Granny charging

[Jomarkh]

What granny charger did you get?

Recently used a 10m 2.5mm ext lead and got 1.9kw. The 45m 2.5mm ext reel gave 1.7kw before tripping the thermal cut out

Hi, did you unwind it fully before using it?

 

[gbrspratt]

What granny charger did you get?

Recently used a 10m 2.5mm² ext lead and got 1.9kW. The 45m 2.5mm² ext reel gave 1.7kW before tripping the thermal cut out

Sorry for the late reply, I bought this: Amazon link

I use. Tough leads 10m extension too

 

[Berkofix]

No worries. Got a weatherproof plug housing and made one from a worn out extension reel.

Hi, did you unwind it fully before using it?

No, I kinda knew it might fail. I had brought several leads to try. I guess the main clue was written on the side of the reel, giving wattage ratings for wound and unwound. The granny charger has a temp sensor in its plug, I was curious to see the failure state. In the end the thermal trip on the reel kicked in first. In theory the granny charger should charge at 10 amps, so 2.3kW ish however depending on the cable I saw 1.9kW or 1.7kW (indicated on the iSmart app).

 

[Peterboat]

UK rated granny EVSE units are usually 10A 2.2kW. If you buy from Ebay or Amazon you can get them at 13A but they will cook the plug or/and socket if on for hours. Heaters etc. are not run continuously as they turn off and on when they reach the set temperature.

I fitted a 16 amp commando socket, twin pole switched, we use it for other things so it's not wasted and it doesn't overheat

 

[stevegus]

There are EV rated sockets with integral RCD that you can get from screwfix and the like.

For the brave “Granny chargers” on Amazon can come as big as 32 amps, which need a 32A “commando” type outlet and a suitable electrician installation.

 

[Coulomb]

In theory the granny charger should charge at 10amps, so 2.3kw ish however depending on the cable I saw 1.9kw or 1.7kw (indicated on the iSmart app)

The iSmart app reports the same as the car, which is net power into the battery after on-board charging losses and 12V loads. That will account for several hundred watts of discrepancy, so you really don't know how much loss is from the extension cord.

 

[Berkofix]

I was curious to see the loss rates, hence used a 45m and 15m 2.5mm² multi-strand extension cord. I do have a bit of 6mm² T+E so could make a lower loss ext lead, however the 20amp UK ring main it's fed from is based on a 2.5mm² Twin and earth ring not a 32amp 6mm² radial.

In the end the Zappi wall charger runs off a 6mm² radial, so the extension lead was an experiment whilst we were staying at an AirBnB.

 

[Everest]

however the 20amp UK ring main its fed from is based on a 2.5mm Twin and earth ring not a 32amp 6mm radial.

I'm not a sparky, but a standard UK ring final will generally support a max current of 32A when wired with 2.5mm² T&E. Unless there are specific reasons for de-rating (e.g. covering a floor area greater than 100m² or a cable length > 100m) then the ring final would normally be protected with a 32A MCB.

 

[stevegus]

Everest, you are absolutely correct that a ring main is 32amps total capacity.

Because its a ring you effectively have TWO 2.5mm feeds (albiet of unequal run length) to any given point on the ring

 

[Berkofix]

I'm not a sparky, but a standard UK ring final will generally support a max current of 32A when wired with 2.5mm T&E. Unless there are specific reasons for de-rating (e.g. covering a floor area greater than 100m2 or a cable length > 100m) then the ring final would normally be protected with a 32A MCB.

My certifying electrician encouraged me to de-rate the 32amp circuits to 20. Has been fine.

 

[Everest]

My certifying electrician encouraged me to de-rate to the 32amp circuits to 20

That's very unusual - why was that? Cable length, multiple circuits running in close proximity or excess insulation around the cable or something else?

 

[Berkofix]

All of the above for a refurb.

I'd imagine its a useful mitigation of risk for the certifying authority.

Its never been an issue, even on the kitchen circuit which you'd think might have a higher draw.