Lighting circuits are now usually 6A instead of 5A. Ring mains are now
usually 32A instead of 30A. However, junction boxes haven't kept up. Is it
technically against the regs to use a 30A junction box on a 32A ring main?
(and similarly with lighting circuits). Obviously, there is very unlikely to
be an actual problem.
Obviously I should have said radial circuit.
5A junction boxes are very common. i.e.
Is it permitted to use these on a B6A lighting circuit?
Hmm, fair point -- but we're splitting hairs here. No-one in their right
mind is going to fault you for using a 30A box on a 32A circuit.
So are compact 20A 4-terminal ones, which are what I'd recommend for
lighting circuits, for ease of wiring.
At most points in most lighting circuits the design current won't exceed 5A
anyway and the question won't even arise. In rare cases where the current
exceeds 5A there might technically be non-compliance, but it's
hair-splitting territory again.
Junction boxes are made to BS 6220, which I've never read. It may be that
this defines the ratings of 5 & 30A, so stopping manufacturers marking them
6 & 32A (or maybe not). It may be that the ratings are notional and allow a
degree of latitude.
I can't help thinking that there are more important things to worry about,
but if you want to pursue this then ask the manufacturers and/or read the
BS -- and let us know what you discover.
Here are some examples of what is required by BS 7671
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