1.5mm ring

[asc]

I noted a ring cct today in 1.5 T+E on a 3036 30a Fuse. What is the largest fusing or mcb that this could safely be used with?.

 

[LoveRocket]

IF it is clipped direct throughout (which it won't be) it COULD in theory be used on a 32a MCB. There's no way it can be on a 30a BS3036 though due to the derating factor.

You can't have a ring circuit on anything other than a 30/32a OPD nowadays, so you're pretty screwed. You'd have to treat it as a radial, in which case it should be OK on a 16a MCB - provided it's not covered in thermal insulation.

 

[ban-all-sheds]

There isn't one.

You cannot have a ring final (I assume it was a socket circuit?) on anything less than a 30/32A OPD.

If the cable isn't rated at at least 20A then you have to increase the size of the cable, you may not reduce the rating of the OPD.

 

[ricicle]

Are you sure it is 1.5mm² ? The regs do allow for the use of the 2.5mm² ring final ensuring that the cable used is able to carry at least 20A for the ring to be on a 30/32A device.
No such regs exist for a 1.5mm² ring final circuit, so if you are feeling mathematical you could ratio the figures to suit. It's late and I'm tired so don't want to do it right now.

 

[LoveRocket]

Are you sure it is 1.5mm² ? The regs do allow for the use of the 2.5mm² ring final ensuring that the cable used is able to carry at least 20A for the ring to be on a 30/32A device.
No such regs exist for a 1.5mm² ring final circuit, so if you are feeling mathematical you could ratio the figures to suit. It's late and I'm tired so don't want to do it right now.

Not true, riccle. All the regs state is that Iz must be 20amps are more for a 30/32a RFC. NO MENTION is made as to a minimum cable size, just a minimum Iz. (and the one two thirds of In rule is no longer in the 17th - it's just an In of 30/32 and an Iz of <=20)

 

[asc]

Thanks for that info, I didn't know you couldn't reduce the protective device to suit the cable. I had thought a 20A plug in mcb might help in the short term, but if it is a no-go I'll forget that.

 

[LoveRocket]

Thanks for that info, I didn't know you couldn't reduce the protective device to suit the cable. I had thought a 20A plug in mcb might help in the short term, but if it is a no-go I'll forget that.

Not on a ring circuit you can't.

That's what the regs say, although common sense says it's CERTAINLY better to have it on a B20 than a 30A BS3036, so definitely do that for the time being. It's just not compliant.

 

[ban-all-sheds]

NO MENTION is made as to a minimum cable size

Do you mean apart from the 2nd sentence in 433.1.5, or including it....?

 

[asc]

yes I am sure of the size - odd though to find this when it had a pir 3 years ago by a well known franchise who do gas and electrical inspections

 

[LoveRocket]

NO MENTION is made as to a minimum cable size

Do you mean apart from the 2nd sentence in 433.1.5, or including it....?

Excluding it.

asc - stick it on your B20 MCB for now.

 

[ricicle]

BAH too slow, just got my regsbook out to prove a point to Mr Rocket as well !

 

[GaryMo]

This is the reg that's being refered to here:

433.1.5

Accessories to BS 1363 may be supplied through a ring final circuit, with or without unfused spurs, protected by a 30 A or 32 A protective device complying with BS 88-2.2, BS 88-6, BS 1361, BS 3036, BS EN 60898, BS EN 60947-2 or BS EN 61009-1 (RCBO). The circuit shall be wired with copper conductors having line and neutral conductors with a minimum cross-sectional area of 2.5 mm2 except for two-core mineral insulated cables complying with BS EN 60702-1, for which the minimum cross sectional area is ].5 mm2 . Such circuits are deemed to meet the requirements of Regulation 433.1.1 if the current-carrying capacity (lz) of the cable is not less than 20 A and if, under the intended conditions of use, the load current in any part of the circuit is unlikely to exceed for long periods the current-carrying capacity (Iz) of the cable.

 

[OwainDIYer]

You cannot have a ring final (I assume it was a socket circuit?) on anything less than a 30/32A OPD.

Yes you can.

It's not a standard circuit, and it's probably rather pointless in terms of available power vs no. of outlets vs. cable use.

It would have to be calculated from first principles though to establish whether a higher rated protective device could be used because it's a ring, rather than a 1.5mm radial. Absent those calcs, I woudln't put it on higher than a 16A MCB.

And in terms of adequacy, a 16A circuit will almost certainly not be adequate to cover the same floor area as a 32A ring, or even a 20A radial, so the circuit would need splitting at mid point and separating into 2 16A radials. Even then it's a bit of a bodge. But splitting it into 2 radials at 16A should be a reasonably workable, if not wholly compliant, interim solution before rewiring.

 

[ban-all-sheds]

The only exemption from the regulations applying to parallel conductors is 433.1.5.

If you don't comply with that then you cannot have any higher rated protective device because it's a ring - you are limited to the Iz of the cable.

 

[ColJack]

didn't we argue this one to death already?

if I want to I can have sockets on a ring fed by a B6 breaker.. as long as the current rating of the cable is equal or higher than the rating of the breaker..

kind of pointless since a radial will suffice but no rule against it.

you cannot however have a ring on a B6 ( etc ) if the rating of the cable is only 4A ( for arguments sake ).

the ONLY such configuration allowed by the regs for a ring is the one mentioned of the 30/32A OCPD protecting cables with a MINIMUM rating of 20A ( and min CSA of 2.5mm² regular or 1.5mm² MICC cables )