How many spare fibres to pull in the feeder

The question comes up on every regional build: the feeder run needs 48 fibres today, so do you buy 48, 96 or 144?

Buy 48 and you have optimised the wrong number. The cable itself is a small share of that section's cost — the duct, the trench, the traffic management and the reinstatement are most of it, and none of them care how many fibres are inside. Going from a 48F to a 144F construction on the same route typically adds a fraction of what a second civils pass would cost, and the second pass is what you are buying insurance against.

What tends to eat the spares

• Take-up above plan. Pleasant problem, same trench.
• Business customers off the same route. A single dedicated-fibre contract can absorb more than the residential plan assumed.
• Backhaul for someone else. Mobile operators and municipal networks will pay for dark fibre on a route that already exists.
• Breaks and repairs. Every repair splice costs you working fibres at that point.

A working rule

Plan 30–50% above day-one demand on distribution, and on the feeder go to the next standard count above that — 48 becomes 96, 96 becomes 144. Standard counts matter: they match the closures and the splice trays your crews already carry.

The exception is genuinely short runs inside a site you own, where going back is cheap. There, buy what you need.