Junction boxes size

Depends what you mean by "size". Conventional junction boxes are usually described by their current carrying capacity - so you'd select one appropriate to the current carrying capacity of the cables you were joining. In practice though it's common to over-size a little as the larger sizes provide more wiring space and so makes life easier for the electrician at minimal extra cost  - e.g. a 20A joint box on a 6A lighting circuit or a 32A joint box on a ring final circuit where the cables are only rated at 20A.


  - Andy.

I'm looking for an answer in the BS7671 regulations, since the only thing i can find is the requirement of no overheating. You know, some countries have regs with requirements about the matter, but i don't seem to find any specification for the matter in the BS7671

Thanks for your answer though, appreciated

Bs7671 is not a prescriptive standard  - it does not tell you how to do anything at all pretty much.  (OK that is a bit sweeping, but you get the idea I hope)

Instead it sets technical aims - e.g. terminals shall be rated for the load they are expected to supply.

How you achieve that , indeed if you have junction boxes at all, or make the joints at the accessories instead, is entirely a matter for you.

It is not in that sense a wiring code.

The 'onsite guide' is closer to a code, as containing many tried and tested recipes to reliably meet the regs in most cases.

the only thing i can find is the requirement of no overheating

That's about the size of it. 134.1.4 'Every electrical joint and connection shall be of proper construction as regards conductance, insulation, mechanical strength and protection'. (and 526.1 which says about the same).


Then there's the general requirements that apply to all sorts of things - including joint boxes - such as


133.1.1 Every item of equipment shall comply with an appropriate standard.

133.2 Every item of electrical equipment selected shall have suitable characteristics appropriate to ... voltage .... current ... frequency ... power ... and conditions of installation.


which covers most bases. There's a bit more in 526 about connections but nothing specifically about 'sizing'.


   - Andy.

If the question is about physical size, I wonder if the OP was hoping to find the equivalent of the USA NEC regulations, where boxes are marked with a volume and then there are tables and formulas to determine the maximum number of wires etc that may be fitted


For example...


(skip to about 4 mins to get to the calculations but note that he shows us the 20.3 cu in markings near the beginning)

In the USA there are specific requirements for the internal volume of junction boxes, and other electrical boxes. Varies according to size and number of conductors.


There is no equivalent requirement in the UK. Certain rather vague and general requirements exist, but nothing comparable to USA code requirements for  XX cubic inches. My personal view is that most UK boxes are much too small to permit of good neat workmanship and to facilitate later inspection or alteration.

That however is my view and not a statutory requirement.