"Let’s preface this discussion by reminding ourselves that no matter how much we know, and how many NEC Articles we can quote by memory, your friendly local AHJ is always right. So be knowledgeable, be persuasive, open possibilities, but always be friendly, professional, and willing to take no for an answer.
The easiest and most cost effective AC connection point for your PV system has always been popping a new breaker into the closest AC main‐ or sub‐panel. This is a load‐side connection, as it’s on the customer’s side of the main service disconnect. But as average PV system size has grown, many dealers have run up hard against the 120% rule. NEC Article 690.64(B)(2) states that the sum of all input breakers amperage may not exceed 120% of the panel’s busbar rating. (Or 100% for those still using the 2005 NEC Code in a commercial/industrial application.) So a common 125A main panel with a 125A main breaker is limited to no more than a 25A PV input breaker. If you’ve got more than about 4,500 watts of PV to land, you’ve got a problem. While it’s possible to install a smaller main breaker, allowing the PV input breaker some more amps, that option can lead to nuisance‐popping of the new under‐sized main. Or you can upgrade the main panel, but that’s an expensive and usually last‐ditch choice. A smarter option, and one that many PV dealers employ regularly, is to install a new Line‐Side AC Connection (also commonly called a Supply‐Side connection).
NEC Article 690.64(A) allows a Line‐Side AC connection to busbars, conductors, or lugs at any point between the customer’s side of the utility meter, and the service disconnect. Many folks call this a “line‐side tap”, which leads to confusion with the “10‐foot tap rule” and Article 240.21(B). Tap rules and Article 240 do not apply here. These aren’t feeders, and service entrance cables don’t have overcurrent protection. Save yourself some trouble and confusion, just don’t utter the word “tap” in the inspector’s presence. You are adding a new supply‐side or line‐side AC connection per Article 690.64(A). This new connection must use 60A rated conductors and a 60A disconnect at a minimum. Maximum size can be anything up to the amperage of the existing service. So if it’s a 200A service, you can install a new 200A supply‐ side connection. The disconnect and fusing can either be a fused disconnect, most common when you’re landing a single large inverter, or a new Solar‐Only breaker panel with appropriately sized main breaker when you’re landing multiple inverters. If you use a fused disconnect, the disco must be minimum 60A rated, although the fuses can be smaller to properly match the inverter. As much as available space allows, this new service disconnect must be accessible, near the service entrance, and grouped with existing service disconnects. (Articles 230.66, 230.70(A)(1), and 230.72(A)).
Okay, you’ve determined you need to install a line‐side AC connection on your next big PV install. How do you actually make the connection to the existing service? The devil’s in the details! This where a good experienced electrician really earns his salt. How you make the physical connection depends on what the client has installed. And because of the WIDE variety of installed service entrance hardware, we’re going to go a bit fuzzy on you here. Basically, you need to connect to the service entrance conductors, busbars, or lugs at some point between the customer’s side of the meter, and the main service disconnect. How you do that in a particular situation depends on your (or your electrician’s) ingenuity. Busbar connectors often make this easy with ready‐made connection points you simply bolt up to. Your electrical supplier will have other devices for conductor attachments, such as insulation‐piercing tap splice connectors, insulated terminal blocks, parallel tap connectors, crimped parallel connectors, and the venerable (but bulky when properly taped) split bolt. Available space with elbow room for installation is always the problem here, particularly with older service hardware. Newer hardware seems to be built with a bit more wiggle room. In all cases you’ll need to pull the meter and disrupt the client’s power while connections are being made and secured. Plan ahead with all tools, cabling, and hardware at hand to minimize disruptions. Some electric companies will let you pull the meter yourself, others will want to be on site and supervise. Check locally.
Line‐Side AC connections are challenging the first few times, but get easier every time. Increasingly as we run into already overcrowded, undersized AC panels, this has become the favored installation path for larger residential and almost all commercial systems."
All credit to Soligent