Reason for new Main Panel?

[Southpasfan]

In my 16.32 three Powerwall install I just got the actual plans.

They include upgrading the main panel from 150 amps to 200 amps. Now, when I look at the plans and diagrams, one wonders why?

My guess is that one calculation, maybe two, are made. The first one would be how much electricity the solar panels and Powerwalls could potentially put back to the grid. In theory (although I don't see when this would happen as powerwalls, as I understand it, are not set up to send elecricity out to the grid) in my case it could be 90 amps from the Powerwalls and 80 amps from the solar, which adds up to more than 150.

But the actual house loads are not changing just because I am installing generation. However, I suppose, in say, Storm watch, the Powerwalls charge from the grid in addition to the house loads. Is that it?

That, conceptually, the Powerwalls are an additional, large, "load" even though we tend not to think of them as a load but as a source.

 

[SoCal Dave]

There is a NEC rule that the bus in a panel cannot exceed 120% of its bus rated value for the power feeding it. This would be the power from the grid and the power from other sources such as solar.

So for my panel for example, my was a 200A panel with a 200A main. 120% would be 240A max. As my main breaker was 200A they could only add a 40A breaker for my 12.24kW solar, but it needed a 50A breaker. They ended up downsizing my main to 175A to ensure we were under the 240A limit.

I would assume with a 16kW system and power walls they are exceeding the 120% limit.

 

[Southpasfan]

There is a NEC rule that the bus in a panel cannot exceed 120% of its bus rated value for the power feeding it. This would be the power from the grid and the power from other sources such as solar.

So for my panel for example, my was a 200A panel with a 200A main. 120% would be 240A max. As my main breaker was 200A they could only add a 40A breaker for my 12.24kW solar, but it needed a 50A breaker. They ended up downsizing my main to 175A to ensure we were under the 240A limit.

I would assume with a 16kW system and power walls they are exceeding the 120% limit.

This is probably a dumb question, but how are the loads (which I think you said were downsized to 175 amps and I would assume means circuits which draw power, like A/C, lights, etc.) something that "feeds" the panel. Aren't they a possible "draw" or "load?"

Aren't the only "feeds" (1) the grid, (2) the panels, and (3) the powerwalls?

 

[BrettS]

This is probably a dumb question, but how are the loads (which I think you said were downsized to 175 amps and I would assume means circuits which draw power, like A/C, lights, etc.) something that "feeds" the panel. Aren't they a possible "draw" or "load?"

Aren't the only "feeds" (1) the grid, (2) the panels, and (3) the powerwalls?

If you google the 120% rule you can get more info. But briefly, the concern is this:

Normally a panel would only be fed from one source and protected with a main breaker. You can put more loads in the panel than the size of the service because it is protected by the main breaker. If, for example, you have 200A service then you will have a 200A panel and a 200A main breaker. You might have loads that total 250A or more, but as long as all your loads aren’t on at the same time then they won’t overload your panel. If you do happen to turn all your loads on at the same time, then that 200A main breaker will trip and protect the panel.

However, with solar the panel is fed from two places... the incoming service as well as the solar panels. So in this case, if you had 250A of loads and you happened to turn them all on at the same time, then you could be drawing 200A from the grid as well as 50A from your solar panels for a total of 250A of power and this would overload your 200A panel. It’s no longer protected by the 200A main breaker because it is also fed from another source.

So the two options here are to downrate the main breaker or upgrade the panel.

 

[Southpasfan]

If you google the 120% rule you can get more info. But briefly, the concern is this:

Normally a panel would only be fed from one source and protected with a main breaker. You can put more loads in the panel than the size of the service because it is protected by the main breaker. If, for example, you have 200A service then you will have a 200A panel and a 200A main breaker. You might have loads that total 250A or more, but as long as all your loads aren’t on at the same time then they won’t overload your panel. If you do happen to turn all your loads on at the same time, then that 200A main breaker will trip and protect the panel.

However, with solar the panel is fed from two places... the incoming service as well as the solar panels. So in this case, if you had 250A of loads and you happened to turn them all on at the same time, then you could be drawing 200A from the grid as well as 50A from your solar panels for a total of 250A of power and this would overload your 200A panel. It’s no longer protected by the 200A main breaker because it is also fed from another source.

So the two options here are to downrate the main breaker or upgrade the panel.

Brett the one who knows all weighs in.

Sheesh, I now know I am going to regret this, but according to my diagram the solar panels do not "feed" the main panel. They have to go through the Gateway first. And they either go to the main panel and out to the grid, or partially out to the main panel and othewise to the home.

So I don't see how the main panel is "fed" from two places. Yes, if it wasn't for the Gateway, I suppose the Powerwalls and the Panels could, at the same time, be sending power out to the grid, but the size of the main panel operates, as I perhaps imperfectly understand it, as a limit as to how much the home can draw, its not even designed with the concept of the home producing power.

So my thought was that with powerwalls, that is the only new item which increases the "draw"

 

[BrettS]

If you are doing a whole house backup then the main panel will only contain your main breaker and a breaker for the gateway. However, if you are doing a partial house backup then all your non backed up loads would also go in the main panel and those could be fed by the solar or the powerwall through the gateway.

Even if you aren’t going to have many (or any) non backed up circuits in the main panel I believe that code still requires that it meet the 120% rule. However, I’m not an electrician, so I could be wrong on this.

 

[cridinger82]

I just went through all this with our local electrical reviewer. Tesla's local engineer and Master Electrician had a big zoom meeting with the city and county. We were over the 120% rule the city said for us and we have a 200amp main. And unfortunately I was in meeting both days they had the meetings. The city approved and stated that our layout through the gateway 2 met some sort of "law" that has to do with the flow of electric, blah blah blah.. and would approve it even though it technically was over the 120% rule. Black magic probably...

 

[cridinger82]

we are doing a whole house backup minus a 20 amp circuit which is in the main/combo meter panel outside.

 

[Southpasfan]

I am just writing this looking at my actual diagram and the power from the solar panels either goes (1) to cover home loads, or (2) to the powerwalls and/or home loads, or (3) out to the grid.

The explanation I found on the 120% rule was good, but it seemed to imply that with solar the additional power (obviously) can allow downstream circuits to draw more power than the power flowing through the main panel breaker from the grid. That's true, but I dont' see how that situation can even happen with a Gateway, or assuming it could happen, how "downstream" power makes the main panel heat up.

Its that latter part that is beyond my technical understanding. Maybe electricity does not "flow" in one direction like water, but all connected lines carry all the power, more like the concept of water "pressure."