Solar Education · Electrical
Solar Backfeed &
Derate Calculations
The 120% rule determines how large a solar system your electrical panel can support. Understanding this calculation explains why some homeowners need a panel upgrade before going solar — and how to avoid it.
The 120% Rule: NEC 705.12(B)
The National Electrical Code (NEC) Section 705.12(B) governs how solar systems connect to your home's electrical panel. The rule is straightforward:
NEC 705.12(B) — The 120% Rule
Main Breaker (A) + Solar Backfeed Breaker (A) ≤ Busbar Rating (A) × 1.20
Example: 200A main + 40A solar = 240A ≤ 200A busbar × 1.20 = 240A ✓
The 20% overage is allowed because solar and grid power are rarely at maximum simultaneously — the panel busbar is unlikely to see both at full capacity at the same time. But the calculation must still be verified and documented on every solar permit.
Key Terms
Busbar rating
The maximum continuous current the panel's internal bus can carry. Usually stamped inside the panel door. Common ratings: 100A, 125A, 150A, 200A, 225A. The busbar rating is often — but not always — equal to the main breaker rating.
Main breaker
The large double-pole breaker at the top of your panel that controls all power to the home. Typically 100A, 150A, or 200A for residential panels.
Backfeed breaker
The breaker in your panel that the solar inverter connects to. Solar power "backfeeds" through this breaker into the panel busbar and then to your loads or the grid.
Derate
Reducing a breaker or conductor rating to account for temperature, conduit fill, or other factors. Solar wire runs in conduit exposed to Fresno's summer heat often require derating — a 10 AWG wire rated at 30A may derate to 24A in a hot conduit.
Real Calculation Examples
These are the four scenarios Robbins Energy encounters most often on Fresno-area solar installations.
Standard 200A Panel — 8 kW System
PassesBusbar
200A
Main breaker
200A
Max backfeed
40A
System size
~9.6 kW max
200A × 1.2 = 240A. 200A main + 40A backfeed = 240A. Exactly at limit.
Standard 200A Panel — 12 kW System
Fails — requires upgradeBusbar
200A
Main breaker
200A
Max backfeed
40A
System size
12 kW requires 62.5A breaker
200A + 62.5A = 262.5A, exceeds 240A limit. Panel upgrade or load-side tap required.
225A Busbar Panel — 12 kW System
PassesBusbar
225A
Main breaker
200A
Max backfeed
70A
System size
12 kW requires 62.5A breaker
225A × 1.2 = 270A. 200A main + 62.5A backfeed = 262.5A. Under limit.
200A Panel — 150A Main Breaker Reduction
PassesBusbar
200A
Main breaker
150A (reduced)
Max backfeed
90A
System size
Up to ~21.6 kW
200A × 1.2 = 240A. 150A main + 90A backfeed = 240A. Allows much larger system.
Conduit Derate in
Fresno Summer Heat
Wire ampacity ratings assume a 30°C (86°F) ambient temperature. In Fresno, conduit runs on south-facing walls or in attics can reach 50–60°C (122–140°F) in summer. NEC Table 310.15(B)(1) requires derating wire ampacity when ambient temperature exceeds 30°C.
Example: 10 AWG THWN-2 wire
What this means for your system
- Conduit runs in attics or on south walls must use the derated ampacity for breaker sizing
- A 30A backfeed breaker may require 8 AWG wire (not 10 AWG) if the conduit run is in a hot location
- Fresno inspectors are familiar with this requirement — undersized wire in hot conduit will fail inspection
- Robbins Energy accounts for conduit location and ambient temperature in every system design
Frequently Asked Questions
What is the 120% rule for solar backfeed?
The 120% rule (NEC 705.12(B)) states that the sum of the main breaker amperage plus the solar backfeed breaker amperage cannot exceed 120% of the panel's busbar rating. For a 200A panel with a 200A busbar, the maximum combined amperage is 240A. If your main breaker is 200A, your solar backfeed breaker can be no larger than 40A — which limits you to roughly a 9.6 kW system on a 240V panel.
What happens if my solar system exceeds the backfeed limit?
If the solar backfeed breaker plus the main breaker exceeds 120% of the busbar rating, the installation fails inspection. The inspector will require either a panel upgrade to a higher-rated busbar, a main breaker reduction (load-side tap), or a reduction in solar system size. In Fresno County and the City of Fresno, inspectors check this calculation on every solar permit.
Can I add more solar panels if I already have a system?
Yes, but the backfeed calculation must be re-run for the expanded system. If your existing system already uses the full 40A backfeed allowance on a 200A/200A panel, adding more panels requires either a panel upgrade or a load-side tap arrangement. This is one of the most common reasons solar additions require a panel upgrade.
What is a load-side tap and when is it used?
A load-side tap (also called a supply-side connection or line-side tap) connects the solar system upstream of the main breaker, bypassing the 120% busbar limitation. It requires a separate disconnect and is more complex to install, but allows larger solar systems on panels that would otherwise be limited by the backfeed rule. It is commonly used for systems over 10 kW on standard 200A panels.
Does adding a battery change the backfeed calculation?
Yes. A battery system with a hybrid inverter that can export to the grid counts toward the backfeed calculation. A battery-only system (AC-coupled, no grid export) typically does not affect the calculation. Always confirm with your installer how the battery is configured and whether it changes the permit requirements.
What panel size do I need for a 10 kW solar system?
A 10 kW system at 240V draws approximately 41.7A. With a 25% safety factor (NEC 690.8), the required breaker is 52A. On a 200A panel with a 200A busbar, the 120% rule allows a maximum 40A backfeed breaker — so a 10 kW system typically requires either a 225A busbar panel, a load-side tap, or a main breaker reduction to 150A.
Need a Panel Assessment Before Going Solar?
Robbins Energy evaluates your panel, runs the backfeed calculation, and tells you exactly what your system can support — before you commit to a design. License #1063682.
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