A 125 mm opening is enough for one connection. It stops being enough the moment there is more behind the back panel than that one.
In practice that is the rule rather than the exception. A multi-socket strip carrying the fridge, the oven and the extractor together. A power supply with a bundle of cables for the under-cabinet lighting. Power, network and aerial outlets side by side on a media wall. The router behind the sideboard that someone has to reach twice a year. What they have in common: whatever sits back there is wider than the hand reaching for it.
Why two separate openings do not solve it
The obvious idea is to cut two service openings next to each other, one left, one right. Geometrically that works — two Ø125 mm holes need enough distance for material to remain between the two frames. That very material is the problem: the web left standing sits right across the middle of what you actually need to reach.
With a socket strip that means reaching the first and the last plug but none of the ones in between. With a power supply it means seeing it but not getting it out. Two separate openings give you two cut-outs — what is needed is an area the hand can move across.
That leaves the larger hole. Hole saws exist in 160 or 200 mm too, but that only moves the problem: a round cover beyond 150 mm is a large, unsupported disc. It bows, it no longer sits flat, and the back panel loses noticeable rigidity across that width — particularly with the 3 to 8 mm panels common in carcass backs. And anyone going beyond that is cutting freehand anyway, and is then left without a component that closes the opening again cleanly.
Two holes, one opening
The route that solves both: cut two Ø125 mm holes at a fixed centre distance and remove the web between them. What you get is a slot with a 62.5 mm radius at each end — one continuous opening you can reach through and move your hand across, instead of two separate places to poke into.
The connector that closes this opening again comes in two lengths. At a 175 mm centre distance the slot measures 300 × 125 mm, at 225 mm it measures 350 × 125 mm. As a rule of thumb from practice: the smaller opening gives access to a four-gang socket frame, the larger one to a five-gang frame.
The centre distances are nothing you have to work out, and they are not derived from whatever sits behind the panel. They are simply the dimensions of the components. What matters is the result: 300 or 350 mm of usable width, into which whatever has to be connected there can be arranged.
Two holes, the web between them removed. The frames are cut away towards the centre.
What matters when cutting
Mark out first, then drill. Mark both centres on the back panel before the panel is fitted. Take the position of the connection point beforehand, and how far it extends — that also tells you which of the two lengths is the right one. A slot either sits right or not at all.
Holes first, web second. Cut both openings with the hole saw, and only then remove the remaining web with a jigsaw. The other way round, the hole saw has nothing left to guide its pilot drill.
Adapt the frames. On both mounting frames the segment facing the centre is cut away so they sit flush against the connector.
The connector bridges the middle section and is screwed in together with the frames.
Why two covers instead of one long lid
The slot is closed not with a single lid but with two round covers and a connector between them. There are three practical reasons for that.
Each cover stays at 125 mm — easy to handle, dimensionally stable, with the same twist lock as everywhere else. You open only the side you need; the other stays shut. And if a cover is damaged or the colour has to change, you replace a standard part, not a special size. The same goes for the length: both connectors work with the same covers, only the middle section differs.
Each side opens on its own. The other stays locked.
What else to watch
Panel thickness. As with a single opening, the range is 3 to 19 mm. With thin back panels it is worth checking that the panel still has support somewhere around the slot — 300 mm, and 350 mm all the more, is a fair span.
Screws. Four Ø3 mm countersunk screws per cover, plus the fixing for the connector. The length depends on the panel thickness.
Version. The connectors fit the RP01125 in both versions, standard as well as Heavy Duty.
Background
The XT01175 and XT01225 connectors exist for this case. They join two RP01125 covers at a centre distance of 175 and 225 mm respectively into one continuous service opening of 300 × 125 mm or 350 × 125 mm — each available on its own or as a set with the two matching covers. The full installation is shown in the instructions with video.
Two related cases where a single opening is enough: the socket behind the wrong cabinet and the recirculating downdraft extractor.