Solar interconnection calculator
Your busbar rating is not your main breaker rating. A 225 amp bus behind a 200 amp main takes 70 amps of backfeed, not 40.
Four methods, and only one of them is unconditional
The 120 percent allowance is the famous one, and it is the one with a condition attached. When that condition is not established the answer is the plain comparison, which is always available. It is not the sum-of-every-device method, which has conditions of its own and on a busy panel is usually the harder test to pass.
The plain comparison
125 percent of the power source output current, plus the rating of the device protecting the busbar, has to fit within the busbar rating.
Always available. No condition attaches to it, which is why it is the answer whenever nothing else can be established.
2017 705.12(B)(2)(3)(a)2020 705.12(B)(3)(1)2023 not quoted2026 not quoted
The 120 percent allowance
The same sum is measured against 120 percent of the busbar rating instead of 100.
The supply and the power source sit at opposite ends of a busbar that carries load between them. Move that breaker and the allowance is gone, which is what the required marking exists to prevent.
2017 705.12(B)(2)(3)(b)2020 705.12(B)(3)(2)2023 not quoted2026 not quoted
The sum of every device
Every overcurrent device rating on the busbar is added up, supply and source alike, leaving out the one protecting the busbar. That total has to fit within the busbar rating.
An alternative rather than a fallback. It ignores where the breakers sit, and on a panel with many loaded breakers it is usually the harder test to pass, not the easier one.
2017 705.12(B)(2)(3)(c)2020 705.12(B)(3)(3)2023 not quoted2026 not quoted
The centre-fed allowance
A centre-fed panelboard may take the 120 percent allowance with the source at either end.
Dwelling units only. A centre-fed panelboard in a commercial building does not get this, and reading it as generally available is a common way to over-permit a panel.
2017 705.12(B)(2)(3)(d)2020 705.12(B)(3)(4)2023 not quoted2026 not quoted
What each busbar gives you, behind a 200 amp main
The column that matters is the last one. A 225 amp busbar and a 200 amp busbar look identical from the outside and differ by 30 amps of backfeed, which is about 5.7 kW of inverter.
| Busbar | Plain comparison | 120 percent allowance | Largest breaker |
|---|---|---|---|
| 100 A | nothing | nothing | nothing |
| 125 A | nothing | nothing | nothing |
| 150 A | nothing | nothing | nothing |
| 200 A | nothing | 40 A | 40 A |
| 225 A | 25 A | 70 A | 70 A |
| 400 A | 200 A | 280 A | 250 A |
Figures read 2026-09-13. A main other than 200 amps changes every row.
How the calculation actually works
Every load-side method in this section is the same shape: add 125 percent of what the sources put out to the rating of the breaker protecting the busbar, and check that total against the busbar. What changes between the methods is the number on the right hand side, and what you have to be able to say about the panel before you may use it.
Two numbers, not one
The busbar rating and the main breaker rating are different facts about the same panel, and a calculator that asks for "panel size" is quietly collapsing them. The busbar rating is on the label inside the door. The main is on the handle. They agree on the commonest panel there is and diverge on the one a solar job tends to land in: a 225 amp bus behind a 200 amp main is the standard solar-ready combination, and reading the main there costs 30 amps of backfeed.
This is why the tool will not compute anything without the busbar rating. Substituting the main would be the exact error it exists to correct, and an answer built on that substitution is worse than no answer.
The code sums a current, not a breaker
The figure that goes into the comparison is 125 percent of the power source output circuit current. It is not the rating of the breaker that lands it. Those diverge whenever the breaker rounds up to the next standard size, which is most of the time.
A 7.6 kW inverter at 240 volts puts out 31.67 amps, so it needs 39.59 amps of busbar headroom and a 40 amp breaker. Where a panel leaves exactly 40 amps that is fine on both readings. Where it leaves 55, a source needing 52.09 amps fits the code arithmetic and its 60 amp breaker does not fit the stricter one. The tool reports both and flags the disagreement rather than picking a side, because the reading that matters is your inspector's.
Four methods, one of them unconditional
The 120 percent allowance is the one with a name, and it is the one that depends on where the backfed breaker sits. When that is not established, the fallback is the plain comparison, which carries no condition at all. It is not the sum-of-every-device method, which has conditions of its own and on a panel with many loaded breakers is usually harder to pass, not easier.
The tool evaluates all four against your panel and shows each one, rather than computing the famous one and stopping. Which method you end up relying on decides whether a permanent field marking is required, so it is worth seeing.
When it does not fit
Three things can change the answer, and they are not equally expensive. Settling a question you have not answered yet costs nothing, so the tool checks that first: if establishing the breaker position would unlock the allowance, it says so before suggesting you buy anything.
After that comes the main breaker. A smaller main leaves more room under every method, and the tool computes the largest standard rating that would work rather than leaving you to search the list. That number comes with a condition attached and the condition is not decorative: the derated main still has to carry the building's calculated load, and nothing here has seen your loads.
Past that the busbar rule stops applying at all, either by connecting ahead of the service disconnect or by using listed equipment that actively limits what reaches the bus. Both are real routes and both are out of scope here.
Where this tool will mislead you
The question here is narrow: whether a busbar can take what you want to put on it. Three neighbouring questions sit outside it, each with a different home. The PV conductors, the disconnect and the grounding belong to Article 690. Feeders and feeder taps are a separate subsection of this one. A battery's charging current is not modelled at all, because the busbar rule concerns what a source puts out.
The fourth thing is not a code question at all. Whether your utility will allow the interconnection turns on facts the electrical code never mentions, like how much generation their feeder is already carrying, and no calculator is going to tell you.
Terminology
- Busbar
- The metal bars inside the panel that every breaker clips onto. Its rating is the current it is listed to carry, and it is on the label inside the door.
- Backfed breaker
- The breaker the power source lands on. Current flows into the bus through it rather than out of the bus, which is the whole reason this section exists.
- Main lug only
- A panel with no main breaker of its own. The device protecting its busbar is upstream, in whatever panel the feeder starts from, so that is the rating this calculation needs.
- Centre fed
- A panelboard whose supply lands in the middle of the bus rather than at one end. It has its own allowance, and that allowance is for dwelling units.
Questions about the 120 percent rule
What is the 120 percent rule?
It is one of four ways the code lets you check a busbar, and it is the one with a name. Where the utility supply and the power source sit at opposite ends of the busbar, you may compare against 120 percent of the busbar rating instead of 100 percent. The extra 20 percent is what makes most residential solar fit at all.
It is not automatic. Put the backfed breaker anywhere else on the bus and the allowance is gone, which is why relying on it means a permanent marking on the equipment saying the breaker must not be moved.
Is the rule based on my main breaker or my busbar?
The busbar, and this is the single most common error. The calculation is 120 percent of the busbar rating, less the rating of the breaker protecting it.
On a 200 amp busbar behind a 200 amp main both give 40 amps. The two readings only diverge once the busbar is larger than the main. On a 225 amp busbar behind a 200 amp main the busbar gives 70 amps and the main gives 40. That is 30 amps of backfeed, or roughly 5.7 kW of inverter, lost to reading the wrong number.
Where do I find the busbar rating?
On the label inside the panel door, usually stated as a bus rating or a panelboard rating in amps. It is frequently not the number on the main breaker handle. Panels sold as solar ready are typically a 225 amp bus behind a 200 amp main for exactly this reason.
Does the tool use my inverter breaker or my inverter current?
The current, because that is what the code sums: 125 percent of the power source output circuit current, plus the rating of the device protecting the busbar. The breaker rating is not in that sum.
They can disagree. A 31.7 amp output needs 39.6 amps of headroom and lands on a 40 amp breaker. On a panel with exactly 40 amps of headroom the design passes on the code arithmetic and fails if someone compares the breaker instead. This tool reports both figures and says so when they disagree, because an inspector may well read it the stricter way.
What if I do not know where the breaker will land?
Then the 120 percent allowance does not apply and the answer here is the plain comparison, which is always available. That is deliberate. Assuming the favourable position is exactly what turns a failing panel into one that looks like it passes.
Where settling that question would be the whole fix, the tool says so before it suggests changing any hardware.
Can I just fit a smaller main breaker?
Often yes, and it is usually the cheapest fix, which is why the tool computes the largest main that would work rather than making you guess.
But a smaller main still has to carry the building. The load calculation under Article 220 is the real constraint and this tool has not seen your loads. Derating a main to make room for solar and then tripping it every winter is a real outcome, not a hypothetical one.
Does a battery count?
Yes. The section governs power production sources generally, not just solar, so a battery inverter feeding the bus counts in the same sum. So does anything already backfeeding it. Add each one as its own source here.
What is not modelled is charging. The busbar rule concerns output.
Why does the tool ask which code edition?
Because the numbering moved in every recent cycle and the arithmetic did not. The allowance that is cited one way under the 2017 code is cited another way under the 2020 one, and the 2023 cycle moved the feeder rules out to their own subsection and renumbered what was left.
So two people can quote the same rule, cite different subsections, and both be right about their own jurisdiction. The tool covers the 2017, 2020, 2023, 2026 editions.
What does this tool not do?
It does not size feeders or feeder taps, which are a separate calculation. It does not size the PV conductors, the disconnect or the grounding, which are Article 690. And it does not tell you your utility will approve the interconnection, because that is an interconnection agreement rather than the electrical code.
Where these numbers come from, and what is not reproduced here
The rules are rules. The 125 percent factor, the 120 percent allowance and the condition each method attaches are short factual statements of what the code requires, stated in dozens of places, and restated on this page in this project's own words. No table text, note text or commentary from NFPA 70 is reproduced anywhere here. The National Electrical Code is copyrighted by the NFPA, who make it free to read online. Free to read is not a licence to republish, and this page does not.
The method conditions and the subsection numbers were read from published code-change material that reproduces this section per edition and quotes the superseded and current wording side by side. Read 2026-09-13. The standard breaker ratings are a list of integers, and the busbar ratings are ratings of manufactured panelboards, published independently by every maker.
The required warning label is the one piece of authored text this subject contains, so it is not reproduced here. The tool tells you that a permanent field marking is required and where its wording lives, and stops there.
Two of the four editions compute but carry no subsection number. The arithmetic has not changed between editions, so the numbers this tool gives are the same in all of them. The subsection numbers for those two have not been read from a source that reproduces the section, and a citation inferred from a neighbouring cycle would be wrong in the one place a citation matters, which is on a permit. They are shown as not quoted rather than guessed.
This is a calculator, not an inspection and not legal advice. If a figure here disagrees with your code book or with something you have measured, tell me and it gets fixed.