44-rated enclosures and fittings (protected from splash)
- Cable entries sealed against water ingress
- GFCI/RCD protection for all circuits within 1.5 m of water sources
- Use of single-core cables in sealed conduit rather than multi-core flat cable in damp locations
Documentation of Routes
Draw cable routes on a site plan before backfilling or plastering. Dimensions from fixed points:
Cable run A: From Panel pos-3, exits panel east face,
runs horizontally to wall joist at 2.3 m height,
turns south along north wall at 2.3 m,
terminates at Workshop socket J3.
All in 20mm PVC conduit, surface-mounted.
This documentation saves hours of investigation when fault-finding years later.