NEC 705

Quick Facts

What Does NEC 705 Cover?

NEC Article 705 establishes requirements for interconnection of multiple electric power sources (PV, ESS, wind, fuel cell, utility) operating in parallel with the primary source.

Key sections

• 705.10 — Identification of power sources (labels on disconnects).
• 705.11 — Source interconnection per IEEE 1547.
• 705.12 — Load-side source connections (the 120% rule).
• 705.13 — Equipment, conductor, and disconnect ratings.
• 705.20 — Source interconnect overcurrent protection.
• 705.22 — Source interconnect disconnecting means.
• 705.30 — Equipment / battery / source overcurrent.

Interconnection Methods

1. Load-side breaker (120% rule)

Most common. Solar OCPD backfed at the end of the busbar, opposite the main breaker.

Inverter_OCPD + Main_breaker ≤ 1.20 × Busbar_rating

Example: 200 A panel, 200 A main:

• 1.20 × 200 = 240 A allowed.
• Existing main: 200 A.
• Max inverter OCPD: 240 − 200 = 40 A.

2. Sum-of-breakers (705.12(B)(3)(2))

Multiple DER sources sharing one panel:

Σ(all backfed OCPDs) + Main_breaker ≤ 1.20 × Busbar_rating

3. Supply-side (line-side) connection

Connect inverter ahead of main breaker, via tap conductors or service-rated equipment. No 120% constraint.

4. Dedicated breaker (705.12(B)(2)(3))

Adding a new breaker (not connected to main) for the DER source. Used when retrofit panels can’t support additional 120% allowance.

5. Service entrance equipment dedicated to PV

A separate service-rated disconnect dedicated to the PV source. Common for commercial.

Worked Example — 120% Rule

Scenario: Existing 100 A residential panel, 100 A main breaker. Customer wants 8 kW solar.

1.20 × 100 = 120 A allowed
Main: 100 A
Max inverter OCPD: 120 − 100 = 20 A
Max inverter AC output: 20 / 1.25 = 16 A continuous
At 240 V → max 3.8 kW

8 kW exceeds 3.8 kW limit. Options:

• Upgrade to 200 A panel.
• Line-side tap (bypass 120% rule).
• Reduce solar system size.

Labels Required (NEC 705.10)

• “WARNING: DUAL POWER SOURCE” at all disconnects.
• “PHOTOVOLTAIC SYSTEM” plaque at the service entrance per NEC 690.56(B).

Permitting Implications

NEC 705 compliance is verified on every solar permit application. SLD must show the chosen interconnection method and corresponding calculation. Plan reviewers flag:

• Violation of 120% rule.
• Missing source labels.
• Incorrect tap conductor sizing.
• Backfed breaker not at end of busbar (where required).

Common Mistakes

1. Backfed breaker placed in middle of busbar (must be at opposite end from main).
2. Forgetting to include main breaker in 120% sum.
3. Multiple DERs without applying sum-of-breakers rule.
4. Line-side tap without service-rated equipment.
5. Missing “DUAL POWER SOURCE” labels.

Best Practices

• Run the 120% calculation explicitly on the SLD.
• For commercial, prefer supply-side or dedicated service for flexibility.
• Label every disconnect per NEC 705.10.
• Document the chosen method (load-side, line-side, supply-side) in design notes.

Standards & Certifications

• NEC 705 — Source interconnection.
• IEEE 1547-2018 — DER interconnection (technical compliance).
• UL 1741-SB — Inverter listing.
• Local utility interconnection rules (Rule 21, NY SIR, etc.).

Key Takeaways

• NEC 705 governs how PV and DER sources interconnect to the existing electrical service.
• The 120% rule limits backfed solar OCPD + main breaker ≤ 120% of busbar.
• Alternative methods: line-side tap, supply-side connection, dedicated breaker, dedicated service.
• NEC 705 labels (dual power source) and NEC 690.56 placard (rapid shutdown) appear on every solar interconnection.
• NEC 2023 expanded provisions for bidirectional storage and clarified source disconnect requirements.