Most domestic jobs need a 20A four-terminal junction box; lighting circuits with a loop-in at the rose often need 30A three-terminal instead. The rating is about the circuit's current, not the number of cables. Get that the wrong way round and you've either overspent or, worse, undersized a connection.
Here's the part that costs people real money though. A junction box you can't reach afterwards is only legal if it's the right kind of junction box. Bury the wrong one under a floor and you've created a fault nobody can inspect.
We've sold these off the counter on Villa Road since 1975. This is the version we give customers who ask.
1. What a junction box is actually for
A junction box does one job: it joins conductors mechanically and protects the joint. That's it. It is not a way to extend a circuit indefinitely, and it is not a fix for a cable that's too short in a place you can't get back to.
Two things decide which one you buy: the current the circuit carries, and how many conductors meet at that point.
- 20A covers lighting circuits and most general connections.
- 30A covers ring final circuit work and heavier connections.
Terminal count is separate. A four-terminal box gives you four common points — enough for a two-way joint plus spare, or a three-plate lighting arrangement depending on how it's wired. A three-terminal box is tighter and often the neater answer at a light.
2. The accessibility rule — this is the one
BS 7671 requires every connection to remain accessible for inspection, testing and maintenance. There are specific exceptions, and the one that matters here is a connection made in a maintenance-free accessory complying with BS 5733 and marked MF.
So: a standard screw-terminal junction box screwed to a joist and then boarded over is a problem. A properly marked maintenance-free box, in the same place, is not.
Trade Tip: Don't take “maintenance-free” from the shape of the box or the colour of the lid — look for the MF marking on the accessory itself. If it isn't marked, treat it as needing to stay accessible. We get asked to confirm this at the counter constantly, and the marking is the only answer that counts. Bring the box in if you're unsure.
3. Screw terminals or push-fit?
Screw-terminal boxes are the traditional answer and they're cheap, robust and repairable. Their weakness is human: an under-tightened screw is the single most common cause of a warm joint we see coming back.
Push-fit connectors are quicker and remove the torque question. They cost more per joint. Both are legitimate; the choice is honestly about how much you trust the person tightening the screws — and if that's you and you're careful, screw terminals are fine.
There's a second consideration people forget: conductor type. Solid twin-and-earth behaves very differently from stranded flex in a screw terminal. Solid cores sit still once tightened. Stranded cores can creep under sustained pressure, and a few individual strands escaping the terminal are a genuine hazard — they're the reason you occasionally find a box with scorching around one terminal and three perfectly clean ones. If you're joining flex, dress the strands properly, twist them lightly, and check the terminal a second time before the lid goes on.
Our position: for anything going into a place you'd rather never revisit, spend the extra and use a maintenance-free solution. Everywhere else, screw terminals in an accessible box, tightened properly.
The Knightsbridge SN8410 is the one that moves fastest here — a 20A four-terminal box in urea, 59 mm across, IP20, rated 230 V, certified to BS EN 60670 and sold in packs of 20. It's the sensible default for lighting work and general accessible joints. There's a black version, the SN8410B, if it's going somewhere visible against a dark surface.
4. When you need 30A and three terminals
Step up when the circuit does. For heavier connections the SN8430B is a 30A three-terminal box, 89 mm across, black urea, sold in packs of 10.
The extra diameter isn't decoration — it's room to dress heavier conductors without forcing them. If you're fighting the lid on, you've bought the wrong size. There's also the SN8420B, a 20A four-terminal box at 79 mm, when you want the larger body without the higher rating.
Trade Tip: Buy the bigger diameter when you're near the limit. The commonest reason a joint fails inspection isn't the box or the rating — it's conductors crushed against the lid because someone saved 40p on the body size. Space is cheap. Rework isn't.
5. Where DIY stops
Replacing a like-for-like accessible junction box on a circuit you've safely isolated is straightforward maintenance. Adding a new circuit, altering a consumer unit, or any work in a kitchen or bathroom zone falls under Part P and is notifiable — that needs a registered electrician who can certify and notify it.
Isolate at the consumer unit, prove dead with an approved voltage indicator, and if any part of what you're about to do changes the circuit rather than repairs it, stop and call someone in. We'd rather tell you that across the counter than sell you a box for a job that needs certifying.
Summary: which junction box
| Job | Rating | Terminals | Stocked line |
|---|---|---|---|
| Lighting circuit, accessible | 20A | 4 | SN8410 (white) / SN8410B (black) |
| Lighting, larger body needed | 20A | 4 | SN8420B, 79 mm |
| Heavier connection | 30A | 3 | SN8430B, 89 mm |
| Concealed, no future access | — | — | Must be MF-marked to BS 5733 — check the marking |
Expert Help in the West Midlands
We've been on Villa Road in Birmingham since 1975, and the trade counter is where this stuff gets settled properly. Bring the box, bring the cable, tell us what circuit it's going on and we'll tell you straight whether it's right — including when the answer is “that one needs an electrician, not a junction box”.
Browse our junction boxes or the full range of wiring accessories, or see our guide to CCTV power cables and junction boxes if it's a camera install rather than mains work.
Frequently asked questions
Can I put a junction box under floorboards?
Only if the connection stays accessible, or the accessory is maintenance-free to BS 5733 and marked MF. A standard screw-terminal box boarded over does not satisfy the accessibility requirement in BS 7671.
What's the difference between a 20A and 30A junction box?
The current the connection is rated to carry. 20A suits lighting and general work; 30A suits heavier circuits. It's determined by the circuit, not by how many cables arrive at the box.
How many cables can go into one junction box?
As many as the terminals take without crowding — but terminal count isn't a licence to keep adding. If conductors are being crushed to get the lid on, move up a body size.