Routing Solar Cable on an Already-Built House: Internal vs External

Cable has to physically get from the roof to the inverter to the distribution board somehow, and on a house that already exists, rather than one being designed from scratch with solar in mind, that route is a real trade-off between appearance and accessibility.

What are the two realistic routing options?

Internal routing runs cable through an accessible roof void or ceiling space, hidden from view for most or all of its length. External routing runs cable along the outside of the roof and down the facade in weatherproof conduit, visible but straightforward to inspect, trace, and service later.

Most installations use some combination of both: internal where a void genuinely exists and is accessible, external where it doesn't, or where the internal path would be significantly longer or more disruptive to reach.

Why do most homeowners lean toward internal routing?

The main reason is simple: nobody wants to see conduit running down the side of their house or along a visible roofline. Internal routing keeps the cable run out of sight for the parts of the house where appearance matters most, typically street-facing walls and prominent roof edges.

The trade-off is accessibility. Cable buried inside a ceiling void or wall cavity is harder to inspect, trace a fault on, or modify later than a run sitting in visible external conduit, since reaching it means opening up a finished surface rather than simply looking at it.

When does external routing actually make more sense?

A house without a genuinely accessible roof void, a common situation on an RC flat-roof home without a traditional attic space, often makes internal routing impractical without significant additional work. External routing in properly rated conduit is the straightforward answer here, run along the shortest practical path to the inverter.

External routing also makes sense on a less visually prominent elevation, a rear or side wall away from the street frontage, where the aesthetic cost most homeowners care about barely applies.

Does the house's construction era change which option is realistic?

Older landed homes built with a traditional timber-truss roof more often have a genuine, walkable roof void, since that structural style naturally leaves open space beneath the roofline. Newer terraces and semi-detached houses built with a reinforced-concrete flat roof, common in developments from the 2000s onward, often have little or no equivalent void to route through.

This is worth raising with an installer early rather than assuming a preference either way is automatically achievable. The house's actual structural era, not the homeowner's aesthetic preference alone, is often what decides which option is realistic in the first place.

What actually protects external cable from Singapore's weather?

Cable routed outside needs conduit rated for sustained UV exposure and moisture, not general-purpose indoor conduit repurposed for an outdoor run. SS 638 specifically requires DC cable to run in metal trunking or conduit for mechanical protection, a requirement that applies regardless of whether the run is internal or external.

Every point where conduit penetrates a wall or roof surface needs proper sealing, the same waterproofing discipline that matters for any other roof penetration. A poorly sealed conduit entry is a more likely failure point over time than the conduit material itself.

Can solar cable reuse existing conduit or trunking already in the house?

Generally no. SS 638 requires DC cable to run in its own dedicated conduit or trunking, separate from other household wiring circuits, so existing conduit installed for general household wiring wasn't sized or rated for solar's specific mechanical protection and segregation requirements.

This matters more for cost expectations than for the routing decision itself. A homeowner who assumes visible existing trunking can simply carry the new cable often finds the actual quote includes a full new conduit run regardless of what's already on the wall.

Does the routing choice affect the cable sizing calculation?

Yes, indirectly. Voltage drop calculations use the actual physical path length the cable travels, not the straight-line distance between the array and the inverter. A routing decision that goes around an obstacle rather than through it adds real length to that calculation.

This is why routing and cable sizing are worth discussing together rather than as separate decisions: a longer external route chosen purely for appearance can push a run into needing a thicker gauge than a shorter internal path would have required.

What should you discuss with an installer before the route is fixed?

Ask to see the proposed route marked on your specific house, not described in general terms, and raise directly which walls or roof sections you'd prefer kept clear of visible conduit. Ask how the choice affects total run length and whether it changes the cable gauge needed.

Run the full roof suitability checklist before requesting quotes, and see Balance of System quality for the wider set of wiring and mounting questions worth asking. Run the Sunnify solar estimate to start planning your own installation.