UK Level 3. Cable sizing.

[rich66]

Just started Level 3 C&G and we're doing cable sizing calcs. Using a previous years exam question.

We ran through a few examples of radial circuits, all well and good. Following the following proceedure:

Work out total load on circuit from plan.
Work out design current. Ib.
Ib<In<It
Find suitable breaker that's bigger than the Ib.
Now find It, tabluated current, by using cable factors Ca, Ci, Cf and Cg.
Once we have It, look up the size of cable for the given reference method of installation, in this case, Single Phase multicore, Method B. using table F5i in the On Site Guide.
Finally, look up volts drop for the selected cable CSA and make sure its less that the allowable limits.

All well and good with the radial examples.

Then we come to a ring final power circuit. We're told that the design current is 23A and so a 32A breaker is to be used.
Applying cable factors totalling 0.8 we get an It of 40A.

If we look up 40A in table F5i in the OSG, we find that we need 10mm2 cable! How can this be right? Everyone uses 2.5mm2 and I would doubt that you could fit two10mm2 tails into a 13A socket terminal!

Even the lecturer was confused by this.

I assume that as the circuit is a ring, the current is halved somewhere in the calcs or is a ring circuit just an oddball requiring a different calc?

Any pointers most welcome! Cheers!

 

[westward10]

For a 32A ring final circuit you don't halve it, Regulations state cables must be rated at 20A.
There was a time when we used 0.67 × 32 which is where we get the rounded down 20A now. 20A is around 0.63 of 32A so applying this to 40A you get 25.5. Not sure if this would be correct but it was applied at one time.

 

[rich66]

Well another week passed and I still don't have a proper answer to where the process is wrong. To reiterate, this is not a question for a real life situation, its a C&G test question.

I fully understand that 2.5mm2 is the norm for Ring Final and that the regs book says that as long as there a 32A breaker, you need a 20A capacity cable, 2.5mm2.

The issue is that we're going through the design grids for lighting and radial circuits, following the Id<In<It rule and taking into account the derating factors. This all works great for radials and lights but makes no sense for Ring Final.

The lecturer can't figure it out either.

Oddly the 'correct' C&G answer to the question (what size cable for a ring final with a 32A breaker) is 4mm2 but I have no idea why.