UK calculating power for a shed

[Mfctitch]

Morning Again
In my last post I was asking whether to TT or not to TT and another question was posse by Strima saying that my allowance of 38 amps was possibly to much.
I'd based this on the fact that I only had a slight idea of what the customer was going to do in the shed/summer house. so allowed for a garage unit with a 32 and 6 amp MCB.

yes this meant that over the 44 metre distance I was going to have to use 10mm SWA.
So I'm asking if you was presented with the same situation how would you come up with an answer.
Thanks for any replies

 

[ChrisElectrical88]

I think finding out what the summer house would be used for is a start. Without knowing the use of the building it’s hard to design the electrical installation. Is it likely to be expanded in future needing more load?
It might just simply need a twin socket, indoor light and outdoor light installed, in which case a single 20A circuit would likely suffice?
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The likely hood of a summer house needing its own independent 6A lighting circuit and then using all 6A is zero, unless there planning on having a cannabis farm growing in there

 

[buzzlightyear]

Put your calculations on here so we can see what you have done.

 

[pc1966]

The lights should be separately protected from my point of view:

1. The accessories and cables are probably only good for 10A or so (though a FCU could provide this).
2. If the ring should trip you don't really want darkness (often a 13A fuse blowing can trigger the magnetic trip).
3. If used as a workshop I would also discuss the option for emergency lighting (to reduce the risk of darkness if RCD trips for all, or supply fails, etc).

Since a typical garage CU comes with a 6A MCB for lights, just use it!

As for the to total consumption that is your "How long is a bit of string?" sort of question and really needs an idea of what it will be used for and what plans they may have for the future. Yes, SWA cable is not a trivial cost but that is also a lot less than the cost of replacing it in the future should the demand increase.

I suspect that more than 20A is unlikely, even for a workshop or summerhouse with electric heater & kettle, but if they decide to add A/C or run a hot tub supply from it then you may well be pushing 30-40A.

 

[davesparks]

Unless they particularly need a lot of power then a 32A supply is usually more than enough for an outbuilding.

32A is usually plenty for a whole house for most of the time so it's very unlikely a shed is going to need more.

 

[Mfctitch]

The lights should be separately protected from my point of view:

1. The accessories and cables are probably only good for 10A or so (though a FCU could provide this).
2. If the ring should trip you don't really want darkness (often a 13A fuse blowing can trigger the magnetic trip).
3. If used as a workshop I would also discuss the option for emergency lighting (to reduce the risk of darkness if RCD trips for all, or supply fails, etc).

Since a typical garage CU comes with a 6A MCB for lights, just use it!

As for the to total consumption that is your "How long is a bit of string?" sort of question and really needs an idea of what it will be used for and what plans they may have for the future. Yes, SWA cable is not a trivial cost but that is also a lot less than the cost of replacing it in the future should the demand increase.

I suspect that more than 20A is unlikely, even for a workshop or summerhouse with electric heater & kettle, but if they decide to add A/C or run a hot tub supply from it then you may well be pushing 30-40A.

How long is a bit of string was my thinking but I've been known to be wrong.
so I went for the 32 and 6 amp option as the only info i had was

They will want to run the small pool pump with is on a 13amp plus they have mentioned heating so that a 3Kw panel heater so I'm assuming a computer and the bits that go with it, maybe a small fridge and a kettle and some lights.

So thought be to go larger that way it gives them scope to expand.
But my thoughts were was I going over the top?
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Unless they particularly need a lot of power then a 32A supply is usually more than enough for an outbuilding.

32A is usually plenty for a whole house for most of the time so it's very unlikely a shed is going to need more.

so 32 amp and 6 amp for lighting is not over the top?

 

[pc1966]

so 32 amp and 6 amp for lighting is not over the top?

With LED lights they are unlikely to go over 1A load, so of the order of 20-30A total I would say.

But when sizing the cables you need to make sure it won't overheat as well as the feed OCPD protecting it from shorts, and for good selectivity you may well have the upstream OCPD above the cable's thermal CCC limit, so then your downstream MCB combinations should be designed to prevent overload.

Lights are unlikely to go above what was installed (so probably 0.5-1A total) but what folk plug in to sockets is beyond your control so the local MCB would need to provide the overload protection for the cable. If left at 32A then you ought to size the cable's thermal limit for ~33A or plan on reducing the MCB to 20A or whatever is though to match the agreed/specified/expected use and feed cable choice.
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But as already mentioned, if they have ideas for a hot tub or similar then you should discuss the costing for the cable to do that future load and let them decide if they want to make it cheapest overall by selecting the right cable once, or pay more if/when they ever do it and need another size of cable added/replaced.