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Discussion Starter #1
Hi all.
What do the regs say about a temporary connected evse to a 32amp commando socket in a garage?

I've read and heard lots about TN systems but not much about true TT systems
My house has a TT earth as my power arrives from overhead cables.

I have a 6mm2 SWA cable to the garage. Will I need a second cable to the garage?
thanks
 

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The basic regs regarding taking TT to an outbuilding say you can but the electrician needs to check a few things to see if the conductor needs to be sized as a bonding conductor. Presumably this happened for the earlier install.

Cable size depends on the length and how it’s installed (clipped or whatever).
 

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What regs apply depends on the declared usage. For indoor usage you should stick to the same type of Earth as the rest of the house.
The question of whether you need an additional cable is dependent on whether you have much other load on other circuit and whether you want to risk faults on the car charging potentially meaning that the other circuits in the garage lose power also, although there's ways around that. For example, if you have a freezer in the garage could you risk it being off overnight?
 

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Discussion Starter #7
Brilliant thanks to the both of you.

Just to answer the questions, its a detached garage, a 6mm2 25 m cable was used and went underground, the DNO fuse is 100amps.
When the work to the garage was done a new earth rod was installed to the main consumer unit. (don't know the results)
The only other load in constant use in the garage is a small chest freezer and that's only about 2-3 amps.

I also compared the TLC link to this one too Simple calculator for electric cable size.

Looks like the cable is sufficient even for the full 32 amps.

Its actually going to be a charge point for a Energica motorbike but that only has a 3kw on board charger so I'll only need actually needn14 amps but I wanted to check in case I ever charge a car at the full 32 amps the EVSE can provide. Although when we eventually get an electric car it will probably be another install on the drive and the rules would have changed.

I just didn't want to get the electrician around again as I already have the 32amp comando socket .
 

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Its actually going to be a charge point for a Energica motorbike
Presumably charging it in the (dry) garage then, not outside. No problem at all really.

Just check that the charger has DC protection ideally. If not then the charging process could 'blind' AC RCDs upstream which is a slight risk.
 

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You also need to think about the arrangements to split the supply at the garage between sockets, lights and the charger supply, protecting the various cable sizes - is there a small consumer unit there or something else?
 

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The only other load in constant use in the garage is a small chest freezer and that's only about 2-3 amps.
So the question is are you willing to accept the EV charger tripping the circuit and you losing the freezer contents? If no, you either need another line into the garage or a separate CU in the garage with the Commando on its own cutout.
 

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Discussion Starter #12
So the question is are you willing to accept the EV charger tripping the circuit and you losing the freezer contents? If no, you either need another line into the garage or a separate CU in the garage with the Commando on its own cutout.
There is currently a consumer unit with 63a RCD and 3 MCBs.

Could the commando be on just an RCBO rather than a separate CU with RCD?
The path would be DNO 100 amp -> main CU 80a RCD -> main CU 40a MCB -> 6mm2 to garage ->
to either A. or B from here.
A.
garage CU 63a RCD -> 1, 2, 3
1. 16a MCB sockets
2. 6a MCB lights
3. 40a RCBO to comando socket
or
B. with larger CU
garage CU 63a RCD No1 -> 1, 2
1. 16a MCB sockets
2. 6a MCB lights
garage CU 40a? RCD No2 -> 3
3. 40a MCB to comando socket (or direct to EVSE as its for internal use only)
 

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Your Sparky wired your property as they did for reasons that fitted with their interpretation of the regs. I suggest that you take their advice.
For my part I would be asking why I had to go with either suggestion and would ask why you couldn't wire the garage CU directly without going through the main CU RCD, and likewise in the garage CU the Commando socket wiring not going via the CUs RCD but instead having its own. That way Earth issues with whatever is plugged into the Commando socket will only cut current to the Commando, not the rest of the garage or house circuits.
Despite all of the above, take professional advice and don't rely on advice from the internet from people not aware of all of the facts.
 

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Could the commando be on just an RCBO rather than a separate CU with RCD?
The path would be DNO 100 amp -> main CU 80a RCD -> main CU 40a MCB -> 6mm2 to garage ->
to ... [garage CU 63a RCD -> ]
The problem with that is two-fold.
A DC problem with the EV charging (if there isn't DC protection) could blind the house and/or garage RCDs, so they no longer protect you.
There is no RCD discrimination since they are all 30mA - a ground fault on the charger will trip all the upstream devices including the main house.

You could rearrange the CU so the garage supply is taken from an MCB or RCBO direct from the mains. The main CU RCD then only feeds the house. At the garage end you then subdivide with RCD/RCBO/MCBs as required.

The catch with rearranging the CU is that the whole lot needs retesting and certifying afterwards. If you are paying £50 an hour it can be cheaper to fit a second small CU to feed the charge-point or garage and leave the house CU (largely) undisturbed as well as bypassing almost all problems of the EV charging affecting the house electrics. And it gives you more spare ways for the future.
 

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Discussion Starter #15
Despite all of the above, take professional advice and don't rely on advice from the internet from people not aware of all of the facts.
Absoluttly , the questioning was to get a better understanding so I can have a more informed conversation with the sparky, which I wish I had done so before he did the original work, true a limited knowledge is dangerous but enough keeps the trades on their toes. :)
Thank you both.
 

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Absoluttly , the questioning was to get a better understanding so I can have a more informed conversation with the sparky, which I wish I had done so before he did the original work, true a limited knowledge is dangerous but enough keeps the trades on their toes. :)
Thank you both.
Quite so. It's similar to buying a car (or anything else really) - You have to understand a bit about it to know what features will fit your needs.
Future changes also need considering as doing or buying something a bit different now could avoid the need to go back and start over a few years down the line - possibly at great cost and inconvenience (like burying a marginal sized cable and putting expensive paving over it).
 
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