Answer this one!

[SPECIAL LOCATION]

433 is the chapter which covers most of the above points AFAIK?

433.2.2 does say... the overload protection device may be fitted ALONG the run of the cable,

providing the cable has no branches or outlets

and should be no longer than 3m.

but 434.2 does prefer protection to be at point of change in C.S.A size.

 

[SPECIAL LOCATION]

Is it not 27A if installed as method 'C', 20A is method 'A' conduit in insulated wall

indeed Ian.

 

[Nicky Tesla]

If the resistance from the first socket to the DB was 1 ohm (for eg)

and the resistance to the DB going around the room and back to the DB was 10 ohm

ie 10 times the distance of the other leg, then the amperage down each leg would be in proportion. 11 parts altogether, 1 part down the first leg, and 10 parts down the other leg

If 11 amps was being drawn through the ring, then 1A would go one way down the 10ohm side, and 10A would go down the 1ohm side,, so as you can see this by no way a 50/50 amperage split,

 

[kme]

Note 1.

I`d have no issues with a spur being derived from the mcb terminals. It is the same as any other spur from a ring. I wouldn`t install one myself, as I would deem it bad design; but wouldn`t code it on a pir.

Note 2.

Due to the job I`m on at the moment, I`ve just wired 3 x 20W 12V downlights to a 6A, C curve MCB. MOST of the circuit is in 2.5, but the last 10M or so is 1.5. The 2 reasons for this are:

A. The flex of the transformers won`t grip into the JB very well with a 2.5 solid conductor.

B. The Zs on the circuit (70M) would be over the disconnection times for the MCB. I don`t need a derating device when I change from 2.5 to 1.5, as the circuit is covered by a 6A MCB!

Note 3.

Nicky - What does your post have to do with the OP? We aren`t discussing the ring itself, per se, just the fact that a spur is supplied from the JB.

 

[Nicky Tesla]

Note 1.I`d have no issues with a spur being derived from the mcb terminals. It is the same as any other spur from a ring. I wouldn`t install one myself, as I would deem it bad design; but wouldn`t code it on a pir.

Note 2.

Due to the job I`m on at the moment, I`ve just wired 3 x 20W 12V downlights to a 6A, C curve MCB. MOST of the circuit is in 2.5, but the last 10M or so is 1.5. The 2 reasons for this are:

A. The flex of the transformers won`t grip into the JB very well with a 2.5 solid conductor.

B. The Zs on the circuit (70M) would be over the disconnection times for the MCB. I don`t need a derating device when I change from 2.5 to 1.5, as the circuit is covered by a 6A MCB!

Note 3.

Nicky - What does your post have to do with the OP? We aren`t discussing the ring itself, per se, just the fact that a spur is supplied from the JB.

i dont know what happened there? :| i was replying to one that said the current was 50/50 around the ring, but werz it gone. im losing it:coat

 

[Gforce]

sorry if im dragging up an old post-maybe im wrong but surely if the boiler is fused at 3 amp the maximum amps running through that particular 2.5 cable would ever be 3 amp before the fuse would blow anyway. The fact that it is spurred from a ring main wouldnt affect it as the current drawn down the ring wouldnt be travelling through the radial at all.

Once again soz if I have got wrong end of stick still new to this.

 

[Andy™]

but surely if the boiler is fused at 3 amp the maximum amps running through that particular 2.5 cable would ever be 3 amp before the fuse would blow anyway.

this is why you can put overload protection at the end of the cable. but the cable must be capable of opening the breaker protecting it if there is a short circuit.

 

[binky]

Checkout the adiabatic equation and sizing circuits by CPC size / loading - its complicated and I've never been very happy with it, but it is legal.

 

[irob000]

When i did an NICEIC certification course, this come up and app your meant to wire any spur from a ring main in 4mm so it is in line with current rating..... But must be honest i'm old school and always use 2.5mm

 

[green-hornet]

a SPUR can be "of suitable current carrying capacity cable" ie a spur from a 10mm cable supplying a light could be wired in 1.00mm cable provided that at the point of connection the cable is protected by a suitable fuse. You do not have to spur off a ring in 4mm, this would be pointless.Remember a spur still maintains the integrety of the ring, provided that a fused spur is used. Zs for the ring is unaffected by the spur,the overcurrent protection for the spur part of the installation is provided by the fuse in the sfu.If you spur off a ring to provide one socket without a fused spur you are well within the limitations to use a single 2.5 twin and earth without any detrimental effects.