What is the 120% rule?

NEC 705.12 limits the total current that can feed a panelboard busbar from the utility and a solar system together. Where the solar backfeed breaker is placed at the opposite end of the busbar from the main, that total may reach 120% of the busbar rating. On a 200 A busbar with a 200 A main, that permits a 40 A solar breaker — which supports an inverter of roughly 7.7 kW.

Do I really need a panel upgrade, or is the installer upselling?

Both happen, and this tool exists to tell them apart. If the arithmetic says you are short, the constraint is real. What is often missing from the quote is the list of alternatives — derating the main, a supply-side connection, a listed power control system, or a smaller inverter — any of which may cost far less than a service upgrade. Ask which were considered and why they were rejected, in writing.

How does derating the main breaker help?

The 120% allowance is the busbar rating times 1.2 minus the main. Lowering the main raises the allowance directly. Going from a 200 A main to a 150 A main on a 200 A busbar takes the allowance from 40 A to 90 A, and the largest supportable inverter from 7.7 kW to 17.3 kW. Whether your house can run on a 150 A main is decided by a load calculation an electrician performs — it is not something to assume.

Why does the sum rule keep returning zero?

Because it is busbar minus main, and on a great many residential panels the main equals the busbar. That is exactly why the 120% rule exists and why the physical position of the backfeed breaker matters: on a 200/200 panel the sum rule offers nothing and the 120% rule offers 40 A.

Should I use the DC system size or the inverter rating?

The inverter AC output. Arrays are routinely oversized relative to their inverters, so a proposal headlined 12 kW DC might carry a 9.6 kW inverter, and it is the inverter that determines the backfeed current. If your proposal only gives a model number, the AC output is on the manufacturer datasheet.

Can I check the busbar rating myself?

You can read it, and you should not go looking for it. The rating is printed on the label inside the panel door and is visible without removing anything. Do not remove the dead front cover to find it — busbars remain energized with the main switched off in most residential panels. If the label is missing or illegible, that is a question for an electrician, not a reason to open the panel.

Does a battery change the answer?

Yes, and this tool does not model it. A battery is an additional power source with its own interconnection requirements, and it frequently arrives with a critical load subpanel that restructures the whole question. Treat a passing result here as applying to the array alone.

What if my jurisdiction is on an older code edition?

The rule exists across cycles but has moved subsection and gained alternatives over time — 705.12(D)(2) in 2014, 705.12(B)(2)(3)(b) in 2017, 705.12(B)(3)(2) in 2020, 705.12(B)(2) in 2023. Listed power control systems became available with the 2020 cycle, so a 2014-edition jurisdiction has fewer routes. The arithmetic in this tool is stable across those editions; the options when it fails are not.

The tool says plausible fit but my installer says I need an upgrade. Who is right?

Possibly both. This screens one rule from three numbers. The installer may be responding to breaker space, panel condition, an obsolete panel type, a manufacturer listing that forbids a backfeed breaker in the required position, or a load calculation you have not seen. Ask them which it is — a specific answer is easy for a competent installer to give and hard to invent.

Is this the same as an EV charger panel check?

No. An EV charger is a load and this rule governs sources. The two do compete for the same panel, and a house planning both should be assessed for both at once, but the arithmetic is different. Our solar and EV charging piece covers where they interact.

HyreSolar tools

Will solar need a panel upgrade?

Screens a proposed inverter against the ratings on your panel label under the NEC 705.12 120% rule. A screen for better questions, not a design.

Checked September 3, 2026. Data as of September 5, 2026. HyreSolar sells and installs nothing.

The 120% rule caps the solar breaker at 120% of the busbar rating minus the main breaker. On a 200 A busbar with a 200 A main, that allows a 40 A breaker, enough for an inverter of about 7.7 kW.

An 11 kW inverter needs 60 A and comes up 20 A short. Derating the main to 150 A raises the ceiling to 17.3 kW on the same panel.

Busbar headroom screen

Three numbers off your panel label and your proposal. A screening check, not a design.

Example values The filled-in numbers are an example, not your home. Type over them with your own.

— Screening verdict

— Largest inverter that fits

— Backfeed breaker needed

— 120% rule allows

— Sum rule allows

— Margin

How this result was worked out

Working it out from the numbers above.

Screens NEC 705.12 arithmetic only. Not a design, not a permit, not electrical advice.

A licensed electrician confirms the label, the load calculation and the enforced code edition. Never open a service panel to gather these numbers.

How to read the verdict

"Plausible fit" means the arithmetic clears, not that the job is approved. A licensed electrician still has to confirm the busbar rating on the label, the physical space for the breaker, the condition of the panel, whether the manufacturer permits a backfeed breaker in that position, and which NEC edition your jurisdiction actually enforces.

This tool checks one gate of several.

"Likely needs a change" does not mean you need a new panel. It means the inverter as proposed does not fit as proposed.

There are at least four ways out before replacement: derate the main breaker, use a supply-side connection, use a listed power control system, or fit a smaller inverter.

A quote that jumps straight to a full service upgrade has skipped that list.

The single most useful output is the maximum inverter size. If a proposal specifies an inverter above your ceiling, either the installer is planning work you have not been told about, or the design needs revisiting. Either way it is a question worth asking before signing.

Comfortable spare capacity: The proposed inverter clears with room. Panel work is unlikely to be needed for the solar itself, though other reasons may still exist.
Zero to a few amps spare: It clears on paper with nothing in hand. Worth confirming the busbar rating carefully, because a panel labeled 200 A is not always a 200 A busbar, and worth asking about future loads.
Short by any margin: As designed it does not fit. Ask the installer which of the four alternatives they considered before quoting a service upgrade, and ask for the reasoning in writing.

Where to find each number

Where each input comes from, so nothing is a guess.

1. Read the busbar rating off the panel label

This is the rating of the metal bar the breakers clip onto, and it is the figure the rule is about. It is not always the same as the main breaker rating.

Where to find it: The printed label inside the panel door, visible without removing anything. Look for a bus or busbar rating in amps. If the label is missing or unreadable, stop and ask an electrician — do not guess and do not remove the cover.

2. Read the main breaker rating

The number stamped on the main disconnect handle. This is what the utility supply is limited to, and it is often but not always equal to the busbar rating.

Where to find it: Stamped on the largest breaker in the panel, usually at the top. A common combination is a 200 A main on a 200 A busbar; another is a 200 A busbar with a 175 A main.

3. Enter the proposed inverter output in kW AC

The AC output rating, not the DC array size. A proposal listing a 12 kW system may pair it with a 9.6 kW inverter, and it is the inverter figure the code cares about.

Where to find it: The equipment page of your proposal, under inverter. If only a model number is given, the AC output is on the manufacturer datasheet.

4. Check the voltage

Residential single-phase service in the United States is almost always 240 V, which is the default. Change it only if you know your service differs.

Where to find it: The panel label. Leave the default unless you have a specific reason.

5. Compare both rules, not just one

The tool reports the 120% rule and the sum rule separately because installers choose between them and they give different answers. The 120% rule requires the backfeed breaker at the opposite end of the busbar from the main.

Where to find it: Nowhere on your paperwork — this is the calculation itself. If a proposal relies on the 120% rule, the physical position of the breaker matters and is an installation detail worth confirming.

6. Take the answer to a licensed electrician

That is the entire purpose of the output. It converts a vague worry into a specific question with numbers attached.

Where to find it: Your installer, or an independent licensed electrician if you want a second view on a quoted upgrade.

How the calculator works it out

1
Size the backfeed breaker
Inverter continuous output current × 1.25, rounded up to the next standard overcurrent device size. This is the breaker the solar needs.
2
Compute the 120% allowance
Busbar rating × 1.2, minus the main breaker rating. Valid where the backfeed breaker sits at the opposite end of the busbar from the main.
3
Compute the sum allowance
Busbar rating minus the main breaker rating. Valid in any position, and frequently zero on a panel whose main equals its busbar.
4
Compare and report the ceiling
The required breaker against the better of the two allowances, plus the largest inverter that would fit.

The formula, in full. backfeed breaker = next standard size ≥ (inverter kW × 1000 ÷ volts) × 1.25. 120% allowance = busbar × 1.2 − main. Sum allowance = busbar − main. Fits if backfeed breaker ≤ the larger allowance.

A worked example, start to finish

A homeowner is quoted an 11 kW inverter on a house with the most common residential panel in America: a 200 A busbar with a 200 A main. The quote includes a $4,000 service panel upgrade, described as required.

Step	Value
Busbar rating	200 A
Main breaker	200 A
Proposed inverter	11 kW AC at 240 V
Inverter continuous current	45.8 A
× 1.25 for the breaker	57.3 A → 60 A standard
120% allowance	200 × 1.2 − 200 = 40 A
Sum allowance	200 − 200 = 0 A
Result	20 A short — but not necessarily a new panel

As designed it does not fit: the solar needs a 60 A breaker and the panel allows 40 A. That much of the quote is honest.

What the quote does not say is that derating the main breaker from 200 A to 150 A raises the allowance to 90 A, which accommodates the 60 A breaker with 30 A to spare and would support an inverter up to 17.3 kW.

Whether a 150 A main is adequate depends on the house’s actual load calculation, which an electrician performs — but it is a question that should have been asked before a $4,000 line item was added.

How the answer moves

A 7.6 kW inverter against different panels, and the 11 kW case. Every figure is computed by the calculator on this page.

Panel and inverter	Breaker needed	120% allows	Sum allows	Verdict	Largest inverter
Base case200 A bus, 200 A main, 7.6 kW	40 A	40 A	0 A	Plausible fit	7.7 kW
11 kW inverter instead of 7.6	60 A	40 A	0 A	Likely needs a change	7.7 kW
Main derated to 175 A	40 A	65 A	25 A	Plausible fit	11.5 kW
Main derated to 150 A	40 A	90 A	50 A	Plausible fit	17.3 kW
150 A busbar, 150 A main	40 A	30 A	0 A	Likely needs a change	5.8 kW
125 A busbar, 125 A main	40 A	25 A	0 A	Likely needs a change	4.8 kW
100 A busbar, 100 A main	40 A	20 A	0 A	Likely needs a change	3.8 kW
225 A busbar, 200 A main	40 A	70 A	25 A	Plausible fit	13.4 kW
400 A busbar, 400 A main, 11 kW	60 A	80 A	0 A	Plausible fit	15.4 kW

Read rows three and four against row two. The panel does not change; only the main breaker does.

Derating a 200 A main to 150 A takes the ceiling from 7.7 kW to 17.3 kW on the same busbar — a 124% increase in allowable inverter size from a breaker swap rather than a service upgrade.

Note also that the sum rule returns 0 A whenever the main equals the busbar, which is why the 120% rule and the position of the backfeed breaker matter so much on ordinary panels.

What moves this number most

Ranked, largest first. A proposal can change any of these without saying anything untrue, so check these inputs first.

1. The gap between busbar and main

The whole game. A 200 A busbar with a 200 A main has 40 A of 120% headroom; the same busbar with a 175 A main has 65 A. Panels are frequently supplied with a main equal to the bus, which is the least generous configuration possible.

2. Whether the main can be derated

The cheapest lever available, and the one most often skipped in a quote. It depends entirely on the house’s calculated load — an electrician performs that calculation and it is not something to estimate from a bill.

3. The inverter AC rating

Not the array DC size. A 12 kW array on a 9.6 kW inverter is a 9.6 kW problem for this rule, which is one reason inverters are commonly undersized relative to arrays.

4. Which NEC edition the jurisdiction enforces

The rule has moved subsection across code cycles — 705.12(D)(2) in 2014, 705.12(B)(2)(3)(b) in 2017, 705.12(B)(3)(2) in 2020, 705.12(B)(2) in 2023 — and the available alternatives changed with it. Listed power control systems, for instance, arrived with the 2020 cycle.

5. What is deliberately absent

The dwelling load calculation, physical breaker space, panel condition and age, manufacturer listing for backfeed position, busbar de-rating for other sources, batteries, and any supply-side option. Each of those can independently decide the answer and none of them can be screened from three numbers.

Common mistakes with this calculation

Assuming the main breaker rating is the busbar rating
They are frequently equal and sometimes not, and the rule is about the busbar. Using the main rating for both will usually understate your headroom. Read the label rather than inferring.
Using the DC array size instead of the inverter AC output
A quote headlined "12 kW" may carry a 9.6 kW inverter. The code cares about what the inverter can push into the busbar, not what the panels can produce on a cold clear day.
Accepting a quoted panel upgrade without asking about a derate
It is the single most valuable question this tool enables. A main breaker swap and a service upgrade are different orders of cost, and the scenario table shows the derate frequently solves the problem outright.
Treating a pass as approval
Clearing the arithmetic is one gate. Breaker space, panel condition, manufacturer listing and the local inspector are others, and any one of them can still require work.
Opening the panel to read the label
The busbar rating is on the label inside the door and is readable without removing the dead front. Do not remove the cover: the line side stays live even with the main off. This is where a screening tool stops and an electrician starts.
Forgetting future loads
An EV charger, a heat pump or a battery all compete for the same headroom. If any of those are plausible within a few years, size the conversation for them now rather than solving for solar alone. See solar and EV charging together.

What this calculator leaves out

The result is a planning estimate. A real proposal accounts for these; this page does not.

A screening check against one code rule.
It is not a design, not a load calculation, not a permit document and not electrical advice.
Does not assess the dwelling load calculation, which is what actually decides whether a main can be derated.
Does not assess breaker space, panel age, panel condition, recalled or obsolete panel types, or manufacturer listing for backfeed position.
Does not model batteries, generators, a second power source, or supply-side connections under NEC 705.11.
Assumes single-phase 240 V residential service unless you change the voltage.
The applicable subsection and available alternatives depend on which NEC edition your jurisdiction enforces, which this tool does not know.
Never open a service panel to gather these numbers.
The busbar rating is on the label inside the door.

The research behind these numbers

How solar interconnection worksWhat the utility reviews and where the panel question sits in the wider approval sequence.Read the research Proposal analyzerNormalize the quote to $/W first, then check whether the panel upgrade inside it is load-bearing.Read the research Solar and EV chargingThe other large load competing for the same busbar headroom, and why solving them separately costs more.Read the research What a critical load panel doesThe subpanel that appears when a battery enters the design, and how it changes the interconnection picture.Read the research Permission to operateThe utility gate after the electrical work is done and inspected.Read the research Incentive finderWhether any program in your state helps with service equipment rather than only the array.Read the research

Questions this calculator answers

What is the 120% rule?: NEC 705.12 limits the total current that can feed a panelboard busbar from the utility and a solar system together.; Where the solar backfeed breaker is placed at the opposite end of the busbar from the main, that total may reach 120% of the busbar rating.; On a 200 A busbar with a 200 A main, that permits a 40 A solar breaker — which supports an inverter of roughly 7.7 kW.
Do I really need a panel upgrade, or is the installer upselling?: Both happen, and this tool exists to tell them apart. If the arithmetic says you are short, the constraint is real.; What is often missing from the quote is the list of alternatives — derating the main, a supply-side connection, a listed power control system, or a smaller inverter — any of which may cost far less than a service upgrade. Ask which were considered and why they were rejected, in writing.
How does derating the main breaker help?: The 120% allowance is the busbar rating times 1.2 minus the main. Lowering the main raises the allowance directly.; Going from a 200 A main to a 150 A main on a 200 A busbar takes the allowance from 40 A to 90 A, and the largest supportable inverter from 7.7 kW to 17.3 kW.; Whether your house can run on a 150 A main is decided by a load calculation an electrician performs — it is not something to assume.
Why does the sum rule keep returning zero?: Because it is busbar minus main, and on a great many residential panels the main equals the busbar. That is exactly why the 120% rule exists and why the physical position of the backfeed breaker matters: on a 200/200 panel the sum rule offers nothing and the 120% rule offers 40 A.
Should I use the DC system size or the inverter rating?: The inverter AC output. Arrays are routinely oversized relative to their inverters, so a proposal headlined 12 kW DC might carry a 9.6 kW inverter, and it is the inverter that determines the backfeed current. If your proposal only gives a model number, the AC output is on the manufacturer datasheet.
Can I check the busbar rating myself?: You can read it, and you should not go looking for it. The rating is printed on the label inside the panel door and is visible without removing anything.; Do not remove the dead front cover to find it — busbars remain energized with the main switched off in most residential panels.; If the label is missing or illegible, that is a question for an electrician, not a reason to open the panel.
Does a battery change the answer?: Yes, and this tool does not model it. A battery is an additional power source with its own interconnection requirements, and it frequently arrives with a critical load subpanel that restructures the whole question. Treat a passing result here as applying to the array alone.
What if my jurisdiction is on an older code edition?: The rule exists across cycles but has moved subsection and gained alternatives over time — 705.12(D)(2) in 2014, 705.12(B)(2)(3)(b) in 2017, 705.12(B)(3)(2) in 2020, 705.12(B)(2) in 2023.; Listed power control systems became available with the 2020 cycle, so a 2014-edition jurisdiction has fewer routes. The arithmetic in this tool is stable across those editions; the options when it fails are not.
The tool says plausible fit but my installer says I need an upgrade. Who is right?: Possibly both. This screens one rule from three numbers. The installer may be responding to breaker space, panel condition, an obsolete panel type, a manufacturer listing that forbids a backfeed breaker in the required position, or a load calculation you have not seen.; Ask them which it is — a specific answer is easy for a competent installer to give and hard to invent.
Is this the same as an EV charger panel check?: No. An EV charger is a load and this rule governs sources. The two do compete for the same panel, and a house planning both should be assessed for both at once, but the arithmetic is different. Our solar and EV charging piece covers where they interact.

Sources and methodology

Figures dated September 5, 2026. Last reviewed September 3, 2026 by the HyreSolar research desk, which writes and checks every tool on this site.

NFPA 70, National Electrical Code — Article 705, Interconnected Electric Power Production Sources (National Fire Protection Association, retrieved September 5, 2026). The code article this tool screens against. NFPA provides free read-only access to the code text. Section numbering varies by edition; your jurisdiction decides which applies.
NEC adoption maps — which edition is enforced where (National Fire Protection Association, retrieved September 5, 2026). Source for the fact that adopted editions differ by state, and that several states delegate adoption to local jurisdictions.
Residential Clean Energy Credit (§25D) (Internal Revenue Service, retrieved September 5, 2026). Relevant to whether service equipment work carried out alongside solar attracts any credit. For property placed in service after 31 December 2025 the credit is $0.

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