Back Box Size (but now about cable size)

[cb9002]

I'm buying stuff to run a 4mm2 radial... what depth back box do people suggest? 35mm?

I normally use 25mm ones for ring mains but I'm wondering if I might struggle to get two 4mm2 T+E and a 2.5mm2 T+E in the box behind a DP switch.

(No need to reopen the FCU/DP switch or is a DP switch rated to 32A arguments!)

Thanks

 

[RF Lighting]

35mm should be OK, but you could use 47mm if you want.

 

[ban-all-sheds]

What's the 2.5mm² doing there?

 

[cb9002]

4mm radial.

2.5mm on the load side of the DP switches run to unswitched undercounter sockets for fridge etc.

 

[securespark]

Which edition are you working to?

 

[cb9002]

errm, 16th I guess, I know nothing about the 17th except the new RCD requirements. What difference does it make? Circuit is RCD protected.

 

[SeanT]

Did you mean DP switches, or switched fused spurs?

 

[cb9002]

DP switches! The fusing will be in the plug behind the appliance.

 

[aptsys]

Get those 2.5mm cables replaced with 4mm then, or downrate the protective device to something suitable for the 2.5mm cable (16/20A?)

 

[cb9002]

Ahh, good point thanks, will use FCUs and flex outlet plates instead then.

Seems strange that I could spur from a ring with 2.5mm but not a radial, when both will be protected with a 32A MCB!

 

[JohnD]

you can

spurs are special.

 

[securespark]

The reason I asked is because the 17th allows 2.5 on a 4mm²/32A radial providing it feeds only one outlet.

[Whispers, "Where's FR?" ]

 

[GaryMo]

Get those 2.5mm cables replaced with 4mm then, or downrate the protective device to something suitable for the 2.5mm cable (16/20A?)

No need.
Overload protection will be provided by the BS1362 fuse in the plug top. Short circuit protection provided by the 32A MCB.

473-01-02
433.2.2

 

[cb9002]

Securespark - thankyou!

Would I have to make any other changes to ensure 17th ed compliance then? The rest of the house is to the 16th, and the circuit is on the RCD side of a split-load board.

Of course I could just stop being a cheapskate and buy some more 4mm cable...

 

[securespark]

You have to decide which edition to install it to. You can do 16th OR 17th, but you must not "mix & match"...