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Charging two cars - replacing existing charge point

Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
2,695
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?

Yes, a priority board will fix this. Cost is around £125, plus installation.
 
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pp61

Active Member
Jan 19, 2016
1,357
60
Switzerland
I did this in the past and agree not difficult. Of course you need to know how to work with electricity but to have the two TWCs talking with each other is simple. Connect the two wires and make one the master via the dip switch settings.
 
B

banned-66611

Guest
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.

But cheaper than the upgrades needed to run 2x 32A chargers I imagine. You seem to have an exceptional electrical system, averaging 89A would cause regular tripping for most people due to peaking. But you have expensive gear like multiple heat pumps and a water treatment system that most don't.

I'd be very hesitant to recommend anyone try to do that without very careful planning. Your idea to use a priority board is much better, if they can live with the limitations.
 

Adopado

Active Member
Aug 19, 2019
3,341
2,514
Scotland
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?

I'm not suggesting upgrading to be able to run 2x32amp chargers simultaneously. I'm just saying it would be preferable to have 2 chargers that either could run at 32amp (if only one operating at a time) or at 16amp (both simultaneously via load balancing). There are several models that offer that capability so no reason to restrict both to 16amps all the time.
 
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SpareHeadOne

Member
Oct 27, 2020
227
134
UK
Without two full EVs at the moment it's difficult to say for sure, but I don't think we'll need the capability to simultaneously charge two cars at 32A, I would expect only one car to get used in long drive situations.

But you're right that having one car use the full 32A if it's the only one charging is probably a better solution, I expect that 90% of the time we'd just stagger the charge sessions overnight.

I guess that part of my problem is I don't yet know the usage pattern or the second vehicle, so I'm definitely leaning towards a solution that would provide more flexibility, even if it's not the cheapest.
 

CMc1

supercharge.info editor
Aug 2, 2019
1,422
1,273
North, UK
We have two EVs and only one charge point. Looking to add commando to use with Ohme.

Both us are are high mileage drivers. We don’t necessarily need 2nd point and can rotate cars between the charge point on alternative days.

Main perk with second charge point would be that both could charge at Agile plunges/low prices and not have faff to swap cars around etc this keeping our pence per mile low and carbon output.

Plus... our future cars will be EVs... family’s cars will end up as EVs so benefits that too.
 

rotor2k

Member
Sep 16, 2019
478
251
London
Lots of good input above, but I would whole-heartedly say that 2 x 16A EVSEs* is a terrible idea. If you have to upgrade the cable then so be it, it sounds like you can do that reasonably easily. However, worst case just stick with the single 32A EVSE and swap the car locations (or get a really long cable). That would *still* be better than 2 x 16A EVSEs, in my humble opinion.

*Sorry for the pedantic use of the acronym EVSE (Electric Vehicle Supply Equipment), but there seem to be a few members who get upset if one uses the word "charger" (a very common-use word that everyone understands, but hey-ho).
 
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miimura

Well-Known Member
Aug 21, 2013
6,155
5,748
Los Altos, CA
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?
For this specific situation with a mini consumer unit in the garage, I would have the second Tesla WC fitted with both set to 16A (or 20A if available and allowed) at installation time with the communication wire in place for load sharing. If at some point in the future you need 32A charging, change the configuration to Master/Slave @ 32A. In the States this is the proper and permitted method. I understand that the non-concurrent EV charging load exception is not specifically allowed in UK code, but the Tesla power sharing will not overload the mini consumer unit or its feed line, so it will operate safely. Obviously, this is an "at your own risk" kind of suggestion.
 
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Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
2,695
UK
Well worth borrowing a peak recording clamp meter, fitting around one of the meter tails and seeing how much power the house actually uses, ideally reading it and resetting it at the end of the day, after all meals have been cooked, and again first thing in the morning, before breakfast has been cooked (for those that have a cooked breakfast).

My guess is that many people will be surprised at just how much headroom they have available. I lived in a house with a 40 A main fuse for a few years (it was on a looped supply feeding a terrace of three houses) and didn't come close to ever using 40 A.. Also worth noting that it is perfectly safe to draw up to the maximum rating of the supply indefinitely, if push comes to shove. It's not exactly recommended, as the DNO applies diversity to the LV network, using the assumption that no house will always draw the full rated current, but it is safe. A 100 main fuse will take several hours to rupture if run at 150 A, and the incoming supply cable is rated for 158 A usually (if it's the very common 35mm² aluminium core, copper sheath, concentric cable). Even at 200 A it will take ten minutes of so to blow a 100 A BS1361 main fuse. I'm not suggesting anyone does this, just pointing out that there is a pretty hefty safe margin built in to the distribution network.


Lots of good input above, but I would whole-heartedly say that 2 x 16A EVSEs* is a terrible idea. If you have to upgrade the cable then so be it, it sounds like you can do that reasonably easily. However, worst case just stick with the single 32A EVSE and swap the car locations (or get a really long cable). That would *still* be better than 2 x 16A EVSEs, in my humble opinion.

*Sorry for the pedantic use of the acronym EVSE (Electric Vehicle Supply Equipment), but there seem to be a few members who get upset if one uses the word "charger" (a very common-use word that everyone understands, but hey-ho).

I agree wholeheartedly, charging at just 16 A is a PITA at the best of times, and as most of the cost is in the labour involved, it seems daft not to provide for 32 A charge points - the cost is likely to be under £20 extra, for a worst case. FWIW, I agree with using EVSE, or charge point, primarily because it removes confusion. A discussion that uses "charger", when referring to AC charging, refers to the units fitted under the rear seat of the Model 3, that are the actual battery chargers that deliver regulated DC to the battery pack, in much the same way as a phone charger delivers regulated DC to a phone battery. An EVSE/charge point is just a power outlet, calling it a charger is like calling an extension lead running a phone charger a charger, or calling a kettle lead a charger, it makes little sense.
 
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Glan gluaisne

Supporting Member
Sep 11, 2019
2,782
2,695
UK
Without two full EVs at the moment it's difficult to say for sure, but I don't think we'll need the capability to simultaneously charge two cars at 32A, I would expect only one car to get used in long drive situations.

But you're right that having one car use the full 32A if it's the only one charging is probably a better solution, I expect that 90% of the time we'd just stagger the charge sessions overnight.

I guess that part of my problem is I don't yet know the usage pattern or the second vehicle, so I'm definitely leaning towards a solution that would provide more flexibility, even if it's not the cheapest.

One option might be to fit a time switch, with a manual over-ride, to the end of the existing cable, together with a changeover contactor to switch power to one or other of the charge points. Not expensive to do, and would allow a second charge point to be daisy chained from the time switch box, using the same cable size (usually 6mm²). The existing cable would be fine to supply power to both with this arrangement.

The time switch, over-ride switch and contactor would need to be in a weatherproof enclosure, but that could be fairly small and neat. That would give either a manual option to use one charge point or the other (but never both together), or a time option, where one charge point was activated one evening, the other the alternate evening, or whatever best suited your charging needs. Easy and fairly cheap to implement, and would retain the option to charge at 32 A from either charge point.
 

gnomen

Member
Mar 22, 2021
5
2
UK
Oh boy! I have just read through this entire thread and my head is reeling. We have just purchased our second electric car, a Tesla Model 3, and we want to have two charge points rather than constant swapping. Is it really necessary to have a PhD in electrical circuits to work it out?

So far, the quotes for the second installation have varied between £1,000 and £2,000, some with and some without the second EVSE grant applied (since the Tesla Wall Box does not qualify for the grant). That's a lot more than our entire PodPoint installation cost 3 years ago.

And there are at least 3 alternative solutions:
  1. A second EVSE with a switch between them to determine which one is active for the next charging session
  2. 2x Tesla Wall Boxes, set up for load balancing in the event both are in use. Simple and elegant, but no grant, so becomes quite expensive
  3. Independent second installation from the meter box, with some kind of load monitoring to ensure the two EVSEs do not draw too much current
I'm no expert in electronics. So have no idea which is the best, safest route. Why insn't the answer simple? If the UK government have their way, most households will eventually have two EVs and need two EVSEs. Seems like early adopters have to sort out the complexities for everyone else.
 

Mrklaw

Member
Mar 5, 2020
228
81
Berkshire
A switch like option 1 - why not just not bother and share the cable?

tesla WC if it does proper master/slave (including with non Tesla cars) that sounds like a good option. assuming the master is a particular cable that still lets you favour car A or car B depending on need and they’ll get 32A while the other waits it’s turn

if both have done long miles over multiple days so you can’t catch yourself up just pop to a nearby fast charger - and think how often it’d be (likely not at all and not worth the fuss/cost of a three phase upgrade etc) - you may be solving for edge cases that won’t be common
 

vardhanr

Member
Nov 16, 2020
5
1
United Kingdom
I have a similar problem, but want to install Tesla Wall Charger in garage that is 60m away from the house. I already have a BP Chargemaster installed (using grant) on the side of the house.

The garage has supply (currently for lights and sockets) but the cable seems to be the same size if not thicker than the one put in by BP for the Chargemaster. I have a 100A supply. Also only have 1 EV at present.

I have had 2 guys look at the problem and one has said we need to speak to the Electricity supplier and check whether there is a loop with my neighbours. The other guy wanted to know about load balancing capabilities of the 2 chargers and since they don't talk to each other says that it cannot be done. Both are on various lists of approved EV charging installers.

Anyone have any suggestions?
Thanks very much
 

Dilly

Active Member
Feb 24, 2020
1,555
1,158
Norfolk
I have two Zappi’s; one in the garage and one outside on the house wall. One for the M3 and the for the Outlander PHEV.
I have a 100amp master fuse.
the Zappi has a host of settings including grid limit and device limit. I assume that for safety, other chargers will have the same settings.
device limit is used when the supply cabling to the charger or it‘s power supply is below the maximum that the charger could supply (under 7Kw). Grid limit will keep the charge under the main fuse limit taking current draw into account. Two chargers don’t need to talk to one another, just need to monitor grid draw.
 

EzekielJM

New Member
Aug 4, 2020
4
3
UK
I have two Zappi’s; one in the garage and one outside on the house wall. One for the M3 and the for the Outlander PHEV.
I have a 100amp master fuse.
the Zappi has a host of settings including grid limit and device limit. I assume that for safety, other chargers will have the same settings.
device limit is used when the supply cabling to the charger or it‘s power supply is below the maximum that the charger could supply (under 7Kw). Grid limit will keep the charge under the main fuse limit taking current draw into account. Two chargers don’t need to talk to one another, just need to monitor grid draw.
While the Zappis have this other chargers often (mostly) don't. I would not assume they can monitor grid draw and limits unless they specify it. that being said i have 1 zappi and when we get our second EV ill be putting a second one to balance etc etc.
 

GRiLLA

Member
Jul 5, 2020
461
446
UK
I have two Zappi’s; one in the garage and one outside on the house wall. One for the M3 and the for the Outlander PHEV.
I have a 100amp master fuse.
the Zappi has a host of settings including grid limit and device limit. I assume that for safety, other chargers will have the same settings.
device limit is used when the supply cabling to the charger or it‘s power supply is below the maximum that the charger could supply (under 7Kw). Grid limit will keep the charge under the main fuse limit taking current draw into account. Two chargers don’t need to talk to one another, just need to monitor grid draw.
To add, this is my plan for adding a second charger. Current charger is a 1st Gen EO Mini Pro, but I'll get a Zappi as the second. The Zappi can monitor the incoming feed and adjust itself to ensure that we stay under 100A. Sure the EO will take precedence as it isn't adjusting, but that's no big deal.
The 1st Gen EO Mini Pro does have a kind of software balancing where a pair of chargers will stay at 16A until they can check with the JuiceNet servers for clearance to a higher figure, but given the device is no longer sold I'm not going down that route.
 

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