Need your help understanding Solar PV, bus bar, the 120% rule?, and utility service implications

[Yikes]

I haven't personally done much electrical system design - - I usually leave that to the electrical engineers. But with more than 9000 structures (including 6000+ single family residences) destroyed in the recent Eaton Fire (Altadena, CA), I'm trying to wrap my head around the implications for the future utility service sizing for the community, assuming current code is not relaxed.
Our CalGreen and Energy codes are currently requiring the following:

1. All replacement homes that previously utilized gas for heating, cooking, clothes drying and and hot water must now be wired to be ready for 100% electric (e.g., heat pumps, induction ranges, etc.).
2. Main panel must have a dedicated space for a 40 amp future EV charger.
3. California will require solar PV and/or battery storage.

I have questions regarding that last one. CEC 705.12(B)(3) says this:

Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar. The busbar shall be sized for the loads connected in accordance with Article 220.​

The dumb questions:
Is the bottom line that the service gets sized for #1 and 2, then upsized again 20% more for #3?
Or is it even more than that - - does it mean that we must first sum both the power draw of the house, and the power supplied by the PV and/or battery, and then make sure that doesn't exceed 120% of the busbar rating.
If the service gets upsized for power from two sources, does that mean the sizing of everything else "upstream" on the utility side needs to get upsized too?

I could easily see how thousands of homes in the 100 year old community that previously had a 60-100 amp service may not have a 200+ amp service, which when applied to thousands of dwelling on hundreds of city blocks would potentially trigger an entire redesign of the Edison utility service to the whole community - -right when they are trying to replace the burned out utility service in-kind. Am I wrong on this?

 

[wwhitney]

Main panel must have a dedicated space for a 40 amp future EV charger.

It has to be the service panel, or it can be any panel in the dwelling?

The dumb questions:
Is the bottom line that the service gets sized for #1 and 2, then upsized again 20% more for #3?

No. The bottom line is that on a residential service, the addition of PV is basically never going to change the service size under the NEC (the conductors from the service point to the service disconnect on the residence). The only way it would is if you had say a 100A service and wanted to install over 24 kW of PV _inverters_ (AC rating; DC panel rating is usually higher), as 240V * 100A = 24 kW. See NEC 705.11. And even then, a PV system that size could have a Power Control System (see NEC 705.13) that is configured to keep the PV from exporting more than 100A, even if the inverters are capable of putting out more than 100A. Which is useful when the difference is used either for immediate onsite consumption, or charging battery storage.

Now PV will affect the infrastructure on the POCO (power company) side of the service point, as they do not size their equipment based on the (generally grossly oversized) NEC requirements. But that would be a different discussion that wouldn't reference the NEC.

Or is it even more than that - - does it mean that we must first sum both the power draw of the house, and the power supplied by the PV and/or battery, and then make sure that doesn't exceed 120% of the busbar rating.

What 705.12(B)(3) means is this: you've got a building's wiring system that has two sources of supply, the utility and the PV inverter. There is some wiring connecting them. That wiring includes some panels. All those panels have two sources of supply, the utility and the PV inverter. We need to make sure those panels' busbars don't get overloaded. One way to do that is that to ensure that the two sources of supply are attached to opposite ends of the busbars. Since current can only flow in one direction at a time (and if you superimpose two different loadings where the current would be flowing in opposite directions, the result is that the busbar sees the difference in the two currents), the current at any busbar section will still be below its rating as long as each source is protected by a breaker not exceeding the busbar rating.

However, that alone is not enough. If you allowed a 100A panel to get 100A from the utility at one end of the busbars, and 100A from PV from the other end, that panel could be supplying 200A of loads. Since the breakers and busbars and other conductors in the panel all produce heat because they have some (small) resistance, that could end up generating heat at twice the rate that the panel was designed and tested for. Since 705.12(B)(3) is an allowance for any panel, with no special testing for two sources, that's not acceptable.

So instead the sum of the two breakers (well, the PV breaker will be at least 125% of the inverter output current, but you're not penalized if you have to use a larger breaker because that's not a standard size, as the rule uses 125% of the inverter output current) needs to be at most 120% of the busbar rating. Because heating depends on the square of the current, that rule will keep the overall heating to at most 104% of the usual case of one source to the panel. [worst case is 100% and 20%, and 100% squared + 20% squared = 104%]. So the NEC says "sure, any panel without any special testing, you can go 4% over on (possible) heating without concern.

If the service gets upsized for power from two sources, does that mean the sizing of everything else "upstream" on the utility side needs to get upsized too?

No, the POCO will used their own sizing methodology based on their extensive historical data. Equipment might easily be sized 1/4 to 1/10 of what you'd expect just looking at the NEC calculations. And then if a transformer burns up too quickly because it's overloaded, they'll put in a slightly larger one.

Cheers, Wayne

 

[Yikes]

Super-helpful, thanks!

A few years ago I had a project in the Coachella Valley, an existing apartment complex of several hundred units built in the 1950s.
We wanted to add solar carports throughout the site. Edison told the owner the service to that city block would need to be upgraded due to adding the solar. I didn't know if that was related.