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Solar-Battery Installation to Metal Garage

John R.

I am proposing to build a new garage-workshop.  Based on current cost estimates, this will be metal-framed and clad, on a concrete base.

The proposal is to install 12 to 14 solar panels on the roof and a Tesla Powerwall in the garage.  And yes, this installation will need a G99.  The panels are DC-coupled to the Powerwall.

The garage will have two separate circuits from the meter area in the house  (as recommended by the installer)

  • Cicuit feeding an EV charger, 3-core SWA
  • Circuit connecting the Powerwall, 3-core SWA

There will also be a local consumer unit in the garage, connected to (b) to feed lights, sockets, power tools etc.

The plan is for the builders to install the cabling to leave tails, and then the installer will install and connect the electrics.  Even though SWA cables will be used, the plan is to install these in a duct for future proofing where this crosses the garden

I’m also likely to install a fibre data cable.  This may also be SWA but I wasn’t proposing to connect the armour, it’s only for mechanical protection.  This is mostly to avoid the latency of a WiFi link.

The supply to the house is TN-S with Zₑ 0.24 Ω 

My only query about this installation is the earth bonding of the garage structure & cladding metalwork.   I’m assuming that the structure’s frame components will need to be earth bonded and perhaps also the cladding panels – does this sound about right?

Would the garage likely need a local earth rod – or is it preferred to use the extended supply earth?

One of the reasons that the earthing is of interest is that the garage may have some ham radio equipment.  My practice is to power such radios from a 12V battery charged by a charger where mains earth is not bonded to the 12V negative rail.  A local clean RF earth is then used, and it’s undesirable to have this close to a mains protective earth rod.

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  • 0

    Not answering your question at all, but have you considered current clamp(s) on the house meter tails, and how the signal will get to the garage?

    A solar + battery system almost certainly needs a current clamp, so that it can match the drawdown on the battery to the needs of the house. If it keeps the current through the meter tails as close as possible to zero, then the house is running on battery not grid power. Without a current clamp, it has no idea how much power to generate.

    Depending on the size of the supply fuse, an EV charger may need a current clamp to make sure it's not overloading the supply. Mine has one as my supply is only 60A. If the supply is being overloaded, it can tell the car to slow down the charge.

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  • 0

    Not answering your question at all, but have you considered current clamp(s) on the house meter tails, and how the signal will get to the garage?

    A solar + battery system almost certainly needs a current clamp, so that it can match the drawdown on the battery to the needs of the house. If it keeps the current through the meter tails as close as possible to zero, then the house is running on battery not grid power. Without a current clamp, it has no idea how much power to generate.

    Depending on the size of the supply fuse, an EV charger may need a current clamp to make sure it's not overloading the supply. Mine has one as my supply is only 60A. If the supply is being overloaded, it can tell the car to slow down the charge.

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  • 0 in reply to Simon Barker

    Hi Simon

    Yes, both the EV charger (which I'm installing for future use, since currently have an IC car on a long lease) will need CTs at the house end.

    There are two approaches to dealing with this 

    (i) install a CAT5 cables and use pairs in that to connect the CTs,

    (ii) Use specialist EV and Powerwall SWA cables that include comms cores.  Doncaster Cables makes these in the UK.  The installer was proposing (i) on grounds of cost, but I rather like (ii) on grounds of simplicity.  I also think that data cables tend to be a bit flimsy and easily damaged.

    https://store.doncastercables.com/powerwall-connect

    I'd add that I remain undecided on the inclusion of a Powerwall Gateway which allows the system to run during mains power outages.  This needs to be reasonably close to the meter and consumer unit, so can't be in the garage!

    Thanks
    John

  • 0 in reply to Simon Barker

    PS - Incoming fuse is 100A.  

    Need to ensure that export doesn't exceed whatever G99 level is allowed although locally there's not much installed solar so I'm hoping for a decent G99 level.  That said, my various spreadsheet models make clear that exporting is generally not a good idea and it's better to hog the battery energy for own use.

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