BS7671 Table 6E1 - Current carrying capacity

[1974stephen]

According to this table, there are different methods of cable installation. Which would be applicable to running a cable through wooden joists under floorboards in a house with no insulation under the floor, and in a vertical run of MMT2 plastic trunking for 2.4m?

I merely ask as my sister in law has a 6.00mm cable feeding an electric shower (power unknown to me), which is protected by a 40A MCB on the RCD side of a split load CU.

They are decorating another room at the moment where this cable rises in the trunking they need to cover it up, but don't want to have to undo it all if the cable needs to be replaced when the bathroom is done, and a larger shower possibly installed.

What would be the max current this cable could have pulled through it according to table 6D2 in this situation?

 

[Adam_151]

take the oppertunity now to draw in a 10mm² , you'll regret it in future if you don't

 

[1974stephen]

I fully accept that pulling in a 10mm is the best option, but what capacity shower can this cable run at max? And of course there's the infamous Part P to take into consideration now if we start running new circuits in to the CU.

 

[Big_Spark]

Stephen..you state Table 6E1..there isn't one..so I can only assume you mean Table 4E1A on page 230..

However this is the WRONG table as it deals with SINGLE CORE 90ºc THERMOSETTING INSULATED CABLES, UNARMOURED, WITH OR WITHOUT SHEATH(COPPER CONDUCTORS)

The Table you should be looking at for T&E is Table 4D2A MULTICORE 70ºC THERMOPLASTIC(PVC) INSULATED AND THERMOSETTING INSULATED, NON-ARMOURED (COPPER CONDUCTORS)

You should be looking at Section 1 (Reference Method 4) and Column 2.

You will see that 6mm² cable has a capacity of 32A whilst 10mm² has a capacity of 43A.

If your wondering why Ref method 4, it's because it is INSIDE and insulating enclosure, namely between wooden joist, floorboards and possibly an insulating ceiling .

You will also note that Table 4D2B gives information of volt drop for the cables, and the column on this that should concern you is No 3.

You need to find out the Power rating of the Shower, that way you will know it's maximum demand (current rating). When you know this, you can then work out the cable you require and also calculate the volt drop for that cable to ensure compliance with the Regulations.

 

[1974stephen]

Thanks Big Spark - excellent reply. I did mean table 4D2A, but I was looking in the OSG which refers to it as 6E1, but quotes that 7671 reference in blue (which I missed!)

So joists and floorboards are classed as insulating materials? I assume that free air means out in the open? But at what point does the void a cable runs in become big enough to be classed as free air? If it's in free air but in a 20000 sq ft warehouse do the walls and roof count as insulation for the cable? (I'm playing devils advocate!)

Any replies appreciated.

 

[ban-all-sheds]

Stephen..you state Table 6E1..there isn't one..so I can only assume you mean Table 4E1A on page 230..

However this is the WRONG table as it deals with SINGLE CORE 90ºc THERMOSETTING INSULATED CABLES, UNARMOURED, WITH OR WITHOUT SHEATH(COPPER CONDUCTORS)

The Table you should be looking at for T&E is Table 4D2A MULTICORE 70ºC THERMOPLASTIC(PVC) INSULATED AND THERMOSETTING INSULATED, NON-ARMOURED (COPPER CONDUCTORS)

6E1 is the table in the OSG, containing the data from 4D2A...

You should be looking at Section 1 (Reference Method 4) and Column 2.

You will see that 6mm² cable has a capacity of 32A whilst 10mm² has a capacity of 43A.

If your wondering why Ref method 4, it's because it is INSIDE and insulating enclosure, namely between wooden joist, floorboards and possibly an insulating ceiling .

I don't think that's right.

Which would be applicable to running a cable through wooden joists under floorboards in a house with no insulation under the floor, and in a vertical run of MMT2 plastic trunking for 2.4m?

The implication of the statement about decorating the wall is that the trunking is surface mounted (Ref Method 3). If you look at the drawings for Method 4 in the regs, it clearly shows the cables embedded in insulation, and in this case there isn't any insulation near, let alone around, the cables. Where they are under the floor, apart from short sections where they pass through joists they are in free air.

That said, Method 3 is only 38A, which is (a) less than the breaker rating, and (b) only enough for a 8.75/9.2kW shower, so the advice to replace the cable with 10mm², now, before it gets buried in the wall, is sound.

Also, Stephen, please read these concerning the rules on concealed cables:

http://www.niceic.org.uk/downloads/C5-43.pdf

http://www.niceic.org.uk/downloads/concealedcables.pdf

http://www.iee.org/Publish/WireRegs...les_concealed_in_thin_walls_or_partitions.pdf

You will also note that Table 4D2B gives information of volt drop for the cables, and the column on this that should concern you is No 3.

Table 6E2 in the OSG.

 

[Big_Spark]

BAS..Okey Dokey on the OSG...didn't have it to hand, and only use the Regs Book for Cable Tables..

Stephen..apologies of my reply about the Table was inaccurate..

BAS..the cables should be derated according to the method of installation, even if the cable passes through only 500mm of insulation on an entire run, you still need to derate for this section..so the part of 4D2A I referred to is the correct one..

Wooden floors do insulate, perhaps not as much as true insulation but it still needs to be taken into account, and as such the cables MUST be derated accordingly.

 

[ban-all-sheds]

But it's not passing through any insulation. It's not even touching any insulated surface....

 

[kai]

A B40 breaker can only offer short-circuit protection for a 6mm cable - provided it's installed on a perforated cable tray throughout.

Overload protection must be provided by a smaller B32 breaker or 30amp fuse downstream of the breaker.

Otherwise the 6mm cable will melt and catch fire, well before the B40 breaker responds.

 

[Big_Spark]

But it's not passing through any insulation. It's not even touching any insulated surface....

So wood is scotch mist is it?

BAS..the region between the floor and the ceiling will have an insulating effect simply by it's very nature, and there will be parts of the cable that pass through INSULATING walls, so the column is still the correct one..you cannot pick and choose to suit.