Loft insulation over electric cables

[touch]

I've been topping up the insulation in my loft and now just got this small corner to finish off:



You can see the old (50mm) insulation in the photo - and large areas of bare plaster with no insulation. The yellow stuff is the new insulation.

There's a few cables in the way here. The 3 smaller cables at the top of the photo are all for lighting and have enough slack to be laid on top of the new insulation.
The large cables are for the shower. I'm not sure what the rating on the shower is but it has a cold water feed and heats the water itself so I guess it uses a lot of power. The shower will only be on for a max of 30 mins a day so there isnt a lot of time for heat to build up in these cables.

So what should I do here?

• Insulate over the top?
• Cut the insulation to leave a space around the cables?
• Some sort of conduit to box the cables in and insulate over the top of that?

 

[Siliconslave]

From doing a bit of reading up on this recently it depends what the wires are. Consensus seems to be lighting ring main is fine as its generally very low load (but make sure you box up any downlighters)

Shower or heater mains cables however run very near to capacity to don't cover them.

 

[koolpc]

Always covered them. Guess that may be wrong but over 30 years of doing so and even this house i now live in has insulation covering the wiring and it was recently renovated (not by me)

 

[phil-k]

it is the same with cables running in wall cavities that have been pumped full of insulation, there must be thousands of homes like that, including mine

 

[touch]

From doing a bit of reading up on this recently it depends what the wires are. Consensus seems to be lighting ring main is fine as its generally very low load (but make sure you box up any downlighters)

Shower or heater mains cables however run very near to capacity to don't cover them.

This is what I've found too. It's just the period of time that the shower is on that makes me think it might not be an issue.
A heater can be on for several hours at a time so I can see how the cables might overheat but my shower is going to be on for around 20mins at a time.

 

[Lumeycom]

https://www.tlc-direct.co.uk/Book/4.3.6.htm

A reasonable excerpt from the regs. The cable should be derated.

 

[divuk83]

I know you are not supposed to insulate over them, but I have always gone over lighting cables (as I think the most powerful light I have is about 10 watts) and gone under/around any cables for sockets/boilers etc. Obviously I avoid going over down lights too.

Dave

 

[touch]

https://www.tlc-direct.co.uk/Book/4.3.6.htm

A reasonable excerpt from the regs. The cable should be derated.

Thanks for this.
It's around 100mm of cable that will be buried in the insulation, so I need to derate by a factor of 0.81

The shower is rated at 10.8kW. So at 230volts, it would need a cable rated to at least 47amps. (Realistically, the shower runs at about 60-70% of max temp so probably doesnt consume anywhere near 10kW)

Derating by a factor of 0.81 means I'd need a cable rated to 58amps.

So, if the cable is 6awg (4.115mm conductor diameter) or higher, it should be ok to cover. I couldnt see any ratings marked on the cable so I'll probably need to find and exposed bit in the switch and measure it.

Does my maths look correct?

 

[Jez]

http://www.cable-ratings.co.uk/

See here for UK cable ratings. Forget the derating thing, just use the first column as thats for an enclosed insulated cable

 

[Lumeycom]

Thanks for this.
It's around 100mm of cable that will be buried in the insulation, so I need to derate by a factor of 0.81

The shower is rated at 10.8kW. So at 230volts, it would need a cable rated to at least 47amps. (Realistically, the shower runs at about 60-70% of max temp so probably doesnt consume anywhere near 10kW)

Derating by a factor of 0.81 means I'd need a cable rated to 58amps.

So, if the cable is 6awg (4.115mm conductor diameter) or higher, it should be ok to cover. I couldnt see any ratings marked on the cable so I'll probably need to find and exposed bit in the switch and measure it.

Does my maths look correct?

The temprature control on an electric shower is water flow not power (unless it has 2 settings in which case its likely to be a pair of heating elements) and power is constant so a 10.8kw shower will always be a 10.8kw shower. There is also no allowance for diversity on a shower circuit so you must treat it as full load.

Please dont use AWG - its daft. Stick to mm2.

'generally' speaking if you look in the shower switch and the earth is solid and the live and neutral are 7 strand then its 6mm2, if the earth is also stranded then its 10mm2 or above. post a pic of the shower switch and someone should be able to tell you by eye alone.
Technically speaking your protection should also match the derating but thats all getting a bit OTT (imho of course).

The link posted by Jez looks good to me.

 

[Nedved11]

If clipped should be fine for lighting circuit. Thing you need to worry about in lofts is polystyrene laying over the cable. Don't do this the two do not mix well! You will be surprised how many times I see this going into people's lofts...

 

[HardwareGeek]

Lighting circuits in 1.0/1.5mm are effectively already derated, running about 1/3 of their max current carrying capacity vs. free air / clipped direct, so approx 15-18A but on a 6A mcb

All other circuits do not have this headroom and you should avoid covering with insulation, cable passing through more than 100mm of insulation should be derated to 1/2 capacity etc

With shower circuits first sign will probably be the shower isolation switch burning out and will seize up so you cant switch it or shower will intermittently loose power because of the bad connection, maybe get a fishy smell from the melting plastic