Advice Needed – Second EV Charger + Tesla Powerwall 3 + Garage Setup + heat pump

Shaya

Hi all,

I’m planning to install a second EV charger and possibly a Tesla Powerwall 3 and heat pump and would really appreciate advice on how best to approach the setup. I’ve already had one quote, but I’m getting mixed messages from different electricians, so I want to sanity-check everything before I proceed.

🏠 Current Setup:

• Detached garage is ~12 meters from the main house fuse box.
• There’s an old MEM 30A fused switch in the garage (very outdated).
• An underground metal conduit already runs from the house to the garage under a brick driveway, but I don’t know if it can be reused (probably too small or unsuitable).
• I currently have one EV charger installed — this would be a second one.

⚡ Planned Install:

Here’s what I’ve been quoted for so far:

Option 2: Install second EV charger + new garage supply

• Supply and install new IP-rated 18th edition consumer unit with SPD at the front of the house to feed garage and EV charger.
• Trench brick driveway to lay two new 6mm² SWA cables:
  
• One for the EV charger
• One for garage power (plus a communication cable for the Zappi charger)
• 
  
• Supply and install Zappi multiphase EV charger
• Install new 6-way consumer unit with SPD in garage for future expansion
• Test and commission everything

Quote: £3,513.82 + VAT

❓My Questions

1. Is 6mm² cable sufficient?
   Some electricians say 6mm² is fine, others recommend 10mm² or even 16–25mm² for future-proofing.
   Since we’re already trenching, I’m wondering if I should just go larger now to avoid having to dig again later.
2. Should I install the Tesla Powerwall 3 in the garage or near the house?
   

• The garage gives me more space, and it would be close to the second EV charger.
• But putting it near the house (next to the main CU and meter) might reduce installation complexity and cost.
• If anyone’s installed Powerwall 3 — what worked best for you?

Is the quote reasonable?
The price seems fair considering it includes the charger, trenching and reinstating block paving, two consumer units, and all cabling. But should I be pushing for larger cable sizes or bundling more work into one dig?

Should I future-proof for three-phase power?
I’m currently on single-phase, but I’m wondering:

• Should I run oversized ducting (e.g. 63mm twinwall) or even install 5-core SWA cable now, in case I upgrade to 3-phase later?
• Would this be overkill, or a smart move if I plan to expand solar, batteries, or EV charging in the future?

🧰 What I’m Trying to Achieve:

• Safe, modern electrical supply to garage
• Support for second EV charger
• Future-proofing for Tesla Powerwall 3 and possibly more loads
• Avoid digging up the driveway again in 2–3 years

FreeBear

Cable size to the garage really depends on what you plan on doing in there. A couple of sockets for some basic DIY, 6mm² would be an overkill. Big wood lathe plus heavy duty extractor and a large compressor, 10mm² or even 16mm² may be needed. If the Powerwall is going in the garage, you should be guided by your electrician.

Although SWA doesn't need to be run in conduit, it is worth doing so - Gives you the option to upgrade or pull an additional cable through at a later date (just make sure to leave a length of rope running through the conduit).

With 2 EV chargers, a Powerwall, and a heat pump, I suspect you'll need to talk to your DNO about main fuses and maximum current draw. If you have electric showers in the house or a hot tub, you could well exceed your main fuse rating.

silvercar

Do you see a need for a second EV charger? Do you really do that much mileage on 2 cars to justify it? With 2 cars we never see the need to charge both at the same time, but if we did, charging one at 7kWH on the EV charger and the other at ~3KWH on a ‘granny’ charger would suffice. 

doodling

Hi,

If you are really wanting to make provision for two 32A chargers, a Powerwall and a Heat pump then the best approach is to run a single submain to the garage, of 16, 25 or maybe even 32mm sq SWA cable, depending on the length of run, the manner in which the cable is installed and the total expected load.

To be honest, I would size the cable for the maximum your DNO is willing to provide (so 80 or 100A).

If you really want to future proof the installation then I would use 5 core SWA to make future installation of three phase easier (and pair the conductors to increase current rating / reduce voltage drop for the single phase case).

Note that, for example, 5 core 25mm sq SWA is around £20 a metre, is a real pain to install because of its stiffness and will require stuff you might not normally see in a domestic environment to terminate it so be aware that making future provision can have a cost.

Qyburn

Are you going for the Tesla gateway, to support operation during a power cut? If so that will need to be located where the electricity supply comes into the house. And you'll need to put in whatever connections are needed between gateway and PW3.

Will you have solar panels feeding the PW3?

Shaya

Qyburn said:

Are you going for the Tesla gateway, to support operation during a power cut? If so that will need to be located where the electricity supply comes into the house. And you'll need to put in whatever connections are needed between gateway and PW3.

Will you have solar panels feeding the PW3?

Thank you for your reply, yes I am hoping to use the tesla powerwall during power cut, are you saying if I put it in garage that would be too far away? the electric come to the front of the house and garage is about 12 meters away , we would also have about 20 panels feeding into the battery yes

Shaya

doodling said:

Hi,

If you are really wanting to make provision for two 32A chargers, a Powerwall and a Heat pump then the best approach is to run a single submain to the garage, of 16, 25 or maybe even 32mm sq SWA cable, depending on the length of run, the manner in which the cable is installed and the total expected load.

To be honest, I would size the cable for the maximum your DNO is willing to provide (so 80 or 100A).

If you really want to future proof the installation then I would use 5 core SWA to make future installation of three phase easier (and pair the conductors to increase current rating / reduce voltage drop for the single phase case).

Note that, for example, 5 core 25mm sq SWA is around £20 a metre, is a real pain to install because of its stiffness and will require stuff you might not normally see in a domestic environment to terminate it so be aware that making future provision can have a cost.

Thank you for your reply,

We currently have 100A installed by DNO, 
Id you think the 5 core might be too costly we may steer away from that, but just doing enough so we dont have to dig the driveway again as that is our main cost at the moment. 

Shaya

FreeBear said:

Cable size to the garage really depends on what you plan on doing in there. A couple of sockets for some basic DIY, 6mm² would be an overkill. Big wood lathe plus heavy duty extractor and a large compressor, 10mm² or even 16mm² may be needed. If the Powerwall is going in the garage, you should be guided by your electrician.

Although SWA doesn't need to be run in conduit, it is worth doing so - Gives you the option to upgrade or pull an additional cable through at a later date (just make sure to leave a length of rope running through the conduit).

With 2 EV chargers, a Powerwall, and a heat pump, I suspect you'll need to talk to your DNO about main fuses and maximum current draw. If you have electric showers in the house or a hot tub, you could well exceed your main fuse rating.

Thank you for your reply, do you think that would be possible to do 12 meters in conduit as its not straight line and 12 meters away, as much as I think that would be the perfect way of doing it for future proof anything im not too sure if it would be possible  

Qyburn

Shaya said:

Thank you for your reply, yes I am hoping to use the tesla powerwall during power cut, are you saying if I put it in garage that would be too far away? the electric come to the front of the house and garage is about 12 meters away , we would also have about 20 panels feeding into the battery yes

It's about what's connected to what, rather than distance per se. The gateway will need to go into the point where the supply comes in, so in the event of a power cut it can fully disconnect and do other stuff to ensure it's safe for the PW3 to supply power. In addition it will need one or more connections to the PW3.

You'll need to look at current documentation. I checked all this when we were designing our system, but that was in the days of the PW2.