Main panel upgrade vs supply-side tap

Adding solar back-feeds current into your main panel, and the busbar can only take so much. NEC 705.12 sets the limit, and when your panel fails it you have three paths: derate the main breaker, upgrade the panel, or move the connection to the supply side. Here is how to decide.

The rule that forces the decision

NEC 705.12(B) — the 120% rule — applies when a solar breaker back-feeds a busbar that also has a main breaker at the other end. The allowance:

• Main breaker + solar breaker <= 120% of busbar rating, with the solar breaker at the opposite end of the busbar from the main.

Worked example on a 200 A busbar:

• 120% of 200 A = 240 A allowed total.
• A 200 A main leaves 240 - 200 = 40 A for solar.
• A 40 A solar breaker back-feeds up to 32 A continuous — plenty for many homes, but a larger system exceeds it.

See the 120% busbar rule reference for the exact code language.

Option 1 — Derate the main breaker

If the calculated load allows it, swap the 200 A main for a 175 A or 150 A main:

• 240 - 175 = 65 A for solar; 240 - 150 = 90 A for solar.
• Cheapest fix — one breaker — but only works if your service load calculation supports the lower main.

This is the first option to check because it is the least disruptive.

Option 2 — Main panel upgrade

Replace the panel with a higher busbar rating (for example 225 A or larger):

• A 225 A busbar allows 270 A total; a 200 A main leaves 70 A for solar.
• A 400 A service with subpanels opens far more headroom.

An upgrade is the right call when the system is large, the load calc won't support a derate, or the existing panel is old, full, or unsafe. It costs the most and often involves the utility.

Option 3 — Supply-side (line-side) tap

A supply-side connection lands ahead of the main breaker, on the utility side of the service disconnect:

• The busbar 120% rule does not apply because you never touch the busbar.
• It fits large systems on a panel that otherwise has no room.
• It requires its own disconnect and fusing and careful conductor sizing, and some AHJs and utilities have specific rules on where and how it may be made.

Choosing quickly

Watch the load calculation, not just the busbar

The 120% rule is only half the story. A main breaker derate is only legal if a NEC load calculation shows the smaller main still covers your home's demand — you cannot simply drop to a 150 A main to make room for solar if the house actually draws more. Run the standard or optional load calculation first, because it can rule out the cheapest option before you commit to it. Likewise, a supply-side tap does not escape every rule: the tap conductors, disconnect, and fusing still have to be sized and located to code and to your utility's service requirements.

Every path has to satisfy your load calculation and the enforced NEC edition, so confirm the approach with your AHJ and utility, and have a licensed electrician or PE review and stamp the interconnection. To see where this lands in the plan set, read what is in a solar permit set or browse the reference library.

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.