One of the most critical — and often misunderstood — parts of PV system design is how the solar system ties into the utility service.
Choosing between a supply-side (line-side) or load-side (breaker-based) connection impacts not only your system cost and complexity, but also code compliance and inspection outcomes.
Let’s break it down in practical terms that every solar professional should understand before submitting a plan set.
What Is an Interconnection?
Interconnection defines where your solar inverter output (AC side) connects into the existing electrical system — either:
- Before the main service disconnect (supply-side)
- After the main service disconnect (load-side)
Each method has unique benefits, limitations, and NEC code implications.
1. Load-Side (Breaker) Interconnection
This is the most common method for residential and small commercial PV systems.
It ties the inverter output into the main panel or subpanel using a dedicated breaker.
Advantages
- Simpler installation
- No service disconnect modification
- Easy to design and permit
Challenges
- Must comply with the 120% rule under NEC 705.12(B)(2)
- Limited by busbar rating and main breaker size
For example:
If a main panel has a 200A busbar and a 200A main breaker, the total backfeed from solar cannot exceed 40A (20% of 200A).
That means you can use a 40A breaker for PV (≈9.6 kW @ 240V), unless you downsize the main breaker or add a PV subpanel.
2. Supply-Side (Line) Interconnection
In this configuration, the PV output ties ahead of the main service disconnect — directly into the service conductors.
This makes the solar system essentially a parallel service source to the utility feed.
Advantages
- Not limited by the 120% rule
- Ideal for larger systems where the main panel can’t handle PV backfeed
- Often used for commercial or high-capacity residential installs
Challenges
- Requires service entrance modifications
- Must comply with NEC 230.82(6) and 705.11
- AHJs and utilities often require utility coordination and sealed metering compartments
- Adds cost and complexity for installation and inspection
When to Choose Which
| Scenario | Recommended Method | Reason |
| Residential 7kW–15kW | Load-side | Simpler and passes most AHJs easily |
| Large Residential / Commercial (15kW–50kW) | Supply-side | Bypasses busbar limitations |
| Meter upgrades planned | Supply-side | Future-ready and cleaner design |
| Tight timeline / retrofit | Load-side | Faster install, minimal disruption |
Common Inspection Red Flags
- PV breaker placed at the top of panel bus (violates 120% rule if not derated)
- Missing tap conductor labeling in supply-side setups
- Utility meter enclosure tapped without utility approval
- Missing or incorrect PDP (Point of Connection) diagram in plan sets
Pro Tip
If you’re unsure, always start your design with a panel schedule review.
Check:
- Busbar rating
- Main breaker size
- Available breaker positions
This helps you determine if you can safely go load-side or need to design a supply-side tap.
Conclusion
Choosing between supply-side and load-side interconnection is not just about convenience — it’s about ensuring long-term system safety, passing inspections without redlines, and aligning with NEC and utility standards.
By planning your connection strategy early, you’ll avoid costly redesigns, save time in permitting, and keep your installation teams focused on clean, compliant work.
Need help reviewing your single-line diagram or verifying interconnection compliance before submission?
Vishtik’s solar design experts can optimize your permit set for your utility and AHJ — ensuring zero rejections and smooth approvals.
Contact us today to streamline your next solar project.
