Options for ring main in wall insulation

[pls1]

We are installing 100mm of insulation to a single skin brick outhouse which will become a utility area. This will comprise of 50mm Kingspan between 2x2 treated battens and then cross battened with another 50mm of insulation. This will then be covered with plasterboard and skimmed.
There will be several appliances in the utility area (boiler, fridge freezer, washing machine) which we were planning to supply by extending the kitchen ring.
Since the house is being partially rewired (new CU, sockets etc) we have several options when it comes to electrics.
I've read about cable derating factors for cable in insulation and the relevant installation methods (specially 102 & 103) but am still puzzled about the best solution in our case.
From what I gather the options are:

1.) Run the ring main behind the plasterboard (touching it) but in front of the insulation. This will be method 102 and 2.5mm will be derated to 21A.
If I keep the cable all at 2.5mm, then a 16A MCB should be used rather than 32A?
Problem with this option is that there might not be a suitable gap between the 50mm insulation and the back of the plasterboard to run the cable.

2.) Run the cable in conduit (20mm round PVC?) and cut a channel in the insulation for the conduit.
Again presumably the MCB will need to be downgraded to 16A since cable is derated to 20A.

3.) Run the circuit as a radial on a 16A fuse (2.5mm)

4.) RUn the circuit in 4mm T&E, which will increase the rating to 17.5A


Not sure which is the best solution.

 

[EFLImpudence]

2.) Run the cable in conduit (20mm round PVC?) and cut a channel in the insulation for the conduit.
Again presumably the MCB will need to be downgraded to 16A since cable is derated to 20A. 18.5
3.) Run the circuit as a radial on a 16A fuse (2.5mm)

4.) RUn the circuit in 4mm T&E, which will increase the rating to 17.5A 25

I think you have misread two of the ratings but it won't make much difference.

Well, you seem to have it sussed.
You either have to limit the current or use larger cable.

However, It is not just a question of using a lower rated MCB but still have lots of sockets and lots of appliances.
For example, if using a 16A MCB then you must plan that no more than 16A will be plugged in.

I would suggest you use 4mm² cable and design accordingly.
I don't know how big the out house is or how many sockets you require but could you have several radial circuits?

 

[JohnW2]

1.) Run the ring main behind the plasterboard (touching it) but in front of the insulation. This will be method 102 and 2.5mm will be derated to 21A. If I keep the cable all at 2.5mm, then a 16A MCB should be used rather than 32A? Problem with this option is that there might not be a suitable gap between the 50mm insulation and the back of the plasterboard to run the cable.

If there is an adequate gap for you to do this, you can have a ring final circuit with 2.5mm² cable and a 32A MCB. The regulation concerning ring final circuits (433.1.5) allows a 32A MCB provided that the current carrying capacity of the cable (with the installation method used) is at least 20A - which would be the case. If you can do this, it would probably be best - since, as EFLI said, a 16A circuit does not allow for a lot of load.

Kind Regards, John.

 

[PrenticeBoyofDerry]

Have you consider using ref method A?
This will allow you to run the cable in conduit.
2.5mm will allow a 20A device to be installed.
4.00mm will allow for 26A, so could install a 25A device if excessive load is a problem.
And depending on the area covered it does not need to be a ring final circuit, could be a radial.

 

[pls1]

So essentially whichever way I do it ring/radial and method 102 or Ref A the max current carrying capacity of the cable is going to be either 20 or 21A for 2.5mm T&E.
Since the outbuilding is small (2x2.5m) and will only have a boiler, fridge/freezer and combined washingmachine/tumble dryer - I might as well run it as a 20A radial (20A MCB) and save on cable.
Once the insulation is in, I can see if there is enough of a gap to run the T&E touching the plasterboard or channel it out and run the cable in conduit.

 

[pls1]

If there is an adequate gap for you to do this, you can have a ring final circuit with 2.5mm² cable and a 32A MCB. The regulation concerning ring final circuits (433.1.5) allows a 32A MCB provided that the current carrying capacity of the cable (with the installation method used) is at least 20A - which would be the case. If you can do this, it would probably be best - since, as EFLI said, a 16A circuit does not allow for a lot of load.

Even though that is the case according to the regs, would it not be better practice to put a 20A MCB on the circuit (even if it is a ring) since the cable will only have a max rating slightly higher than 20A.

 

[JohnW2]

Even though that is the case according to the regs, would it not be better practice to put a 20A MCB on the circuit (even if it is a ring) since the cable will only have a max rating slightly higher than 20A.

Some do not like ring final circuits (for this, and other, reasons), but they are extremely common and the whole point of them is that they allow circuit (and MCB) to be designed for a current nearly double the current carrying capacity of the cable. The actual current which flows in the two legs of the ring depends upon how the loads are distributed around the ring (which is one of the reasons some people don't like rings) - but if you take the 'best case' of 32A worth of load all being connected at the centre of the ring, only 16A would travel in each leg. The vast majority of ring finals are wired in 2.5mm² cable, and the highest current carrying capacity it has (when 'clipped direct', Method C) is 27A; more commonly it will only be 20/21A, and it is perfectly acceptable practice to have 32A ring final circuit in such a situation.

As you've said, you could save on cable by having a 20A radial circuit, but only you know whether 20A is always going to be enough; unless Method A or C, even 4mm² cable would not be enough for a 32A radial.

Kind Regards, John.