What Size Solar Breaker Can You Back-Feed Into a 225 A Panel With a 125 A Main?
Short answer: 125 A. On a 225 A busbar with a 125 A main, NEC 705.12(B)'s 120% allowance is 270 A, leaving 145 A for the solar breaker — which supports roughly a 24 kW inverter at 240 V.
The rule, applied to this panel
NEC 705.12(B)(3)(2) — the "120% rule" — lets the busbar rating × 1.2 be shared between the main breaker and the sum of back-fed supply breakers, provided the solar breaker sits at the opposite end of the busbar from the main.
- Busbar rating: 225 A
- 120% allowance: 225 × 1.2 = 270 A
- Main breaker: 125 A
- Remaining for solar: 270 − 125 = 145 A
Every breaker size on this configuration
| Back-fed breaker | Main + back-fed | 120% allowance | Headroom | Result |
|---|---|---|---|---|
| 15 A | 140 A | 270 A | 130 A | allowed |
| 20 A | 145 A | 270 A | 125 A | allowed |
| 25 A | 150 A | 270 A | 120 A | allowed |
| 30 A | 155 A | 270 A | 115 A | allowed |
| 35 A | 160 A | 270 A | 110 A | allowed |
| 40 A | 165 A | 270 A | 105 A | allowed |
| 45 A | 170 A | 270 A | 100 A | allowed |
| 50 A | 175 A | 270 A | 95 A | allowed |
| 60 A | 185 A | 270 A | 85 A | allowed |
| 70 A | 195 A | 270 A | 75 A | allowed |
| 80 A | 205 A | 270 A | 65 A | allowed |
| 90 A | 215 A | 270 A | 55 A | allowed |
| 100 A | 225 A | 270 A | 45 A | allowed |
| 125 A | 250 A | 270 A | 20 A | allowed |
Before you rely on this
Passing the arithmetic is necessary but not sufficient. The solar breaker must be at the opposite end of the busbar from the main — that physical placement is what the 120% allowance is premised on, because it keeps the busbar from carrying both sources across its full length. The panel also needs the 705.12(B)(3) warning stating the busbar is fed from multiple sources and the breaker must not be relocated. A reviewer who sees correct arithmetic and no placard will still red-line the set.
If 125 A is not enough for your inverter, the options below apply just as they would to a failing calculation.
Where the 120% rule does not get you there, these are the code-compliant routes:
- Derate the main. Dropping to a 110 A main frees headroom for a solar breaker, and is usually the cheapest fix — provided the NEC 220.82 calculated load actually supports the smaller main. Run the load calc before promising it.
- Supply-side (line-side) tap per 705.11, landing ahead of the service disconnect. The busbar is never back-fed, so the 120% rule does not apply at all. This is the standard answer when a large inverter meets a small panel.
- Feed-through / sum rule. 705.12(B) also permits the sum of all breakers to stay within the busbar rating, which is more restrictive but occasionally the cleaner path.
- Panel upgrade to a larger busbar — the most expensive option, and the one to price against a supply-side tap rather than assume.
Related
Computed with OneLine Studio's NEC engine — the same deterministic code tables that size a real permit set, not an estimate. Conductor ampacity from NEC 310.16, correction factors from 310.15(B)(1), fill adjustment from 310.15(C)(1). Always confirm the enforced NEC edition and any local amendments with your AHJ.
FAQ
What is the largest solar breaker I can back-feed into a 225 A panel with a 125 A main?
125 A. The 120% allowance is 270 A, the main uses 125 A, leaving 145 A — so 125 A is the largest standard breaker that fits.
What size inverter does that support?
Roughly 24 kW at 240 V single phase. A back-fed breaker must be at least 125% of the inverter's continuous output current, so a 125 A breaker supports about 100 A of continuous output.
Why does the breaker have to be at the opposite end of the busbar?
The 120% allowance assumes the two sources feed from opposite ends, so no single section of busbar carries the full combined current. Relocating the breaker defeats that assumption, which is exactly why 705.12(B)(3) requires a permanent label warning against it.
Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.
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