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Charging two cars - replacing existing charge point

SpareHeadOne

Member
Oct 27, 2020
227
134
UK
No. Each wall connector needs to be on its own dedicated circuit.

Drat.

That scuppers that plan, I'm hoping to avoid running another cable because if I do that then I may as will just get any old charge point installed and leave the existing one in place.

Now I've done some more digging, the C-TEK Chargestorm Connected may be suitable, according to EV warehouse the model they sell takes a 16-32A single phase feed to provide two charge ports, so on the face of it my existing cabling may be OK.

I just wish it wasn't an eye-watering £2k!
 

Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
2,695
UK
Actually, I don't believe there's any reason why you couldn't run two charge points from the same circuit, as far as the regs are concerned. The supply cable would need to be increased to 10mm² (assuming the cable doesn't run inside insulation), both charge points would need to be set to below 32 A (say, 30 A) and the over-current, RCD and open PEN protection for the cable would need to be increased to 63 A. An external junction box would be needed, as the terminations inside the TWC won't take two 10mm² cables (but will just take a single 10mm² cable I believe).

The arrangement would just be a 10mm² radial running from the main incoming supply to an external wiska box, where it splits out to be two 10mm² runs, one to each charge point. The protection at the feed end of that cable would be as normal, an open PEN device, and either a 63 A MCB and 63 A Type B RCD, or the equivalent in one of the all-in-one connection boxes (not sure if they do these with a 63 A rating, though).
 

phil4

Member
Sep 8, 2020
291
160
UK
Actually, I don't believe there's any reason why you couldn't run two charge points from the same circuit, as far as the regs are concerned. The supply cable would need to be increased to 10mm² (assuming the cable doesn't run inside insulation), both charge points would need to be set to below 32 A (say, 30 A) and the over-current, RCD and open PEN protection for the cable would need to be increased to 63 A. An external junction box would be needed, as the terminations inside the TWC won't take two 10mm² cables (but will just take a single 10mm² cable I believe).

The arrangement would just be a 10mm² radial running from the main incoming supply to an external wiska box, where it splits out to be two 10mm² runs, one to each charge point. The protection at the feed end of that cable would be as normal, an open PEN device, and either a 63 A MCB and 63 A Type B RCD, or the equivalent in one of the all-in-one connection boxes (not sure if they do these with a 63 A rating, though).

On the basis the TWCs would share the 32A, why would you need a 64A MCB,RCD etc?
 

Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
2,695
UK
On the basis the TWCs would share the 32A, why would you need a 64A MCB,RCD etc?

A bit of code running on a device doesn't meet the required proof that both could not be pulling the full rated current at the same time. The regs have to use the rated current for the equipment, not something set in a bit of software/firmware. The installer can only go on the rating label, not a setting enabled within the device firmware, when determining the safe size of cable and the over-current protection needed for that cable. The same goes for things like cookers and hobs, but an exception is specifically made for those, in that diversity can be applied. Diversity cannot be applied to a charge point at the moment, although with the advent of variable rate charge points that may change at some future date.
 

phil4

Member
Sep 8, 2020
291
160
UK
A bit of code running on a device doesn't meet the required proof that both could not be pulling the full rated current at the same time. The regs have to use the rated current for the equipment, not something set in a bit of software/firmware. The installer can only go on the rating label, not a setting enabled within the device firmware, when determining the safe size of cable and the over-current protection needed for that cable.

You are absolutely right about regs... My point is though, surely as you don't want them pulling more than 32Amps, and indeed if they malfunctioned and did that, you'd want the fuse to blow, why wouldn't you put a 32Amp breaker in? And how is putting a lower rated breaker in not acceptable, since with it in it can't draw more than 32Amp?

I am, without a shadow of a doubt being naïve, stupid and have it wrong. But in my little world (my head) I'd love to understand.
 

Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
2,695
UK
You are absolutely right about regs... My point is though, surely as you don't want them pulling more than 32Amps, and indeed if they malfunctioned and did that, you'd want the fuse to blow, why wouldn't you put a 32Amp breaker in? And how is putting a lower rated breaker in not acceptable, since with it in it can't draw more than 32Amp?

I am, without a shadow of a doubt being naïve, stupid and have it wrong. But in my little world (my head) I'd love to understand.

The regs also require that installations be designed such that nuisance tripping of over-current or earth leakage protection devices be minimised. Fitting a protection device rated at about half the maximum current of the installed equipment doesn't really meet that requirement. It would be safe enough for a time, but nuisance tripping isn't just a nuisance, it also causes wear and tear on the protection device. One fairly common fault found is an MCB that has tripped and been reset a few times, and ends up refusing to reset due to mechanical wear - they aren't designed like switches, for lots of use.


That's an older version of the regs. The current version states this (the note at the end clarifies multiple charge points):

722.533 Devices for protection against overcurrent

722.533.101 Each charging point shall be supplied individually by a final circuit protected by an overcurrent protective device complying with BS EN 60947-2, BS EN 60947-6-2 or BS EN 61009-1 or with the relevant parts of the BS EN 60898 series or the BS EN 60269 series.

NOTE: The electric vehicle charging equipment may have multiple charging points.

In this case, as I read it, the final circuit is the cable running from the supply to the junction box where multiple charge points could be connected.
 

phil4

Member
Sep 8, 2020
291
160
UK
The regs also require that installations be designed such that nuisance tripping of over-current or earth leakage protection devices be minimised. Fitting a protection device rated at about half the maximum current of the installed equipment doesn't really meet that requirement. It would be safe enough for a time, but nuisance tripping isn't just a nuisance, it also causes wear and tear on the protection device. One fairly common fault found is an MCB that has tripped and been reset a few times, and ends up refusing to reset due to mechanical wear - they aren't designed like switches, for lots of use.

Thanks, that makes perfect sense. So in that regard, does that also mean you need to be running back suitable wiring for 64Amps all the way to the main consumer unit, and popping a 64A in there too?

Where does that leave the 100Amp main fuse? If I tot up all my fuses, they'll be well over 100Amp.
 
B

banned-66611

Guest
Actually, I don't believe there's any reason why you couldn't run two charge points from the same circuit, as far as the regs are concerned. The supply cable would need to be increased to 10mm² (assuming the cable doesn't run inside insulation), both charge points would need to be set to below 32 A (say, 30 A) and the over-current, RCD and open PEN protection for the cable would need to be increased to 63 A. An external junction box would be needed, as the terminations inside the TWC won't take two 10mm² cables (but will just take a single 10mm² cable I believe).

63A just for chargers at a residential property is no trivial matter. Depending on what else they have in the house it might not be possible to supply that without very expensive upgrades, in fact the odds are against it.
 
B

banned-66611

Guest
Thanks, that makes perfect sense. So in that regard, does that also mean you need to be running back suitable wiring for 64Amps all the way to the main consumer unit, and popping a 64A in there too?

Where does that leave the 100Amp main fuse? If I tot up all my fuses, they'll be well over 100Amp.

Exactly. You would be better off with 2x 16A chargers if you are worried about cost, otherwise you will need an expensive dual charger or serious upgrades to your house's electrical supply.
 

Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
2,695
UK
Thanks, that makes perfect sense. So in that regard, does that also mean you need to be running back suitable wiring for 64Amps all the way to the main consumer unit, and popping a 64A in there too?

Where does that leave the 100Amp main fuse? If I tot up all my fuses, they'll be well over 100Amp.

Installations almost always have a total maximum load that may seem to exceed the supply rating, if all the fuse/over-current device ratings are added up. In practice, there are rules of thumb used to estimate the likely maximum load, using a principle called diversity. Diversity assumes that not all loads will be on at the same time, or if they are on they will not be drawing their full rated current. A set of commonly used diversity rules are given in the IET On-Site Guide, from which I've cut and pasted the table below (assuming fair use applies and the IET won't get cross . . .):

OSG Diversity.jpg


Although charge points aren't yet mentioned in this 2018 table, it has been confirmed that no diversity is allowable for them - like water heaters and electric heating, the assumption is that they will operate at full current all the time they are active.

Finally, the main purpose of a fuse or other over-current protection device is just to protect the cable it supplies, not the equipment at the end of that cable. Over-current devices have to be sized to protect the size of the cable, as de-rated for its particular installation conditions. This means, for example, that a single run of 2.5mm² T& E, run as a radial, a cable that has a free air current rating of about 27 A, may need to be protected by a 16 A MCB if it is run under 200mm of loft insulation, as it will be derated by a factor of 0.63 because of the insulation.
 
  • Informative
Reactions: Bistoptom and phil4
B

banned-66611

Guest
Installations almost always have a total maximum load that may seem to exceed the supply rating, if all the fuse/over-current device ratings are added up. In practice, there are rules of thumb used to estimate the likely maximum load, using a principle called diversity. Diversity assumes that not all loads will be on at the same time, or if they are on they will not be drawing their full rated current. A set of commonly used diversity rules are given in the IET On-Site Guide, from which I've cut and pasted the table below (assuming fair use applies and the IET won't get cross . . .):

View attachment 622389

Although charge points aren't yet mentioned in this 2018 table, it has been confirmed that no diversity is allowable for them - like water heaters and electric heating, the assumption is that they will operate at full current all the time they are active.

Finally, the main purpose of a fuse or other over-current protection device is just to protect the cable it supplies, not the equipment at the end of that cable. Over-current devices have to be sized to protect the size of the cable, as de-rated for its particular installation conditions. This means, for example, that a single run of 2.5mm² T& E, run as a radial, a cable that has a free air current rating of about 27 A, may need to be protected by a 16 A MCB if it is run under 200mm of loft insulation, as it will be derated by a factor of 0.63 because of the insulation.

Okay, but as I already pointed out this is missing the critical point that if they have 2x 32A chargers with no derating then they have at best about 33A left over for everything else. And as phil4 says it all adds up to well over 100A, and homes in the UK have 100A fuses and you need expensive upgrades which might not even be granted to get more.

This just isn't going to work.
 

Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
2,695
UK
Our installation is a really good example of some crazy looking over-current protection device ratings, all in the meter cabinet. It sort of illustrates how things add up to give some very high numbers, if the fact that those over-current devices are only protecting their cables is ignored.

We have a 100 A incoming main fuse, with the 25mm² tails from the meter going via a 100 A DP isolator switch to a pair of Henley blocks. From there, another set of 25mm² tails goes to a fused DP switch, with an 80 A fuse, that supplies a ~15m run of 25mm SWA to the house consumer unit. Another set of 25mm² tails runs to a 4 way consumer unit that just contains two 63 A DP MCBs. One of those feeds a 10mm² run of SWA to our detached garage, the other feeds a run of 10mm² SWA to an outdoor distribution board that contains half a dozen outdoor circuits, like our water supply and treatment systems, outdoor power outlets and lights and one of the charge points.

Adding that lot up, and ignoring all the downstream protection devices, the apparent load on that 100 A supply could be 80 A + 63 A + 63 A = 206 A. In reality, the maximum demand rarely exceeds about 50 A, as many of the loads are only on for a short time, and not at the same time as other loads.
 

Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
2,695
UK
Okay, but as I already pointed out this is missing the critical point that if they have 2x 32A chargers with no derating then they have at best about 33A left over for everything else. And as phil4 says it all adds up to well over 100A, and homes in the UK have 100A fuses and you need expensive upgrades which might not even be granted to get more.

This just isn't going to work.

As already mentioned, a priority box fixes this problem, but anyway it doesn't really exist with the TWC, as they can be set to load share, as they effectively have a priority box built in. This doesn't mean that the cable can be derated - the regs have yet to allow for diversity for car charge points, even if they do have priority load management.
 

HenryT

Member
Jan 29, 2020
486
382
Manchester
Drat.

That scuppers that plan, I'm hoping to avoid running another cable because if I do that then I may as will just get any old charge point installed and leave the existing one in place.

Now I've done some more digging, the C-TEK Chargestorm Connected may be suitable, according to EV warehouse the model they sell takes a 16-32A single phase feed to provide two charge ports, so on the face of it my existing cabling may be OK.

I just wish it wasn't an eye-watering £2k!


I think that is the same one I was pointing you to in post #12 above.
 
B

banned-66611

Guest
Adding that lot up, and ignoring all the downstream protection devices, the apparent load on that 100 A supply could be 80 A + 63 A + 63 A = 206 A. In reality, the maximum demand rarely exceeds about 50 A, as many of the loads are only on for a short time, and not at the same time as other loads.

That seems very optimistic, if two cars get plugged in at the same time it is immediately over 60A alone. The idea of not turning everything on at once only really works for short term stuff showers and kitchen appliances, not car chargers that run for hours on end.

Your priority box idea isn't great either. They are designed for things like showers that may peak at much higher currents but then fall back to low levels, say 3kW. Again, because car chargers are running for hours on end and because short temporary interruptions can cause some vehicles to decide there is a fault and not to charge I wouldn't want to rely on that.
 

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