Power Sharing Question that I can't find the answer to

[SkoshOfSean]

I know this has been discussed. I have searched all this forum and others and looked at the information from Tesla but I am still confused.

I want to install two Tesla Universal Wall Chargers in my garage. I will have a sub panel and can only safely install one 60 AMP breaker.

I thought the point of power sharing was that I could have two Wall Chargers share that one 60 AMP breaker. But from what I am reading I have to install 2 60 AMP breakers and when set up the wall chargers will coordinate and never draw more than 48 amps.

But there are still two 60 amp breakers which is over my capacity. What makes this safe and legal other than relying on software and wifi? Or is there something I am completely missing?

Thanks for any help.

 

[Acps110]

You can share a 60 amp breaker with 2 Wall Connectors, but they should be on their own breakers.

Example, 60 amp breaker feeding a sub panel, then two 60 amp breakers in the sub panel each feeding 1 Wall Connector. So you would end up with three 60 amp breakers. Because the feed is only 60 amp, Power Sharing must be enabled so that both Wall Connectors in total don’t pull more than 48 amp continuously between the 2 of them.

Do you not have space in your sub panel for 2 - 240 volt breakers? If not, you need a bigger sub panel, but can still keep the feed as whatever it is now. How big is the feed to your sub panel? 60 amp?

Power Sharing is set up to limit the total draw to whatever the feed is, not the individual breakers hooked up to the Wall Connectors. When only using 1 Wall Connector, and the other is unplugged, the power sharing setup will be able to give the full 48 amp continuous to 1 car regardless of which Wall Connector is used.

 

[MP3Mike]

But there are still two 60 amp breakers which is over my capacity. What makes this safe and legal other than relying on software and wifi?

The software is what makes it safe and legal. If one of the Wall Connectors loses communication, either because of being turned off or the WiFi goes down, all of the Wall Connectors revert to a "safe" mode where they only provide limited power, so that it is not possible to draw too much. (Total available amps divided by the number of Wall Connectors.)

 

[SkoshOfSean]

Thank you for the reply.

Your example of a 60 amp breaker feeding a sub panel that has two 60 amp breakers gets to the heart of my confusion.

Wouldn't it be bad practice to install two non-power sharing 48 amp chargers to those breakers? Charging 2 cars at the same time would obviously trip a breaker.

What makes installing two Tesla wall chargers with power sharing any safer other than software and the hope that their wifi connection is stable?

I have only spoken with one electrician so far. He admitted to having no experience with power sharing, but he did not understand how it would be safe to having those two separate circuits.

My original plan was to have a 100 or 150 amp sub-panel installed with a single 50-60 amp circuit that would feed the two wall chargers. That way I would have expansion flexibility when I eventually upgrade my home to 400 amp service.

 

[SkoshOfSean]

The software is what makes it safe and legal. If one of the Wall Connectors loses communication, either because of being turned off or the WiFi goes down, all of the Wall Connectors revert to a "safe" mode where they only provide limited power, so that it is not possible to draw too much. (Total available amps divided by the number of Wall Connectors.)

I was wondering about that. Is that documented anywhere? That information might help my electrician understand.

 

[MP3Mike]

What makes installing two Tesla wall chargers with power sharing any safer other than software and the hope that their wifi connection is stable?

Because as I said, if communications goes down the Wall Connectors go in to safe mode. If you have 48A to share, and have 4 Wall Connectors, and one of the 4 goes down or loses WiFi they all revert to only providing a maximum of 12A, so that it is not ever possible for the total draw to go above 48A.

 

[MP3Mike]

I was wondering about that. Is that documented anywhere? That information might help my electrician understand.

I don't think it explicitly spells it out, but the manual says this:

And there is this error, that tells you when that is happening:

 

[SkoshOfSean]

Thanks. So if you do this setup without linking the two wall charges to power sharing then it would not be safe? I guess there is something in my head that is dubious of over provisioning an electrical system but relying on software to make it safe.

Though I guess that is true if they were simply sharing the same circuit as well.

 

[Acps110]

Thank you for the reply.

Your example of a 60 amp breaker feeding a sub panel that has two 60 amp breakers gets to the heart of my confusion.

Wouldn't it be bad practice to install two non-power sharing 48 amp chargers to those breakers? Charging 2 cars at the same time would obviously trip a breaker.

What makes installing two Tesla wall chargers with power sharing any safer other than software and the hope that their wifi connection is stable?

I have only spoken with one electrician so far. He admitted to having no experience with power sharing, but he did not understand how it would be safe to having those two separate circuits.

My original plan was to have a 100 or 150 amp sub-panel installed with a single 50-60 amp circuit that would feed the two wall chargers. That way I would have expansion flexibility when I eventually upgrade my home to 400 amp service.

By having the 60 amp breakers in series, you are providing a 60 amp ”pathway” from the main panel all the way to the car. Yes, it would be very bad practice to oversubscribe the sub panel with two 60 amp devices without any kind of a Power Sharing feature enabled.

To your point of 100 amp sub panel with 2 wall connectors… You would still install 2 Wall Connectors on their own 60 amp breakers and then setup the Power Sharing to limit to 100 amp total / 80 amp continuous.

Going back to your main 200 Amp panel, you would need to do a load calculation to see how much capacity you have left over at the main feed coming in from the street. If there is only 80 amp capacity there, then setup the Power Sharing to max out at 80 amp, and your 100 amp breaker just serves as a disconnect.

As to why this is legal, the National Electric code says that devices must be installed in accordance with their manufacturer’s instructions. Since Tesla allows Power Sharing, you aren’t oversubscribing. The Power Sharing is setup at installation time, but is behind a password protected web page to prevent unauthorized or unsafe changes. If any random user pulls up and plugs in, they don’t even have to know about the Power Sharing for their car to get the right amount of amps.

 

[SkoshOfSean]

Thank your for all the help. I think that clears up all my confusion!

 

[SkoshOfSean]

Actually, I have one more question. If I want two power sharing Wall Connectors sharing 60 amps off two breakers in a. sub panel, is there a reason for the breaker not to be connected to a 100 amp breaker at the main panel? Right now I have 60 amp capacity but I am hoping to upgrade to 400 amp service at some point and I would like to plan for that now if I can.

 

[Acps110]

Actually, I have one more question. If I want two power sharing Wall Connectors sharing 60 amps off two breakers in a. sub panel, is there a reason for the breaker not to be connected to a 100 amp breaker at the main panel? Right now I have 60 amp capacity but I am hoping to upgrade to 400 amp service at some point and I would like to plan for that now if I can.

Your wiring feed from the main panel to the sub panel would need to be up-sized appropriately to handle 100 amps. This would be a good future-proofing step towards your ultimate goal of upgrading the house to 400 amp service.

By powering the sub panel from 100 amp breaker, you would have a combined 80 amp continuous for the 2 wall connectors. (60 amp allows 48 amp continuous.) This will bump your amps up per car from 24 to 40 continuous, or 48 continuous if only 1 car.

 

[Rocky_H]

Thanks. So if you do this setup without linking the two wall charges to power sharing then it would not be safe? I guess there is something in my head that is dubious of over provisioning an electrical system but relying on software to make it safe.

Well, it sounds like you are thinking of this as some unreliable, hacked together kludge that Tesla came up with, so you're thinking it's flaky. I'll say that it's a confirmed enough solid practice in the entire industry that the National Electrical Code actually has specific written provisions that spell out the allowance for intentionally over subscribing a line if it is implementing a manufacturer's load sharing system. So this is a real thing that the industry trusts and relies on.

 

[SkoshOfSean]

Your wiring feed from the main panel to the sub panel would need to be up-sized appropriately to handle 100 amps. This would be a good future-proofing step towards your ultimate goal of upgrading the house to 400 amp service.

By powering the sub panel from 100 amp breaker, you would have a combined 80 amp continuous for the 2 wall connectors. (60 amp allows 48 amp continuous.) This will bump your amps up per car from 24 to 40 continuous, or 48 continuous if only 1 car.

Thank you everyone for your help. Just to clarify my last question, is there a reason I should not properly wire the sub panel for 100 amps even though I only currently have 60 amp capacity. It would seem to me if I am only putting in the two 60 amp breakers to be shared that it would be fine. But when I upgrade my service then I can replace the 60 amp breakers without having to otherwise rewire.

 

[vlee]

Thank you everyone for your help. Just to clarify my last question, is there a reason I should not properly wire the sub panel for 100 amps even though I only currently have 60 amp capacity. It would seem to me if I am only putting in the two 60 amp breakers to be shared that it would be fine. But when I upgrade my service then I can replace the 60 amp breakers without having to otherwise rewire.

You can wire it up for 100 amps and even use a 100 amp breaker (either main breaker or backfeed breaker) in the subpanel. Since you only have 60 amp capacity, you can put a 60 amp breaker on your load center (main panel) to feed the subpanel. That is enough to prevent the whole thing from drawing more than 60 amps.

 

[Acps110]

Thank you everyone for your help. Just to clarify my last question, is there a reason I should not properly wire the sub panel for 100 amps even though I only currently have 60 amp capacity. It would seem to me if I am only putting in the two 60 amp breakers to be shared that it would be fine. But when I upgrade my service then I can replace the 60 amp breakers without having to otherwise rewire.

Yes, you can use a 100 amp breaker and appropriate sized wire to feed the sub panel before you upgrade your main panel. The two 60 amp breakers will be attached to the Wall Connectors and won’t be upgraded later as that is the highest size breaker that a new Wall Connector supports.

Upon initial installation, you set the Power Sharing to 60 amp max to fit within your existing capacity. Don’t lose the documentation on how to setup the Power Sharing!

Later, when upgrading your main panel, none of your sub panel wiring will need to be changed. Just move the 100 amp breaker and its wiring into the new main panel. Then go back into the Power Sharing and change it to 100 amp max.

 

[SkoshOfSean]

Thank you thank you everyone. I feel like can talk with my electrician now and have an idea what is going on and decide to go with a different electrician if I need to! ;-)

 

[Proflig8tor]

The universal charger is a high quality splitter.