Hi,

I recently bought a cottage with a detached 10m x 10m shed. The shed has power via what I believe is 2.5mm SWA cable which appears to be fed from a 13A socket in the kitchen. Essentially this is a spur from the main ring and as I understand it, it should be limited to 13A. However, this is terminated in the shed with a secondary consumer unit with two MCBs. A 32A MCB feeding several 13A outlets in the shed, plus a 6A MCB feeding the fluorescent lighting. Not ideal... Fortunately we don't have anything in the shed which draws a lot of current and until we get this wiring sorted out we're being careful not to draw more than 13A at a time.

We also have an electric car, a BMW i3, and I want to get a fast charger (7.4kW) installed but clearly we can't go pulling 32A+ from the shed with the present arrangement. So, I have decided to run a cable directly from the main consumer unit in the house to the secondary CU in the shed. Don't worry, I'm not planning to commission it myself but I want to lay the cable and have it ready so that the electrician who comes to install the car charger can check and commission the new cable and CU at the same time.

With all that said, my question is basically what cable to buy - mainly whether I need 3 cores or if 2 is sufficient, and whether to go for 16mm or if 10mm would be plenty. The cable route will be from the main CU through a stud partition into a perpendicular stud partition for about 3m where I intend to run the cable through a series of holes drilled in the middle of each stud behind the skirting board until it meets the exterior wall of the house where it will go through the wall and turn downwards into some conduit. The conduit will extend down to the appropriate depth (say 0.5m) and I'll probably just run the cable direct in the soil until it meets the exit conduit next to the shed where it will go through the shed wall to the secondary CU. Total length is around 15m.

Here are the ratings for direct in soil at 20C ground temp (I'm in Scotland so I suspect 10C will be closer).

Cores 2 3+
6.0mm² 53A 44A
10.0mm² 71A 58A
16.0mm² 91A 75A

I don't begrudge the cost of the cable but I know SWA is quite hard to work with so smaller is easier but I want to do this job once and forget about it so if it needs to be 16mm then fine. But I'm thinking, especially if I only need 2 cores that 10mm might be more than enough. I'm also unclear if I need 3 core or if the steel sheath can be used as an earth.

Advice welcome - thanks,
Craig.

ps. Main CU has a 100A fuse and is pretty modern, fitted circa 2008 when the house was completely renovated. I believe it has TN-C-S earthing.

[Edited on 12/10/2020 by craig1410]

I have just gone through something similar.

7.5kw@230V is 32A. Technically a 2.5mm cable would do the job in the kind of temperature and installation conditions you describe.... However, I suspect the duty cycle on a car charger is pretty high so better to spec a slightly larger cable to avoid heating over prolonged use.

Your 7.5KW car charger would be more than fine on a 6mm twin SWA. (assuming its single phase).


Connect the SWA into the CU WITHOUT an RCD but attach it to its own MCB of a suitable rating. You don't need an RCD as its an SWA....

In the garage you would then need a small consumer unit WITH an RCD to split the load into sockets, lights and EV charger.

[Edited on 12/10/20 by tegwin]

[Edited on 12/10/20 by tegwin]

Hey @tegwin, thanks for the advice.

I agree, the duty cycle of the EV charger will be pretty high, especially since our energy supplier, Octopus, gives us cheap electricity between 00:30 and 04:30 so this is when our EV will be pulling power. On the plus side, this is a time of day when not much else will be drawing power via our shed. That said, I don’t want to have to do mental arithmetic when using my electric fan heater while welding just because the car is charging during the day.

So, while I agree 6mm is fine for the car charger, I’m thinking that stepping up to 10mm gives me more margin for those times when the car is charging while I’m working in the garage and drawing significant power via the 13A sockets. I certainly don’t want any surges affecting the EV charger. I know this is probably overkill but are there any downsides other than cable cost and more difficult cable manipulation?

You said the charger would be fine on “a 6mm twin SWA” so can I just double check that you meant 2-core? If so, where does the shed get its earth from? Does it come via the SWA sheath or does the secondary CU derive earth from the neutral wire just like at the primary CU? I’ve never really used SWA before other than for some patio lights so I should probably read up on it a bit...

Oh, btw my charger will be single phase as that’s all I’ve got available out here in the sticks! We’ve got an 11kV 1-ph pole outside with a transformer hanging off of it.

Thanks again.

Better to use three core. That way you don't have to worry about the earthing capacity. SWA can be earthed through the armour, but you do have to calculate it, because the armour is not always sufficient.

Firstly I'm not a electrician. Only done 18mths apprentice 40 years ago.

My thoughts would be bigger is better until it doesn't work mechanically.

More copper = less losses.

If 10mm is straight forward then go for it.

Think like a Victorian, make it last for ever.

Hi guys,

Yes, I think if 3 core is recommended then I'm probably better off going for 10mm since that little table of ratings shows that 3 core cable has a lower overall rating:

Cores 2 3+
6.0mm² 53A 44A
10.0mm² 71A 58A
16.0mm² 91A 75A

So with 58A of capacity worst case (my ground/air temp is lower than standard and part of the cable is above ground) that should be plenty to run at 32A for a full charge of the car (4-5 hours) without the temperature getting anywhere near the 90C cable rating.

One important point I failed to mention is that I hope to install a 2 post electro-hydraulic lift at some point once I've tested my concrete depth and strength. I think these take 20-30A but obviously the duty cycle is very low (maybe 45 seconds of use then many minutes of rest) but at least if the EV is charging outside and I use the lift inside then 58A is still sufficient. I should probably do proper calculations as per the wiring regs so check for voltage drop but 15m isn't a massively long cable length so hopefully will be okay.

Further comment welcome, especially if my conclusions are in any way suspect
Thanks again.

Ultimately, if you're having an electrician sign off on it, they have to be happy with the circuit design so a quick phonecall should give you a minimum cable size, even if in our combined wisdom, we think it lacks margin.

I can't see the electrician raising any issues with oversize cable, but set against doing the bulk of the work and cost before asking their advice, you'd be far better making sure you keep them happy from the outset.

From my understanding, I'd be looking to run 10mm² 2 core SWA so you can get a 63A breaker in the shed, meaning you can charge the car at the same time as you weld, with fan heaters running and a couple of halogen floodlights for that awkward weld in that dark corner.....with the tumble drier on (though you might not need the fan heaters if you have the tumble drier).

Also see this link for reasoned discussion on using the armour as the CPC.

Edit: If you're looking to put the ramp in there too, 16mm 2 core is still smaller diameter than 10mm 3 core so you'd gain significant headroom on current carrying capacity for no detriment to workability of the cable.

[Edited on 13/10/20 by indykid]

To calculate cable size

http://www.doncastercables.com/technical-help/

For 32A 2.5mm is sufficient for 15m. If you may have a higher but intermittant duty in the future I would plonk in a 6mm2 3 core personally or 10mm2 if you think you will draw more than 60A continuous.

Don't forge you have to inform your DNO that you are connecting a car charger as they have to check you will not create too much noise on the phase you are connected to/capacity of your supply. When I did they confirmed all was well within a couple of day - there was no charge.
https://www.ukpowernetworks.co.uk/electricity/electric-vehicle-charging-point/charging-point-installer#notifyingUKPowerNetworks

Regards
Hugh

To calculate cable size

http://www.doncastercables.com/technical-help/

For 32A 2.5mm is sufficient for 15m. If you may have a higher but intermittant duty in the future I would plonk in a 6mm2 3 core personally or 10mm2 if you think you will draw more than 60A continuous.

Don't forge you have to inform your DNO that you are connecting a car charger as they have to check you will not create too much noise on the phase you are connected to/capacity of your supply. When I did they confirmed all was well within a couple of day - there was no charge.
https://www.ukpowernetworks.co.uk/electricity/electric-vehicle-charging-point/charging-point-installer#notifyingUKPowerNetworks

Regards
Hugh

I also got my electricity supplier to change the main house fuse (the sealed one you can't play with) from 40A to 100A. I worked out that charging my car plus cooking the Sunday lunch would be enough to blow the old one.

The people who manage the electricity supply (name escapes me) arrived, looked at the size of the tails going to the meter, checked the inspection labels on my distribution panel and then changed the fuse. No charge.

if you use a small cable, it warms up... but that energy is lost
a thicker cable will give lower losses

Check with your electrician. Also for how he wants the cable laid etc
I would suggest a 100mA RCBO or RCD to protect the SWA to te shed, with 30mA RCD in the shed consumer unit.
It may need a TT stake for the shed earth depending on all sorts. As to earthing for EVs, there are quite specific requirements.

One thing to look at is the main board loading, or use a clamp meter to get max demand on the existing installation. That may limit what you can do, although there are fudges

Hi guys,

Apologies for not replying to the last few comments - I thought I had but apparently not!

I’ve decided to go with 10mm 3-core XLPE SWA for a couple of reasons.

Firstly it gives me the option to use the 3rd conductor as an additional earth in the event that the armour is insufficient. Also, by going to 3-core, the SWA overall diameter is increased which has a knock on effect that it increases the cross sectional area of the armour itself. So this gives an overall boost to earthing capacity which may be important in my situation. As most of you will be aware, the steel in the armour conducts poorly compared to the copper conductors and so you need something like 8x as much steel as copper cross sectional area to get the same conductivity.

The second reason is due to the fact my house has a TN-C-S earthing system which makes things complicated when you want to “export” the earth from the house to an outbuilding. The alternative to exporting an earth is to install an earth rod for the shed in a TT earthing arrangement. I’d rather not have to do this if it can be avoided so by having the extra earth capacity in the cable, I keep my options open to some extent. It’ll still depends on a few factors and might require permission from my supplier so we’ll see what happens. I’ve got a book arriving tomorrow which should help me understand all the ins and outs of equipotential bonding zones which will help me understand things better. My shed has an H-beam steel frame which is driven into the ground and then covered by a galvanised steel skin so you’d think I could use this as a TT earth but I expect a proper rod will be necessary if I have to go down this route. I’m assuming that the metal shed frame will be considered an “extraneous conductive part” which is a shame because if it was just a wooden shed with no water or gas pipes coming in, then my house earth probably could just be exported without any issues. That’s my understanding anyway. Please correct me if I’m wrong.

As suggested by @cliftyhanger I will install an 100mA RCBO in the main CU to protect the SWA cable regardless of whether the earth is exported or isolated because the SWA armour will be connected to the house earth in any event. My main CU only has a single RCD way left but it’s 30mA, so I’ll install the RCBO in a spare non-RCD way. I have a consumer unit arriving tomorrow which has a 100A isolator switch and two separate 63A RCD circuits, each with 3 ways for MCBs. That should give me plenty of scope for what I need.

I spent today removing the skirting board and lower 3.5” of plasterboard from a stud wall to allow me to route the SWA through the studs just above the foot plate to the outer wall adjacent to the shed. I was going to run T&E internally and then join to SWA for the outdoor section but decided to just use SWA from end to end otherwise I was going to have to buy 25m of T&E just to use 5m worth. City Electrical Factors are quite good in that you can buy SWA in whatever length you need rather than having to buy 25 or 50m drums. I my case I need 16m total.

Anyway, once I have the cable and second CU in place, I’ll get an electrician out to quote to hook it all up. Hopefully the fact I’ve done all the non-electrical donkey work will make it easier (read “cheaper”) but I expect that will depend on the electrician.

Thanks again for all the replies.

Update: The more I read about TN-C-S and the hazards associated with exporting the earth, the more I'm thinking a separate TT earth in the shed might be preferable. The metal shed frame and skin is probably all electrically connected together so maybe I could just bond the frame to the main earth but it might be dangerous to assume that all parts of the steel shed are connected electrically to a low enough resistance. Also, the floor is concrete so the risk of a dangerous shock current is much higher than if the floor was wooden. These risk factors all push me in the direction of a TT earth in the shed. Anyway, once my cable is in place I'll get a couple of sparky's round to quote to complete the job.

[Edited on 15/11/2020 by craig1410]