Cable size/power needed for AC unit?
Cable size/power needed for AC unit?
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Discussion

whatxd

Original Poster:

475 posts

122 months

Sunday 17th August 2025
quotequote all
Another AC thread I know, but I'm currently doing a self build and am at the first fix stage. There's no budget for it at the moment, and it's not part of the SAP/EPC, but when it's all signed off in 6 month's time and VAT reclaimed, I may indulge in an external AC unit. It would just be a retro fit, there's no ducting or anything going in.

Question is, what kind of cable would I need to run now to where the proposed unit would be going? This is by no means a guarantee to happen, just something I'd like to leave the door open for in the future.

b0rk

2,407 posts

167 months

Sunday 17th August 2025
quotequote all
Single phase or three phase?

Assuming single phase the AC outdoor unit is likely to need a 32a feed so depending on run length and any insulation. You’ll need between 6mm dia and 10mm dia twin and earth.

You’ll could get your electrician to terminate this in an outdoor isolation switch ready for the later AC install.

Get it wired on a separate breaker at your consumer unit.


You do realise that a split unit needs control cables to the controller and paperwork to the indoor unit. The supply cable will be the least of your issues.

whatxd

Original Poster:

475 posts

122 months

Sunday 17th August 2025
quotequote all
I honestly don't know, I'm thinking it would go on the side gable, pumping into the open plan area and my bedroom above. I appreciate your input and will pass this on to my electrician

b0rk

2,407 posts

167 months

Sunday 17th August 2025
quotequote all
There will be supply and return pipe work to get the gas from the outdoor unit to the indoor fan coil.

There will also be a power cable to the indoor fan coil.

You may want to consider where this will go if you are still at the first fix stage.

caziques

2,796 posts

189 months

Sunday 17th August 2025
quotequote all
b0rk said:
Single phase or three phase?

Assuming single phase the AC outdoor unit is likely to need a 32a feed so depending on run length and any insulation. You’ll need between 6mm dia and 10mm dia twin and earth.

You’ll could get your electrician to terminate this in an outdoor isolation switch ready for the later AC install.

Get it wired on a separate breaker at your consumer unit.


You do realise that a split unit needs control cables to the controller and paperwork to the indoor unit. The supply cable will be the least of your issues.
A standard 2.5mm cable can have a 16 amp breaker. 16 amps is around 4kW, or around double the current draw of a very large domestic unit.


119

16,173 posts

57 months

Sunday 17th August 2025
quotequote all
We have a 3.5 kw unit in one room and it is plugged into a 13a socket!

Accelebrate

5,536 posts

236 months

Monday 18th August 2025
quotequote all
Work out how many BTUs you need for the space you intend to cool. There are calculators online...

https://www.airconcentre.co.uk/pages/btu-calculato...

If you can share the BTUs and a rough cable length from where it will be cited to your consumer unit, you'll get some more accurate responses in here.