Garage Sub Board SWA sizing

[charlie76]

Evening folks. Quick question, I am installing a garage sub board which will have TT earthing. It has a 40A 30mA RCD main switch and I will be putting a B16 and B6 in the board. Would you size the SWA for the breakers i.e. 22A or for the main switch 40A in case someone swaps out an MCB in the future?

 

[mickfred]

Can't tell if your agreeing with me or taking the Mick . Murdoch

 

[charlie76]

Thanks to those that managed a civilised opinion.

@davesparks / @westward10 it was a question of bare minimum or future proofing, no design required or critic on whether or not I am capable of calculating the CCC of a cable or what title I give myself in my forum profile. I have a degree in electrical engineering so I am more than capable of carrying out the the design calcs to run a cable from a house to a garage...

 

[kenny7askew]

im sure you are very talented but you are not helping your argument mate by having a degree in electrical engineering and asking strangers on the internet about the best way to wire a garage

 

[Phil Thompson]

I'm all about the future proofing, i see it all the time, just a light and a socket, next thing there's a 6ft chest freezer and she's got her washing machine and tumble dryer out there too, and a heater in the winter for the dog

 

[davesparks]

'Future proofing' is impossible, the future is unknown by its very nature, you can't proof against an entirely unknown factor.

 

[charlie76]

im sure you are very talented but you are not helping your argument mate by having a degree in electrical engineering and asking strangers on the internet about the best way to wire a garage

I'm not sure I did ask about the best way to wire a garage. It could have been anything being wired it was the principle. You see if you read back over this thread you get those that understand the query and reply with something constructive and those that either don't understand it or just enjoy being cantankerous who reply with utter garbage which does not in any way add to the debate.

I don't have an argument that requires any help either, mate, but thanks for the advice. And as for asking strangers questions on the internet, I was of the understanding that that is what forums are all about...

 

[charlie76]

'Future proofing' is impossible, the future is unknown by its very nature, you can't proof against an entirely unknown factor.

Dave come on. I'm asking would you size the cable for the design current or the rated current of the main switch in the board given that its 25M buried run, its a garage.

 

[DavidSpark]

I'd discuss it with the client and explain the differences and what they want to do. As mentioned by others, the cost per meter is minimal at those sizes and it gives piece of mind.

If you're really worried about it, why not lay a length of ducting for the cable to be pulled through and then in future it's a case of pulling a new cable through should they decide to turn the place into a disco.

 

[Darkwood]

R&m electrical website first one that popped up on the internet Mr Westwood . Sat in Mickey D's carpark with a coffee waiting for the dogging to start . Sorry if I miss quoted can't read my regs book with this gimp mask on

@mickfred

If you have any issue with any members response then please report it to a member of staff using the report option or the message system, responses like this only encourage a war of words and thus the thread gets dragged down and off course.


@charlie76
Your opening post can read a few ways in all honesty hence the responses, you supply circuit design will initially be calculated through design load, the rating of the front end rcd or main switch will only need to be higher or equal to this, I get the impression you are asking whether you should future proof the installation in case of additions or alterations are made in which case I would recommend it unless it is clear this will not be a future issue.
I think bringing the rating of the RCD into the post and asking if you should upsize to match it is why you saw some of the responses, your front end fusing device based on design current will determine the current rating required of the RCD, not the other way around, I think your following posts addressed what you meant, regarding advice, the higher the front end protection and cable size can be will be beneficial for discrimination of devices under fault conditions but given it's domestic it is usually the case that full discrimination is impossible to achieve and only partial at the best, the wrong device tripping first is of little inconvenience as as oppose to commercial or industrial.

@members responses

- the OP addressed what he was actually asking part way through the thread, the following posts should reflect on this and not be still hanging on the initial post, all he wishes to know is whether to future proof the system or just keep to bear minimum for the max load 22 amps. Personally and ccounting for install methods I would be popping a 40amp breaker in anyway and thus designing the cable install to suit.

 

[HandySparks]

The rating of the main switch is pretty irrelevant. If the board you picked happened to have an 80A RCD, you wouldn't necessarily rate the supply to match it.

It's all a matter of judgement. If it's a 4ft x 4ft shed, then it's unlikely to have much heavy machinery installed. If it's nowhere near the road, then car charging isn't a consideration.

I'd go for an installation to meet the customer's requirements plus any future load they feel the need to allow for, after discussion. If the underground part of the distribution cable is ducted, then the risk of cable damage by poor backfill is much reduced and the ease of replacing or adding a cable improved.

 

[Ian1981]

1.5mm 25a ,2.5 mm 33a, 4mm 43a you could use any of these for current carrying cap (not sure about volt drop etc ) and could use 25a 32a or 40a MCB at your main board but your right for the cost difference of about 20quid why not put a 4mm in

Looks like your figures are ccc of swa running at 90 degrees rather than the recommended 70 degrees max operating temperature to match the accessories.
Perhaps your Gimp mask needs bigger eye holes mate

 

[mickfred]

Sorry darkwood think your getting me mixed up as am not the OP. And Ian and westward are probably right I have quoted from the wrong table . Night vision goggles and me regs book next time . And for the record the OP questions was not that bad

 

[Octopus]

Can't tell if your agreeing with me or taking the Mick . Murdoch

Agreeing

If it was me I would be stating the maximum design current on the estimate AND invoice...

 

[Darkwood]

Sorry darkwood think your getting me mixed up as am not the OP. And Ian and westward are probably right I have quoted from the wrong table . Night vision goggles and me regs book next time . And for the record the OP questions was not that bad

My apologies .. it was late and had been a long day , I have sub divided said post but the quoted post and response still stands, can we refrain from such responses please .

 

[Lou]

I am closing this thread now.