Lighting Wiring Consideration.

Why was the old type 3/029 cable only rated at 7.8 amps ?

I would have expected at least 10 or 12 amps based on cross sectional area, which is similar to modern 1.5mm. I would expect a rating of less than modern cables, but still at least 10 or 12 amps.


The fuse wire often used to protect domestic lighting circuits was sometimes  described as being "five to ten amps" which I understood to mean a continual loading of 5 amps, and short term use up to 10 amps, when plugging the clothes iron into the lamp socket over the kitchen table.

Further to the funny current rating, in my Grandfather's ICS (International Correspondence Schools) library of building and construction techniques, from the same sort of era, that sadly went west when he died, was a section introducing  the then new-ish  electricity, including a table of cable ratings that based cable ratings on 1000 amperes per square inch, as a pure scaling of current with cross-section, although there was a foot note that more recent developments suggested that this rule tended to overheat larger conductors, and underestimate the capacity of thinner conductors they being better able to cool. I guess at that time some of the subject was was still in the air, and temperature  testing had just not been done on enough types of cable.

It also had wonderful illustrations of wiped joints in plumbing and cutting bricks and many other things as well - the electrics was just one chapter.

It may be that Zooms's table of current ratings dates or at least was copied from, an era when the detail of how to deduce the current ratings of smaller cables was still a matter of doubt.


I guess the way to see is to compare a number of other cross-sections in the same book, and see if the ratings look like they are in proportion to area

I'm sure there were cotton wrapped dry paper insulated singles available as well, which is what I was imagining, are they on another page by chance ? Clearly however they would not have been suited to immersion before  testing at a pressure of 1000V, so indoors only !!

M.

Why was the old type 3/029 cable only rated at 7.8 amps ? I would have expected at least 10 or 12 amps based on cross sectional area, which is similar to modern 1.5mm. I would expect a rating of less than modern cables, but still at least 10 or 12 amps.

A couple of guesses - firstly the assumption would be for overload protection by rewireable fuses (course protection in old money) - these days we'd use the 0.725 factor. Then given the natural rubber insulation rather than PVC they probably preferred a slightly lower conductor temerature - so de-rate a little more again.


  - Andy.

Yes coarse protection v close protection was quite a by-word a few years back & HBC become HRC etc etc

Don't forget... 3036 fuses' to both the neutral and live!!


It's ok, for insulation, as the wooden CCU will not transfer any current outside of the CCU as the wood, unless damp / wet, is ok!! 


If needed, I actually have BRAND NEW imperial MICC pots, and glands, from the mid 60's, still in their boxes, haha, (TRUE too!!).