one for the pro's to debate...
- Thread starter ColJack
- Start date
Sponsored Links
433.1.3 is a qualifier to the basic 433.1.1, which states the overall requirements for how to ensure the correct co-ordination between conductor overload and protective device.
i.e. you start with 433.1.1 defining the Ib <= In <= Iz and I2 <= 1.45Iz relationships.
433.1.2 says that where the protective device is a <list of types> if you do Ib <= In <= Iz you'll automatically have done I2 <= 1.45Iz.
433.1.3 says that if you have a BS3036 you need to go for In <= 0.725Iz in order to achieve I2 <= 1.45Iz, i.e. it's the special consideration for that design which you need to make to ensure compliance with 433.1.1.
433.1.4 has the special consideration you need to make for 20° buried cables to ensure compliance with 433.1.1.
And our old chum 433.1.5 has the special considerations for ring finals, including those using 30A BS3036s, which you need to make to ensure compliance with 433.1.1.
Like I've always said, it's a kludge designed to regularise an established practice, which was brought into being for reasons of expediency, and would not otherwise comply. If radials were all we had ever had, and 433.1.1 - 433.1.4 were the only regs we had ever had which prescribed the correct co-ordination between conductor overload and protective device, do we really think that a wiring committee would say "tell you what, let's allow undersized ring finals and write a special regulation which basically says ignore the previous 4 regulations and do this because it will be OK"?
They should be consigned to history. Simply remove 433.1.5, then as circuits get modified the protective devices have to be replaced with 20 or 25A ones. Would make the whole issue of spurs much easier too.
And there would always be the option to find a logical place to split the ring and turn it into two radials. (You could even leave the cpc as a ring, as per 543.7.2.1(ii)(c), if you want to retain the resilience it offers.)
You beat me to it 
Sponsored Links
- Joined
- 11 Jan 2004
- Messages
- 43,883
- Reaction score
- 2,872
- Country
But what if you then put that cpc on a plane on a conveyor: would it take off?
Only if as it got thinner the voltage decreased and the current increased to compensate, and if you touched your titanium bodied camera to the neutral.
A great mass debate!
Surely the point is that regs require all circuits to be 'designed' but in order to save us all reinventing the wheel each time the IET have set out 3 standard circuits for us to reuse.
Their statement is if you stick to the requirements of these designs no futher calculations are required. so taking a spur from ring at source is ok
Type A1.
However for a radial type A2 on 2.5 it must be fused at 20A or less.
4 pages of discussion over what is laid down in simple terms with drawings in appendix 15......come on fellas!
Surely the point is that regs require all circuits to be 'designed' but in order to save us all reinventing the wheel each time the IET have set out 3 standard circuits for us to reuse.
Their statement is if you stick to the requirements of these designs no futher calculations are required. so taking a spur from ring at source is ok
Type A1.
However for a radial type A2 on 2.5 it must be fused at 20A or less.
4 pages of discussion over what is laid down in simple terms with drawings in appendix 15......come on fellas!
So whilst carrying out a PIR you come across an installation where a 30/32A OPD supplies a socket or FCU, do you:
a) Say that can't be - that circuit arrangement isn't listed in apprendix 15 (informative)
b) Look further into BS7671:2008 (normative) to see if it's a valid circuit arrangement
Depending on you answer, would you note the arrangement as an observation? What code and recommendation would you give?
Anyway, it's discussions like this that make this forum interesting.
- Joined
- 27 Jan 2008
- Messages
- 24,923
- Reaction score
- 2,882
- Location
- Llanfair Caereinion, Nr Welshpool
- Country
I would read the regulations as
to mean as a branch not at end of line.
However I have quoted what you have written because of the 3 meters rule. Very basic I know but spurs have a maximum of 3 meters so to original question.
Yes but maximum of 3 meters in length.
The cable type and routing is of course important and with Ali-tube cable being designed for partition walls even if not as yet widely used one must consider the use of cable rated to 90 degrees even if you don't want to include mineral insulated cable.
Since 2.5mm cable (90 deg) is rated at 33A clipped direct the fact that the regulations say we can use 2.5mm on a spur does not mean it says anywhere we can over load cable as we could be using Ali-tube clipped direct and we as the electrician must therefore decide when we fit cable below the rating for the automatic disconnection device if we are doing something dangerous.
Back to
and I am afraid many electricians don't consider that they just do it that way because they have always done it that way.
The BS7671:2008 was not written by one man. And I am sure we all agree it still has errors (RCD on SELV for example) and as a result I can't understand why people have such faith in inspectors from their club. They can miss read a regulation just the same as us.
134.1.1 says we should follow manufactures recommendations who have produced a cable specially for our new walls so if we don't use Ali-tube are we in breach of regulations? Especially now "Rule of thumb" has become history! No I am sure we can continue to use twin and earth but I do think some thought should be given to the run and it should no longer be. I've always done it that way so why change now!
Eric, you're missing the last part of 433.2.2 - at least ONE of the following conditions, (i) OR/AND (ii).
If compliance is by (i) then you don't necessarily have to comply with (ii) which mentions 3M.
- Joined
- 27 Jan 2008
- Messages
- 24,923
- Reaction score
- 2,882
- Location
- Llanfair Caereinion, Nr Welshpool
- Country
OK you can comply with 434 instead but that says:-
Oh dear back to 3 meter rule I see!!!!!
Yes there are some special places where it is allowed to use no protection but this hardly applies to a house.
Oh dear back to 3 meter rule I see!!!!!
Yes there are some special places where it is allowed to use no protection but this hardly applies to a house.
that's only ONE section of 434 and the one that deals with the placement for the device for fault protection at that....
and your reading of the other reg is also flawed..
the important word there is NOR...
this means that the outlets don't HAVE to be on the branches....
and your reading of the other reg is also flawed..
the important word there is NOR...
this means that the outlets don't HAVE to be on the branches....
Eric, are you saying that a spur from a ring final including when originating from the consumer unit should be less than 3 meters?
I'm not sure I agree with you.
But not at all for double sockets, as there is a branch before the protective device (fuse plug).
I was thinking it would all be covered by 433.3.1 (ii) but having read it again I think it could not apply to socket outlets as overload current is still possible.
So to answer the original question
... I would like to change my answer from a blanket yes to yes up to 3m (single), and no (double).ColJack said:
mfarrow, are you saying that a spur from a ring final including when originating from the consumer unit should be less than 3 meters?
If not then what's the difference?
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
