Ring Fused Spurs and overload protection (433.1.204)

[Steve T89]

Just a quick question from a new qualified sparky.

So in 433.1.204 when it says the Iz of the cable must not be less than 20A for a ring final circuit.
Does this include a radial fused spur taken off the ring final?

If you install it in 2.5mm in trunking then the Iz is immediately down to 20A even before you add in any other factors. (I will be installing this in trunking with a 1.5mm lighting cable, so it will be less)

Will I have to install using a 4mm cable, or seeing as the spurred part of the circuit is protected by a 13a fuse, does that then comply with 433.1.1? (Treat it as a radial, not a ring from that point on?)

 

[telectrix]

if you're fitting a 13A FCU, your 1.5mm spur will be compliant. fit as many sockets as you want (within reason).

 

[Steve T89]

if you're fitting a 13A FCU, your 1.5mm spur will be compliant. fit as many sockets as you want (within reason).

Thanks Telectrix, I think you got back to me 1st the last time I asked a question...and it was the most helpful answer!

The spurs going to be in 2.5mm, but it's sharing the trunking with a separate light circuit.
I was thinking it would be close with the correction factors and 4mm seemed a bit OTT!

Sadly they only want one socket, but it's all practice!

Thanks again.

 

[Pretty Mouth]

Just a quick question from a new qualified sparky.

So in 433.1.204 when it says the Iz of the cable must not be less than 20A for a ring final circuit.
Does this include a radial fused spur taken off the ring final?

If you install it in 2.5mm in trunking then the Iz is immediately down to 20A even before you add in any other factors. (I will be installing this in trunking with a 1.5mm lighting cable, so it will be less)

Will I have to install using a 4mm cable, or seeing as the spurred part of the circuit is protected by a 13a fuse, does that then comply with 433.1.1? (Treat it as a radial, not a ring from that point on?)

Hi Steve. How are you getting 20A Iz for 2.5mm in trunking? Should be 23A, ref method B.

As @telectrix says, 1.5mm will generally comply for a fused spur from a ring. Have a look at P505 in your regs book and reference it with chapter 7 OSG for more info.

 

[Spoon]

If you install it in 2.5mm in trunking then the Iz is immediately down to 20A even before you add in any other factors. (I will be installing this in trunking with a 1.5mm lighting cable, so it will be less)

If you look at page 409 of the BBB then it states '(enclosed in conduit in an insulated wall)' which is Method A for 20A
As @Pretty Mouth has said, should be 23A.

Also if you look at Note 9 of Table 4C1 it states 'If, due to known operating conditions, a cable is expected to carry not more than 30 % of its grouped rating, it may be ignored for the purpose of obtaining the rating factor for the rest of the group.'

 

[Steve T89]

Hi Steve. How are you getting 20A Iz for 2.5mm in trunking? Should be 23A, ref method B.

As @telectrix says, 1.5mm will generally comply for a fused spur from a ring. Have a look at P505 in your regs book and reference it with chapter 7 OSG for more info

Yes, you're completely right. I was looking at Ref Method A. I think I've been at this too long without a break!

I was running it in 2.5mm T&E as it was going to run through a wall and back entry into an outside socket for the garden.

I can't believe I was looking at page 505 earlier too.

Many thanks for your help. Glad I'm getting this right now at the design stage rather than having to go back and change something!

Where did you find the ref meth B for T&E, can't find it the regs or OSG?

 

[Steve T89]

If you look at page 409 of the BBB then it states '(enclosed in conduit in an insulated wall)' which is Method A for 20A
As @Pretty Mouth has said, should be 23A.

Also if you look at Note 9 of Table 4C1 it states 'If, due to known operating conditions, a cable is expected to carry not more than 30 % of its grouped rating, it may be ignored for the purpose of obtaining the rating factor for the rest of the group.'

Yes I must have seen conduit and jumped to conclusions, I was looking at Ref Meth A.

Well that makes it a lot more achievable! I was even considering thermosetting for a moment.

Great tip about 4C1 too, I'm going to do my cable calculations tomorrow now (got caught up on this!) so I'll prove that too.

I want to do this job as thoroughly as possible as its going to be a back up to get registered.

Thanks for the advice, Spoon (and everyone else!), much appreciated!

 

[littlespark]

Is it only one socket they want? If so, there's no need to fuse down to 13A for the spur.

 

[Steve T89]

Is it only one socket they want? If so, there's no need to fuse down to 13A for the spur.

Yes one double socket, but it's one of those IP66 ones on an outside wall, I thought they needed an FCU?
If not, I'll save me the bother and them the cost!

 

[littlespark]

See what the other guys think, but i normally fit a dp switch inside to completely isolate, but with a SFCU, they have the option to extend from that one socket to another