JDW: 
 

Are 5amp junction boxes acceptable on a 6amp lighting circuit?

I personally wouldn’t use them as they are too small, but if they are already fitted are they acceptable? 

Oh yes. The rating is only nominal. You say “already fitted.” Fitted to what?

Z.

Yes fine in my view, to require that 5 amp junction boxes be replaced when a 5 amp fuse is replaced with a 6 amp MCB is IMHO over pedantic.

The load probably wont exceed 5 amps, and with the general use of low energy lamps is unlikely to exceed even half that.

And in most situations the junction box wont be carrying the entire load of the circuit in any case.

And even if a full 6 amps did pass, I doubt that the terminals would even get warm. Most of the current will pass from one wire to the other wire when these are forced into close contact by the screws. Very little current will pass through the metal parts of the terminals. 

I would take the “5 amp” rating as meaning “not big enough to take the size cables used on power circuits” Rather than meaning “liable to overheat at over 5 amps”

Zoomup: 
 

JDW: 
 

Are 5amp junction boxes acceptable on a 6amp lighting circuit?

I personally wouldn’t use them as they are too small, but if they are already fitted are they acceptable? 

Oh yes. The rating is only nominal. You say “already fitted.” Fitted to what?

Z.

I worked with an electrician who always used them on lighting circuits with 6a breakers, is this common practice on domestic properties?

Yes.

I worked with an electrician who always used them on lighting circuits with 6a breakers, is this common practice on domestic properties?

If common practice is anything like what I do, then it'd be more like avoiding junction boxes like the plague.

I much prefer to keep terminals accessible - looping in at lights or switches - is much preferred.

Junction boxes might sometimes be needed for additionals/alterations to existing circuits - though to be honest I'd probably go for a plastic box and some wago connectors these days. I can't remember when I last used a traditional round JB - certainly more than a decade ago.

    - Andy.

AJJewsbury: 
 

I worked with an electrician who always used them on lighting circuits with 6a breakers, is this common practice on domestic properties?

If common practice is anything like what I do, then it'd be more like avoiding junction boxes like the plague.

I much prefer to keep terminals accessible - looping in at lights or switches - is much preferred.

Junction boxes might sometimes be needed for additionals/alterations to existing circuits - though to be honest I'd probably go for a plastic box and some wago connectors these days. I can't remember when I last used a traditional round JB - certainly more than a decade ago.

    - Andy.

Sometimes the small circular junction boxes were good as they fitted through a hole in the ceiling allowing a new downlighter to be wired in. Also they were flame retardant and not just made of flimsy soft plastic.

Z.

JDW: 
 

Are 5amp junction boxes acceptable on a 6amp lighting circuit?

I personally wouldn’t use them as they are too small, but if they are already fitted are they acceptable? 

Is it actually possible to overload them?

It wouldn’t be in my house, my lighting circuit loads are less than one amp.

Around here, for many years, folk would pick either “5A ” or “30A” junction boxes for either lighting or power. Very often the “5A” ones were actually 20A ones anyway.

When the MCB became more common than the fuse in consumer units they were 5A and 30A then after a while became 6A & 32A . Nobody asked for or expected 6A and 32A junction boxes

While a radial socket or shower circuit may be called upon to deliver the full 32 A , there will be almost no lighting circuits on a 6A MCB that take anything like the full 6A, and probably in many cases not even an amp. 

Arguably a 20A JB should be fine on a ring of sockets, as 20A cable would be accepted though I prefer to see 32A.

Given the true load is fixed, and low, the rating of a lighting box is a bit of an irrelevance, as the real question relates to how many wires can be inserted without it failing to do up properly and is it in a sensible place to inspect. 

In recent years I have been moving towards screwless fittings as vastly more reliable than things with little grub screws that someone either fails to tighten or cross threads so they feel tight but are not gripping.

Mike.

 

Given the true load is fixed, and low, the rating of a lighting box is a bit of an irrelevance, as the real question relates to how many wires can be inserted without it failing to do up properly and is it in a sensible place to inspect. 

Do they ever get inspected, most seem to be under floorboards!