AleisterCrowley wrote:From what I read, it (shower cable) may present a fire risk if routed under the insulation (mine isn't)
News to me, as a lot of power cables are embedded /buried
I'm sure the wiring regs cover this. If only we knew a highly skilled electrician.... (CHRIS!!)
You choose cables on the basis of current carrying capacity.
The current carrying capacity is down to two factors - resistance (thickness and length) and the ability to dissipate the heat caused by that resistance.
The thickness bit of the resistance factor is understood by most people - they know that if they want to wire a 32A ring main, they need to use 2.5mm sq twin and earth, rather than the 1mm or 1.5mm they might use for a lighting circuit.
Less well understood is the length factor - a long run requires the derating of the cable. (e.g. treating a 2.5mm cable as having a rating of 20A rather than 32A).
Even less well understood is that a cable generates heat, which needs to be dissipated. You can get heat dissipation where you might have thought it impossible. Sticking a cable between two layers of insulation means you really have to derate the cable, as the heat dissipation becomes negligible.
When I reinsulated my loft the lighting cables were across the insulation. I worked on the principle that a cable nailed to a joist dissipated a lot more heat than a cable sandwiched between insulation. I also worked on the principle that an upstairs lighting circuit designed to take four 100W lightbulbs, wasn't going to struggle with 4x 10W CFL/LED bulbs. So I moved the wiring onto the joists and put the insulation down.
This approach will NOT work for a shower cable.
There are many shower installations that were designed and the cable sized for a 7.2kW shower.
Then the shower needs replacing and the homeowner thinks "I don't want a tepid dribble, I want a 8.4kW or 10kW shower".
A naive homeowner or a feckwit installer then looks at the current install, sees 6mm cable and thinks "Woohoo - we're good for 10kW here".
They don't realise that it's 6mm instead of 4mm because the cable run goes halfway around the world to get to the bathroom and needs to be thicker because of the length (more L = more R = more heat)
So before they've started, the cable in the loft is already pushing the envelope, but given that one side is lying flat in free air, and they don't have a teenager who takes 20minute showers, they get away with it.
Sandwich the cable between two layers of insulation...
I would suggest that a shower cable running on top of insulation needs to be rerouted, noting that if the drops to the shower unit and consumer unit are retained, care needs to be taken when using junction boxes to connect the replacement cable - otherwise you replace the risk of overheating cables with the risk of a high current resistive connections causing smouldering/burning/fire in the junction boxes ("Bright and tight" is the rule).
For the record, I don't have an electric shower - that circuit was repurposed to power for a shower pump and the original MCB was replaced with a 6A jobby (looks a bit incongruous with 4mm cable!)
PochiSoldi
(I am not a qualified electrician disclaimer applies here..)