mixing cable sizes

[gjh]

the cable for my lighting circuits is 1mm at the consumer unit, however at some point upstairs it turns into 1.5mm cable...

is it nessisary to replace the 1mm cable [i really hope not!!].

As doing both lighting circuits in 1mm cable [its only a small house, 4 rooms on each floor] should be OK, then I assume that having a higher rated cable present for part of the circuit should not be a problem...especially as the cable at the consumer unit is the lower rated of the two so no misleading if someone should ever decide to extend one of the circuits.

any thoughts?

 

[pna]

Not sure from your message if you are planning to change anything or just asking if your current setup is dodgy.
Don't think there is anything wrong with part of a circuit being in 1.5mm cable as long as the fuse is linked to the current carrying capacity of the 1mm cable.

 

[felix]

Pna is correct. The consumer unit fuse (five or six amps) is adequate to protect 1mm cable. You can always use larger stuff. It costs more but it's perfectly safe. I knew somebody who wired up a whole house, lights and all, with 6mm cable! Reason - he had access to a free supply of the stuff.
There is one genuine reason why somebody would use 1.5mm cable on a lighting circuit. It might be a very long run and they were concerned about voltage drop.

 

[ban-all-sheds]

2 x 6mm² T/E in sockets?

6 x 6mm² cores and 3 x 2.5mm² cpcs into ceiling roses?

3 x 6mm² T/E into 3-gang light switch boxes?

Ye gods.

 

[felix]

I did wonder whether the effort involved in manipulating that stuff was worth the savings he made on cable. I also asked myself whether he mistook 4mm for 6mm - or whether he was just a compulsive liar! I haven't seen him for twenty years so I guess I'll never find out.

 

[gjh]

pna, I dont intend to change anything, I just wanted to check whether I should change things as at the moment I have [relatively] easy access to most of the cables whilst renovating. I just didnt want to be told by a sparky in 6 months time that I need to replace the 1mm cable when I get the electrics certified. The 1mm cable is connected to a 6 amp fuse, although the 1mm cable does run under the insulation in the loft as well, is this a problem [or should this be another subject?].

 

[towman]

I'm no expert - but being in insulated material will affect the rating of the cable. However, if anyone can point to a house with insulation that doesn't cover the cables .....

Of course you could always extend all the cables so they can sit on top of the insulation!!! Or make sure there are gaps where the cables go - but that would rather negate the purpose of the insulation!!

An aquaintence of mine was recently required by BC to have 275 mm of insulation in the roof space - wonder how that would squre up with Part P?

 

[felix]

If you look in the right catalogue (Farnell I think) you can find 1mm cable rated at 11 amps. It's not special stuff, just bog standard PVC insulated flat twin and earth. Since this rating fits the three halfs power law quite well I'm inclined to believe it. Your 1mm cable is well up to the job, even when covered with insulation, though I would run it either under or over the stuff, not in the middle.