That circular junction boxes have a current rating puzzles me.   The terminals hold the two wires in close contact so that no current flows through any part of the box.   They should be rated only by the size of cable they will accommodate.

Hello JDW.

I think there's a bit of history involved in your initial question, which dates back to the rewireable fuse days, 5A, 15A, 20A & 30A (metric values?!) When we harmonised round about 1980 the main IEC MCB equivalents were 6A, 16A and 32A (i.e. not metric!), so those values were introduced in the UK to harmonise. Close enough for no existing hardware changes to be necessary. Where found, an existing 5 amp junction box on a 6 amp MCB will be fine, all other things being equal.  The small size makes cable sheathing enclosure a common failure with these - most guys installed 20 amp JBs for just that reason; the actual terminals themselves being similar in size.

Regards,

           Colin Jenkins.

JDW: 
 

Another I have seen a few times is strip connectors (choc blocks) in loft and floorboard spaces without an enclosure. Would that fail an inspection? 

Definitely. 

I think there's a bit of history involved in your initial question, which dates back to the rewireable fuse days, 5A, 15A, 20A & 30A (metric values?!) When we harmonised round about 1980 the main IEC MCB equivalents were 6A, 16A and 32A (i.e. not metric!), so those values were introduced in the UK to harmonise. 

The change from “round numbers” (5, 10, 30 etc) to what we have now (6, 16, 32 etc) is less to do with metrication and more to do with “preferred numbers”. It's an old idea dating back to a certain Charles Renard and ropes for French military balloons, but the underlying principle is now used very widely. Basically it's the idea of being able to serve a wide range of requirements with the minimum waste.

The idea is that every rating is within a certain fixed percentage of the previous one - so nothing need ever be over-sized by more than that amount. The choice of percentage varies but will be a compromise between having to manufacture and hold stock of lots of different values against the oversizing in the actual installation. Often the percentage is chosen so that the numbers fit exactly into a ‘decade’ - i.e. the sequence produced between 10 and 100 is the same as between 1 and 10 but multiplied by 10.

The value for electric stuff is usually around 58% - (actually the 5th root of 10) - dividing each decade into 5, which gives values (slightly rounded) like 1.0, 1.58, 2.5, 3.9, 6.3, 10, 15.8, 25, 39, 63, 100 and so on. Which (when rounded again slightly) gives us the familiar 1.0, 1.5, 2.5, 4, 6, 10, and 25mm2 cable sizes.

Using the 10th root of 10 (about 26%) gives ten values per decade: e.g. 1.0, 1.26, 1.58, 2, 2.5, 3.2, 3.9, 5, 6.3, 7.9, 10, 12.6, 15.8, 20, 25, 32, 39, 50, 63, 79, 100 and so on. From which (again with a bit more rounding) the familiar MCB/fuse sizes of 6A, 10A, 13A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A can be spotted.

   - Andy.

Similar in electronics and preferred values sus as E6, E12, E24 etc

Hello Andy. Thank you your input. This topic is quite a bit more complicated than I thought! Regards,

Colin Jenkins.