Helpful video reference. Gary Hayers of GSH Electrical covers the height requirements for catenary wire installations in his video "Catenary Wire - What Height Must it be Installed at?". Gary is a UK-qualified electrician who produces domestic wiring training content for the BS 7671 18th Edition. The video explains why the different clearance heights apply and how to measure correctly from the lowest point of the cable span.
1. When is a catenary wire needed?
An overhead catenary span is used to supply power to a detached garage, outbuilding, garden office or similar structure when burying a cable is not practical. Common reasons to choose overhead rather than underground:
- The ground between the buildings is paved with slabs, tarmac or other hard surfacing that makes trenching impractical.
- The span crosses a driveway and a conduit under the surface would be at risk from heavy vehicles.
- Soil conditions (tree roots, rock) make trenching difficult or expensive.
Burying SWA cable is usually the preferred method because it avoids the height requirements and the risk of physical damage to an overhead line. If you have a choice, go underground. When the overhead route is the right answer, the catenary wire provides the mechanical support so the cable itself does not need to carry any tension load.
2. Identify what the span will cross
Before you calculate the required height, identify every area that the span passes over between the two buildings. The BS 7671 clearance figures are based on what could approach the cable from below:
- Pedestrian-only areas: a garden, a footpath, a yard or any area where only people on foot can go. No vehicles can reach it.
- Vehicular access areas: a driveway, car park, farm track or yard where cars, vans or agricultural vehicles can travel. Even a domestic driveway where only private cars park counts as vehicular access.
- Public roads: any road over which public vehicles have a right of way.
If the span crosses more than one category, the most demanding clearance applies to the lowest point of the span across that area. In practice, if any part of your span crosses a driveway or road, that clearance requirement governs the overall installation height.
3. Apply the correct minimum clearance height
BS 7671 Table 52.3 sets out the minimum clearances for overhead wiring. These figures are measured from the ground directly below the cable to the underside of the cable at its lowest point in the span, including any sag at mid-span and any cable attached to the catenary wire:
- Pedestrian access only: 3.5 m minimum. This applies to footpaths and garden areas where vehicles cannot enter.
- Vehicular access (driveways, car parks, farm tracks): 5.2 m minimum. This applies where cars, vans, tractors or other vehicles can travel underneath.
- Public roads: 5.8 m minimum. This applies where the span crosses any road over which the public has a right of way.
These are the clearances at the lowest point of the installed cable. The fixing points at each end of the span must be higher than these figures to account for the sag of the catenary wire and cable between the supports.
A common mistake is to fix the eye bolts at 3.5 m height on a pedestrian garden span and assume the job is done. If the span is more than a few metres long, the mid-span sag can bring the cable noticeably lower than the fixing points. Measure the clearance at mid-span, not at the fixing points.
4. Calculate fixing height to account for sag
The catenary wire will sag under the weight of the wire itself plus the attached cable. A rough working figure for a domestic-scale span is to allow 1% to 2% of the span length as sag at mid-span. For a 10-metre span that is 100 to 200 mm of sag. For a 20-metre span it is 200 to 400 mm of sag, and so on.
Add this sag figure to the required clearance height to find your minimum fixing height:
- Required clearance (for example 3.5 m on a pedestrian garden span) plus estimated mid-span sag (for example 150 mm on a 10 m span) = minimum fixing height of 3.65 m at the eye bolts.
In practice, most electricians add a more generous safety margin than the minimum calculation and fix the eye bolts higher. This is especially important if the span may carry festoon lights, additional cables or anything else added later, which will increase the load and sag.
5. Fix the support points
At each building, fit an eye bolt, catenary bracket or straining clamp into solid masonry - not into render, timber cladding or a cavity. Wall plugs and screws are not sufficient for a loaded catenary wire; use anchor bolts with a shear and pull-out rating appropriate for the anticipated load.
The catenary wire exerts a horizontal tension force on the fixing points as well as a downward force from the cable weight. The fixings need to handle both. In masonry, a pair of M10 through-bolts or resin-anchor bolts set into brick or block in good condition will typically be adequate for a domestic span of up to 15 metres. For longer spans or where the wall construction is uncertain, a structural engineer's input is sensible before fixing.
6. Tension the catenary wire
Thread the galvanised stranded steel catenary wire through the eye bolts or clamps at each end and attach a tensioning device (a turnbuckle or a commercial catenary tensioner) at one end. Take up the slack until the sag reaches the calculated or desired level, then lock off the tensioner.
Do not over-tension. A taut wire vibrates in wind, puts high stress on the fixing points and will not accommodate the slight movement of the two buildings relative to each other over time. A small amount of sag is correct - the wire should look gently curved, not perfectly straight.
7. Attach the electrical cable and earth the catenary wire
Once the catenary wire is in place and tensioned, attach the electrical cable to it using suspension clips, cable ties or P-clips at intervals of no more than 250 to 300 mm. Space them evenly along the span to share the load. The cable should hang slightly below the catenary wire, with the catenary taking the mechanical strain and the cable following along.
At each end, form a drip loop: allow the cable to curve downward and then back up before it enters the wall gland or trunking entry. This prevents rainwater from running along the cable sheath and into the building or fitting.
The catenary wire itself is metallic and must be earthed if a fault in the cable could make it live. Connect it to the main earthing at both buildings using earth clamps and a conductor sized for the circuit protective conductor requirements. Verify continuity of the catenary earth as part of the final circuit test.
When to call us
Richard can supply and install overhead catenary spans to outbuildings, garages and garden offices across Sandwich, Deal and east Kent, making sure the clearance heights are correct and the installation is certified under Part P. Call for a fixed-price quote before any work starts.
Need an outbuilding or garage supply in east Kent?
Richard can install overhead catenary spans and buried SWA cable runs to outbuildings across Sandwich and east Kent, with Part P certification included.
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