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I had a Tesla Charge Connector installed in my garage and rather than dealing with a complete panel replacement (the old 100amp service is maxed out), the contractor put in a large panel in the garage.

He then put a 100amp breaker in to supply the previous 'main panel' in the basement. I think this would technically make this a 'sub panel' and I understand that there are specifics regarding grounds and neutrals in a main vs sub panel.

Two questions: 1) could this have been done without access to the panel in the basement (ie only from the stack and 'new' 200 amp main panel and 2) is there a way I can easily check to see if this is properly done? (other than calling in a contractor to check)

I guess I forgot a very practical question: Can I add a 'sub panel' to the panel in the basement? If yes, Can I tie the neutrals together?

Thanks Doug '

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    Normally the main panel will be the one and only place where the equipment grounds are connected to neutral. Downstream, at any subpanels for example, the equipment grounds should NOT be connected to neutral. Your former main panel in the basement would have needed some modification to accomplish this, so should have been accessed.
    – Armand
    Jul 3, 2023 at 22:27
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    As always here, it's a big help to post photos of the panels in question (with deadfront removed if you feel comfortable with that).
    – Armand
    Jul 3, 2023 at 22:29
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    Neutrals and grounds are only bonded/connected together at the main panel. Usually this is at the meter panel or the first panel connected to the meter. Bonding/connecting neutral to ground can be as simple as a screw. Bigger problem might adding more circuits to an already maxed out panel, by sub panels.
    – crip659
    Jul 3, 2023 at 22:33

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Contractors don't do service upgrades. Licensed electricians do. Unfortunately the EV thing has created high demand which has been filled by less fully qualified people.

I'm sorry you got a service upgrade for your Tesla before talking to us, that could have been avoided simply by non-traditional thinking. The EV charge protocols were designed very smartly by people who understood that a service upgrade is a show-stopper for most EV buyers.

I find nothing wrong at all with the strategy of doing a 200A service by installing a new 200A panel and feeding the legacy panel as a subpanel. However you are correct that the interconnect between new and old panel must be 4-wire with separate neutral and ground - though the ground can be retrofit. Should be #8 copper or #6 aluminum.

Neutrals and grounds must be separated in the old panel. This cannot be done remotely, but it's not terribly hard. Add an accessory ground bar that screws directly into the box, move all grounds to it, and then remove the neutral-ground bonding strap. I see no conceivable way to do this without 20 minutes in front of the panel. Also, it would be a real time-saver to look at the panel labeling and see the listed model numbers of accessory ground bars - those models are designed to bolt right up to pre-tapped holes on the box. You can tap your own 8-32 or 10-32 holes, but why work?

Also, when the old wire is used as a feeder on a larger-than-100A service, it reduces wire capacity somewhat. #2 aluminum is now good to 90A. #4 copper is now good for 85A. The Load Calculation and breaker size must be appropriate for this.

You might ask your AHJ (permit issuing authority) if they gave your installer a waiver on the 4-wire requirement. They might have argued since they're not disturbing the panel it's "grandfathered" to be a 3-wire subpanel since it was installed (as a main panel) prior to the 4-wire mandate in NEC 1999. That seems a little fishy, though, and that mandate was put there for a reason.

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  • Agree...I use my existing 30A clothes dryer plug to charge my Tesla, without service upgrade. That will give me 200 Miles to drive the next day.
    – Traveler
    Jul 4, 2023 at 0:37
  • @Ruskes True. But for those who prefer hardwired EVSE - both because it is more reliable, designed for extended service life and avoid having to plug/unplug things every day - that is not an option. But there are other options. And 30A being enough is correct, I just wouldn't do that with a cord/plug connection. Jul 4, 2023 at 1:38
  • @manassehkatz-Moving2Codidact we do not use clothes dryer for 9 months in a year so not plug in plug out,
    – Traveler
    Jul 4, 2023 at 1:47
  • @Ruskes That does make a difference. Typical 30A receptacles are not designed for frequent plug/unplug - but sitting plugged in, the Tesla EVSE is as safe as the dryer would normally be. Jul 4, 2023 at 1:49
  • Thanks for all the feedback. The contractor is a licensed electrician. Here in Ontario, simply being a qualified electrician is no longer enough to do this work. They have to be under the auspices of an established contractor. I suspect this is to ensure liability insurance needs are met. The homeowner can do their own work, provided that it's inspected and is up to code. I don't know if the line to the basement is 4 conductor. I doubt it because he didn't enter the house. We were away at the time. But I doubt he even mentioned this to the inspector who did the inspection. Jul 8, 2023 at 21:22

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