The reason I said "might" in that initial post is just because someone who is qualified to do so must perform load calculations to ensure that it is possible to downsize the main OCPD and still serve the load adequately and safely. However, if that load analysis indicates that you can downsize the main OCPD rating, then you can add a PV OCPD with a rating equal to or less than the amount by which you can reduce the main OCPD rating. As long as the sum of the main OCPD and the PV OCPD is less than or equal to 100% of the busbar rating, then there is no restriction on the PV OCPD location. You can use this strategy with any panel, and it may be the only approach that an AHJ will allow with a center-fed panel.
If you go back a couple Code cycles (2005 and earlier), then there was no restriction on the PV breaker location within the context of the 120% allowance. That's because the 120% allowance was an exception that only applied to residential applications, and we all know that there are few continuous large loads in a residence. Breaker location only became an issue when the 120% allowance was opened up to include commercial services in 2008. This history is interesting in part because it sheds light on the intent of the Code changes. Applying the 120% allowance to center-fed panel in a residential setting is actually consistent with the intent of the Code going back 20 years or so. Once breaker location enters into the picture, it creates a problem interconnecting in a center-fed panel in a residential setting?a problem that did not exist previously. Arguably, this is more of a language problem than a safety problem.
The safety concern that breaker location is intended to address is really unique to commercial services and commercial load profiles. If you assume continuous loads AND full loading of the busbar, then you want the main OCPD and the PV OCPD at opposite ends of the busbar so that there is no single spot on the busbar that is ever overloaded. This solution is applied to residential systems under the 2008 Code onward, which makes sense as a best practice. It also makes sense because it unifies the way that the 120% allowance is applied in the field. But previously the 120% allowance was considered safe in residential settings regardless of PV breaker location, and the nature of residential load profiles did not change. What changed is that the CMP introduced this concept of "at the opposite (load) end from the input feeder location or main breaker location." With a center-fed panel, the best you can do is to install a PV OCPD "as far as possible" from the main OCPD. However, you can never literally get the main OCPD and the PV OCPD at opposite ends of the busbar.
In practice, a literal interpretation of the breaker location requirement in center-fed panel in residential settings will disallow some proposed installations that are safe and consistent with decades of Code intent and interpretation. Presumably this is why the CPM has proposed adding an allowance in NEC 2014 for an engineer to supervise the decision about whether the 120% allowance can be applied to center-fed panel. It appears that they are trying to fix an unintended consequence of the changes that were made in 2008 to 690.64(B)(7) [now 705.12(D)(7).
It is an interesting study in how the sausage gets made....
