Diversity..??

[Trazor]

I,m no electrician, but after reading many threads on this forum,I keep reading that diversity has to calculated and allowed for.

Why is it not possible to just uprate every cable, to the point where diversity could be ignored, or am I not understanding things correctly.?Apart from the extra cost.

 

[davy_owen_88]

Why is it not possible to just uprate every cable, to the point where diversity could be ignored, or am I not understanding things correctly.?Apart from the extra cost.

An example: if you didn't allow for diversity on socket circuits you would have to assume every socket outlet is supplying 13A and fit cable according to the number of outlets - 5x single sockets would require 10mm² cable - at about £2.50/metre it would soon get expensive when its common to go through a 100 metre reel quite easily when wiring up a house.

Considering most houses have upwards of 20 double sockets your talking big bucks and even bigger cables. Imagine having 16mm² running between sockets, most single brick walls would be hard done by to accommodate suitable back-boxes and all sockets would need to be re-designed to accommodate 2x 16mm² cables... A single ring circuit per floor would no longer be sufficient, your talking more like 1x ring per room, and 2 or more for a kitchen...

 

[big-all]

I,m no electrician, but after reading many threads on this forum,I keep reading that diversity has to calculated and allowed for.

Why is it not possible to just uprate every cable, to the point where diversity could be ignored, or am I not understanding things correctly.?Apart from the extra cost.

ok look at it this way without diversity you would need 50% bigger cables 50% more sockets
50% more power stations
50 more capacity overall just in case

you would probably rule out most propertys older than 10 years old as they cant cope with 100% load all the time

you uprate the cable in the house you have to uprate the consumer unit the tails the street cable the sub stations the power stations

every thing has to be 50% bigger just in case

 

[Trazor]

Thanks for the quick answers.

As a non electrician, I did not realise just how much of a jump in cable size would be required, and the knock on effect.....

 

[ricicle]

It's the fact that if you added up the ratings of all the fuses/mcb's in your CU then this could be well over 100A.It is highly unlikely that all your circuits will be fully loaded at anyone time, and even if they did peak it could take several minutes for the main fuse to operate.
The cost would be greater if you demanded the local supplier gave you 125A or 150A.And imagine if they had to base their network on every domestic property using over 100A - it would just be totally impracticle.

The factory I work in has dozens and dozens of machines rated at 100A per phase, but not all of them are calling for heat/doing work at the same time so the ammeters in the main switchroom do not register several thousand amps, more like around 1500A

 

[JohnD]

As it happens, I had a new meter fitted in my house yesterday, and asked the fitter to show me the Company Fuse (the fuseholder is marked 100A but often they contain smaller fuses).

I have circuits with the following MCBs:

32A Ring (Ground floor)
32A Ring (1st floor)
32A Ring (2nd floor)
6A (Ground Lights
6a (1st lights)
6a (2nd lights)
16A (Immersion)
32A (Cooker 1)
32A (Cooker 2)
6A (Alarms)
20A (Outdoor sockets and lights)
-----
200A total MCBs

The company fuse is 60Amps and has never blown.

 

[tim west]

Why is it not possible to just uprate every cable, to the point where diversity could be ignored

You are thinking the exact opposite of what diversity is there for, Diversity makes the assumption that not all outlets may be used at the same time or at their full capacity, therefore the calculation allows some free way in deciding the size of conductor used
Because of this people should be aware that they are not trying to overload a circuit that may not be capable of supplying the current needed.
Used to happen years ago with the extensive use of multiway socket outlet adaptors. Still possible with the use of socket outlet strips 4 ways plugged to 4 ways and so on putting strain on the circuit as a whole.

 

[JohnD]

... Still possible with the use of socket outlet strips 4 ways plugged to 4 ways and so on putting strain on the circuit as a whole.

No it isn't, because 4-way strips are protected by a 13A fuse, so no matter how many you plug into each other, that is the limit of total current drawn.

 

[tim west]

... Still possible with the use of socket outlet strips 4 ways plugged to 4 ways and so on putting strain on the circuit as a whole.

No it isn't, because 4-way strips are protected by a 13A fuse, so no matter how many you plug into each other, that is the limit of total current drawn.

Read again, I said on the circuit as a whole you could load each 4 way strip plugged into the ring circuit up to 13A with enough plugged into each outlet then the ring circuit could end up being overloaded. Diversity applies to the fixed installation. Also don't forget that plugtop fuses are he same class as rewireable fuses and can carry as much as 3 times their rating in an overload situation.

 

[JohnD]

In the same way that you could overload the circuit by putting a 3kW fan heater into every socket. The use of 4-way strips does not add to this overload. You would not be putting more than 13A through any of them.

 

[tim west]

In the same way that you could overload the circuit by putting a 3kW fan heater into every socket. The use of 4-way strips does not add to this overload. You would not be putting more than 13A through any of them.

yes but a lot of people use these strips gradually building up an overload situation.

 

[plugwash]

Its really heaters and white goods that you have to watch in domestic installations. I think you are pretty unlikely to cause an overload on a ring with lamps TVs PCs games consoles AV equipment (some amplifiers being the exception ) etc unless the number gets really excessive (running a server farm, growing cannibis etc).

besides its not as though overloading is the end of the world, i remember seeing a table of the effects of different temperatures on cable lifetime and the bottom line is that short duration moderate overloads aren't really a big deal.