18.3 Aerial Conductors & Underground Wiring
Key Takeaways
- EPC 27 covers aerial conductors, underground wiring and specialist cables under AS/NZS 3000 and local supply authority requirements
- Aerial spans are limited by conductor type and support strength, and must maintain minimum clearances above ground, roads, driveways and structures
- A catenary support system carries the mechanical load so the cable itself is not stressed in tension
- Underground cables must be of a type suitable for direct burial or be enclosed, and must be laid at the required depth with marking tape or cover protection above them
- Clearances and separation from other services such as gas, water and communications must be maintained, and existing services must be located before excavation
Aerial Conductors & Underground Wiring
Quick Answer: Aerial wiring is governed by span length, support strength and clearance; underground wiring is governed by cable type, depth, mechanical protection and separation from other services. Both must satisfy AS/NZS 3000 and the local supply authority. Locate existing services before you dig, every time.
Why This Is a Critical Capability
An electrician running a submain to a shed, a workshop or a second dwelling is doing exactly this work. EPC 27 is critical because the failure modes are severe: an aerial span at insufficient height contacts a vehicle or a person, and an underground cable at insufficient depth is struck by the next person with a shovel.
Aerial Conductors — Types and Application
| Type | Description | Typical application |
|---|---|---|
| Insulated aerial cable (aerial bundled) | Insulated cores twisted, often with a bare or insulated catenary core | Most common modern consumer mains and submains aerials |
| Catenary-supported cable | Standard cable supported by a separate steel catenary wire | Spans where the cable has no self-supporting strength |
| Bare conductors | Uninsulated, spaced on insulators | Legacy installations and distributor network — much greater clearance requirements |
For consumer installations, insulated or catenary-supported systems are the normal answer. Bare conductors within a consumer's installation are largely historical and attract far larger clearance requirements.
Span Limitations and Clearances
Span length is limited by:
- the mechanical strength of the cable or catenary;
- the strength and stability of the supports at each end;
- the permitted sag, which must not allow the conductor to fall below minimum clearance under maximum temperature and wind loading.
Clearances must be maintained above ground and above anything the span crosses. The categories to name are:
| Crossed feature | Clearance principle |
|---|---|
| Ground, not accessible to vehicles | Sufficient to be clear of persons and equipment |
| Driveways and areas accessible to vehicles | Substantially greater, to clear trucks and machinery |
| Public roads | Greatest clearance, to the supply authority's requirement |
| Roofs and structures | Clearance above, below and beside, so the cable cannot be reached from a normally accessible position |
| Swimming pools | Aerial conductors over a pool are heavily restricted |
Always take the actual values from the current AS/NZS 3000 clause and the local supply authority requirements on the day. The examinable knowledge is the hierarchy — pedestrian areas need less than vehicle areas, which need less than public roads — plus the discipline of measuring at maximum sag, not at the pole.
Supporting Poles, Posts and Struts
- Select the pole material and section for the loading — conductor tension, wind, and any future addition.
- Set the pole to sufficient depth and consolidate the backfill; a leaning pole reduces clearance across the whole span.
- Use struts or stays where the pull is one-directional, such as at a termination.
- Confirm the structure being used as a support can take the load. A private pole attached to a light timber fascia is a defect.
- Provide safe access for maintenance and correct earthing where a metallic pole is used.
Catenary Support Systems
A catenary wire carries the mechanical load so the cable does not:
- Terminate the catenary at each end to purpose-made anchors, not to the cable glands.
- Tension the catenary so sag stays within clearance limits at maximum temperature.
- Attach the cable with hangers at appropriate intervals so no tension reaches the cable itself.
- Earth the catenary where required, since a steel wire spanning between structures can otherwise become a hazard.
- Provide a drip loop at each entry so water does not track along the cable into an enclosure.
Underground Wiring — Cable Categories
Acceptable underground arrangements depend on the protection provided:
| Category | Typical arrangement |
|---|---|
| Direct buried | Cable specifically suitable for direct burial, at full required depth, with protective cover and marking tape |
| Enclosed in conduit | Cable in heavy-duty conduit, which may permit reduced depth in some situations |
| Under a slab or paving | Enclosed and protected; the surface treatment does not itself substitute for depth |
| Under a roadway or driveway | Greater depth and full mechanical protection, usually heavy-duty conduit |
Depth requirements increase with the likelihood of disturbance: a garden bed differs from a driveway, which differs from a road crossing. Take the values from the current clause; know the principle that depth follows risk of excavation and vehicle loading.
Marking is required so the next person finds the cable before their shovel does: orange marking tape at the specified height above the cable, or a protective cover slab, or both, depending on the arrangement.
Clearances From Other Services
Underground electrical cables must be separated from other services. The reasons are specific and examinable:
- Gas — an electrical fault near a gas line is a fire and explosion risk; a gas leak into a cable trench is equally hazardous.
- Water and sewer — excavation for repair of one service must not damage the other; water in a trench accelerates cable degradation.
- Communications — induced voltage and physical damage during maintenance.
Where the required separation cannot be achieved, a barrier or additional mechanical protection is used, and crossings are made at the specified separation with the electrical cable normally in conduit.
Before You Excavate
- Obtain service location information and have the site scanned or potholed.
- Hand-dig in the vicinity of known services.
- Treat any unidentified cable as live until proven otherwise.
- Record the as-installed route and depth so future work is safer.
- Reinstate marking tape and covers before backfilling — this is the step most often skipped and it is a defect.
What is the primary function of a catenary wire in an aerial cable installation?
Aerial span clearances are checked at which condition?
Which factor most directly determines the required burial depth of an underground cable?
What must be reinstated before backfilling an electrical cable trench?