NEC 705 Interconnection Point Selection (Load Side vs Supply Side)

Why this matters

The interconnection point is the single largest design decision on a residential or commercial PV install. Pick wrong and the customer fails AHJ inspection, the utility refuses to grant permission to operate, or the install passes inspection and overloads the service-entrance conductors invisibly until something melts. NEC Article 705 governs interconnection of power-production sources to a premises wiring system, and the choice between load-side and supply-side connection is the first call the designer makes.

The two options

NEC 705.12 defines the two acceptable connection methods for a parallel power source on a utility-fed premises:

• Load-side connection (705.12(B)): the PV output lands on a breaker installed in the main service panel or a downstream subpanel. The PV current and the utility current combine inside the panel busbar.
• Supply-side connection (705.12(A)): the PV output lands ahead of the service main disconnect, typically via a tap into the service-entrance conductors between the meter and the main breaker. The PV current does not pass through the service-panel busbar.

Each method has a place. Picking by habit instead of by the math is the most common design error in residential solar.

When load-side is the right call

Load-side is preferred when:

• The service-panel busbar rating is large enough to accept the PV breaker per the 120 percent rule (see separate KB article on the math)
• The PV system is sized at or below the panel busbar capacity for back-feed
• The customer wants the simplicity of a single panel with one PV breaker visible

Most residential installs under 10 kW AC on a 200 amp panel work as load-side connections. The PV breaker installs at the opposite end of the busbar from the main breaker, properly marked, and the 120 percent rule passes.

When supply-side is required

Supply-side is required when:

• The PV system size exceeds what the 120 percent rule allows on the existing panel busbar
• The service panel is fully loaded with branch breakers and there is no physical slot for the PV breaker at the opposite end
• The customer's panel is a known recalled brand (Federal Pacific Stab-Lok, Zinsco) that should not have new circuits added regardless of capacity
• The system is on a service that does not have a discrete service panel (some commercial installs and some older residential meter mains)

Supply-side requires a fused or breakered disconnect ahead of the tap point, sized per NEC 705.31 to the PV inverter output. The tap itself uses listed tap connectors rated for the conductor sizes involved.

The mechanical execution of a supply-side tap

The supply-side tap is conducted between the utility meter base and the service main. The sequence:

1. Pull a service-disconnect-required permit; most AHJs require a utility coordination call before opening the meter
2. Schedule the utility cut so the meter is pulled while the work is conducted
3. Install the supply-side tap kit (typically Polaris or Burndy listed tap connectors) onto the existing service-entrance conductors
4. Run the new PV-disconnect conductors to a service-rated disconnect listed for use as service equipment
5. From the disconnect, run to the inverter AC output (or the inverter AC combiner)
6. Restore the meter and verify the utility-side voltage and PV-side voltage independently before energizing

This work is hot or near-hot every time. PPE per NFPA 70E Article 130 is mandatory. A residential solar shop without a service-disconnect process should not be conducting supply-side taps; subcontract to a licensed electrician with the documented process.

Marking and labeling

NEC 705.10 requires permanent plaque labeling at the service equipment identifying:

• The PV system location
• The PV system AC output rating
• The PV system disconnect location

For load-side connections, an additional label at the PV breaker reading "PV SYSTEM AC DISCONNECT" with red background and white letters. For supply-side, a label at the service main reading "DUAL POWER SOURCE - SECOND SOURCE LOCATED AT [location]."

Labels are inspector-visible. The first thing the inspector does is open the meter base or service-panel cover and look for the labels. A missing label is a fail; the install does not pass.

Utility-specific requirements layered on NEC 705

NEC 705 is the floor. Most utilities layer additional requirements:

• A visible-open lockable disconnect within sight of the meter, accessible to utility personnel without entering the building (most West-Coast utilities, all California IOUs)
• A revenue-grade meter at the PV output for net-metering settlement (some Midwest and Southeast utilities)
• A specific labeling format and color scheme (varies by utility)

Pull the utility's current interconnection handbook before the design phase, not after. A design done to NEC alone and submitted to a utility with stricter labeling or disconnect requirements gets rejected at PTO request.

Inverter listing and grid-support functions

NEC 705.40 requires the inverter to be listed to UL 1741 or UL 1741 SA (Supplement A for grid-support functions including voltage and frequency ride-through). California Rule 21 and Hawaii Rule 14 require SA-listed inverters with active grid-support functions enabled in the field setup. Verify the inverter and verify the field setup, not just the model number on the shipping carton.

Common design errors

• Designer assumes load-side because the prior job was load-side. The 120 percent rule must be checked per panel. A 200 amp panel with a 200 amp main and a 60 amp PV breaker passes; a 100 amp panel with a 100 amp main and a 50 amp PV breaker does not.
• Supply-side tap done without service-rated disconnect. A standard branch disconnect is not listed for service-equipment use. The inspector will fail the install. Spec a disconnect with the appropriate listing in the cut sheet.
• Labeling done in pencil or with non-permanent stickers. Engraved phenolic plaques or laser-etched anodized aluminum are the only acceptable label media in most jurisdictions.

References

• NEC 2023 Article 705 (Interconnected Electric Power Production Sources), specifically 705.10, 705.12, 705.30, 705.31, 705.40
• NFPA 70E 2024 Article 130 (work involving electrical hazards) and Annex D (arc-flash hazard analysis methods)
• UL 1741 / UL 1741 SA (Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources)
• IEEE 1547-2018 (Standard for Interconnection and Interoperability of Distributed Energy Resources)