Boxing in of cables

[JohnW2]

In real-world terms, I wonder how often any of this really gets taken into account in domestic premises, particularly given that boxing is very often 'retrofitted' after the electrical installation has been designed and installed

Simple answer - never.

Quite, that's what I assumed, and I agree totally that it is the common sense approach.

This isn't usually a problem, since most circuits in a house will never be used at anything like their maximum capacity, and for many circuits the cables are substantially oversized.

As I said, I personally agree that this is the common sense approach. However, in many people's eyes the name of the game is unfortunately not 'common sense' but, rather, 'compliance with the regulations'.

In terms of the regs, the problem with the common sense approach is obviously that it is taking account of how much load the circuits/cables really are likely to bear in practice, whereas 5.1.1 & 5.1.2 of Appendix 4 of the regs seem to make it pretty clear that they are expecting the required cable size to be determined on the basis of the rating of the overload protection device protecting the cable concerned (or of each and all the cables concerned, when grouped), which will obvioulsy often be a lot more than the common sense estimate of load.

It therefore strikes me that most people are being sensible, but ignoring the word of the regs - which is a slightly odd situation!

Kind Regards, John.

 

[JohnW2]

That's pretty tight 'grouping'. may I ask if this had any impact on cable sizing?

No - there's actually quite a lot of room for cables in 5000mm²....

Oh, are you talking about 50mm x 100mm trunking? I've never used galvanised trunking, and for some reason conjoured up a mental image of something much smaller than that. If it's really that big, I withdraw my comment and add an apology for the confusion - as you say, that's not very tight grouping at all.

The only one which carries significant current is the shower cable, and that of course is of short duration - 98% of the time it's carrying no current.

Fair enough. However, as I just wrote to falameport, if one wants to be compliant with 5.1.1 /5.1.2 of Appendix 4, it seems that one has to design for the MCB rating, even if current is carried for only 2% of the time - or am I missing some 'let out' in the regs?

The rest are lighting and non-kitchen sockets. We have no immersion heater, and the only electric heating is about 1kW worth of thermostatically controlled towel rail & panel heaters in bathroom/WC.
Cooker and kitchen circuits don't run through them.

A very modest installation. Your electricity bills would undoubtedly put mine to shame

Kind Regards, John.

 

[ban-all-sheds]

Oh, are you talking about 50mm x 100mm trunking?

Mine are in 2 runs of 50mm galvanised trunking

Sorry - I meant 2 parallel runs.

I've never used galvanised trunking, and for some reason conjoured up a mental image of something much smaller than that.

I'm using it because hand on heart I couldn't say that when boxed in the route would be a "safe" zone.

However, as I just wrote to falameport, if one wants to be compliant with 5.1.1 /5.1.2 of Appendix 4, it seems that one has to design for the MCB rating, even if current is carried for only 2% of the time

You should, and I have.

- or am I missing some 'let out' in the regs?

You're missing 533.2.1, but that's really a get-out-of-jail free card for loads with a known cyclic nature that can't otherwise be accommodated.

The day that a teenager spends an hour in the shower, or someone turns it on, goes to answer the phone, and gets hopelessly sidetracked you'll be glad you didn't put your 10kW shower on 1.5mm² because you thought it would only be on for 10 minutes at a time.

A very modest installation. Your electricity bills would undoubtedly put mine to shame

Well we do have kitchen circuits, and Mrs Sheds apparently believes that washing lines are the work of the Devil, salvation only possible by using the tumble drier, so it's not all sweetness and non-recessed-spot light...

 

[JohnW2]

- or am I missing some 'let out' in the regs?

You're missing 533.2.1, but that's really a get-out-of-jail free card for loads with a known cyclic nature that can't otherwise be accommodated.
The day that a teenager spends an hour in the shower, or someone turns it on, goes to answer the phone, and gets hopelessly sidetracked you'll be glad you didn't put your 10kW shower on 1.5mm² because you thought it would only be on for 10 minutes at a time.

I've often looked at 533.2.1 and wondered, but have always decided that nothing domestic (certainly nothing I have) is sufficiently regularly/predictably cyclic to come into this category, so haven't actually spent too much time thinking about it.

However, now that I have been prompted to think about it, I'm really not totally sure what it is saying - but, in any event, it seems to be primary about the rating of the overload protection device, not directly about Iz.

Well we do have kitchen circuits, and Mrs Sheds apparently believes that washing lines are the work of the Devil, salvation only possible by using the tumble drier, so it's not all sweetness and non-recessed-spot light...

Ah, I live with an interesting variant of that - everything, indeed, ahs to be dried in the tumble dryer, but then has to go on the washing line for a day or two 'to be aired'

Kind Regards, John.