4mm in walls is <32 yet we use 32A mcb

[ontimeelectics]

I know this has been around but how is it reconciled that 4mm in plaster (with or without capping) can be used with 32A MCB when tables say cable rating is less than 32A?

Also:

I can't get my head around why a single socket spur on a single socket spur on ring is different to one double socket spur on a ring which is allowed. Surely two kettles in each arrangement would draw 26A down spur 21 cable?

Thanks.

Craig

 

[davesparks]

The British standard for twin sockets rates them at 20A not 26A, they are not designed to handle 26A!
The test for standards compliance is a load of 13A on one side and 7A on the other.

And a kettle is only on for a couple of minutes at a time so is pretty much irrelevant to the cable rating as it is such a short term load. Electric heaters, immersion heaters or tea urns are the real problem as far as 13A sockets go.


Where have you got the idea that it is acceptable for a cable to be rated at less than the OCPD from?? This is only allowed in the case of fixed loads which by their nature are not subject to overloading.

 

[telectrix]

4.0mm T/E ref. method C 37A table 4D7. what table are you using to get <32A?

 

[HappyHippyDad]

4.0mm T/E ref. method C 37A table 4D7. what table are you using to get <32A?

Where is table 4D7 Tel?

Hasn't the chap got a point though? 4mm is a bit close to the 32A mark. The only time (almost) that the CCC exceeds the 32A is ref method C, so if it is enclosed in conduit on a wall or trunking it has a CCC of 30A and shouldn't be on a 32A MCB? Or if it is running above the floorboards and they have insulation in then it will have less than 32A CCC.

 

[davesparks]

Where is table 4D7 Tel?

Hasn't the chap got a point though? 4mm is a bit close to the 32A mark. The only time (almost) that the CCC exceeds the 32A is ref method C, so if it is enclosed in conduit on a wall or trunking it has a CCC of 30A and shouldn't be on a 32A MCB? Or if it is running above the floorboards and they have insulation in then it will have less than 32A CCC.

What's wrong with being close to the 32A mark?

 

[HappyHippyDad]

What's wrong with being close to the 32A mark?

Well nothing Dave if the CCC is a bit over, but what about if it's a bit under?

4mm is always being used as a radial on a 32A MCB so I guess I'm missing something, but if its in conduit or trunking, or has any insulation around it then its CCC will be less than 32A.

 

[HappyHippyDad]

Just one last thing to the OP as I need to sleep!!

See P.320 Table 4A2 (57).. This shows that cables buried in wall are ref method C, so CCC would be 36A and fine on a 32A MCB.

As for having a single socket spurred off a single socket spur.. I can see your point about it being the same as one double spur in terms of what could actually be plugged in. But I think you have to look a bit further and think what could happen in the future, those singles could easily get changed for doubles by the next (DIY) owner.

 

[telectrix]

Where is table 4D7 Tel?

Hasn't the chap got a point though? 4mm is a bit close to the 32A mark. The only time (almost) that the CCC exceeds the 32A is ref method C, so if it is enclosed in conduit on a wall or trunking it has a CCC of 30A and shouldn't be on a 32A MCB? Or if it is running above the floorboards and they have insulation in then it will have less than 32A CCC.

sorry. my typo. should have read table 4D5.( for flat cables), and as you say in a later post, cables in plaster, method C.

 

[Darkwood]

Probably got one of those dodgy fake reg's books :willy_nilly:

 

[gazj82]

Personally I have never used 4mm. Even back when I worked for someone else, we always just missed 4mm out and went straight to 6mm.

Unlike 1.0mm and 1.5mm, always used 1.0mm when I first started then for some reason we changed to 1.5mm, now I don't ever seem to ask for 1.0mm, I take it that it is still readily available?

 

[davesparks]

What a waste of money!

 

[driverman]

Personally I have never used 4mm. Even back when I worked for someone else, we always just missed 4mm out and went straight to 6mm.

Unlike 1.0mm and 1.5mm, always used 1.0mm when I first started then for some reason we changed to 1.5mm, now I don't ever seem to ask for 1.0mm, I take it that it is still readily available?

Yep still available

 

[gazj82]

Yep it probably is wasteful. It's just another size to stock I guess that's why my last employer (my Uncle) never bothered. Not sure why one day he decided to start using 1.5mm instead of 1.0mm. Maybe it was spec on a job we did and we never changed back.

 

[shanky887614]

Yep it probably is wasteful. It's just another size to stock I guess that's why my last employer (my Uncle) never bothered. Not sure why one day he decided to start using 1.5mm instead of 1.0mm. Maybe it was spec on a job we did and we never changed back.

we use 1mm 3c sometimes usually because wholesaler hasnt got any 1.5

 

[Engineer54]

All ring and radial final circuits on our project are constructed with 4mm conductors. The vast majority of which within a metal containment system....