With reference to 433.1.204 and cable as installed has min capacity of 20A if protected by a 30A or 32A. If the protective device is reduced to 20A how is the new minimum as installed capacity calculated or arrived at ? I've been looking in the Electrical Installation Design Guide, but the answer is avoiding my eyes.
I chose the 1m example as one that almost never occurs, unless there is a socket in the cupboard with the consumer unit.
I agree, I can't imagine that many sparks in the heart of battle (or any other southern town for that matter ) bother to think about spreading the sockets around the ring, although there are rules of thumb, like twisting the ring when you have a situation like a long corridor so that sockets 1,3,5 are one one leg of the ring, and sockets 2,5,6 are on the other.
Even if you give it no thought at all, the risk of an overloaded ring in a domestic setting is very small - partly as to have 30 A of load plugged in at once is very rare, and to have it all sweating away within a very small distance of each other is rare still - unless the building has air conditioning it will become insufferably hot pretty quickly.
Also the cable is not at risk as much as it may appear -
1) most houses are not a 30degree ambient - the loft in summer maybe, but then that is not when you start lugging electric fires about. A 20degree ambaint is more likely.
2) a 70C rated cable does not rise up and seize you round the neck and throttle you at the instant it reaches 71C. Indeed pour bioling water over a cable offcut, or if on site drop it into you mate's coffee, and it does not melt the insulation like butter. Yes it gets a bit more flexible, but in the wall of a house it is not being flexed. You need to be well over 100C, more like 120-130 C before the plastic actually starts to move of it's own accord. the current to reach that sort of temp relates to perhaps twice the intended heating per unit length, and so about 150% of the supposed max current.
There will be a small temperature effect that raises the resistance of the most overloaded section, but it is small effect compared to the variations in cable length and so on..
Repeated cycling of hot and cold to well above 70C may cause the plastic to age prematurely, but only in the sense of a 20 year life instead of a 50 year one, not instant failure.
This is how we find showers wired in 2.5mm that have been there for years that look perfectly fine.
Mike
PS I am pro ring by the way - a perfectly sensible solution to getting power over a large area with low voltage drop and more flexible cables than the 4mm2 radial, and also allows a self test that earthing goes all the way to each point, even if you are not sure where the far point is. (bit like the foundation electrode having 2 terminals - you can verify the continuity, and you know both are connected to some metal)
It needs understanding, but if you can't grasp ohms law and cable sizes, you really should not be wiring things up except to a preset recipe anyway. Which is fine for the bulk of situations.
I chose the 1m example as one that almost never occurs, unless there is a socket in the cupboard with the consumer unit.
I agree, I can't imagine that many sparks in the heart of battle (or any other southern town for that matter ) bother to think about spreading the sockets around the ring, although there are rules of thumb, like twisting the ring when you have a situation like a long corridor so that sockets 1,3,5 are one one leg of the ring, and sockets 2,5,6 are on the other.
Even if you give it no thought at all, the risk of an overloaded ring in a domestic setting is very small - partly as to have 30 A of load plugged in at once is very rare, and to have it all sweating away within a very small distance of each other is rare still - unless the building has air conditioning it will become insufferably hot pretty quickly.
Also the cable is not at risk as much as it may appear -
1) most houses are not a 30degree ambient - the loft in summer maybe, but then that is not when you start lugging electric fires about. A 20degree ambaint is more likely.
2) a 70C rated cable does not rise up and seize you round the neck and throttle you at the instant it reaches 71C. Indeed pour bioling water over a cable offcut, or if on site drop it into you mate's coffee, and it does not melt the insulation like butter. Yes it gets a bit more flexible, but in the wall of a house it is not being flexed. You need to be well over 100C, more like 120-130 C before the plastic actually starts to move of it's own accord. the current to reach that sort of temp relates to perhaps twice the intended heating per unit length, and so about 150% of the supposed max current.
There will be a small temperature effect that raises the resistance of the most overloaded section, but it is small effect compared to the variations in cable length and so on..
Repeated cycling of hot and cold to well above 70C may cause the plastic to age prematurely, but only in the sense of a 20 year life instead of a 50 year one, not instant failure.
This is how we find showers wired in 2.5mm that have been there for years that look perfectly fine.
Mike
PS I am pro ring by the way - a perfectly sensible solution to getting power over a large area with low voltage drop and more flexible cables than the 4mm2 radial, and also allows a self test that earthing goes all the way to each point, even if you are not sure where the far point is. (bit like the foundation electrode having 2 terminals - you can verify the continuity, and you know both are connected to some metal)
It needs understanding, but if you can't grasp ohms law and cable sizes, you really should not be wiring things up except to a preset recipe anyway. Which is fine for the bulk of situations.
