10.5kw Shower on 30a Jointbox

[paulh53]

Went to relocate an existing 10.5Kw shower last week, And discovered that the 10mm2 T&E cable had been jointed with a 30a joint box under the floorboards, This shower takes about 45a, So was well over the top for the jointbox, The shower was supplied from a 50a MCB/30ma RCD,
The jointbox had been in service for over 5 years, and showed no signs of heat damage or arching,
The reason is jointbox terminals do not carry the actual load current,
But merely compress two overlapping conductors together, So why is the jointbox rated at 30a? When it could probably carry 80a, or more,
Obviously the physical size of the terminals denotes maximum csa of conductors, But this did not stop the previous installer from jointing the cable,
I obviously replaced the entire length of cable,

Would love to hear your opinions on this subject,

 

[Adam_151]

They will have been designed with a certain safety factor in mind, but it is far from advisable to make use of this, also a shower isn't in use for too long at a time, which would have stopped it freom getting too hot

 

[paulh53]

They will have been designed with a certain safety factor in mind, but it is far from advisable to make use of this, also a shower isn't in use for too long at a time, which would have stopped it freom getting too hot

I have five teenage kids, And believe me, My shower sometimes runs for 2Hrs or more when they heard themselves through the shower one at a time,
Also like I said, The jointbox terminals don't carry the actual current, They just provide security & seperation of conductors,

 

[mapj1]

Indeed the maker's current rating makes huge assumptions about the number of cores and what cross-section. The ultimate fire limited current rating with any particular cable configuration depends critically on the flatness of overlapping conductors, the tightness of the screws, cleanliness of the ends when fitted etc.
In many cases I suspect its more to do with if you can get 2 or 4 cables of 4mm in (30A) or 6mm ( 45 amp) under the screws.
Also the environment (hot/cold humid or dry) makes a huge difference to the change in contact resistance over time.
If you remember the old screwits (sometimes called wire nuts and still popular in the US) these have no metal in them at all, they just screw onto the wire ends and produce a twist join with enough core to core overlap to maintain the required cross-section. In some developing countries (like India for example) the use of twisted joins with no support at all is commonplace, and providing it is done well (two pairs of pliers, plenty of tension) it seems to hold up, even though it looks so wrong to us soft hearted westerners. (particularly when singles are used, and the joins are uninsulated but rely on being staggered for avoiding short circuits)

I even remember seeing a twisted only splice in the middle of an 11KV overhead line, and it was HORRIBLE - a real hedgehog of a thing, and it had apparently been working for years then, and probably is still today.

What I'm saying is I'm not at all surprised your joint box worked OK, with care and a bit of polishing some of the most apparently under-rated things can be pressed into service for many years before failing, if ever they do.
However, it doesn't mean its a good idea in general, particularly if the conditions of use are not predictable.
There is a corollary, that even though the rating says its OK, for other factors like screws not tight, inadequate crimp pressure or or dirt or oil on the cable or something, an apparently 'in spec' part may still fail prematurely.

regards M.

 

[Damocles]

I don't see why two cables could not be spliced using a suitable plastic collar and screw. Not that I would regard this as sensible, just possible. Some junction boxes have more space for bigger cables than others.

 

[SparkyTris]

Just a thought: for a product such as a JB it is probably cheaper for a manufacturer to tool-up for a maximum specification, then simply emboss different batches with whatever "rating".