I have three electrical cables (not flex - running to an immersion) tied together and run through joists in my loft space. I want to insulate between the joists.

I know it is not a good idea to bury cables, particularly of a high rating. But it would be costly in terms of time and money to re-route the cables.

Is is acceptable to run the cables in some sort of conduit or sheathing to maintain an air gap between the cable and the insulation?

The point is, can they cool, or the resistance heat they produce just builds up? Oversizing within insulation is to reduce the resistance to design load, hence reduce the heat produced.

Posted By: djh
Isn't threading them through conduit about as time consuming as laying them on top of the insulation?

I was assuming that wouldn't be easy since they currently go through the joists, and that the conduit might be something like sections of the trunking with a removable side?

As to how that affects the calculations, IANAE...

Yes, they go through the joists. The joists are about 150mm deep and they go right through the middle.

Sorry, I am not an electrician hence my trepidation re. the immersion coil. If they were just going to downlighters I would re-wire myself but I'm not so sure about this.

Posted By: Andrew_Doran
What size are the cables (2.5mm^2 ?) and what rating of fuse or breaker are they fed from?

EDIT are the cables PVC twin&earth or singles?

Sorry but I can only give a layman's description. They are 10mm wide, flat, grey. At least one of them goes into an MCB with B20 on it - that means 20A?

Posted By: SteamyTeaI think this is notifiable work, so you need to be Part P certified or get someone in that is.

Even if there's no re-wiring?

Possibly, there are 'installation methods' to take into account.

I also think that if you disconnect and reconnect a water heater then it is classed as major work. Not 100% sure on that though (I did my Part P a few years back and it has changed since).

Quite simply you are going to struggle without rerouting what sounds like twin and earth.

Posted By: SteamyTeaPossibly, there are 'installation methods' to take into account.

I also think that if you disconnect and reconnect a water heater then it is classed as major work. Not 100% sure on that though (I did my Part P a few years back and it has changed since).

Thanks. Part P or not I don't fancy it!

Posted By: Ed DaviesWould it be feasible to stuff some insulation under the wire (e.g., foil-faced PUR) but leave them freely in contact with the air above? Ie, break the mineral wool or whatever main insulation say 75 mm each side of the cables but make up for the gap with a bit of higher-grade insulation under.

I am with Ed on this one, just put 75mm pir/pur under the wires say 100mm wide and leave above free to air, fill all other voids with wool as described. No touching electrics, no part P, no problem.😀

There's probably a calculation one can do for the vacuum temperature of a conductor of a particular material of a particular diameter carrying a particular current. If this is within tolerable limits then greater losses caused by contact with insulation material would lower the temperature - even if convection provides greater losses.

Another approach could be to bung a couple of temperature sensors in hot spots along the cable (the most insulated places), either for monitoring or even connecting them to a thermal cut-out of the immersion. You can stress test the cable (immersion on for 6 hours during a hot summer's day in a cold tank) and if it never rises more than 20C above ambient then perhaps that's safe enough?

Posted By: gravelldhttp://electrical.theiet.org/wiring-matters/15/current-ratings.cfm?type=pdf" rel="nofollow" >http://electrical.theiet.org/wiring-matters/15/current-ratings.cfm?type=pdf

So what do these current ratings mean? Given a 10mm cable...

You misunderstand. The data in that link refers to the cross sectional area of the copper cores (in mm^2) not the size of the outer grey insulation. You need to find out exactly what the wires are.

In a typical house you might find 1 or 1.5mm^2 wire used for lighting and 2.5mm^2 for ring mains. You might also find 6, 10, or 16mm^2 feeding something like a cooker or shower. Perhaps also cables for a PV array.

Many of these wires are capable of carrying more current than they are actually used for. For example I think a 2.5mm^2 wire can carry up 27A or 54A in a ring without insulation. However that document says if the wire is on top of a joist and under 440mm of insulation it should be derated to carry around 16A or 32A for a ring. However many rings are protected by a 32A breaker in the consumer unit so the current is already limited to 32A. In which case no action is required when covering that ring with insulation.

Other sizes of cable may need to be derated by changing the breaker to a smaller one. Sometimes that's not possible. For example a 12kW shower draws 52A. If it was on a 16mm^2 wire the depth of insulation might mean limiting the current to 45A. If you fitted a 45A breaker the 52A shower would probably trip it.

Posted By: CWatters
You misunderstand. The data in that link refers to the cross sectional area of the copper cores (in mm^2) not the size of the outer grey insulation. You need to find out exactly what the wires are.

In a typical house you might find 1 or 1.5mm^2 wire used for lighting and 2.5mm^2 for ring mains. You might also find 6, 10, or 16mm^2 feeding something like a cooker or shower. Perhaps also cables for a PV array.

Many of these wires are capable of carrying more current than they are actually used for. For example I think a 2.5mm^2 wire can carry up 27A or 54A in a ring without insulation. However that document says if the wire is on top of a joist and under 440mm of insulation it should be derated to carry around 16A or 32A for a ring. However many rings are protected by a 32A breaker in the consumer unit so the current is already limited to 32A. In which case no action is required when covering that ring with insulation.

Other sizes of cable may need to be derated by changing the breaker to a smaller one. Sometimes that's not possible. For example a 12kW shower draws 52A. If it was on a 16mm^2 wire the depth of insulation might mean limiting the current to 45A. If you fitted a 45A breaker the 52A shower would probably trip it.

Thanks for that.

So is the upshot of that that I need to check the wire is 2.5mm (twin)? And if so, and if the breakers are 16A or 32A I would be OK to cover? As above it looks to be 20A but will check all cables with my voltage tester pen thing, turning off the MCBs and testing if they are still live.

Posted By: gravelldSo is the upshot of that that I need to check the wire is 2.5mm (twin)?

mm². The squared bit is important but unfortunately electricians habitually leave it out leaving the rest of us not quite sure what is being talked about in odd cases. E.g., are connectors which are not typically used in domestic situations being specified for wire sizes in mm (diameter) or really mm² (cross-sectional area).

It'll almost certainly be twin and earth. Live (phase) and neutral wires, each of the specified cross-sectional area, in their own coloured insulation with a bare copper earth wire (usually slightly smaller cross-section, I think) all wrapped in an outer PVC layer of insulation.

Posted By: djhGiven your lack of confidence, I'd suggest that you really do need to get a qualified electrician to have a look and sign off whatever you end up doing.

Right. To be clear: my lack of confidence is in my inexperience to perform wiring on a high draw appliance. If I see credible evidence that leads to a solution to this however, I would have confidence in that.

Sure, thanks to you all for advice. I think I'll do that. I might see if I can construct something with EPS (not touching cable) to keep the loft roll back.

Frankly, if a mains wire is generating so much heat that it requires convection cooling to avoid a fire hazard, then it's undersized in the first place.