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Extension lead for 3pin granny charging


Well-Known Member
Aug 19, 2019
Hi guys. Does anyone know if this would be safe to use to extend the Tesla 3 pin cable for home granny charging?

Masterplug Weatherproof Outdoor Single Inline Socket Extension Lead with ‘H’ Frame Cable Tidy, 15 Metres, Orange

It’s 13amp so think it’s ok as long as it’s unwound when used and has a covered plug. Interested in your collective views tho!

Did you look at the "customer images" for this item in the reviews? Aside from a decent quality plug/socket I would be interested to know the size/thickness of the individual cores ... 1.5mm would be good.
Zoomed in, got the specs




Active Member
Aug 16, 2019
North West
Most people seem to do the Tough Leads or www.industrialextensionleads.co.uk which I would also recommend you do.

Tesla recommend you do not use any extension leads.

That said, if that’s all I’d had yes I’d use it as long as there was an RCD on the Consumer Unit or the wall socket. No idea if you can close the weatherproof cover over the Tesla UMC plug. It’s not 1.5mm2 (arguably over spec’d ;)) but for 15m heat/voltage drop wouldn’t worry me.

Note the warning not to leave unattended when risk of exposure to moisture.

Outdoor Power - IP Rated - Portable Power - Masterplug UK
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The issue with 13A plugs/extensions seems to be the risk of overheating the fuse in the plug (and consequently getting a poor contact on the live pin of the connection also). I'm wondering of the UMC has any thermal monitoring of the 13A plug - I'm sure I've seen reference to this concept somewhere.

If you need to use an extension, running it at below 10A makes some sense (provided you can spare the charge time) - and remember that there is a potential impact on any RCD upstream which protects other circuits in the case of a fault.
Don't cut corners for the sake of a few penny's

Electric vehicle leads with inline RCD option and socket tester

I can vouch for these guys too. Ordered a cable three weeks ago. 10m with RCD. Have used it already. Left it to charge for 12 hours. No overheating at all. Actually the socket of the cable was cool all the time. Just keep it fully unfolded while in use.
By the way it was also raining. Kept the UMC under the car and all was fine.


Well-Known Member
May 8, 2019
The part of the equation that gets forgotten is the socket. If own/control the one you plan to plug into and its old then swapping that for a new good quality one and/or making sure the connections inside are well connected and clean is probably worth doing whether or not you use an extension lead. Also better to use one on the ring main than a spur if that is an option.

I have been using a 2.8kw ( about 12amps) pool heater for the last few weeks for upto about 5 hours a night using a relatively cheap 5metre 1.5mm 13amp extension lead. I tested it carefully in the day time before running it overnight and the extension lead plug became very slightly warm to the touch but that was it so while I am sure issues are possible at the UMC's 10amp I think there would have to be a wiring issue for there to be any kind of problem.
Has the umc got low voltage protection? That would give some protection against volt drop due to length. I would not be concerned with a 13A extension as charger is limited to 10A but continuous for several hours typically. 1.25mm sq is typical have to make your own for a 1.5mm sq cable. If you make your own I recommend you should have crimped ends on cables. Ebay sell a great crimper for about £15 with crimps/ferrules.


Well-Known Member
May 8, 2019
I use a 20m lead from Industrial Extension leads to charge my M3 at a remote farmhouse in Snowdonia. It uses 2.5mm cable for an extra safety margin and to minimise voltage drop. It works perfectly, which is just as well as the charging infrastructure locally is appalling.
The advice I had from an electrician was that for a 10m cable 2.5mm is overkill and you are just wasting money and weight to no benefit. You may be right to get 2.5mm for such a long run, I don't know, but for anyone considering 10m or less I would not bother.
For the record a 10m 1.5mm cable fits nicely in a waterproof box just big enough to take the whole UMC while charging so that works out quite nicely.

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