25mm vs 16mm vs 10mm SWA Overkill for outbuilding?
Anonymous user 06/10/2025 - 8.38 AM
Hi, I wanted to seek some friendly advice after I received 5 electricians quotes for the same work and got varying responses. Two quotes suggested 10mm SWA, one said 16mm and the other two said 25mm.
So is a 3-core 25mm SWA or 16mm SWA overkill to run 25metres from my house to an office/gym outbuilding that will contain a 10.5kW Aqualisa aquastream electric shower, a Stiebel Eltron DCE-X 10/12 Premium - 238159 (Single Phase 13kW) Instantaneous Water Heater, that will supply the shower, bathroom wash basin, and kitchen sink.
There will be 8 low voltage down lights, and twelve double sockets, to support my exercise bike, rowing machine, small fridge/freezer, a large TV, kettle, microwave and small hi-fi, desktop computer with 3screens, Wi-Fi router, and other occasional use electrical items.
I'm also looking to budget for a 2kW panel heater. Whilst the building will be constructed to the appropriate standards with insulation to meet and exceed current standards, I'll still need something to take away any chill on the coldest of winter days.
Whilst the outbuilding may be constructed to dwelling standards, this is by no means a home. I'm really only going to be using the shower sparingly, and the most I would expect is if we have visitors around for a garden party, and they might use the toilet and wash basin. For Monday to Friday use, it's purpose will be for an office and gym for me.
Are you a tradesperson and able to answer this question?
Hi. 25 mm swa is only way to install , as your load is high enought! Would be proper so just dont worry about the cable that will be safest for the installation:)
You can reach 100 amps with your equipments, and devices, and appliances so that 25mm should be fain the cable designed to that high load!
The only way to determine the correct answer to this would be to do the cable calculations otherwise you may be selecting a cable larger than required and that will add quite a bit to the price.
Like others have said it is likely Volt Drop will be your biggest issue in determining the size of the cable.
I am unsure why you require a 10.5Kw Shower when you will be installing an Instantaneous water heater that can supply a shower function. That would take out 10.5Kw and would likely put you in the realm of a 16mm (if you went with a lower rated shower) depending on other cable factors and the VD being ok.
According to Siebel that heater can supply 1 shower and 1 basin.
A design change could be adding a small instantaneous water heater just for the kitchen sink. That could shave off 5kW if you went with a 5kW Heater (no 10.5kW Shower)
Alternatively do away with the Siebel. Keep the shower but down rate to 8kW and add a small instantaneous water heater for the basin and kitchen sink.
Loads of options but you seem to have gone overkill on the ratings for the water heating and a shower you will only use sparingly.
If you do still need the loadings as stated then I have to question if this has been factored into to your overall Maximum Demand at the property?
Hi
You need a 25mm swa cable to run. As it’s a long run 25Meter there will be voltage drop if you run 16mm will not be enough. And you are running cable once then run it properly as you will be burying the cable.
You are installing 10.5kw shower a Stiebel Eltron DCE-X 10/12 Premium - 238159 (Single Phase 13kW) and 2.5kw electric heater and other appliances which builds up load more than 25kw
So all you need a strong cable to support that load.
And you may need a few more stuff in the future so 25mm cable is good enough.
As stared . You need to do all the appropriate cable current capacity /grouping , installation calculations and consideration as to volt drop for the cable length amd how you will earth the unit as it may require TT installation as not within th e properties equipotential zone of the house . This Is a job for a qualified electrician amd notification to BC under part P too
