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Wall Connector/Charger Daisy Chaining on Separate Breakers

captjoemcd

Member
Nov 29, 2019
258
288
California
Hey all!

I'm working through the plans for adding two wall connectors to my garage for our soon to be delivered X and upcoming S and have run into what seems to be an obvious solution except for perhaps limitation of the wall connector setup.

The goal is obviously max charging speed within code (ie "official" safety limits)

I'll be adding a new flush-mounted subpanel in the garage specifically for these two chargers, using 1/0, 1/0, 1/0, 2 aluminum SER through the attic and a 100A breaker in the main panel, allowing for continuous 80A. It'll go through some insulation in parts, hence the need to derate to 60C column on NEC Table 310.15(B)(16).

The wall charger instructions suggest/specify a single 100A breaker, which require #3 AWG copper, which then goes to a splice box, and then (I believe) #3 AWG copper to each of the wall chargers. I haven't seen the #3 from splice box to charger specifically called out anywhere, but it seems logical.

My plan had been to run conduit from the subpanel to each of the two wall connectors with #6 AWG copper, each fed from a 60A breaker, supplying 48A of charging each, which I believe is the max the S and X will be able to draw. The wall chargers would be set to 60A internally. The two chargers would need to use the daisy-chain communication to ensure that no more than 80A is drawn at any time across the two devices (otherwise they would each pull 48A. The benefits would be the ability to turn them off individually at the panel, much smaller/easier to install wire, and no bulky junction box, enabling flush mounting the chargers on the wall, which I much prefer.

The wrinkle I came across reading through the forums is that I think the master wall connector would need to be set to 100A. Otherwise it would think it has only 60A to distribute across the two chargers.

2 questions - is the 100A setting for the master correct? and does a single wall charger ever pull more than 60A?

From what I can tell, the answers are Yes, 100A is the correct setting, and No a single wall charger doesn't ever pull more than 60A. Which unfortunately means that even though its overkill, the whole system needs to be wired for 100A and my plan won't work.

I could use 50A breakers, 6/3 romex, and set the chargers to 50A (40A delivered), but that doesn't meet the performance goal. I'm fairly certain that my main panel won't support a 125A subpanel due to max stab ratings, so going higher on the subpanel is out too.

Thanks!
 

captjoemcd

Member
Nov 29, 2019
258
288
California
One other thought came to mind - I could wire the master charger with 100A breaker and #3 AWG since it'll be right next to the subpanel and set it to 100A, and put the slave on a separate 60A breaker, wire it with #6 AWG and set it to 60A. I think that would meet all the needs, although I'm not sure how code would look at that arrangement. d
 
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mswlogo

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Aug 27, 2018
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MA, NH
I don’t know the exact details but I thought you’re supposed to run ONE line, one circuit breaker to the largest circuit you can handle in parallel to both wall connectors. Then set each to that rating. One wall connector could pull the whole load if the other is not used. If both are used it will balance it out so the total never exceeds the max circuit.

It saves you from running two circuits and maximizes the “peak load” on one.

I don’t know the details of max loads of new S and X. Even if S or X can’t use that peak load I think that’s the way it was intended to be setup. So you still might need to set both to 80A. So that it knows the total load is 80A.

It might be easier to run two small circuits in your case and not link them.

It’s not balancing the load to the (sub) panel. It’s. Balancing the load on a single circuit. Maybe it can do what your thinking as well but I’m not sure that would meet code (two (or more) 60A breakers on a 100A panel).
 
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captjoemcd

Member
Nov 29, 2019
258
288
California
Yeah I'm probably overthinking it and should just wire the whole thing with #3 on a single breaker and use the splice box as suggested in the instructions, even though neither charger will pull anywhere near the rated continuous 80A
 
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dhrivnak

Active Member
Jan 8, 2011
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NE Tennessee
One other thought came to mind - I could wire the master charger with 100A breaker and #3 AWG since it'll be right next to the subpanel and set it to 100A, and put the slave on a separate 60A breaker, wire it with #6 AWG and set it to 60A. I think that would meet all the needs, although I'm not sure how code would look at that arrangement. d
I believe this is how it is supposed to be run.
 

mswlogo

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Aug 27, 2018
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I believe this is how it is supposed to be run.

And that would be wrong.

All Slaves are set to F, slave. You can’t set a Slave to 60A and Master to 80A. The diagram is pretty clear cut. One Breaker.

49143290963_595100b8c1_b_d.jpg
 

dhrivnak

Active Member
Jan 8, 2011
4,495
3,762
NE Tennessee
And that would be wrong.

All Slaves are set to F, slave. You can’t set a Slave to 60A and Master to 80A. The diagram is pretty clear cut. One Breaker.

49143290963_595100b8c1_b_d.jpg
Yes I stand corrected. Bothe the master and Slave can run at 80 amp but with the communications enabled if both cars try to charge each will only be at 40 amps. When one car finishes then the other will jump to 48 amps. Just one 100 amp breaker.
 
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boaterva

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Apr 2, 2016
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And note there is no ‘set to 100’. The max load is 80 on a 100 amp circuit. By code definition. 80% of rated max capacity.

I have two of them set up just like this and that’s how the Tesla certified electricians installed them. See sig for photos but it matches the diagram above.
 

captjoemcd

Member
Nov 29, 2019
258
288
California
And note there is no ‘set to 100’. The max load is 80 on a 100 amp circuit. By code definition. 80% of rated max capacity.

Fair point. I should've said "set for" 100A breaker, ie rotary switch position D, which is max output current 80A. Anytime I said "set" or "setting" or "wired for" above, I was referring to breaker size or conductor rating without taking into account the 80% max continuous current.
 

SSedan

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Jul 24, 2017
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Greenville Wisconsin
I would consider running copper to the sub-panel.
IMO the reason they don't want aluminum is it's tendency to heat up and expand under sustained load, with the 100apm sub-panel being fed by aluminum and your ability to keep it at 80amps for a VERY long time with two cars you are just moving that heat failure point to the box instead of the connector.
The concern with heat is not melting at the sustained 80% but rather the loosening the heat/cool cycles can create at the terminals.


also I think you will find little use for being able to charge both cars at max. I am all for having a wall connector at max capacity available that way you have it when needed but the chances of meeding it on both cars in Cali seems slim.
 
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captjoemcd

Member
Nov 29, 2019
258
288
California
also I think you will find little use for being able to charge both cars at max.

gah! now there's the pretty compelling argument to just run 2 50A breakers and 6/3 Romex which will be orders of magnitude easier installation and setting them both to 40A with no daisy chaining, which also leaves the option of installing NEMA 14-50s when I move out and taking the chargers with me. maybe it's coin flip time.
 

SSedan

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Jul 24, 2017
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Greenville Wisconsin
gah! now there's the pretty compelling argument to just run 2 50A breakers and 6/3 Romex which will be orders of magnitude easier installation and setting them both to 40A with no daisy chaining, which also leaves the option of installing NEMA 14-50s when I move out and taking the chargers with me. maybe it's coin flip time.

Having a full 48amp available on occasion would be good. Remember a current UMC is only good for 32amps.
 
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captjoemcd

Member
Nov 29, 2019
258
288
California
Having a full 48amp available on occasion would be good. Remember a current UMC is only good for 32amps.
Right - I was saying I'd install the wall chargers at 40A on a 50A breaker then have the option of swapping them out for 14-50's when selling the house in ~2-3 years when we move. So there's the rub, 40A vs 48A for the next two years :) Also, one will be 24' and the other will be 8.5'. I could also do the 24' on a 60A breaker to reach both and the 8.5' on a 40A breaker to get 48A and 32A continuous. Like @electracity said, I'm overthinking this.
 

boaterva

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Apr 2, 2016
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The original setup for the sharing protocol was more useful when we had cars with 72 amp chargers (like my X does). Then being able to share a large circuit was a cool idea. It’s also good for commercial installs or the large garage (like Erik and David who have 3 or 4 HPWCs in their garage, see their YouTube on the subject).

Now that all cars have lower max draw chargers, running two circuits may indeed be easier and cheaper from a copper cost.

Another reason I did the shared circuit install was for backup: either HPWC can charge either (any!) car at max power.
 

mswlogo

Well-Known Member
Aug 27, 2018
6,124
4,737
MA, NH
I would consider running copper to the sub-panel.
IMO the reason they don't want aluminum is it's tendency to heat up and expand under sustained load, with the 100apm sub-panel being fed by aluminum and your ability to keep it at 80amps for a VERY long time with two cars you are just moving that heat failure point to the box instead of the connector.
The concern with heat is not melting at the sustained 80% but rather the loosening the heat/cool cycles can create at the terminals.


also I think you will find little use for being able to charge both cars at max. I am all for having a wall connector at max capacity available that way you have it when needed but the chances of meeding it on both cars in Cali seems slim.

I hate Aluminum too and avoid using it when I can, even if it cost way more. So much easier to route and terminate too. Once you go over a certain size it can be hard to find though, which come to think about it, was for a 100A circuit I had trouble finding large copper gauge ;)
 

Rockster

Active Member
Oct 22, 2013
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4,621
McKinney, TX
Yeah I'm probably overthinking it and should just wire the whole thing with #3 on a single breaker and use the splice box as suggested in the instructions, even though neither charger will pull anywhere near the rated continuous 80A

This. And set the switches in both wall chargers to 80 amps, since that's the maximum that the circuit can deliver. The car will ask for the amount that it's capable of receiving (or set to receive if you've dialed it down in the car) and the two chargers will share the 80 amps. This is how mine looks:

Wall chargers_wall open.jpg
 
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