[KJA2001]

My garage was built in 2010 and I lacked foresight. It is supplied by a 20m long 4mm three core SWA cable, buried in ground. I have run two EV charge points for the last few years but both current cars draw only ~3.3 kW. I am about to swap one car to something that will draw 7kW so I now need ~48A which exceeds my hardware capabilities. I would prefer to future proof to allow two 7kW charge points at a total of 64A. The original cable route is no longer available (expensive paving) but I have the option of a new 24m route. Rather than bin the existing cable and install an expensive and difficult to handle 10mm or 16mm SWA I thought I might run a new 6mm SWA connected in parallel with the old 4mm one. That would give me a greater current capacity than a single 10mm cable. I am aware of guidance regarding matched cable types and lengths in parallel power cable setups and issues with unmatched cable currents, reactance issues if cables are not in matched electromagnetic environments etc but most of this pertains to three phase and very high current situations. Does any of this matter for my proposed setup?

 

[freddym]

Is this for real?

Do you have any electrical background?

Who is designing the circuits and their protection?

 

[mikegs]

Rather than bin the existing cable and install an expensive and difficult to handle 10mm or 16mm SWA I thought I might run a new 6mm SWA

24m of 3-core 6mm is about £70. 10mm is about £112 and 16mm is about £160.
Is £40/£90 really worth saving given the total cost and effort of the project and the bodge you'll end up with?

Also you may only need 2-core (cheaper) as at that distance a TT island may be better anyway.

 

[Danny8oy]

I think you would be better off running one charge point with one feed and the other for the new one on separate circuits there is no way any competent spark would wire 2 runs into one like that.

 

[dk6780]

As above. Convert the current cable to run everything else in the garage apart from the EV charging and run a new cable to carry 70 amps which ought to be 10mm at that length.
Displaying ignorance, but how is the OPs suggestion different from a ring main with a spur off of it?

 

[Danny8oy]

As above. Convert the current cable to run everything else in the garage apart from the EV charging and run a new cable to carry 70 amps which ought to be 10mm at that length.
Displaying ignorance, but how is the OPs suggestion different from a ring main with a spur off of it?

Im no expert but the danger is if one run was damaged and had a large rcd on both potentially one run could overload without tripping. At least that’s how I read it.

 

[tom66]

What happens if one run fails? 64A on 4mm. Your expensive paving might not look so important when you have a fire in your consumer unit.

Leave the old 4mm circuit, put 10mm in on the long run, and disconnect the 4mm. Or configure the 4mm to just provide sockets or lighting for the garage and run it as an independent circuit.

 

[KJA2001]

I think you would be better off running one charge point with one feed and the other for the new one on separate circuits

Why didn't I think of that?

if one run was damaged and had a large rcd on both potentially one run could overload without tripping.

What happens if one run fails? 64A on 4mm. Your expensive paving might not look so important when you have a fire in your consumer unit.

Good point. Thanks chaps.

 

[Hilltop Dave]

Future proof - install a new 10mm^2 cable on your long run, which could supply two 32A charging points at a pinch, and keep the existing 4mm^2 cable for the garage supply and one (16A?) charging point. Any more than this and you will need the supply to your house upgrading (unless you already have a 3-phase supply, in which case make the new 10mm^2 cable 4-core to be able to take 3-phase power to the garage)

 

[A_Camera]

Future proof - install a new 10mm^2 cable on your long run, which could supply two 32A charging points at a pinch, and keep the existing 4mm^2 cable for the garage supply and one (16A?) charging point. Any more than this and you will need the supply to your house upgrading (unless you already have a 3-phase supply, in which case make the new 10mm^2 cable 4-core to be able to take 3-phase power to the garage)

You should NOT supply 2x32A charging points with a single pair of 10mm^2 cable (Line + Neutral + PE). That's not enough for more than maximum 40A. Minimum 25mm^2 is recommended for single phase.

 

[ColA]

It is common in large current three phase installations to use parallel cables connected to a single distribution device but these would be the same specification and run adjacent. It just makes the cabling more manageable. Once you have different cable sizes and types then each one has to be protected in accordance with its current carrying capability which depends upon size, type and installation method.

 

[Weebull]

As I understand it, even ignoring the point about one failing that others have made (which is a very good point), the current would take the easiest path between the two points. That might not be easy to determine, especially with cable runs which are taking different paths, and so are different lengths. You could end up in a situation where you think you have 48A worth of capacity, but it's all being forced down the old connection because it's the most direct. It wouldn't be until the cable heated up, and became higher resistance that the current would change how it's flowing, but the cable heating up is what we're trying to avoid.

 

[Hilltop Dave]

For a domestic ring main, the circuit breaker must not be rated at more than 1.5 times the cable rating (but this allows for a couple of heavy loads connected to sockets near to one end of the ring).

 

[HandyAndy]

If you do run a new cable in, probably worth running a cat-6 (1 gigabit) or cat-6a (if you think you'll ever need 10 gigabit!) or whatever's the recommended ethernet cable these days, to support future smart whatevers, security cameras, that you may want later. Might become an office one day, you never know...

 

[A_Camera]

As I understand it, even ignoring the point about one failing that others have made (which is a very good point), the current would take the easiest path between the two points.

Not really true. Ohms law is what should be applied, the current will be proportionally divided between the two cables, so if everything is fine then in theory, parallel cables are OK. The problem is safety, if one cable fails all current will go through the other cable up to the limit of that cable, at which the cable will melt if the current is too high, and might even cause fire. Maybe legally it's OK to use parallel cables in UK, but it is definitely illegal as a DIY job, a qualified electrician can explain local or national regulations, but the least thing to do safety wise is to have separate fuses, one for each cable, to make sure that if one cable fails, the other fuse breaks the circuit even for the other cable. I am sure that at least here in Sweden, I could not get any serious and qualified electrician to do such job though, I think that if nothing else, common sense would stop them from doing it. Personally I would not even suggest such a solution or ask them about this type of work.

 

[A_Camera]

For a domestic ring main, the circuit breaker must not be rated at more than 1.5 times the cable rating (but this allows for a couple of heavy loads connected to sockets near to one end of the ring).

This is not about ring mains wiring. The OP planned for parallel use of a 6mm^2 and a 4mm^2 wire to get 10mm^2 so he could load the circuit with what 10mm^2 allows. That's NOT ring wiring. It would be a ring if he would have used the 6mm^2 extra wire as redundancy for the 4mm^2 and would NEVER load the circuit with more than what is safe for the 4mm^2.

 

[A_Camera]

If you do run a new cable in, probably worth running a cat-6 (1 gigabit) or cat-6a (if you think you'll ever need 10 gigabit!) or whatever's the recommended ethernet cable these days, to support future smart whatevers, security cameras, that you may want later. Might become an office one day, you never know...

It's a good advice, but ONLY if run it through separate conduits (pipes). Anyway, future proving through an extra unused pipe is also a good idea, but there are rules about how to mark and separate different type pipes and cables, and which sort of piper are for wich sort of use and so on. Those are for safety reasons, not because some bureaucrat invented them.

 

[Hilltop Dave]

Future proof - install a new 10mm^2 cable on your long run, which could supply two 32A charging points at a pinch,

You should NOT supply 2x32A charging points with a single pair of 10mm^2 cable (Line + Neutral + PE). That's not enough for more than maximum 40A.

I was surprised when I looked at my old 16th Edition (1991) regulations [the most recent that I have] that for multicore armoured pvc insulated cables clipped direct (table 4D4A) at an ambient temperature of 30 deg C, for a single, on its own, 2-core cable the rating for 10mm^2 is 67A. For Twin+Earth (table 4D2A) the clipped direct rating is given as 63A and the enclosed in conduit or trunking rating (for a single cable) is given as 52A. (It is the buried in an insulating wall rating which is only 43A)

Even considering the voltage drop on a 24m run, allowing a voltage drop of 6V gives a maximum load of 56A at the quoted 4.4mV/A/m.

 

[Kingpleb1]

If you do run a new cable in, probably worth running a cat-6 (1 gigabit) or cat-6a (if you think you'll ever need 10 gigabit!) or whatever's the recommended ethernet cable these days, to support future smart whatevers, security cameras, that you may want later. Might become an office one day, you never know...

This but use the shielded version to avoid interference over the run being so close to Alternating power.

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Also make sure what ever ducting it is in has clear entry and exit points and is decently wide so can take future stuff.

 

[stevelup]

Shielded network cable is almost universally a bad idea. Unless correctly terminated and you use shielded and grounded patch panels, it can actually include more EMI into the cable than without, and there are then issues with earth loops etc - especially in this scenario where we're likely talking about two buildings with separate earthing systems.

It's fine to run normal (unshielded) ethernet cable alongside SWA.

Or, if you want to go belt and braces, stick a fibre in. It's cheap as chips these days, completely isolated and won't blow up everything in your house if there's a nearby lighting strike