What is the 83% rule?

NEC 310.12 lets the conductors of a one-family dwelling service or feeder be sized at 83% of the rating of their overcurrent device rather than 100%, where the run carries the entire dwelling load, the system is single-phase 120/240V, and the rating is 100A through 400A. On a 150A feeder that means a conductor rated at least 124.5A instead of 150A.

Does the 83% rule stop applying when solar taps the feeder?

No. This is the most common misconception about it. NEC 310.12 is a conductor sizing allowance and it keeps applying. What changes is that Article 705 adds a separate, additional test for the portion of the feeder on the load side of the connection, and an 83%-sized conductor usually cannot pass the first option in that test.

Do I even have to tap the feeder?

Often not, and this is the first thing to check. NEC 705.12(A)(2) only applies when the connection is made somewhere other than the end of the feeder farthest from the overcurrent device protecting it. Backfeeding a breaker in the existing downstream loads panel is that far end, so the feeder is never re-tested and you are under the busbar rules instead.

Can I use the next standard size up for the overcurrent device?

Treat it as unavailable. 705.12(A)(2)(b) calls for a device rated not greater than the ampacity of the feeder and offers no round-up. This is a reading rather than settled text, because where the NEC means to bar 240.4(B) it usually says so directly, as 240.21(B) does for tap conductors. Design to the strict reading and confirm with your AHJ before relying on anything looser.

Why would a 150A service end up with a 125A main?

Because when the conductor already exists you size the breaker to the wire, not to the service. A feedthrough conductor sized at 83% of a 150A device may only be rated 130A, so the device protecting it lands at 125A, the next standard size down.

Which NEC edition are these section numbers from?

NEC 2023, with the 2020 numbers noted alongside. In 2023, 705.12(A) covers feeders and feeder taps and 705.12(B) covers busbars. In 2020 the feeder rule was 705.12(B)(1) and busbars were 705.12(B)(3). The 83% allowance itself was Table 310.15(B)(6), renumbered to Table 310.15(B)(7) in 2011, restated as the 83% factor in 2014, and moved to 310.12 in 2020.

NEC Compliance

What a solar feeder tap actually entails the full sequence, from the 83% conductor to the plan set

Landing a power source on a dwelling feeder is usually treated as one decision: pick a connection point and size the tap conductors. It is not one decision. It is a sequence of about a dozen, and the ones that get skipped are rarely the ones a designer expects.

The conductor between a meter-main and the loads panel was almost certainly sized under the 83% allowance in NEC 310.12. That conductor is where the sequence gets hard, and what it needs is widely misunderstood. This page walks the whole thing: what still applies, what Article 705 adds, the two ways to size the overcurrent device, the physical and load-side checks that move the answer, and what belongs on the plan set.

A technical reference for solar designers and electricians. Section numbers are NEC 2023 with the 2020 equivalents noted. Last updated August 2026.

The short answer: what actually changes

The 83% rule (NEC 310.12) lets the conductors of a one-family dwelling service or feeder be sized at 83% of the rating of the overcurrent device protecting them, instead of the full 100%. On a 150A feeder, that means a conductor rated at least 124.5A rather than 150A.

When a power source connects to that feeder, two things are true at once:

1.310.12 still applies. It is a sizing allowance based on how a dwelling actually uses power. Nothing about solar changes the conditions it asks for.
2.Article 705 adds a second test for the part of the feeder on the load side of the connection. The 83%-sized conductor usually fails the first option in that test by a wide margin, so you take the other one.

Three common readings of this are wrong, and each one points at the wrong fix. The 83% allowance does not expire, because nothing about solar changes the four conditions 310.12 asks for. The existing conductor does not become a tap conductor, because 240.21(B) governs new conductors spliced in at the connection rather than the run that was already there. And nothing is revoked, because 310.12 is a sizing allowance based on dwelling load diversity, not a concession that a second source can withdraw.

The trade has exactly two clean answers, and which one you use depends on one thing: whether the conductor already exists. Buying new wire means sizing the wire up to the breaker. Living with wire that is already in the wall means sizing the breaker down to it.

The whole sequence

Twelve decisions, in the order they come up on a real job.

01Does it have to be a mid-feeder tap at all? Often not, and this changes everything downstream.
02What is the existing conductor, actually? Size, material, and verified rather than assumed.
03Does the feeder pass 705.12(A)(1)? Ampacity at least 125% of source output, wherever the connection lands.
04Pick the 705.12(A)(2) path: upsize the conductor, or put a device on it.
05Size that device to the conductor's corrected ampacity, not to the service rating.
06Check the device against the Article 220 calculated load. It now protects the whole dwelling.
07Size any new tap conductors under the right 240.21(B) tier, with the 705.12(A)(3) basis and its termination requirement.
08Protect whatever they land in per 408.36.
09Make the enclosure physically legal: working space, outdoor rating, splice box size.
10Interrupting rating, grounding and bonding at the new equipment.
11Utility and AHJ constraints on equipment next to the meter.
12Write the sizing chain onto the plan set, so a reviewer does not have to reconstruct it.

On this page

1.The short answer
2.The whole sequence
3.First: does it have to be a tap?
4.What Article 705 requires
5.The two clean resolutions
6.The sizing trap
7.A worked 150A example
8.The inserted panel, settled and contested
9.What else changes the ampacity
10.The checks that are not about ampacity
11.Frequently asked questions
12.The short version

First question: does it have to be a tap at all?

Before pricing any fix, check whether you have the problem. NEC 705.12(A)(2) [2020: 705.12(B)(1)] only applies when the connection is made somewhere other than the end of the feeder farthest from the overcurrent device protecting it. One end is exempt, and it is the far end.

Four ways to never touch the feeder:

Backfeed a breaker in the existing downstream loads panel. That is the far end. The feedthrough conductor is never re-tested, and you are under the busbar rules instead, where the 120% rule applies against a panel that usually has bus to spare. On the example later on this page, that is a 70A breaker at the opposite end of the loads panel bus and the entire problem disappears.
Backfeed a spare space in the meter-main combination panel itself, where one exists. Also busbar rules, also skips the feeder.
Connect on the supply side under NEC 705.11. On a meter-main combination this usually loses: there is typically no listed lug space between the meter and the main, the enclosure is a listed assembly that was not evaluated for a field tap, and many utilities will not permit work inside the sealed section. Worth ruling out deliberately rather than assuming. Our interconnection method selector walks the options with edition-aware citations.
Upgrade the service. What actually gets built when a battery or an EV charger is in scope anyway.

Each of these avoids the conductor question entirely, and on a typical dwelling at least one of them is available. Everything below applies only when the connection genuinely has to land mid-feeder.

What Article 705 requires

A meter-main combination panel holds the utility meter and the main breaker in one enclosure. It feeds the house through feedthrough lugs, and the conductor from those lugs to the downstream loads panel is a feeder. That conductor is the subject of everything below.

Its ampacity, the current it is rated to carry continuously, is what every decision below turns on.

Why it was sized small

310.12(B) is narrower than most people quote it. All four conditions have to hold:

A one-family dwelling, or an individual dwelling unit of a two-family or multifamily dwelling
Single-phase, 120/240V
Rated 100A through 400A
The feeder supplies the entire dwelling load

310.12(A) is the service-conductor version of the same allowance. 310.12(C) adds that a dwelling-unit feeder is never required to exceed those ampacities.

So on a 150A main, 1 AWG copper at 130A in the 75 degree C column of Table 310.16 is compliant. Note how little margin that is: 130 against 124.5 is 4.4%.

What 705.12(A) asks for

Three numbered items, and the first one is easy to miss because it applies no matter where the connection lands:

705.12(A)(1): the feeder ampacity must be at least 125% of the power source output circuit current. On dwelling solar this is usually comfortable, but it is a test and it belongs on your list.
705.12(A)(2) [2020: 705.12(B)(1)]: where the connection is not at that far end, the load-side portion must satisfy either(a) ampacity at least the primary overcurrent device rating plus 125% of the source output current, or (b) an overcurrent device at the load side of the connection point, rated not greater than the feeder's ampacity.
705.12(A)(3) [2020: 705.12(B)(2), and genuinely different]: for tap conductors sized under 240.21(B)(2) or 240.21(B)(4), the basis becomes the feeder's overcurrent device plus the ratings of the power source overcurrent devices. Two things to notice. The 2020 text used 125% of output current instead, so do not mix editions. And the 10-foot tap of 240.21(B)(1) is outside this provision's scope in 2023.

Whatever the conductors land in still needs its own overcurrent protection under 408.36. A main breaker in a newly inserted panel can do double duty, satisfying both (b) and the panelboard requirement.

The two clean resolutions

One input is fixed and the other is yours to choose. That is the whole decision.

The conductor is	Fixed input	What you do	150A example
New, you are buying it	The breaker	Size the conductor up to the breaker	150A device, 1/0 Cu
Existing, it is in the wall	The conductor	Drop the breaker down to the conductor's ampacity	1 AWG Cu, 125A device

Lead with the second one. A 130A conductor protected at 125A is compliant under plain 240.4. It needs no interpretation of 705.12, no argument about which segment is which, and no assumption about what a future electrician will do. When the authority having jurisdiction is unknown, it is the answer that cannot lose.

One thing the table does not say, and you must check separately: that 125A device now protects the entire dwelling. Run an Article 220 calculation against it. If the calculated load exceeds 125A you cannot use this resolution, and you are pushed to new conductors or a service upgrade. Our load calculator will get you there.

The sizing trap

705.12(A)(2)(b) calls for an overcurrent device rated not greater than the ampacity of the feeder.

Not the next standard size above it. On a 130A conductor that puts the device at 125A, the next standard size down, not 150A.

Designers reach for the 240.4(B) round-up by reflex because it applies almost everywhere else in Article 240.

This is a reading rather than settled text. Where the NEC means to bar the 240.4(B) round-up it usually says so out loud, and 240.21(B) does exactly that for tap conductors. 705.12 carries no equivalent sentence.

The strict reading is the prevailing one and it is what an examiner is most likely to apply. Design to it. If a job genuinely turns on the difference, ask the AHJ before you draw it rather than after.

A worked 150A example

150A meter-main combination panel
Feedthrough feeder is 1 AWG copper, 130A at 75 degrees C, sized under 310.12(B)
Roughly 64A of microinverter output, connected mid-feeder

All three options, including the one that fails:

Option	The math	Verdict
(a) Upsize the conductor	150 + (1.25 × 64) = 230A needed	The 130A wire fails by a wide margin. 4/0 Cu is exactly 230A at 75 degrees C, so any correction factor pushes it to 250 kcmil. Usually dies on lug range and raceway fill before cost.
(b) Device sized to the wire	Device ≤ 130A, so 125A	Works, subject to the load calculation. The conductor stays.
Derate the upstream main	130 − (1.25 × 64) = 50A main	Not a viable dwelling service. Derating only pencils at small ratios of solar to service, and it costs a load calculation, a main that may not be field-replaceable, and a utility appointment.

That 230A figure is why (a) rarely wins on an existing dwelling. The upstream main is counted at its full rating rather than at the calculated load, so the requirement lands far above anything an 83%-sized conductor was going to carry.

The inserted panel: what is settled and what is contested

A common build for option (b) inserts a new feedthrough panel: combination panel, then the tap where solar joins, then the new panel with its own main, then out through its feedthrough lugs to the pre-existing loads panel.

What everyone agrees on

The conductors from the tap to the new main are new wire, sized to that main. For a 150A main that is 1/0 copper at 150A. The new main then satisfies (b) against that conductor exactly.

Where the readings disagree

If you keep a 150A main in the inserted panel, the existing 130A conductor downstream of it is protected at 150A. There are two readings and they do not agree:

The objection. 705.12(A)(2)(b) caps the device at the ampacity of the load-side portion, and the downstream run is still on the load side of the connection. 150 over 130 is a violation, and upsizing only the tap-side segment does not cure it.
The counter-reading. The inserted device terminates the tapped feeder. Downstream of it you have a distinct dwelling main power feeder protected at 150A, for which 310.12(B) gives that 1 AWG conductor the same justification every non-solar dwelling relies on.

The counter-reading is defensible and it is rejectable. If the jurisdiction is unknown, use the 125A main. It needs no interpretation at all.

If you do rely on the counter-reading, put the whole chain on the drawing rather than leaving an examiner to reconstruct it: the existing conductor's size and material, its ampacity at 75 degrees C, that it was sized per 310.12(B), that it supplies the entire dwelling load, and that no power source connects downstream of the new main. Without that note the default reading is a 130A conductor protected at 150A, and you are writing a response letter.

Do not backfeed the inserted panel

Worth separating from the first section of this page, because the same verb points in opposite directions. Backfeeding the existing loads panel is the good move. Backfeeding the inserted panel is not.

A 125A bus with a 125A main leaves roughly 25A of busbar allowance under the 120% rule. The example on this page needs 70A, so it fails immediately. A new panel with open spaces sitting next to the array homerun is a standing invitation, which is why it is worth a note on the drawing.

Keep it a pure pass-through

The counter-reading also requires the inserted panel to carry no branch circuits. Land circuits in it and the downstream feeder stops supplying the entire dwelling load, 310.12(B)'s fourth condition fails, and that conductor reverts to ordinary sizing against the calculated load.

A note on the one-line diagram is not enough on its own, because the person who breaks this in ten years will never see your plan set. A permanent field-applied label at the enclosure per 110.21(B) survives; a PDF in a permit office does not.

Better still, notice that the inserted device does not have to be a panelboard. An enclosed circuit breaker or a fusible safety switch satisfies (b), is usually smaller and cheaper, and makes branch circuits physically impossible for the life of the building. That moots the whole concern.

What else changes the conductor's ampacity

Everything above assumes 130A. Five things commonly make it something else, and each one moves the device with it.

Verify the conductor before you specify anything. It lives inside a sealed enclosure and behind a wall, and most designs start from an assumption. If it cannot be confirmed before permit, draw it as existing, size to be verified in field, with the device not to exceed the verified ampacity.
Aluminum. 2/0 aluminum at 135A is at least as common in this role as 1 AWG copper. 1/0 aluminum is 120A, which puts the device at 110A rather than 125A. Terminations and connectors must be listed for the material.
Termination temperature, 110.14(C). The ampacity you compare the device against is the termination-limited one. A 60 degree C marked terminal drops 1 AWG copper from 130A to 110A, and the device with it.
Ambient and conduit fill. A 4.4% margin does not survive much. A hot attic or a fourth current-carrying conductor in the raceway can pull 130A below 125A. Check fill and grouping if PV conductors end up sharing it.
The existing feeder may be three-wire. The inserted enclosure sits downstream of the service disconnect, so its neutral must be isolated and an equipment grounding conductor is required. Confirm the existing run has one before you plan to reuse it.

The checks that are not about ampacity

Steps 9 through 11. These decide whether the design can be built at all, and they are the ones most commonly discovered on site rather than at the desk.

Working space, 110.26. A new enclosure needs 3 ft of depth, 30 in of width and 6.5 ft of headroom clear, plus the dedicated space above it. Meter-mains live in side yards next to fences, hose bibs, gas meters and condensers. Confirm this on the site survey, because it is what turns the inserted-enclosure option from cheap to expensive.
Interrupting rating, 110.9. The new device sits a few feet from the service. On a short lateral from a modern transformer the available fault current can exceed the 10 kAIC rating of a residential load center. One- and two-family dwellings are exempt from the 110.24 field marking, not from the requirement itself.
Grounding and bonding. The inserted enclosure is downstream of the service disconnect, so its neutral is isolated, grounds and neutrals land on separate bars, and an equipment grounding conductor is required. Bonding it because it looks like a main panel puts objectionable current on the grounding path.
The splice itself. Feeder splices need connectors listed for the conductor material and any mixed combination per 110.14(B), in an accessible enclosure sized for the bending radius per 314.28, and rated for the location. A 4/0 pull box is not small.
Utility rules. Some utilities restrict what may be mounted adjacent to or downstream of their metering, or require clearance from the meter. The interconnection application's one-line has to reflect the new equipment either way.

Frequently asked questions

What is the 83% rule?: NEC 310.12 lets the conductors of a one-family dwelling service or feeder be sized at 83% of the rating of their overcurrent device rather than 100%, where the run carries the entire dwelling load, the system is single-phase 120/240V, and the rating is 100A through 400A. On a 150A feeder that means a conductor rated at least 124.5A instead of 150A.
Does the 83% rule stop applying when solar taps the feeder?: No. This is the most common misconception about it. NEC 310.12 is a conductor sizing allowance and it keeps applying. What changes is that Article 705 adds a separate, additional test for the portion of the feeder on the load side of the connection, and an 83%-sized conductor usually cannot pass the first option in that test.
Do I even have to tap the feeder?: Often not, and this is the first thing to check. NEC 705.12(A)(2) only applies when the connection is made somewhere other than the end of the feeder farthest from the overcurrent device protecting it. Backfeeding a breaker in the existing downstream loads panel is that far end, so the feeder is never re-tested and you are under the busbar rules instead.
Can I use the next standard size up for the overcurrent device?: Treat it as unavailable. 705.12(A)(2)(b) calls for a device rated not greater than the ampacity of the feeder and offers no round-up. This is a reading rather than settled text, because where the NEC means to bar 240.4(B) it usually says so directly, as 240.21(B) does for tap conductors. Design to the strict reading and confirm with your AHJ before relying on anything looser.
Why would a 150A service end up with a 125A main?: Because when the conductor already exists you size the breaker to the wire, not to the service. A feedthrough conductor sized at 83% of a 150A device may only be rated 130A, so the device protecting it lands at 125A, the next standard size down.
Which NEC edition are these section numbers from?: NEC 2023, with the 2020 numbers noted alongside. In 2023, 705.12(A) covers feeders and feeder taps and 705.12(B) covers busbars. In 2020 the feeder rule was 705.12(B)(1) and busbars were 705.12(B)(3). The 83% allowance itself was Table 310.15(B)(6), renumbered to Table 310.15(B)(7) in 2011, restated as the 83% factor in 2014, and moved to 310.12 in 2020.

The short version

310.12 never stops applying. Article 705 adds a second test on the load-side portion of the feeder, and an 83%-sized conductor usually fails its first option.

Check first whether you need a mid-feeder tap at all, because a backfed breaker in the existing loads panel avoids the question entirely. If you do need one: new wire gets sized up to the breaker, existing wire gets the breaker sized down to it. The second is the answer that needs no interpretation.

Draw the tap, and see the conductor and device math as you go.

Solar Design Lab sizes conductors and overcurrent devices as you wire, and runs live NEC checks while you draw. A device that outruns the conductor below it, including the contested case on this page, is flagged at design time rather than at plan review. Free to use, no card required.

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Section numbers are NEC 2023 with 2020 equivalents noted. Article 705 was reorganized in 2023 and the 83% dwelling allowance has moved twice since 2008, so confirm every citation against the cycle your jurisdiction has adopted before putting it on a plan set. Where this page identifies a contested reading it says so; those are decisions for the authority having jurisdiction, not settled code text. Local amendments and utility requirements vary. This is an educational technical reference, not a substitute for project-specific engineering.