CABLE DERATING

[LEEF]

I wonder if any bright sparks out there can shed some light

During the course of speaking to a building regs chap about knocking down a wall in my kitchen, he mentioned amongst other things that they get people to insulate the first floor ceiling void these days.

Me trying to do my bit for the environment and my energy bill, thought I'd insulate my kitchen ceiling after the wall comes down and before the new ceiling goes up.

Now the actual question (finally! I here you mutter), is about cable derating.

From my very limited knowledge a cable fully encased in insulation (as a cable would be as it passes through the centre of the joist (as they do)means they should be derated by 50%.

I'm wondering if insulating the ceiling might mean rewiring the entire house (all cables originate from the pantry CU and pass through the kitchen ceiling)?

 

[JohnD]

is the kitchen a single-storey extension?

what is the joist depth?

 

[DESL]

insulate the first floor ceiling void these days.

I assume you mean the ground floor ceiling void

 

[LEEF]

Sorry, mean't ground floor ceiling void

 

[DESL]

Ok so next question

where and how many cables are there running through this void?

Would be possible create an air space around them and still insulate.

 

[JohnD]

I was thinking, apply the insulation from underneath, so the cables are not covered. Fibreglass will squash down. You could tack some ply under them like a cable tray to be sure.

If it is an internal floor the insulation is AFAIK thinner than required in a loft.

 

[LEEF]

All the cables from the CU run through the void, they're all bunched initally and then run off in two directions (mainly), one across the joists/room and one with the joists down the length of the room.
Theres a cooker radial (10mmT/E 30A MCB) approx 20ft
A single socket ring 2.5T/E 30A MCB (no idea on length)
x2 lighting circuits each 1.5T/E 5A MCB's (no idea on length), I'm 100% certain loft insulation is already covering these cables in the attic?
Immersion heater radial 2.5T/E 15A MCB approx 20ft.
Shower radial 10mmT/E 30A MCB approx 15ft.
Electric heater radial 2.5T/E 15A MCB to the main bedroom (This has been disconnected and replaced with a double socket to which a TV & Video are plugged in).

How far would you need to pull the insulation away from the cables for derating not to come into play?
With regards to the ply, the joists are 150mm, the space between the ceiling board and the cables is 50-75mm
Do you need the ply to isolate the insulation? what if the insulation (I was considering 100mm) was a loose fit underneath (50mm) clear on the sides and above, would you still need to derate by 50%?

 

[JohnD]

IIRC the derating rule says "surrounded by insulation."

You could put a barrier below the cable, and leave the top exposed; or above the cable, and leave the underside exposed. Either way they would only have insulation one one side (either above or below them), they would not be touching it, and otherwise exposed to the air and the ceiling or floor.

The ply between the cable and the insulation will prevent the insulation from sagging or creeping to engulf the cables.

What is above this kitchen? Another room? A loft? A flat roof?

p.s any cables that run "along" the joists might have enough slack to be moved so they can be clipped to the sides or tops of the joists, not touching the insulation.

 

[Steve3948]

IIRC the derating rule says "surrounded by insulation."

You could put a barrier below the cable, and leave the top exposed; or above the cable, and leave the underside exposed. Either way they would only have insulation one one side (either above or below them), they would not be touching it, and otherwise exposed to the air and the ceiling or floor.

The ply between the cable and the insulation will prevent the insulation from sagging or creeping to engulf the cables.

What is above this kitchen? Another room? A loft? A flat roof?

p.s any cables that run "along" the joists might have enough slack to be moved so they can be clipped to the sides or tops of the joists, not touching the insulation.

JohnD is correct, most builders will put in the insulation up from under before tacking the ceiling, therefore cables are above the insulation, no need to de-rate.

When it's a refurb and the ceiling is existing but the floor above is up, I get the builder to tuck the insulation under the cables.

Cables not normally a problem, not like Low Voltage Downlight issues.

 

[LEEF]

Above the ceiling is a bathroom, the landing and a bit of a bedroom. Boards up on the landing and in the bedroom are a possibility but the bathroom would be a nigh on impossible. Got to be careful when removing boards on the landing as the bathroom walls are solid and sitting directly on the floor boards! House was built in the fifties, maybe this was OK then but doesn't seem like a good idea to me.
The ply does it need to be fixed? or can it be a 4-5" wide strip sitting on the insulation following underneath the cables?
Not enough slack in the cables to move more than an inch!
If the cables are sitting on ply I don't have to derate but if they sit on the insulation you have to derate?

 

[Steve3948]

If sitting on the insulation NO, only when surrounded.

 

[LEEF]

Above the ceiling is a bathroom the landing and a bit of a bedroom. Boards up on the landing and in the bedroom are a possibility but the bathroom would be a nigh on impossible. Got to be careful when removing boards on the landing as the bathroom walls are solid and sitting directly on the floor boards! House was built in the fifties, maybe this was OK then but doesn't seem like a good idea to me.
The ply does it need to be fixed? or can it be a 4-5" wide strip sitting on the insulation following underneath the cables?
Not enough slack in the cables to move more than an inch!
If the cables are sitting on ply I don't have to derate but if they sit on the insulation you have to derate?(more than is feasible with existing cables)

 

[JohnD]

the reason for the ply is just to make sure the cables don't sink into the fibreglass over time, and the fibreglass doen't shift so it engulfs the cables. You only need to tack them lightly to the joists - they will not be carrying any weight. You could use scraps of plasterboard if you prefer. As you know that the cables will not be surrounded by insulation you don't need to derate them.

But as you say it's an internal ceiling with a room above, what is the point of the insulation? I have it over my integral garage ceiling, as it's an unheated space. It's only 50mm there (much thicker in the loft where it divides a heated space below from an unheated space above)

 

[LEEF]

Sorry Steve didn't see your reply when I tacked some thing to the end of my last answer.

I have a couple of more quick questions while your good enough to give me the benefit of your wisdom(s).
I’m also replastering all the walls and planned on putting all the cables into conduit (to make rewiring possible in the future), this isn’t going to cause derating issues also is it?

Finally, I have a new Induction Hob (6.9kW/32A) and a built under oven 3.6kW/16A to replace a free standing cooker. As mentioned above I currently have a 10mm cable & 30A MCB. If I connect the new hob to the cooker outlet covered by the 30A MCB is it likely to trip as the Hob spec is a 32A or do appliances not actually pull the MAX specified?
The oven I suspect you will suggest running a new radial rather then connecting to the existing cooker switch ?
Would you suggest a 2.5mm, 6mm or 10mm? cable, going to a 20A DP switch? before finishing at another cooker outlet?
What’s the damage likely to be to then get a part ‘P’ spark to connect to the CU?

 

[LEEF]

The only reason I was considering it was because the building regs chap said they now get people to insulate even internal ceiling voids (unless I got the wrong end of the stick), insulation don't cost a packet and if the ceiling is down already?
Got plenty of plaster board so would probably go with that instead of ply.