Interesting Conundrum 1.5 power

[UNG]

I think VD is, as you mentioned earlier, where it may fail. We're probably starting at 240V rather than 230, so perhaps not a big deal real world, but still.

What does it matter whether you start at 240v or 230v, ok at 240v you have an extra 0.5v before you hit maximum allowed volt drop within the installation but if you are on the limit then potentially you are running equipment possibly below it's optimum voltage

 

[swaRRR]

The only way 1.5mm c.c.c is above 16A is if its clipped direct in open air. Bury it in a wall, or run it through insulation and youre down to 14 approx.

2.5mm ran in a wall is only rated at ~18.5a yet everyone puts them on 20a breakers when doing a radial. Shouldn't your advice to the homeowner then apply to basically every building in Britain with a radial in it for sockets?

In fact even ring final's buried in walls are rated at less than 32a too. Why does nobody contest this? I'm genuinely curious. Technically if you're going to bury cable in walls then radials should be done in 6mm shouldn't they? Or am i missing something?

 

[littlespark]

I generally put 2.5mm radials on a 16A UNLESS the install method allows a 20A

I really should have the BBB at hand to check my CCC but 2.5 on an rfc has a combined ccc of 52A. 26A on both legs. Much more than the 32A ocpd.

32A will just cover a 4mm radial… depending on method… but 6mm would be fine on 32A.

 

[Pretty Mouth]

What does it matter whether you start at 240v or 230v, ok at 240v you have an extra 0.5v before you hit maximum allowed volt drop within the installation but if you are on the limit then potentially you are running equipment possibly below it's optimum voltage

I did say real world rather than regulations world.

If you calculate 5% of nominal 230V, you have a voltage drop limit of 11.5V. This will give you 218.5V at the furthest point of use, and is deemed to be the lower limit of acceptable.

However, typically the supply voltage is actually 240V, and often higher still. At 240V, you have to drop 21.5V to reach the same acceptable limit. That's an extra 10 volts to drop, not 0.5. A very significant buffer, no?

For that reason, I find it difficult to take voltage drop in domestic installations tremendously seriously for the most part.

 

[Pretty Mouth]

2.5mm ran in a wall is only rated at ~18.5a

Which table/ref method are you taking this from?

 

[swaRRR]

Which table/ref method are you taking this from?

Cable Ratings dot co dot uk i think

Some NIC site i've just been looking at states 'For a single cable likely to be totally surrounded by thermally insulating material for a length of 0.5 m or more, the current-carrying capacity must be taken as 0.5 times the current-carrying capacity for that cable clipped direct to a surface (Reference Method C).'

So ring finals should be only able to carry the 'normal' clipped current of a single 2.5mm cable at ~27a.

I just want to know why 16a + 1.5mm is apparently borderline dangerous but 32a + 2.5mm isn't? Eager to learn.

 

[Pretty Mouth]

Cable Ratings dot co dot uk i think

Some NIC site i've just been looking at states 'For a single cable likely to be totally surrounded by thermally insulating material for a length of 0.5 m or more, the current-carrying capacity must be taken as 0.5 times the current-carrying capacity for that cable clipped direct to a surface (Reference Method C).'

So ring finals should be only able to carry the 'normal' clipped current of a single 2.5mm cable at ~27a.

I just want to know why 16a + 1.5mm is apparently borderline dangerous but 32a + 2.5mm isn't? Eager to learn.

It's better you take this info from the tables in the regs. The statement from NIC is true, but it's only part of the picture, there's a lot in between.

1.5mm T+E has a rating of 16A when installed in a stud wall with insulation, so long as it not completely surrounded by the insulation, and is touching the plasterboard. Installed this way, it would be adequately protected against overload by a 16A breaker, so not dangerous at all.

Ring finals are an unusual circuit. Invented after WW2, they were a pragmatic solution to reduce the amount of copper needed to wire a house.

Each point on the ring has 2 paths back to the fusebox. The current is shared proportionally between the paths, more of it taking the shorter route. Fully loaded at 32A, it's unlikely that either leg will carry more than 20A, and if overloaded, the cable should be protected by the 32A breaker, even though the cable rating as installed may be only 20A.

In theory, you can overload one leg of a ring by putting lots of heavy loads a short distance along one leg from the fusebox, but I've yet to see this happen in the real world. After 80 years of them being in use, if this was really a problem we would know about it by now.

 

[timhoward]

I just want to know why 16a + 1.5mm is apparently borderline dangerous

I don’t think anyone said exactly that. The speculation is about whether it passes through insulation.
If the 1.5mm is in insulation in a wall the table in the regs (table 4D5) says it’s current carrying capacity is only 10 amps.
That would rate it at less than it’s over protective device which contravenes the regs.

If on the other hand it’s all touching the surface it’s rated at 16A as @Pretty Mouth explains above.

It feels like a poor design to plan from the outset for circuits to be this heavily loaded, especially in the kitchen / utility room.

 

[Dustydazzler]

Do other countries have similar de-rating factors for cables touching insulation ?
1.5 clipped is 19 amps but our regs then say if it passes through insulation you effectively have to reduce its ccc by half , which imo does seem a bit OTT

 

[pc1966]

For that reason, I find it difficult to take voltage drop in domestic installations tremendously seriously for the most part.

Like many rules and regulations, there has to be a defined point for acceptable or not. Our regs happen to say 5% drop (3% lights but that is another debate...) from the nominal 230V for calculation and if it less then OK, if not then you need to alter the design (usually thicker cable, as typically load is fixed and route has no shorter paths).

What you can get away with in practice is a lot more, but that is not really the point of compliant design!