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Charging two cars - replacing existing charge point

Adopado

Active Member
Aug 19, 2019
3,355
2,528
Scotland
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.

I think you are confusing two aspects of this discussion. The 2 charge points will be capable of running at 32 amps individually at different times but not both at the same time. They will load share to manage 32amp total at any one time if both are running simultaneously. As Glan explained the regulations require that the load is treated as if it is going to be just a simple added total ... even though this is not what the charge points will be drawing.
 
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Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
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UK
I think you are confusing two aspects of this discussion. The 2 charge points will be capable of running at 32 amps individually at different times but not both at the same time. They will load share to manage 32amp total at any one time if both are running simultaneously. As Glan explained the regulations require that the load is treated as if it is going to be just a simple added total ... even though this is not what the charge points will be drawing.

Spot on!

The key issue is that, when sizing the cabling, and hence sizing the over-current protection required, no account can be taken for the effective 50% diversity that a pair of TWCs provide (because no diversity is allowed to be applied to a car charge point installation). However, when determining the total load presented to the supply (different part of the regs) it's acceptable to make a common sense assessment, and take into account the inherent demand limiting provided by the TWC data link capability (or something like a priority board). There's no set formula for assessing the total demand, there's some guidance, but in the main it comes down to using the guidance and applying common sense. The same does not apply to cable and protection ratings, they should be rated for the maximum that could be carried.
 
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banned-66611

Guest
I think you are confusing two aspects of this discussion. The 2 charge points will be capable of running at 32 amps individually at different times but not both at the same time. They will load share to manage 32amp total at any one time if both are running simultaneously. As Glan explained the regulations require that the load is treated as if it is going to be just a simple added total ... even though this is not what the charge points will be drawing.

Yes, I understand that. But then you need a load balancing charger. I thought the goal was to save money and speed wasn't a big issue, so two 16A chargers would be cheaper and adequate.
 

Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
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UK
Yes, I understand that. But then you need a load balancing charger. I thought the goal was to save money and speed wasn't a big issue, so two 16A chargers would be cheaper and adequate.

The TWC is a load balancing charge point (no such thing as an AC "charger", they don't exist). It's pretty cheap, too, compared to the cost of other charge points, so a pair of them are probably the cheapest way to provide two 32 A capable charge points with demand limiting.
 

SpareHeadOne

Member
Oct 27, 2020
227
134
UK
Yes, I understand that. But then you need a load balancing charger. I thought the goal was to save money and speed wasn't a big issue, so two 16A chargers would be cheaper and adequate.

No, my original query was to allow charging of two vehicles without running another cable if possible - because where my cables run is very tight in places.

Re-using the existing cable would be ideal from a disruption point of view, but if it has to be upgraded to something slightly thicker then that would be OK too at a push.

I've asked here because installers will sometimes tell you something can't be done because they can't be bothered to do it, so I figured that I would get answers telling me what is possible allowing me to ask the right questions and weed out those installers.

OK, so I don't want to be forking out £10k for a solution as that feels disproportionate, but I just want to figure out my options at this stage so I can decide if I want to spend the money.

TWC Gen 3 (for example) looks interesting because if the possibility that it will be able to "talk" to my PowerWall in future and charge the car with excess solar during the summer. That option will look more attractive if I end up having to bite the bullet and put an additional cable in because I'll then swap the current Rolec for a second one when/if the Mrs gets her car.
 
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banned-66611

Guest
No, my original query was to allow charging of two vehicles without running another cable if possible - because where my cables run is very tight in places.

Re-using the existing cable would be ideal from a disruption point of view, but if it has to be upgraded to something slightly thicker then that would be OK too at a push.

I've asked here because installers will sometimes tell you something can't be done because they can't be bothered to do it, so I figured that I would get answers telling me what is possible allowing me to ask the right questions and weed out those installers.

OK, so I don't want to be forking out £10k for a solution as that feels disproportionate, but I just want to figure out my options at this stage so I can decide if I want to spend the money.

TWC Gen 3 (for example) looks interesting because if the possibility that it will be able to "talk" to my PowerWall in future and charge the car with excess solar during the summer. That option will look more attractive if I end up having to bite the bullet and put an additional cable in because I'll then swap the current Rolec for a second one when/if the Mrs gets her car.

Ah in that case have you looked at "commercial" units? Something like a Rolec SecuriCharge?

Rolec EV-SecuriCharge Double Charger 18th Edition 16A 3.6kW EVSC0050 | RS Electrical Supplies

Dual 16A sockets from a single 32A supply, no cable upgrades needed. Not smart or anything like that but you might prefer that, the simple ones tend to be more robust and reliable.

Chances are the cost of upgrading to use the TWC would take many, many, many years to pay off from solar savings, especially if you are on a good tariff like Octopus.
 

Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
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UK
Given that you already have an existing circuit, if you feel competent you could do part of the work yourself, perfectly legally. For example, replacing the existing cable (which is probably 6mm SWA) with 10mm SWA may be a DIY proposition, just need to make sure that the supply is dead, and proven to be so, before starting. There's a fair bit of labour in laying cable, as it's a bit time consuming, so there's money to be saved. TWCs are £460 each, so not that expensive as far as charge points go.
 

SpareHeadOne

Member
Oct 27, 2020
227
134
UK
Ah in that case have you looked at "commercial" units? Something like a Rolec SecuriCharge?

Rolec EV-SecuriCharge Double Charger 18th Edition 16A 3.6kW EVSC0050 | RS Electrical Supplies

Dual 16A sockets from a single 32A supply, no cable upgrades needed. Not smart or anything like that but you might prefer that, the simple ones tend to be more robust and reliable.

Chances are the cost of upgrading to use the TWC would take many, many, many years to pay off from solar savings, especially if you are on a good tariff like Octopus.

I hadn't looked at commercial, good point. I think I more or less assumed they would be stupidly expensive because they're commercial.

That looks like one for the shortlist, or something like that.
 
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banned-66611

Guest
I hadn't looked at commercial, good point. I think I more or less assumed they would be stupidly expensive because they're commercial.

That looks like one for the shortlist, or something like that.

It's because they are both very simple (no smart features, it's basically just an RCBO and simple control circuit in a box) and because businesses don't get so many subsidies so there has been more price pressure on them.

They do a 32A one as well (i.e. 2x 7kW chargers) but of course you would need to have suitable wiring and a 64A breaker, probably not a good idea in a domestic setting.
 

Adopado

Active Member
Aug 19, 2019
3,355
2,528
Scotland
Dual 16A sockets from a single 32A supply, no cable upgrades needed. Not smart or anything like that but you might prefer that, the simple ones tend to be more robust and reliable.

But then when charging only one car you remain on half the potential speed. This is a real issue if taking advantage of the short off-peak tariffs, such as Octopus Go, because 4hrs often wouldn't give you enough time to charge.
 
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banned-66611

Guest
But then when charging only one car you remain on half the potential speed. This is a real issue if taking advantage of the short off-peak tariffs, such as Octopus Go, because 4hrs often wouldn't give you enough time to charge.

Yes. But if it costs £10k to upgrade the electrics just to support 2x32A chargers, and you have issues with exceeding the 100A supply to your house, are you actually going to save any money vs. a couple of 16A chargers?

16A for 4 hours is something like 60 miles of range. Don't know what the OP does every day but 60 miles would cover most people's daily usage.
 

Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
2,695
UK
My experience of having to charge at 16 A for a time, when I first got my Model 3 and found it was ultrasensitive to the CP voltage swing (it needed a tighter tolerance than specified in IEC 61851, and so was unreliable charging from my charge points until they got fixed) was that it was a bit of a pain. Coming back from even a relatively short day out would often mean charging for two nights to get back to 90%. Even at 32 A the car didn't fully charge back to 90% in a single 7 hour overnight charge after our Christmas break this year, it needed about an hour on the second night to finish the charge and do the cell balancing stuff it tends to do during the taper phase at the end of some AC charges.

Running two 30 A / 32 A charge points with priority charging, either using TWCs or by just fitting a priority board (like this one from Garo: Garo 1 Row Shower Priority Metal Board | Express Electrical) allows the priority vehicle to charge at full power, with the second vehicle being able to charge at full power as soon as the priority vehicle has either finished charging, or isn't being charged.
 
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banned-66611

Guest
Sounds like a fault with the car if it doesn't meet IEC 61851, you should have asked Tesla to fix it.

A priority board would work but you have to remember that only one car will charge, so if you need one to be ready in the morning better put it on the right charger. Obviously it depends on your usage pattern which will be of more use to you.

Sounds like you are an unusually high consumption user though. 7 hours at 16A will net you around 100 miles range, so if a "relatively short day out" was covering more than that you either drive extremely inefficiently or have different ideas about what a short day out is.
 

SpareHeadOne

Member
Oct 27, 2020
227
134
UK
Yeah, I have a rough rule-of-thumb that I get around 9%/hour from my current 32A setup, about 60% in the cheap rate window at a push.

Fortunately it's pretty unusual for me to have to do distances of over 150 miles on consecutive days so I'm not expecting it to be a problem as I can charge over two nights. I guess that if I need more charge then I'll have to just take the hit on using peak rate - I should be able to fully charge the car in around 11-12 hours in the worst case.

A 16A charger could definitely complicate things in those situations, worst case I would need ~24 hours for a full charge or four consecutive nights; if I'm really stuck there are two Fastned sites quite near me that opened this year so I do have the additional option of paying their mad 39p/kWh to use the 50kW CCS too.
 

26ct2143

Member
Nov 22, 2020
180
73
Burton-on-Trent, UK
The 'normal' scenario (guessing here as I'm not you):
Both cars are low (30% ?), you plug both in with the aim of getting cheap rate electric.
So maybe they start charging at midnight, they will both want to charge flat out 32amp.
The supply is 32amp, so whether you have fancy (expensive) load balancing chargers or not, they will get 16amp each.
May as well save some money and just have 2x 16amp 'basic' chargers.

Or you upgrade the supply cable to 63amp, then just have 2x 32amp basic chargers.
But what is your total incoming supply rated to? some are 63amp, so 2 cars flat out plus house consumption, probably going to be a problem.

I'm fairly sure some 32amp chargers have switches inside to set them to 16amp. This could be helpful, so you set them to 16amp, but in the future you could swap 1 or both to 32amp if you upgrade the cable.
Meaning you have options for the future.
 

KennethS

Supporting Member
May 3, 2019
485
340
UK
As expected, there are some extremely knowledgeable posters on here and I am always learning from the wisdom of others. I had hoped that adding a second charge point inside my garage would be made easier following the installation of the first, but it seems that will not be the case. My existing Tesla WC is fed from a mini consumer unit in the garage that is cabled to the house consumer unit via a 40A breaker. The cabling from house to garage looks very robust, thick and shielded. I had hoped to be able to add another Tesla WC in the master/slave configuration and rely that used together they would never pull more than 32A. It sounds like the diversity rules will disregard this (and I suppose I can understand the concern). So will a priority board potentially solve this?
 

VanillaAir_UK

Supporting Member
Jun 17, 2019
7,312
4,815
Surrey, UK
The supply is 32amp, so whether you have fancy (expensive) load balancing chargers or not, they will get 16amp each.
May as well save some money and just have 2x 16amp 'basic' chargers.

The Tesla chargers works as master and multiple slaves. The master gets priority for the load, the slaves get the remainder. So you have the flexibility is charging full whack, then when it finishes the slaves pick up - this also has the benefit of handling any taper that may be operation. Or you could limit the charge rate on the master car, and slave get the remainder.

So you don't end up being locked into a limitation chosen prior to installation. Cost of flexibility, I suspect a couple of £££ (if that) depending on where you source the units from.
 

VanillaAir_UK

Supporting Member
Jun 17, 2019
7,312
4,815
Surrey, UK
It sounds like the diversity rules will disregard this (and I suppose I can understand the concern). So will a priority board potentially solve this?

I believe that instead of using diversity tables, you can also monitor the load over an extended period of time. This is certainly the case with whole house loads and the DNO fuse, but whether it could be applied to several circuits I am sure I will be enlightened.

The other solution is for the installer to limit each TWC to (say) 16A to keep within the 32A total load. Any adjustments made to the TWC settings after the installer has gone and true loads are known are then down to the user :rolleyes: Of course an aggregating MCB or equivalent would be needed to ensure that combined current keeps within expected/safe limits.
 

Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
2,695
UK
Sounds like a fault with the car if it doesn't meet IEC 61851, you should have asked Tesla to fix it.

If you read back through this forum you will see that it was indeed a failure to comply with IEC 61851, was reported to Tesla, and Tesla took many months to getting around to sorting it out (I reported the non-compliance in December 2019, it was finally resolved in June 2020). I wasn't alone in spotting this non-compliance, several charge point manufacturers got caught out by it (notably Ohme, IIRC) and to the best of my knowledge Tesla never even acknowledged there was an issue, even after they fixed it in June this year.

A priority board would work but you have to remember that only one car will charge, so if you need one to be ready in the morning better put it on the right charger. Obviously it depends on your usage pattern which will be of more use to you.

It's unlikely that two users in the same household would be driving hundreds of miles each per day, but if they do, then neither a 16 A charge point, a priority board solution, or a priority sensing charge point solution would meet their requirement, they would need a supply capable of running two 32 A charge points at the same time, in all probability, and that might mean upgrading to a 3 phase supply.

In our case, we can comfortably run two 32 A charge points, the hot water heating and the electric central heating overnight from a 23 kVA single phase supply. The heat pump draws a maximum current of ~9 A when running flat out to provide about 6 kW of heating (it rarely ever runs flat out, though). The hot water heating uses 2.8 kW, so draws ~12 A for maybe 2 to 3 hours overnight, if there's been no excess PV generation the day before. The residual loads (water treatment disinfection, MVHR system and sewage treatment plant air pump) draw about 250 W of background 24/7, so ~1.1 A. If it's one of the nights when the water system backwashes it's filtration then that will draw about 650 W for maybe 15 minutes, when the borehole pump is running, so ~2.8 A. That gives a grand total, with two cars charging at 32 A, and everything else that could be drawing power overnight running, of about 89 A, comfortably within the 100 A rating of the main fuse.

If push comes to shove, then, as @VanillaAir_UK mentions above, any installer that's concerned about possibly exceeding the maximum allowable demand can just fit a peak reading clamp meter to the tails and monitor the house, before fitting the charge point(s) anyway. That's an acceptable way of determining the headroom available within the installation, and only need take two or three days of monitoring to satisfy the requirement.

Sounds like you are an unusually high consumption user though. 7 hours at 16A will net you around 100 miles range, so if a "relatively short day out" was covering more than that you either drive extremely inefficiently or have different ideas about what a short day out is.

We're keen NT members, so often visit NT properties an hour or twos drive away, and never bother to charge either on route or at our day trip destination (mainly because few NT places yet have charge points). It's not at all unusual to drive 100 to 150 miles on a day out, and in this cold weather that tends to eat up a fair bit of battery capacity. Getting close to 300 Wh/mile seems a struggle in this cold weather, although 250 Wh/mile seems achievable in warmer weather. Right now, the car's only managing around 340 Wh/mile, which doesn't seem untypical for the time of year, and means a 100 mile journey would use around 34 kWh. At only a 16 A charge rate, that means over 9 hours charging, longer than the 7 hour Economy 7 off-peak period, and more than double the Octopus Go 4 hour off-peak period.
 

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