Incomers and service fuses

[ban-all-sheds]

Recent years have seen a proliferation of the numbers of circuits in people's homes, what with conservatories, outdoor lighting, garden pond/water feature pumps, powered outbuildings, shower circuits, separate kitchen/utility room circuits (in addition to the cooker circuit).

When you add them all up, even with diversity you're going to come to more than 100A. Presumably this means that the main switch on the CU should be rated at more than 100A? They seem few and far between - even 16-way CUs still come with 100A switches.

In theory should you reduce the capacity of breakers (e.g. drop rings to 20A) to keep below the 100?

If you run with a bigger main switch, or multiple CUs, what's the largest service fuse that (typically) a supplier will provide for a 230V single-phase supply?

 

[sterose]

...When you add them all up, even with diversity you're going to come to more than 100A. Presumably this means that the main switch on the CU should be rated at more than 100A?

Obviously diversity is not being applied properly, because it if drew over 100amps the fuse would simply trip. I find it risible that any house could draw over 100amps at once - even with every single appliance switched on.

 

[Big_Spark]

Ban, that very issue is actually under discussion by a panel in the IEE with reps on from the Industry, NICEIC, Government and the supply companies. Many new installs in the London region are now 125A as a result of this problem.

According to the IEE they hope to have new guidlines published within 6 months so that all new installs, nationally, will be 125A, and that when meters are changed in premises, where practical to do so, supplies will be upgraded accordingly..although suppliers are apparently upset by this proposal due to cost.

 

[Big_Spark]

Obviously diversity is not being applied properly, because it if drew over 100amps the fuse would simply trip. I find it risible that any house could draw over 100amps at once - even with every single appliance switched on.

It is very easy for this to occur, and it does happen. It is also a bit pressumptious to assume that diversity is not being applied properly.

 

[AdamW]

suppliers are apparently upset by this proposal due to cost.

Dunno why, means they get to sell more of the sparky stuff to us for powering our 24kW heated ponds

 

[ban-all-sheds]

Obviously diversity is not being applied properly, because it if drew over 100amps the fuse would simply trip. I find it risible that any house could draw over 100amps at once - even with every single appliance switched on.

Do you actually know what diversity means?

 

[ban-all-sheds]

..According to the IEE they hope to have new guidlines published within 6 months so that all new installs, nationally, will be 125A, and that when meters are changed in premises, where practical to do so, supplies will be upgraded accordingly..although suppliers are apparently upset by this proposal due to cost.

That I can understand, as people will rarely actually draw 125A, so they won't get much more money in.

And is the expectation that we will all fit multiple CUs, or are the manufacturers ready to unleash products with 125A incomers?

 

[andemz]

Obviously diversity is not being applied properly, because it if drew over 100amps the fuse would simply trip. I find it risible that any house could draw over 100amps at once - even with every single appliance switched on.

Do you actually know what diversity means?

Can you explain to me then what diversity means, i am only aware of this in a social research capacity

 

[Big_Spark]

Ban, most modern consumer units can handle 125A so long as the Main Switch is uprated, and as many are simnply DIN rail mounted in the panel this should be an easy task.

The problem, of course, will be for those with cheaper brand consumer units as the busbars are likely only rated at 100 A, or that of the Isolator, in which case a board change would be required if you needed to uprate your board.

However, just because you have a shiny new 125A head, does not mean you will need to uprate your Main Isolator, so long as the fuse in your head is only rated at 63A, 80A or 100A depending on your isolator.

 

[ban-all-sheds]

Can you explain to me then what diversity means, i am only aware of this in a social research capacity

Diversity is the concept that not all circuits will be loaded to their maximum simultaneously, and that you therefore make certain assumptions about how much load to allow for each circuit when assessing the total requirement. These "assumptions" are not a substitute for proper design, so if you were doing something out of the ordinary - e.g. you take in washing so in the utility room there are 2 washing machines, a tumble drier and an iron on continually all day, then the assumptions would not be valid.

It's also important to note that they should not be applied to individual circuits, so although the "rule" for a cooking appliance is 10A plus 30% of the remainder plus 5A if the control unit has a socket, that does not mean that a 10kW cooker should be treated as if it was a 20A device, only that when estimating the total requirement of your house you assign a consumption of 20A to the cooker circuit, or 25A if there's also a socket. The circuit should still be 6/10mm cable on a 45A breaker.

You can read more about it here .

 

[andemz]

another clear answer thanks

 

[securespark]

fwl

125A incomers means 35mm2 tails.....

 

[mildmanneredjanitor]

35mm2!

Just out of interest.

What are most domestic supply cables rated at then?

 

[Millennium_Boy]

Modern houses have 25mm meter tails with a 100A main fuse.
Older houses had a 16mm meter tail with a 60a fuse.
Very Old houses had 19/.044 meter tails with a 60a fuse in BOTH the Live AND Neutral of the main supply intake.

 

[ban-all-sheds]

35mm2!

That's only 1mm larger in diameter than 25mm - no big deal.