DIY Doctor UK DIY Forums

Hi,

I am in the process of buying a new house from a builder.

It has a detached garage, I requested and paid for power to be installed in the garage, this is what I requested to be installed in the garage

1) a 32 AMP circuit to charge an electric car,
2) 3 double sockets
3) Lighting to the garage

When I went to see the house lately the electrician has install T&E cable from the house CU to the outside wall, a cable run distance of approx 15m, then underground armoured cable from the wall point to the garage CU. A cable distance of approx 8m

The House CU has a 32 amp MCB, which I stated is not right, as the car charger draws near 32 amps when it starts to charge, if I turned on a light or an appliance it would blow.

The electrician has now replaced the 32 amp MCB with a 40 amp.

This still does not seem right, can someone tell me what cable and MCB should be used for this type of install? Or am I just being paranoid?

Any calcs or help you can give would be appreciated.

Thanks

Neil

24 meters at 40 amp is the limit for 6mm sq to comply with lighting volt drop of 3% or 6.9 volts. It would not allow any internal wiring volt drop so really it would need 10mm sq cable for the volt drop. Installation method C does allow 47 amp for 6mm cable but installation method 100 is down to 34 amp so it is possible you could use 10mm flat twin and earth in house and 6mm to garage but my java script program does not allow two cables and even with one cable the way it is configured does not make it easy.

Assuming incoming supply loop impedance of 0.35 ohms the feed to garage should be 0.52 ohms to be within the volt drop figures.

There are different rules to charge outside to inside PME (TN-C-S) is permitted for inside charging but for outside charging it depends on the spec of the car, normal is to use a TT supply outside.

Because 6mm cable would be OK at 32A and not at 40A I would be checking the cable specs. As long as wire wound ballast fluorescent fittings are not used the volt drop in real terms would not be a problem. But it would not comply with regulations.

Thanks for the info,

At this distance what would be the maximum AMPS for a 10mm sq cable all the way?

Thanks

Neil

Installation method changed the amps a cable can carry and also there is a limit to size of MCB that will fit in a consumer unit. The old LoadMaster did a 70A but modern units normally stop at 45A so one has a jump from 45A to 100A, the latter using a henley block so only fuse in the DNO supply.

So 66 meters of 10 mm sq is the limit at 100A.

In the main homes are supplied with 60, 80 or 100A. After 100A you can't use a consumer unit in theroy they can be 125A but in practice not seen one over 100A. When the supply is requested you have to tell the DNO what size supply you require. The fuse carrier always says 100A what ever size fuse is inside.

If one has something special like a welding set you have to inform the DNO and they can say no. Again one can request a 3 phase or split phase supply to get extra power. Some fast chargers for cars need a 125A three phase supply one would need permission from the DNO to use one of these.

For the Tesla it would seem there are three stages of charge 11 kW, 50 kW and 120 kW for domestic 230 Vac single phase supply that is 47.8A, 217A, and 522A clearly the latter two would not come from a single phase supply so not domestic.

I selected Tesla as it is around the biggest electric car. 47.8A is higher than the 45A limit for most MCB's as we move to moulded breakers the sky is the limit I have worked with 500A versions and if you remember Juristic Park the original they had to pump up the breaker before resetting that is actually how it is done.

This seems to be the standard method.
Rolec WallPod EV Charger
EVWP2020
IEC 62196 Socket
32 Amp - 7.2Kw
Full Mode 3 EV Charging
Built-In RCD
They are expensive at around £330 however to quote from Energy Saving Trust.
"Funding is currently available towards the cost of home charge points for electric cars from the Office for Low Emission Vehicles (OLEV)."

"Who does the installation?
Installations need to be carried out by OLEV-accredited suppliers. A full list of OLEV-accredited suppliers can be found here. Please see below a list of OLEV-accredited suppliers operating in Scotland: "

Because it is specialist work I have only glanced at what is required out of general interest. As with fitting solar panels unless one is registered one can't get the government grants.

So if I was working as a domestic installation electrician I would sub-contract this work to some one who is registered. Even though I have a degree in electrical and electronic engineering and have been an electrician for 50 years I would not do this work myself.

I was going for the tesla 32 amp charger,

but my concern would be that when the charger is running I still need to draw full power from the sockets,

so given a 32 amp charger, three double sockets and a lighting circuit I thought I would need a peak current draw of 50 amps? Is that reasonable?

Thanks

Neil

Looking are other rechargeable transport fork lifts, mobility scooters, wheel chairs etc, the charger takes full power for may be 1 hour then quickly tails off. So likely there would not be a problem.

I was taught lead acid and with them the better chargers are either stage or pulse and with the stage chargers there is a marked reduction in power as it hits the second stage.