On a 200 A busbar with a 200 A main breaker, the solar connection gets 40 A. That 40 is 125% of the inverter's output current, so the inverter itself tops out at 32 A. Under the 120% rule, it also has to sit at the far end of the busbar.
Those numbers come from 705.12, the rules for connecting a power source on the load side of the service. Everything here is the 2023 Code, so check which version your jurisdiction enforces.
705.12 starts with a permission: the output of an interconnected power source is permitted to connect to the load side of the service disconnecting means, at any distribution equipment on the premises.
Once two sources feed the same equipment, the feeders and busbars have to meet 705.12(A) and (B). The currents are based on the maximum circuit currents calculated in 705.28(A).
The output of an interconnected electric power source shall be permitted to be connected to the load side of the service disconnecting means of the other source(s) at any distribution equipment on the premises.
The handbook explains which current counts. The required ampacity is based on the inverter output circuit current, rather than on the overcurrent device inside the inverter. The Code then applies the 125% to that current.
The required ampacity of a feeder or bus connected to the interactive inverter is based on the inverter output circuit current, rather than on the overcurrent protective device (OCPD) in the inverter.
Start with feeders, under 705.12(A). Where the power source connects to a feeder, the feeder ampacity has to be at least 125% of the power-source output circuit current.
The feeder ampacity is greater than or equal to 125 percent of the power-source output circuit current.
Location on the feeder matters. If the connection is anywhere other than the opposite end from the primary overcurrent device, the feeder downstream of the connection needs protection one of two ways. Either its ampacity is at least the primary breaker's rating plus 125% of the source current, or a breaker on the load side of the connection is rated no higher than the feeder ampacity.
Taps off that feeder get the same treatment. For taps sized under 240.21(B)(2) or (B)(4), the tap ampacity must be at least one-third of the sum of the feeder breaker's rating and the ratings of any power source breakers on the feeder.
the ampacity of taps conductors shall not be less than one-third of the sum of the rating of the overcurrent device protecting the feeder plus the ratings of any power source overcurrent devices connected to the feeder.
Now the busbar, under 705.12(B). These methods apply to equipment with no specific listing and instructions for combining sources. One of them has to be used.
The first is the plain one. 125% of the source current, plus the rating of the breaker protecting the busbar, must not exceed the busbar rating.
For power source connections to distribution equipment with no specific listing and instructions for combining multiple sources, one of the following methods shall be used to determine the required ampere ratings of busbars:
The second is the one everybody calls the 120% rule. It applies where the primary source and the other source are at opposite ends of a busbar that contains loads. Then 125% of the source current, plus the rating of the breaker protecting the busbar, must not exceed 120% of the busbar rating.
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 busbar ampere rating.
It comes with conditions. The busbar still has to be sized for the connected loads under Article 220. And a permanent label goes next to the back-fed breaker: warning, power source output, do not relocate this overcurrent device. The label has to meet 110.21(B).
Run the numbers one step at a time. The busbar is rated 200 A. 120% of that is 240 A. That is the most the main breaker and the solar connection can add up to. The main breaker takes 200 of it, so 40 A are left for the solar side. The rule counts that 40 as 125% of the inverter's current, so the inverter can be up to 32 A. The handbook's table shows the same math: a 100 A busbar with a 100 A main leaves 20.
A larger busbar is the first way to raise the 40 A. Keep the 200 A main, and put it on a 225 A busbar. 120% of that is 270. Take away the 200, and 70 A are left, enough for a 56 A inverter. A smaller main breaker is the second way. A 175 A main on the 200 A busbar leaves 65, if the Article 220 load calculation still supports the smaller main.
The third method counts every breaker. The sum of the ratings of all the overcurrent devices on the panelboard, load and supply, leaving out the breaker protecting the busbar, must not exceed the busbar ampacity. That breaker must not be rated above the busbar.
That one has its own label: equipment fed by multiple sources. The total rating of all overcurrent devices, excluding the main, shall not exceed the ampacity of the busbar. The handbook notes that service equipment has no limit on the number or rating of breakers. This method caps their total rating on the panelboard.
The sum of the ampere ratings of all overcurrent devices on panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, shall not exceed the ampacity of the busbar.
Here is the sum-of-ratings method on a 125 A panelboard that feeds only a few circuits. 42 A solar breakers and 31 A load breaker add up to a 110 A. That is under the busbar's 125, so it complies. The rule does not say where on the bus those breakers sit, and the breaker protecting the busbar must not be rated above a 125.
The last three are narrower. A center-fed panelboard in a dwelling can take the connection at either end, under the same 120% limit. Feed-through lugs are covered as well, with the feed-through conductors sized under 705.12(A).
Anything outside those configurations is allowed on switchgear, switchboards, and panelboards when it is designed under engineering supervision, with available fault-current and busbar load calculations. The informational note adds that some equipment is specifically listed for combining sources, and these methods are for equipment without that listing.
Connections shall be permitted on switchgear, switchboards, and panelboards in configurations other than those permitted in 705.12(B)(1) through (B)(5) where designed under engineering supervision that includes available fault-current and busbar load calculations.
705.13 adds another option: an energy management system, under 750.30, is permitted to limit the current on busbars and conductors fed by one or more power production or storage sources.
An EMS in accordance with 750.30 shall be permitted to limit current and loading on the busbars and conductors supplied by the output of one or more interconnected electric power production or energy storage sources.
So take three numbers out of this. 125% of the inverter's output current is what you count. 120% of the busbar is the ceiling, for opposite-end connections, and for center-fed dwelling panels. The solar allowance comes down to two numbers: the busbar rating and the rating of the breaker protecting it. A 200 A main leaves 40 on a 200 A busbar, and 70 on a 225.
Everything here is the 2023 NEC. Your jurisdiction may enforce a different Code year; check which version applies where you work.