For ref this is the cable I was going to buy from Scrwfix: Prysmian 6943X Black 3-Core 6mm² Armoured Cable 50m Drum - Screwfix

 

Hi, 6mm is the minimum you can use.
You will need to check if the voltage will drop over 35m with 6mm or if you should use 10mm.

 

10mm cable for use over that distance for sure.

 

Would it be beneficial to get those cable with ethernet built-in? Most EV charge points these days need additional signals for CT clamps.

 

Voltage drop will be around 8V for 6mm² over 35m. That's a loss of over 250W in the cable when charging at 32A. The percentage drop is only a bit over 3.5%, so is inside the 5% limit, but I'd be inclined to up the cable to 10mm², just to reduce the wasted power. Every 4 hours of charging is going to waste a kWh or more, so adding to your charging costs.

 

Crickey! I did my 20m run using 6mm² thinking that the drop was likely insignificant. Just done the calculation and if I did it correctly my voltage drop is around 5V (equates to about 160W) so that isn't too bad but more by luck than judgement in my case! My electrician didn't do that calculation I am certain of that. I certainly didn't.

 

It's a good point, as anyone designing an installation is required to do a voltage drop calculation, it's in the regs. The limits are fairly high, though, as there is an assumption that high current loads won't usually be on for hours on end. A power circuit is allowed to have a voltage drop of up to 5%, a lighting circuit (or combined power and lighting circuit) is allowed to have a voltage drop of 3%.

These limits don't account for wasted power, though. A 5% voltage drop may be within the regs for a charge point, but it's a power loss of nearly 370W when charging at 32A. For an 8 hour overnight charge that's going to waste nearly 3kWh, enough to drive around 12 miles. Over the lifetime of an EV that will add up to quite a bit of wasted energy, and increase the running costs a bit. Doesn't take long to recover the higher cost of thicker cable.

 

6mm² armoured, and armoured ethernet cable, is perfect and can be buried directly. 35m is at the long end for 6mm² so you might want to lay 10mm² instead.

You can get "EV Ultra" cable that contains both power cores and twisted pairs for data. However while it is neater, it costs more than the two cables you mention.

 

I made this for anyone wondering about power loss. It's aimed at EVSE to car but you can use it for DB to EVSE. 7kw = 3x6mm^2. 3kw = 3x2.5mm^2. 22kw = 5x6mm^2. 11kw = 5x2.5mm^2
6mm^2 cables run at 32A
2.5mm^2 cables run at 16A

 

I made this for anyone wondering about power loss. It's aimed at EVSE to car but you can use it for DB to EVSE. 7kw = 3x6mm^2. 3kw = 3x2.5mm^2. 22kw = 5x6mm^2. 11kw = 5x2.5mm^2
6mm^2 cables run at 32A
2.5mm^2 cables run at 16A
View attachment 177425

The figures seems slightly low to me for 6mm² SWA cable feeding a charge point. There's an SWA voltage drop calculator here: https://swacable.net/swa-cable-calculator-2/

According to that, a 35m run of 6mm² SWA will have a voltage drop of 8.85V at 32A. That gives a cable power loss of 283.2W. Looking at your plot I think it may be for 6mm² multistrand flex, which has a slightly lower resistance per metre than SWA.

Edited to add:

Looking at the BATT Cables data (attached), they give the conductor resistance as being 3.08 ohms/km, so the loop resistance for 35m would be 0.2156 ohms, giving a power loss of 220.7W at 32A, which is different to the SWA Cable calculator. Not at all sure why there should be differences like this, TBH.

 

Should be in a pipe at least 450mm underground

Electrical Installations in the Garden

Elecsa offer support and guidance for electrical garden installations. Electrical installations installed in a garden, including luminaires and their associated cables, should be securely fixed and pro

professional-electrician.com

May be easier to attach it above ground to a wall / fence.

 

SWA is fine for direct burial in terms of the regs. Definitely a good idea to put it inside a duct if there is a likely need to replace it at a future date, but only if the duct run is fairly straight. Pulling SWA through bends in underground ducting is a PITA.

 

Pulling unarmoured cable through a straight duct is tough too! I'm used to fibre telecom cable, with an outside diameter of 6mm. I pull that through up to 100m of duct without too much trouble. But getting 6mm2 UltraEV cable, outside diameter about 25mm, and much less flexible, through 7m of straight duct was much more difficult! It took two of us, one pushing the other pulling to get it through. Still easier than digging though.

 

I'd skip the duct and just bury the thing, it'll be a hell of a job pulling the cable, and direct contact to earth will sink any heat away.
Charger wise, get a WiFi one or better still lte connected.... Ethernet cables are garbage these days, and a pain to protect underground, especially if you go the conduit route as I've found they trap water.