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?

 

[davesparks]

I would recommend getting an electrician to do the job as you clearly don't know the very basics of designing a circuit.

 

[Ian1981]

Size it for your intended final load taking into account any correction factors cc,cs,ca,cd etc.
Or if you want to future proof it then stick an unnecessary large sized cable in at a unnecessary cost of course.

 

[charlie76]

Cheers @davesparks that was most unhelpful. @Ian1981 I'm not sure it is unnecessary cost, bear in mind there is a 25M trench dig for the cable run. I'll give the client the option. I'm sure they'd rather pay a bit more now than 2 years time decide they want another circuit, EV charge point, 16A outlet for welding or the like and need the garden dug up again. We are not talking huge figures here lets put it into perspective.

 

[alamski]

Since you mentioned car charging port, that is something you definitely want to think about now rather than later as that will have a drastic difference on the swa cable size.

 

[mickfred]

If it was me I would look to go slightly bigger as it gives the client a bit of wiggle room in the future .Sod law if the bigger cable is installed they will never use it

 

[westward10]

I am somewhat perplexed that someone who has a profile which indicates to being an electrician has posed such a rudimentary question, hence DSs response and the possible lack of others.

 

[charlie76]

Gents, with respect I think you are missing the point of my question. I am not asking for help calculating the cable size etc etc. Forget any of the figures I have written in my question they are irrelevant.

Lets re-word:

I have been asked to install a sub-board which has a capacity of 40A but the design current is 22A. The feed to the board is trench dug so any future upgrade would be expensive. Would you suggest sizing the cable now to the capacity of the board in case of future upgrade or size it for the design current?

 

[Bradley6969]

Gents, with respect I think you are missing the point of my question. I am not asking for help calculating the cable size etc etc. Forget any of the figures I have written in my question they are irrelevant.

Lets re-word:

I have been asked to install a sub-board which has a capacity of 40A but the design current is 22A. The feed to the board is trench dug so any future upgrade would be expensive. Would you suggest sizing the cable now to the capacity of the board in case of future upgrade or size it for the design current?

Gents, with respect I think you are missing the point of my question. I am not asking for help calculating the cable size etc etc. Forget any of the figures I have written in my question they are irrelevant.

Lets re-word:

I have been asked to install a sub-board which has a capacity of 40A but the design current is 22A. The feed to the board is trench dug so any future upgrade would be expensive. Would you suggest sizing the cable now to the capacity of the board in case of future upgrade or size it for the design current?

Give him the option? Can't go wrong

 

[mickfred]

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

 

[davesparks]

Gents, with respect I think you are missing the point of my question. I am not asking for help calculating the cable size etc etc. Forget any of the figures I have written in my question they are irrelevant.

Lets re-word:

I have been asked to install a sub-board which has a capacity of 40A but the design current is 22A. The feed to the board is trench dug so any future upgrade would be expensive. Would you suggest sizing the cable now to the capacity of the board in case of future upgrade or size it for the design current?

Ask the person who specified the 40A board?

I'm still at a bit of a loss as to the point of this thread, you're not making much sense from an electrical design point of view.

Who specified the 40A capacity and why? How did you establish the design current?

 

[westward10]

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

Where did you get those ratings from.

 

[westward10]

The main switch is 40A, design it to that.

 

[Octopus]

Given the cost difference between 2c. 4mm and 2c 6mm is well under £1 per meter I would include 6mm in the pricing

 

[mickfred]

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]

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.