PV Interconnection Method Selector

The default load-side interconnection: solar lands on a backfed breaker in the existing main service panel. The panel busbar is normally protected only by its main breaker (main ≤ bus). Adding solar as a second source from the opposite end of the bus is allowed because loads tap off BETWEEN the two sources, so no single bus segment ever carries both the full main and the full solar feed. NEC permits this when (main OCPD) + (PV breaker) ≤ 1.20 × busbar rating. The PV breaker is the next standard size at or above 125% of the inverter output current (690.8) and must sit at the OPPOSITE end of the bus from the main with a permanent "DO NOT RELOCATE" label, that placement is what makes the 20% headroom valid.

The canonical 120% case: on a Separate MDP service, solar lands on a backfed breaker in the existing main panel bus. No separate AC disconnect is added, the breaker itself is the OCPD. The bus is normally bounded by its main breaker; the PV breaker enters from the opposite end and loads tap off between the two sources, so no segment carries both at once. The connection is legal when (main OCPD) + (PV breaker) ≤ 1.20 × busbar rating, with the PV breaker sized at the next standard size ≥ 125% × inverter output (690.8) and placed at the opposite end of the bus from the main under a permanent "DO NOT RELOCATE" label.

Same as Backfeed Breaker - Main Panel, but the existing main breaker is DERATED to a lower rating so the 120% sum clears. When (main OCPD) + (PV breaker) > 1.20 × busbar, swapping the main to a smaller standard size shrinks the first term until the inequality passes, the derated main still protects the busbar and the service conductors (240.4). The PV breaker still enters at the opposite end of the bus under a "DO NOT RELOCATE" label. The catch: the derated main must still be large enough to serve the dwelling's Article 220 calculated load, you cannot derate below what the loads actually draw.

On a meter-main COMBO (meter + service main breaker + distribution bus in one enclosure) with no separate feedthrough panel, solar lands on a backfed breaker directly on the combo's distribution busbar. The combo's own main breaker bounds that bus; the PV breaker enters from the opposite end and loads tap off between the two sources. The 120% allowance applies to the COMBO busbar: (main OCPD) + (PV breaker) ≤ 1.20 × busbar rating, PV breaker sized at the next standard size ≥ 125% × inverter output (690.8), placed opposite the main under a "DO NOT RELOCATE" label.

Same as Backfeed Breaker - Meter-Main, but the meter-main combo's main breaker is DERATED so the 120% sum clears on the combo bus. When (main OCPD) + (PV breaker) > 1.20 × busbar, swapping the combo main to a smaller standard size shrinks the first term until the inequality passes. The derated combo main still protects the busbar and service conductors (240.4), and must still serve the dwelling's Article 220 calculated load. The PV breaker enters at the opposite end of the bus under a "DO NOT RELOCATE" label.

Solar lands on a backfed breaker in the meter-main COMBO bus, with an existing downstream feedthrough panel (with its own main breaker) carrying the dwelling loads. The 120% allowance applies to the COMBO busbar where the PV breaker actually lands: (combo main OCPD) + (PV breaker) ≤ 1.20 × combo busbar. The downstream feedthrough panel is untouched, it keeps its own main and is fed normally; it is not where solar interconnects, so its busbar is not part of the 120% calc here. The PV breaker enters opposite the combo main under a "DO NOT RELOCATE" label.

Solar lands on a backfed breaker in the meter-main COMBO bus, and a NEW downstream feedthrough panel (with its own main breaker) is installed to host the dwelling loads. The 120% allowance applies to the COMBO busbar where the PV breaker lands: (combo main OCPD) + (PV breaker) ≤ 1.20 × combo busbar. The new feedthrough panel is a downstream sub-distribution panel, the service N-G bond and grounding electrode stay at the meter-main combo (250.24(A)(5)); the new panel is fed with a 4-wire feeder and a separate EGC, NOT re-bonded. The new panel + its main are NEW equipment. The PV breaker enters opposite the combo main under a "DO NOT RELOCATE" label.

Solar lands on a backfed breaker in the meter-main COMBO bus with an existing downstream feedthrough panel, and the combo main breaker is DERATED so the 120% sum clears. When (combo main OCPD) + (PV breaker) > 1.20 × combo busbar, swapping the combo main to a smaller standard size shrinks the first term until the inequality passes. The derated combo main still protects the busbar and conductors (240.4) and must still serve the dwelling's Article 220 calculated load. The PV breaker enters opposite the combo main under a "DO NOT RELOCATE" label; the existing downstream feedthrough panel is fed normally.

Solar lands on a backfed breaker in the meter-main COMBO bus, the combo main is DERATED to clear the 120% sum, and an existing downstream feedthrough panel with its own main breaker carries the loads. When (combo main OCPD) + (PV breaker) > 1.20 × combo busbar, swapping the combo main to a smaller standard size shrinks the first term until the inequality passes. The derated combo main still protects the busbar and conductors (240.4) and must still serve the Article 220 calculated load. The downstream feedthrough panel keeps its own main and is fed normally; the PV breaker enters opposite the combo main under a "DO NOT RELOCATE" label.

Solar lands on a backfed breaker in a downstream MLO (main-lug-only) FEEDTHROUGH panel rather than the meter-main combo. The 120% allowance applies to the FEEDTHROUGH panel busbar, but an MLO panel has no main breaker of its own to form the first term, so the bounding OCPD is the upstream device protecting the feed to that panel (the combo main or the feedthrough feeder breaker). The calc is (upstream feed OCPD) + (PV breaker) ≤ 1.20 × feedthrough busbar. The PV breaker is sized at the next standard size ≥ 125% × inverter output (690.8) and lands at the opposite end of the feedthrough bus from the feed-in lugs under a "DO NOT RELOCATE" label.

Solar lands on a backfed breaker in a downstream FEEDTHROUGH panel that has its OWN main breaker. Because the panel has a main, the 120% calc is the clean classic form on the FEEDTHROUGH busbar: (feedthrough panel main OCPD) + (PV breaker) ≤ 1.20 × feedthrough busbar. The PV breaker is sized at the next standard size ≥ 125% × inverter output (690.8) and lands at the opposite end of the feedthrough bus from its main under a "DO NOT RELOCATE" label. The upstream meter-main combo is not where solar interconnects, so its bus is not part of this calc.

Solar lands on a backfed breaker in a downstream MLO feedthrough panel, and the UPSTREAM main breaker (the device feeding that panel) is DERATED so the 120% sum clears on the feedthrough bus. An MLO panel has no main of its own, so the upstream feed OCPD forms the first term: (upstream feed OCPD) + (PV breaker) ≤ 1.20 × feedthrough busbar, derating the upstream device shrinks the first term until it passes. The derated upstream main still protects the feedthrough conductors and bus (240.4) and must still serve the Article 220 calculated load. The PV breaker lands opposite the feed-in end under a "DO NOT RELOCATE" label.

Solar lands on a backfed breaker in a downstream feedthrough panel that has its OWN main, and the UPSTREAM main breaker (feeding that panel) is DERATED. The 120% calc itself is the classic form on the feedthrough bus: (feedthrough panel main OCPD) + (PV breaker) ≤ 1.20 × feedthrough busbar. The upstream derate is applied so the upstream feeder/service side stays coordinated, the derated upstream device still protects the feedthrough conductors (240.4) and must still serve the Article 220 calculated load. The PV breaker lands at the opposite end of the feedthrough bus from the panel's main under a "DO NOT RELOCATE" label.

On a Separate Disconnect service (a standalone service disconnect ahead of the distribution panel), solar lands on a backfed breaker in the downstream feedthrough panel. The 120% allowance applies to that FEEDTHROUGH panel busbar, bounded by the OCPD protecting the feed to it (the upstream service disconnect main, or the panel's own main if present): (feed OCPD) + (PV breaker) ≤ 1.20 × feedthrough busbar. The PV breaker is sized at the next standard size ≥ 125% × inverter output (690.8) and lands at the opposite end of the feedthrough bus under a "DO NOT RELOCATE" label.

On a Separate Disconnect service, solar lands on a backfed breaker in the downstream feedthrough panel, and the UPSTREAM service main disconnect is DERATED so the 120% sum clears on the feedthrough bus. The feedthrough bus is bounded by the OCPD feeding it; derating the upstream disconnect main shrinks the first term of (feed OCPD) + (PV breaker) ≤ 1.20 × feedthrough busbar until it passes. The derated disconnect main still protects the feedthrough conductors and bus (240.4) and must still serve the Article 220 calculated load. The PV breaker lands at the opposite end of the feedthrough bus under a "DO NOT RELOCATE" label.

On a meter-main DISCONNECT service (combined meter + service main breaker + N-G bond + GES, no load distribution of its own), solar lands on a backfed breaker in the downstream MLO feedthrough panel that the disconnect feeds. The 120% allowance applies to that FEEDTHROUGH panel busbar, and since it is MLO with no main of its own, the bounding OCPD is the meter-main disconnect's main feeding it: (disconnect main OCPD) + (PV breaker) ≤ 1.20 × feedthrough busbar. The PV breaker is sized at the next standard size ≥ 125% × inverter output (690.8) and lands at the opposite end of the feedthrough bus from the feed-in lugs under a "DO NOT RELOCATE" label.

On a meter-main DISCONNECT service, solar lands on a backfed breaker in the downstream feedthrough panel that has its OWN main breaker. Because the panel has a main, the 120% calc is the clean classic form on the FEEDTHROUGH busbar: (feedthrough panel main OCPD) + (PV breaker) ≤ 1.20 × feedthrough busbar. The PV breaker is sized at the next standard size ≥ 125% × inverter output (690.8) and lands at the opposite end of the feedthrough bus from its main under a "DO NOT RELOCATE" label. The meter-main disconnect upstream is not the interconnection point, so its rating does not enter this calc.

On a meter-main DISCONNECT service, solar lands on a backfed breaker in the downstream MLO feedthrough panel, and the meter-main disconnect's main breaker is DERATED so the 120% sum clears on the feedthrough bus. The MLO panel has no main of its own, so the disconnect main forms the first term: (disconnect main OCPD) + (PV breaker) ≤ 1.20 × feedthrough busbar, derating the disconnect main shrinks the first term until it passes. The derated disconnect main still protects the feedthrough conductors and bus (240.4) and must still serve the Article 220 calculated load. The PV breaker lands at the opposite end of the feedthrough bus under a "DO NOT RELOCATE" label.

On a meter-main DISCONNECT service, solar lands on a backfed breaker in the downstream feedthrough panel that has its OWN main, and the meter-main disconnect main is DERATED. The 120% calc itself is the classic form on the feedthrough bus: (feedthrough panel main OCPD) + (PV breaker) ≤ 1.20 × feedthrough busbar. The upstream derate keeps the meter-main disconnect coordinated with the feeder, the derated disconnect main still protects the feedthrough conductors (240.4) and must still serve the Article 220 calculated load. The PV breaker lands at the opposite end of the feedthrough bus from the panel's main under a "DO NOT RELOCATE" label.

Backfeed breaker on the meter-main combo bus, qualified by the SUM RULE instead of the 120% rule. Where the 120% rule allows main + PV ≤ 1.20 × busbar, the sum rule is tighter: the sum of ALL overcurrent devices supplying the busbar (main + PV) must not exceed the busbar ampere rating, no 20% headroom. Used when the 120% math fails but the conductors/busbar still have margin at 100%. Its tradeoff cuts both ways: tighter ampere budget, but it does NOT carry the 120% rule's opposite-end placement requirement, so the PV breaker can sit anywhere on the bus. The existing downstream feedthrough panel (with its own main) is unchanged.

Same sum-rule backfeed on the meter-main combo bus as Backfeed with Sum Rule - Meter-Main, but a NEW downstream feedthrough panel (with its own main) is installed as part of the scope. The sum rule still governs the combo busbar (main + PV ≤ busbar, no 120% headroom); the new feedthrough panel is added downstream as a separate enclosure to host the dwelling loads. The new panel + its main are NEW equipment; the service-entrance N-G bond stays at the combo (250.24(A)(5)).

Load-side tap on an existing downstream feedthrough panel (with its own main), qualified by the SUM RULE on that panel's busbar. The tap brings the PV source onto the feedthrough panel; the sum of the panel's main + the PV contribution must stay within the panel busbar rating (no 120% headroom). This is the sum-rule alternative to Load Side Tap - Feedthrough with Main, chosen when the panel bus can't satisfy the 120% allowance but does hold at 100%. A fused AC disconnect protects the tap conductors (705.12 feed-through + 240.21(B)).

Load-side tap qualified by the SUM RULE, but a NEW feedthrough panel (with its own main) is installed downstream to host the loads. The sum rule governs the new panel's busbar (its main + the tapped PV ≤ busbar, no 120% headroom). Same as Load Tap with Sum Rule but the downstream panel is new equipment rather than existing. A fused AC disconnect protects the tap; the new panel is a sub-distribution panel (no N-G re-bond, 250.24(A)(5)).

On a meter-main COMBO (one enclosure: meter + service main breaker + distribution bus), solar interconnects as a LOAD-SIDE TAP on the combo's feedthrough lugs through a fused AC disconnect. No separate downstream feedthrough panel is modeled, the tapped feedthrough conductors carry on to the dwelling loads. The tap is on the load side of the service main (NEC 705.12 load-side connection). A tap does not increase the feeder's load (PV is a source offsetting utility draw); post-tap the feeder is evaluated under NEC 705.12(A), Feeders and Feeder Taps. It complies via (A)(2)(b): an OCPD ≤ the feeder ampacity at the load-side connection. The 310.12(B) 83% main-feeder allowance was the pre-solar basis for the feeder size. The fused disconnect provides the OCPD the feedthrough conductors lack at the tap point.

On a meter-main combo, solar taps the feedthrough conductors that feed an EXISTING downstream distribution panel, and that panel already has its OWN main breaker. The tap is on the load side of the combo main (NEC 705.12 load-side / feed-through), through a fused AC disconnect. A tap does not increase the feeder's load (PV is a source offsetting utility draw); post-tap the feeder is evaluated under NEC 705.12(A). Because the downstream panel's existing main is an OCPD ≤ the feeder ampacity at the load-side connection, the feeder is compliant via 705.12(A)(2)(b) without adding equipment, this is the clean case. Distinct from Load Side Tap - Feedthrough (No Main) (no downstream panel) and Load Side Tap - Meter-Main Disconnect (downstream panel is MLO and a main must be ADDED).

Same load-side feed-through tap on a meter-main combo, but here a NEW feedthrough panel (with its own main breaker) is INSTALLED downstream as part of the solar scope, rather than relying on an existing downstream panel. Solar taps the feedthrough conductors through a fused AC disconnect on the load side of the combo main. A tap does not increase the feeder's load (PV is a source offsetting utility draw); post-tap the feeder is evaluated under 705.12(A) and is compliant via (A)(2)(b), the new panel's main is an OCPD ≤ the feeder ampacity at the load-side connection. Use this when there is no suitable existing downstream panel, or the design intentionally adds a dedicated feedthrough/distribution panel. The new panel + its main are NEW equipment.

Load-side feed-through tap on a meter-main combo (no separate downstream panel, like Load Side Tap - Feedthrough (No Main)), but the upstream combo MAIN breaker is DERATED to a smaller rating. A tap does not increase the feeder's load; post-tap the feeder is evaluated under 705.12(A). Derating the primary main lowers the (A)(2)(a) threshold (primary OCPD + 125% PV) the feeder must meet, so the existing (smaller) feedthrough conductors can satisfy (A)(2)(a). Use this when the existing feedthrough conductors are sized smaller than the original main and upsizing them isn't practical, derating the main lowers the threshold the conductors must meet instead. Solar still taps through a fused AC disconnect on the load side.

Load-side feed-through tap on a meter-main combo feeding an EXISTING downstream panel that has its own main (like Load Side Tap - Feedthrough with Main), AND the upstream combo main is DERATED. A tap does not increase the feeder's load; post-tap the feeder is evaluated under 705.12(A). The existing downstream main already satisfies (A)(2)(b); derating the combo main additionally lowers the (A)(2)(a) threshold (primary OCPD + 125% PV) where the design wants margin on both paths. Combines two levers: the existing downstream main is an OCPD ≤ the feeder ampacity at the load-side connection (A)(2)(b), and the derated combo main lowers the (A)(2)(a) threshold, together keeping the feedthrough conductors compliant under 705.12(A).

A combined meter-main disconnect is the service disconnect, it holds the meter, the service main breaker, the neutral-to-ground bond, and the grounding-electrode connection. It has no load distribution of its own; its feedthrough lugs carry the dwelling's power through feedthrough conductors to the downstream main distribution panel. Solar interconnects here as a LOAD-SIDE TAP on those feedthrough conductors (NEC 705.12(B)(3)(6)), through a fused AC disconnect. The tap lands on the meter-main disconnect's feedthrough lugs, upstream of the downstream panel's main breaker. The key NEC consequence is protection coordination, not overload. A tap does not increase the feeder's load, the feedthrough still serves the same subset of dwelling loads, and PV is a source that offsets utility draw. Before solar, the feedthrough conductors are the dwelling's main power feeder and may be sized at 83% of the service rating under NEC 310.12(B). The moment solar taps in, the feeder is evaluated under NEC 705.12(A), Feeders and Feeder Taps. Because the tap is not at the opposite end of the feeder from the primary OCPD, 705.12(A)(2) applies: the load-side portion must be protected by an OCPD ≤ the feeder ampacity at the load-side connection (A)(2)(b), or the feeder ampacity must be ≥ the primary-source OCPD + 125% PV (A)(2)(a). That is why the downstream feedthrough panel must be CONVERTIBLE and carry a MAIN BREAKER ≤ the feedthrough conductor ampacity, that main is the (A)(2)(b) OCPD at the load-side connection. An MLO downstream panel has no such OCPD, leaving the feeder without a 705.12(A)(2)(b) path, so a main breaker is added as part of the solar scope.

On a meter-main DISCONNECT service (combined meter + service main breaker + N-G bond + GES, no load distribution of its own), solar taps the disconnect's feedthrough lugs and a NEW feedthrough panel (with its own main) is installed downstream to host the dwelling loads. The tap is on the load side of the disconnect main, through a fused AC disconnect. This is the NEW-panel counterpart to Load Side Tap - Meter-Main Disconnect (which converts an existing MLO downstream panel). A tap does not increase the feeder's load; post-tap the feeder is evaluated under 705.12(A) and is compliant via (A)(2)(b), the new panel's main is an OCPD ≤ the feeder ampacity at the load-side connection. The new panel + its main are NEW equipment.

The derate variant of Load Side Tap - Meter-Main Disconnect. On a meter-main DISCONNECT service, solar taps the disconnect's feedthrough lugs feeding the dwelling's existing downstream panel, which was MLO, so a main breaker is ADDED (convertible panel, shown (N)), AND the meter-main disconnect's main is DERATED. A tap does not increase the feeder's load; post-tap the feeder is evaluated under 705.12(A). The added downstream main is an OCPD ≤ the feeder ampacity at the load-side connection (A)(2)(b), and derating the disconnect main additionally lowers the (A)(2)(a) threshold (primary OCPD + 125% PV) where the design wants margin on both paths. Use when the design wants both levers, the added downstream main plus a lowered (A)(2)(a) threshold on the (smaller) existing feedthrough conductors.

On a Separate MDP service (meter → main panel), solar connects on the SUPPLY (line) side: it taps the service-entrance conductors between the meter and the main panel, AHEAD of the service main breaker, through a FUSED AC disconnect. Because the connection is on the line side of the service disconnect, the panel-busbar rules of 705.12 (120% / sum) do NOT apply, this is governed by NEC 705.11. The fused disconnect supplies the overcurrent protection the tapped service conductors otherwise lack, and becomes a (PV) service disconnect: neutral-ground bonded with a grounding-electrode connection. The limiting math is the SERVICE ampacity (sum of supply-side sources), not a busbar.

Same supply-side (line-side) connection as Supply Side Connection, but the tap is made inside a WIRE TROUGH (auxiliary gutter) installed between the meter and the main panel rather than as an in-panel tap. The trough gives clean, accessible space to land the line-side tap connectors and route the PV feeder to its fused disconnect, useful when the existing meter/panel has no room for the tap. NEC 705.11 governs; the trough must meet wiring-space fill (312.8).

A NEW fused service main disconnect is installed between the meter and the existing main panel; solar taps the line (supply) side of that new disconnect. Installing a service disconnect ahead of the panel demotes the existing panel to a downstream feeder/sub panel and MOVES the service-entrance role (N-G bond + grounding electrode) to the new disconnect (NEC 250.24(A)(5)). This is the supply-side answer when the existing panel busbar can't accommodate a backfeed breaker (120% fails) and a clean line-side tap point is wanted: the new fused disconnect both creates the tap point and provides the service OCPD. The new disconnect + its fuses are NEW equipment.

Identical topology to Supply Side + Fused Main, but the new service main disconnect is BREAKER-protected (a main breaker) rather than fusible. The OCPD for the service comes from the new disconnect's main breaker instead of fuses. Installer/AHJ choice driven by stock and preference; both are NEC 705.11-compliant. The new disconnect becomes the service disconnect (bond + GES move to it), the existing panel is demoted, and solar taps the line side of the new disconnect through its own fused PV disconnect.

A new fused MLO (main-lug-only) service disconnect is installed ahead of the existing panel, and solar taps its line side. Used when the EXISTING main panel has no main breaker, historically a "six-handle" service where up to six disconnects served as the service disconnecting means (NEC 230.71). Rather than rely on that arrangement, a new fused MLO disconnect establishes a single, clearly fused service disconnect; the existing panel becomes a downstream feeder panel. Solar connects supply-side at the new disconnect through its own fused PV disconnect.

Solar lands on a BACKFED BREAKER in an MLO (main-lug-only) panel that has NO main breaker. Electrically this is a SUPPLY-SIDE connection, not a 120% install: with no main breaker on the bus there is nothing to bound a 120% calc, and each breaker in an MLO service panel is itself one of the service disconnecting means (NEC 230.71, the "six-handle" rule). So the PV breaker is treated as an added service disconnect under 705.11, and the limit is the service ampacity (sum of all the disconnects/sources), NOT a busbar 120% figure. This is the classifier's "MLO override": a backfed breaker on a main-less bus is reclassified from oneTwentyRule to supplySlide.

On a Separate Disconnect service (meter → main disconnect → downstream panel), solar taps the line (supply) side conductors AT the existing main disconnect, through a fused PV disconnect. The connection is ahead of the existing service main, so it is a 705.11 supply-side connection, no 705.12 busbar rule. The existing main disconnect remains the service disconnect (keeps its bond/GES); the PV fused disconnect provides OCPD for the tapped conductors. Distinct from Supply Side Connection (Separate MDP) only by where the line-side conductors live, at a standalone disconnect rather than between meter and main panel.

Same supply-side connection at the existing main disconnect as Supply Side - Main Disconnect, but the line-side tap is made with a LUG or POLARIS (insulated multi-tap) connector on the service conductors rather than an in-enclosure tap kit. The Polaris/lug connector must be listed and rated for supply-side use (the conductors are unfused at that point). NEC 705.11 governs; the connector method is a means of making the tap, not a different interconnection, the fused PV disconnect still provides OCPD and the existing main disconnect stays the service disconnect.