8Kw 125A panel question.

[bakin1]

I've been getting bits and pieces of info for a few days now, but I just can't come to a complete understanding. Here is my scenario.
Trying to install an 8kW system.
Currently have an ancient 125A main panel. No sub panels. I do already have a EV charger if that matters.
Tesla stated that the panel needs to be upgraded. Engineering sheet shows 200A. I'd like to avoid upgrading the service as the lines are underground.

Question. Is it possible to just upgrade to a solar ready 125A panel? Seems like there are some with plenty of "space" for breakers.
The inverter is 7600W/40A. So, my math tells me yes as 100A(main) + 40A(solar) is less than 150A(125AX1.2).
Is this correct? Am I missing anything else? IE. Am I limiting how much excess can go back to the grid?

Thanks!

 

[BrettS]

Keep in mind that a panel upgrade and a service upgrade are two different things. You can put in a 200A panel with 125A service. So unless they are telling you that you also need a service upgrade (and I’m not sure why they would) then I believe that they are just intending to replace your panel.

 

[SoundDaTrumpet]

@BrettS Except in PG&E land the service must match the service (entrance) panel.

@bakin1 Your panel must be really ancient. Send some pictures for us to verify. Also, your neighbor might have the label if it hasn’t fallen off yet.

Most ancient underground work is direct burial cable without a conduit. If you have a nice short run to the PG&E Chrissy box without hardscape, it would be worth doing.

 

[gpez]

Most ancient underground work is direct burial cable without a conduit. If you have a nice short run to the PG&E Chrissy box without hardscape, it would be worth doing.

Which also means its possible the utility will upgrade it at their expense if it's due for replacement. I'm not in PG&E territory but my home was being served by a nearly 50 year old direct burial cable that was tripping quite literally weekly which they replaced with a new conduited cable.

 

[bakin1]

Here is the panel. House was built in 1972 I think.
@BrettS Yeah, not sure if they would allow me to just upgrade the panel to 200A and not upgrade the line. I can try to find out, however, I thought that was a no-no as mentioned by @SoundDaTrumpet
I'm in Sacramento but not with PG&E, I have SMUD. They still require the homeowner to pay for the trenching though. They'll only supply the wire. I'd like to avoid trenching if possible. I'd have to remove a big area of concrete if I do decide to trench.

As far as my last question, If I'm able to stick with the same service line(100A), does that limit how much power goes back to the grid?

Thanks for all the quick replies!

 

[Vines]

Green Meter Adapter Might be an option in this case, if you have underground service and your utility allows.

If you feel comfortable opening the deadfront of that service panel, it might instead be able to take a line side tap.

GMA is similar to a line side tap, but is special hardware that allows you to backfeed up to 60A of PV into the meter socket.

 

[Vines]

Other solutions like upgrading the panel are drastic, so take some more pictures of the meter area first, on top of the panel. The more pictures the better.

 

[mswlogo]

You could upgrade to a new 125A panel with plenty of slots over what you have.

I have 100A panel that’s new (I think it’s like 32 slots) but still pretty full (no twins though) and the solar company tapped the mains to back feed it. 7.6Kw Solar.

The main breaker is somewhat independent of the panel (depends on the brand and series). But You can get the same panel with 100A,150A,200A main breaker. So if you upgrade your service you just replace the main breaker.

Solar is not increasing your load. So 200A shouldn’t be required.

I’d recommend replacing that panel no matter what. It’s not that expensive.

 

[SilverSp33d3r]

The upgrade to the panel is to avoid the “120%” rule. If they upgrade your panel and put back a 125 amp main it will allow you to pull 125 amps from the grid and the 40 amps from the inverters without overloading the bus on the panel.

 

[wwhitney]

You could do a load calc and see if 100A is sufficient. If so, you could downsize the main breaker to 100A, then you'd have headroom for up to 9.6 kW of inverters.

You could also go with a smaller solar system. A 4.8 kW inverter would be OK with your main panel as is. With a 1.5 DC/AC ratio, you could put 7 kW of panels on it.

On the other hand, if the equipment is 50 years old, it's about due for a replacement.

Cheers, Wayne

 

[bakin1]

You could do a load calc and see if 100A is sufficient. If so, you could downsize the main breaker to 100A, then you'd have headroom for up to 9.6 kW of inverters.

You could also go with a smaller solar system. A 4.8 kW inverter would be OK with your main panel as is. With a 1.5 DC/AC ratio, you could put 7 kW of panels on it.

On the other hand, if the equipment is 50 years old, it's about due for a replacement.

Cheers, Wayne

thanks my thinking too. I currently have a 125a panel with 100a service without issue. It was confusing to me as Tesla shows 200a in their docs. I’m still trying to get then to confirm that a new 125A panel will work, but no luck yet. Worst case scenario, I upgrade my 50 year old panel for $3K and a Tesla says they can’t install for whatever reason.

 

[mnsweeps]

I had a 200 amp panel with a 200 amp main bus ..It came with house built in 2003 so its not old. I was still asked to upgrade panel to a 200 amp / 225 amp bus which I did last week..

 

[mswlogo]

You could do a load calc and see if 100A is sufficient. If so, you could downsize the main breaker to 100A, then you'd have headroom for up to 9.6 kW of inverters.

You could also go with a smaller solar system. A 4.8 kW inverter would be OK with your main panel as is. With a 1.5 DC/AC ratio, you could put 7 kW of panels on it.

On the other hand, if the equipment is 50 years old, it's about due for a replacement.

Cheers, Wayne

Solar SUBTRACTS from the grid load, it never adds. There is no reason to reduce anything amp capacity on anything. Now if he was adding a PowerWall you could argue that, that can be additional load on the grid.

My guess is Tesla is rightfully recommending a new “service” and 200A is fairly standard these days. If you don’t have electric heat it’s rarely needed that you need 200A.

 

[bakin1]

Solar SUBTRACTS from the grid load, it never adds. There is no reason to reduce anything amp capacity on anything. Now if he was adding a PowerWall you could argue that, that can be additional load on the grid.

My guess is Tesla is rightfully recommending a new “service” and 200A is fairly standard these days. If you don’t have electric heat it’s rarely needed that you need 200A.

Yup, no electric heat. It does make sense that Tesla recommends or assumes 200A. However, there shouldn’t be an issue if I replace what I currently have with a 225A, correct?

 

[mswlogo]

Yup, no electric heat. It does make sense that Tesla recommends or assumes 200A. However, there shouldn’t be an issue if I replace what I currently have with a 225A, correct?

I assume you meant 125A. But If you’re asking me, sure, I think that would be fine. But no idea what Tesla will do. The installers are often not engineers and just following recipes in a book.

 

[wwhitney]

Solar SUBTRACTS from the grid load, it never adds. There is no reason to reduce anything amp capacity on anything.

The issue is that having multiple sources on a single busbar introduces the possibility of overloading the busbar.

A typical panel supplied only by the grid has a single breaker on the source of supply whose rating does not exceed the busbar rating. There may be many distribution breakers whose total rating exceeds the busbar rating, but in the unlikely event all those branch circuits are drawing enough current to actually exceed the busbar rating, the single supply breaker will protect the busbar.

Now when you add a second source of supply, that is no longer true. You could draw current from the utility supply and from the second source of supply, and the total current flowing out on the branch circuits could exceed the busbar rating without tripping either of the supply breakers.

The allowable compromise is commonly called the "120%" rule. The two sources have to be on opposite ends of the busbar, and the sum of the two source breaker ratings must not exceed 120% of the busbar rating. While Kirchoff's law would allow full sized breakers at each end of the busbar without exceeting the ampacity of the busbar at any cross section, the resulting heat dissipation inside the panel would exceed what the panel is designed for, so the limit is 120%.

The upshot is that if you have a 125A utility breaker on a 125A bus, you can only add a 25A solar breaker on the opposite end. If, however, you only need 100A of power from the utility, then you can decrease the utility breaker to 100A and use up to a 50A solar breaker.

Cheers, Wayne

 

[mswlogo]

The issue is that having multiple sources on a single busbar introduces the possibility of overloading the busbar.

A typical panel supplied only by the grid has a single breaker on the source of supply whose rating does not exceed the busbar rating. There may be many distribution breakers whose total rating exceeds the busbar rating, but in the unlikely event all those branch circuits are drawing enough current to actually exceed the busbar rating, the single supply breaker will protect the busbar.

Now when you add a second source of supply, that is no longer true. You could draw current from the utility supply and from the second source of supply, and the total current flowing out on the branch circuits could exceed the busbar rating without tripping either of the supply breakers.

The allowable compromise is commonly called the "120%" rule. The two sources have to be on opposite ends of the busbar, and the sum of the two source breaker ratings must not exceed 120% of the busbar rating. While Kirchoff's law would allow full sized breakers at each end of the busbar without exceeting the ampacity of the busbar at any cross section, the resulting heat dissipation inside the panel would exceed what the panel is designed for, so the limit is 120%.

The upshot is that if you have a 125A utility breaker on a 125A bus, you can only add a 25A solar breaker on the opposite end. If, however, you only need 100A of power from the utility, then you can decrease the utility breaker to 100A and use up to a 50A solar breaker.

Cheers, Wayne

I see your point. You’re effectively adding more supply to the buss.

But keep in mind most modern panels are 200A busses (probably not the OP) and they just downgrade them with smaller mains. Thanks for the clear correction. My panel came with 100A main but it clearly says 200A max on the door.

That might be the reason why they back fed my system (100A) by taping the mains before the main breaker and not using a breaker on the buss. Panel was full but room could have been easily made. They have an external 40A breaker over at the Inverter.

 

[bakin1]

Just wanted to post a quick update incase anyone stumbles across this post.
Per Tesla, its not that they can't install on a 125A panel, its the fact that in my area(Sacramento), SMUD did not use conduit when installing service in the 70s. So, Tesla's requirement is for the conduit, not 200A.

 

[wwhitney]

I see no reason for Tesla to care if your service lateral is direct buried or in conduit. That is nonsensical.

Cheers, Wayne

 

[Vines]

Just wanted to post a quick update incase anyone stumbles across this post.
Per Tesla, its not that they can't install on a 125A panel, its the fact that in my area(Sacramento), SMUD did not use conduit when installing service in the 70s. So, Tesla's requirement is for the conduit, not 200A.

That is a crazy excuse in my opinion. Nobody here cares about whether the service wiring is in conduit, overhead, or direct buried. I cannot fathom why Tesla does either. Maybe ask Tesla why the direct bury conductors are an issue?