by ericmark »
Thu Nov 26, 2009 8:22 pm
Cables have moved from Imperial to Metric and 7/0.052 is closest to 10mm² at 9.4mm². So for Reference Method 100# (above a plasterboard ceiling covered by thermal insulation not exceeding 100 mm in thickness) instead of 45A it would be around 40A. Not only less cross sectional area, but also cross rubber insulated, so likely 60ºC instead of 70ºC but this is a bit of a guess as I don’t have any books that old.
Modern cables are in the main PVC so rated at 70ºC but there are also Ali-tube or other XLPE (Cross-Linked Polyethylene) or LSZH (Low Smoke Zero Halogen) cables. These are rated at 90ºC so are likely to be 58A in a similar installation.
However although you may have method 100# you could be looking at method #102 (36A) or methods #103 (32A) and to give ratings of cables without knowing installation method would be very miss-leading.
We are not allowed to give links or post pictures on this forum but I am sure you can find Batt or Eland cables web site to get more information.
The problem today is insulation and it is near impossible to tell post fitting what the cable runs through. So really only safe way is never to up-rate any protective device and if it had a 32A type B you leave it with 32A type B.
The circuit breakers and fuses for time a shower is used will take some overload but they also tend to weaken over time so the closer to original sizes the better.
There is also mineral insulated cable used a lot with stone cottages where it can run even hotter but then one must also look and see if the surrounding materials can take the heat. My book give about 7 different values on basic chart and then has a load of corrections to obtain the correct value. But on a personal thought I would not want to run any cables close to maximum and showers are available at below 10Kw and where the routes are unknown one must ask is it worth all the problems to fit large showers or is it better to replace like for like?
Especially when one realises unless like for like you need to pay LABC their £100+ fee under Part P if DIY.
Sparx is of course correct in all he says so no point in me repeating.
