Electrical / runnable MCP tool
Run the Solar Backfeed Breaker Calculator (NEC 705.12 120% Rule) in your AI assistant
Through the free Intry MCP server, Claude, ChatGPT, or Cursor runs this exact calculator and hands back a verdict and the cited source, not a guess. Tool name: solar_backfeed_breaker.
What you get back: the max backfeed breaker, the supportable inverter continuous output (breaker / 1.25), and a verdict.
How it computes the answer
Computed from the NEC 705.12(B)(2) 120% busbar allowance, rounded down to a standard 240.6(A) breaker and held to the busbar rating; where the backfeed is NOT at the opposite end of the bus, 705.12(B)(1) governs instead at 100% and gives a smaller breaker than this.
What it cites
Your assistant calls the same engine the interactive calculator calls, with the inputs listed under Inputs. These are the sources that engine reads.
- NEC 705.12(B)(2)
- The 120 percent allowance, and it is a CONDITIONAL rule rather than a general one. NFPA 70-2023, p. 70-639 and p. 70-640, read from the printed folio: it applies "where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads", and then "the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the busbar ampere rating." Two further requirements print in the same paragraph and the tool repeats both: the busbar must be sized for the connected loads per Article 220, and a permanent label reading "WARNING: POWER SOURCE OUTPUT DO NOT RELOCATE THIS OVERCURRENT DEVICE" must be applied to the equipment. Whether the two sources really sit at opposite ends is a site fact the tool cannot see, so it asks rather than assumes.
- NEC 705.12(B)(1)
- Same folio, p. 70-639: "The sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed the busbar ampere rating." That is 100 percent of the busbar, not 120, and it is the rule that applies when the backfeed breaker is NOT at the opposite end of the bus from the main. It yields a smaller breaker than the figure this tool prints, which is why the tool names it in the site-condition note it attaches to every answer rather than leaving the reader to find it.
- NEC 408.36
- The bound that holds whatever the main breaker reads. NFPA 70-2023, p. 70-305: "a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supply side of the panelboard." Read strictly that requires the panelboard to be protected by a device at or under its rating, so the sentence that caps the backfeed device itself is the second one in 705.12(B)(3), p. 70-640: the rating of the overcurrent device protecting the busbar shall not exceed the rating of the busbar. Together they are why no figure here may exceed the busbar rating whatever the main reads, and a mistyped 0.5 A main on a 200 A busbar is refused rather than answered with the 225 A breaker the arithmetic alone would allow. It cannot catch every mistyped main: a 16 A main on a 200 A bus is as unlikely as a 15 A one and only the 15 A case is refused, so check the main against the rating stamped in the panel.
- NEC 240.6(A)
- Standard overcurrent-device ratings, p. 70-124. The raw allowance is rounded DOWN to a real breaker, because a computed 55 A of headroom is not a size anyone can buy: on a 200 A busbar with a 200 A main the engine returns 240 minus 200 = 40 A and a 40 A breaker, supporting 32 A of continuous inverter output at the 125 percent factor.
These are NEC limits applied to the two ratings you enter, and both of those come off the panel label rather than from us. The tool bounds the BREAKER, which is the conservative direction: a breaker at or under the allowance guarantees the code inequality holds for the inverter current beneath it. Whether the backfeed breaker actually lands at the opposite end of the busbar from the main, whether the busbar is sized for its connected loads under Article 220, and whether the required warning label is applied are all site conditions only you can settle. Your AHJ and the utility interconnection agreement have the final say.
The full receipt, with every source and how each is cross-checked, is on the interactive Solar Backfeed Breaker Calculator.
Ask your assistant
The one rule that makes it reliable: name Intry in your question. That is what tells Claude to run the solar_backfeed_breaker tool and answer with the real number, the verdict, and the citation. Leave Intry out and Claude may just guess from memory, which is exactly what you are trying to avoid. Type something like:
Inputs
| Field | Required | Accepts | What it is |
|---|---|---|---|
| busbarAmps | yes | number | Panel busbar / bus rating in amps (e.g. 200, 225). |
| mainOcpdAmps | yes | number | The main breaker (busbar overcurrent device) rating in amps (e.g. 200). |
Add Intry to Claude
One paste and Claude can run this tool and every other Intry calculator, right from your phone.
- Claude opens with the Intry connector already filled in. Review it and click Add.
- (If a field is blank, paste the link below into the URL field.)
- In any chat, tap + then Connectors and switch Intry on. Then name Intry in your question (“use Intry to…”) so Claude runs the tool instead of guessing.
https://www.intrysys.com/api/mcpNo login, no API key, and it works on every Claude plan, right from your phone.
Using ChatGPT, Cursor, or a dev tool? See the developer setup on the main page. The endpoint is https://www.intrysys.com/api/mcp.
Every answer comes with the cited NEC section, so you can check it or show an inspector. The numbers come from the same calculator we run on intrysys.com, and we re-check every one automatically before anything goes live, so the number Claude hands you matches the code.