An installer has a signed 8 kW job and a plan set drawn for string inverter equipment. Then the customer asks for microinverters, or the distributor swaps the inverter, or the installer is moving its whole business to a different platform. Someone has to tell the permit coordinator which sheets are now wrong. Most will say “just the SLD.” That is close, but it leaves out the calculations sheet, the conductor schedule and the rapid shutdown evidence.
This guide draws one illustrative 8 kW house both ways and lists every sheet that changes. It is written for the US installer operations manager or permit coordinator. The goal is to scope a platform switch before the AHJ (authority having jurisdiction) sees a mismatched set.
Quick answer. A microinverter plan set and a string inverter plan set share the site plan, structural sheets and utility interconnection math. They differ on the single-line diagram, the calculations sheet, the rapid shutdown evidence, the conductor schedule and the labels. Microinverters need a branch circuit count check against the datasheet. String inverters need a cold-temperature Voc check and an inverter voltage window check.
The two designs below are labelled illustrative. They use a generic 400 W module and published manufacturer limits. They are not product recommendations, and neither is an approval prediction for any AHJ.
What stays the same and what changes between the two plan sets?
Think of the plan set as two groups of sheets. One group describes the building and the array. The other describes the electrical conversion and wiring. Only the second group changes.
| Plan set item | Same for both? | Why |
|---|---|---|
| Site plan, roof access and fire setbacks | Same | Driven by the array footprint, not the inverter |
| Structural and attachment sheets | Same | Same modules, same racking, same roof loads |
| Utility interconnection and busbar math | Same method | Driven by AC output current. See the NEC 705.12 guide |
| Single-line diagram | Changes | Different devices, different conductor groups |
| Calculations sheet | Changes | Branch count check versus cold Voc and voltage window |
| Rapid shutdown evidence | Changes | Built into the microinverter, added to the string design |
| Conductor and conduit schedule | Changes | AC trunk and branch wiring versus DC homerun |
| Labels | Partly changes | DC side labels exist only where a DC circuit leaves the array |
| Equipment schedule and datasheets | Changes | One inverter versus 20, different listing evidence |
A plan set that comes back for a mismatch often has one sheet updated and another left behind. Treat the five changing items as a set, and check all five whenever the inverter model changes. The cost of a late equipment swap is covered in the post-stamp equipment swap guide.
What does the illustrative 8 kW house look like both ways?
The house has one roof plane of 20 modules, a 240 V split-phase service and an 8.0 kW DC array. Both designs use the same module.
| Illustrative module (not a product) | Value |
|---|---|
| Power at STC | 400 W |
| Vmp, Imp | 40.5 V, 9.88 A |
| Voc, Isc | 48.9 V, 10.45 A |
| Voc temperature coefficient | -0.27 % per °C |
| Vmp temperature coefficient | -0.30 % per °C |
| Lowest expected ambient temperature | -10 °C |
| Hot cell temperature for window check | 65 °C |
The two equipment choices use public manufacturer documents.
| Design A: microinverters | Design B: string inverter | |
|---|---|---|
| Equipment | 20 Enphase IQ8PLUS-72-2-US | 1 SMA Sunny Boy 6.0-US (240 V) |
| Source | Enphase data sheet DSH-00207-7.0, July 2026 | SMA data sheet SBxx-US-DS-en-41 and manual SBxx-US-1XP-41-BA-en-11 |
| DC array | 8.0 kW, 20 modules | 8.0 kW, 20 modules |
| AC rating | 20 x 295 VA = 5,900 VA | 6,000 W |
| DC to AC ratio | 8,000 / 5,900 = 1.36 | 8,000 / 6,000 = 1.33 |
| Rapid shutdown | Built into each microinverter | Module-level array disconnect switches added |
The manufacturers appear only as the source of the published limits. The same drawings can be built from other makes, and the same checks apply.
How does the single-line diagram change?
The single-line diagram tells the reviewer what is installed and how it is connected. The two architectures produce two different pictures.
Design A, microinverters. The array block shows 20 modules, each with its own microinverter behind it. The diagram shows two AC branch circuits of 10 microinverters each. They run on trunk cable to a junction box, then to an AC combiner with two 20 A breakers. From there the path goes to an AC disconnect and the point of interconnection. There is no DC homerun and no DC disconnect on the diagram, because no DC circuit leaves the array. The Enphase data sheet describes an ungrounded array with no additional DC side protection required.
Design B, string inverter. The array block shows two strings of 10 modules, each with a module-level shutdown device. The diagram shows the DC homerun from the roof junction box to the inverter. It also shows the inverter’s integrated DC disconnect, the AC output, the rapid shutdown initiation device and the AC disconnect. It also shows the array boundary and the shutdown devices, because the reviewer must see how the 80 V limit is met.
Three items on the SLD differ in kind, not just in number:
- The conductor groups. Design A has AC branch circuits on the roof. Design B has DC source circuits on the roof and an AC output circuit at the inverter.
- The protection devices. Design A has branch breakers at the combiner. Design B has the inverter’s input protection, and fusing only where strings are combined.
- The rapid shutdown representation. Design A needs a note and the equipment listing. Design B needs the shutdown devices drawn, their count per string and the initiator shown.
The Midway City, Utah plan review checklists show the same split. Its string inverter checklist asks for the number of modules per DC string and the number of strings. Its microinverter checklist asks for the number of modules per AC circuit, and notes that the count per circuit cannot exceed the datasheet. Both are AHJ-published and were compiled from the 2020 NEC, so confirm the equivalent items in your adopted edition.
How does rapid shutdown compliance differ under NEC 2023?
Both designs must comply with NEC 690.12, and neither is exempt. They get there differently. The detailed rules, boundary definitions and initiation device requirements are in our NEC 690.12 compliance guide and the 2017, 2020 and 2023 comparison. This section covers only what changes between the two plan sets.
What does the NEC 2023 text allow inside the array boundary?
The NEC itself is login-gated, so this guide relies on IAEI Magazine (J. A. Kraft, January 2023) and Mike Holt’s 2023 NEC material. Inside the array boundary, NEC 2023 690.12(B)(2) allows two main routes:
- 690.12(B)(2)(1): a listed PV hazard control system (PVHCS), installed per its listing instructions. The listing standard is UL 3741. IAEI reports that the plans must include the PVHCS certificate that itemizes the specific equipment.
- 690.12(B)(2)(2): DC conductors inside the boundary controlled to 80 V or less within 30 seconds of initiation. This is the route taken by module-level power electronics (MLPE), such as microinverters and DC optimizers.
The UL 3741 design guide covers the PVHCS route in depth.
How does Design A document it?
The Enphase IQ8 data sheet (DSH-00207-7.0, July 2026) says the product is UL Listed as PV rapid shutdown equipment. It says the product conforms with NEC 2014, 2017, 2020 and 2023 section 690.12 for AC and DC conductors when installed per the manufacturer’s instructions. The plan set cites that statement and the datasheet. Midway City’s microinverter checklist adds a point on initiation. A readily accessible AC disconnect outside the house can serve as the initiation device, because utility-interactive microinverters stop on loss of grid. That note comes from a 2020 NEC checklist, so check the initiation rules for your edition.
How does Design B document it?
A string inverter alone does not complete the job. The SMA manual for the Sunny Boy US-41 series says the inverter is listed as PV rapid shutdown equipment (PVRSE) to UL 1741. It adds that a complete system consists of the inverter, PV array disconnect switches and an initiation device. Four statements in that manual change what the plan set must carry:
- The sum of the standby voltages of all array disconnect switches in a string must be below 30 V. On a 10-module string that is an average of 3 V per switch. The manufacturer’s own limit is a design input, not a note.
- The array disconnect switches must disconnect the array from the inverter within 15 seconds of initiation.
- The inverter’s rapid shutdown function is disabled by default. It must be enabled and set to the operating mode that matches the switches.
- If the function is enabled and no array disconnect switches are installed, the inverter can be damaged.
That last point matters for the commissioning notes on the plan set. The SLD must show the devices, and the install must match.
Can a string inverter use the PVHCS route instead?
For a pitched residential roof, the public listings we reviewed do not make this the common path. The SMA UL 3741 page names the Sunny Tripower CORE1 and Sunny Tripower X 30, both described as commercial products, with racking partners listed. The Fronius page says the UL 3741 certification covers the Symo Advanced only. It applies to listed flat roof systems with the inverter inside the 1 ft array boundary, and it excludes the Primo and GEN24. Neither page mentions a residential configuration, and neither page carries a date.
We could not open UL Product iQ, so treat this as a statement about public pages, not about every listing. If a PVHCS is proposed, the certificate must match the exact inverter, racking and module list, as IAEI notes.
What about datasheet edition drift?
The Enphase IQ8+ data sheet (July 2026) cites NEC 2023. The SMA data sheet (file last saved February 2022) says its rapid shutdown solution fulfills UL 1741, NEC 2014 and NEC 2017. The Enphase IQ8M and IQ8A data sheet (December 2022) cites NEC 2014, 2017 and 2020. A reviewer in an NEC 2023 jurisdiction may ask for evidence that a datasheet written for an older edition still applies. Pull the current datasheet and the current listing, and put the revision on the equipment schedule. The edition question itself is covered in the NEC edition by state guide.
How many microinverters can go on one branch circuit?
The branch limit comes from the manufacturer, and it reproduces from the 125 percent rule. The Enphase IQ8+ data sheet lists a maximum continuous output current of 1.23 A and a maximum of 13 units per 20 A (L-L) branch circuit. The IQ8 (IQ8-60-2-US) lists 1.0 A and 16 units. The data sheet footnote says limits may vary and to refer to local requirements.
The check below shows why those numbers are what they are. This is our arithmetic, not a quote from the data sheet.
| Check | Arithmetic | Result |
|---|---|---|
| IQ8+ at the datasheet limit | 13 x 1.23 A x 1.25 | 19.99 A, within a 20 A breaker |
| IQ8+ at 14 units | 14 x 1.23 A x 1.25 | 21.5 A, over 20 A |
| IQ8 at the datasheet limit | 16 x 1.0 A x 1.25 | 20.0 A, within a 20 A breaker |
| Design A, 10 units per branch | 10 x 1.23 A x 1.25 | 15.4 A, within a 20 A breaker |
Design A uses two branches of 10 rather than one of 13 and one of 7. Balanced branches leave margin if a module is added later, and each branch has a short, even trunk run. The SolarAPP+ sample checklist (NEC 2020, approved July 2023) sets the branch overcurrent device at 20 A maximum for microinverter systems. It also requires one module per microinverter.
Total Design A output: 20 x 1.23 A = 24.6 A. At 125 percent that is 30.75 A, so the point of interconnection breaker is the next standard size, 35 A.
How do you size the string inverter design?
Design B needs two checks that Design A does not. The first is the maximum voltage in cold weather. The second is the inverter’s operating voltage window. Both are covered with a full worked method in the string sizing math guide. Here they are applied to the same house.
NEC 2023 690.7(A) sets the maximum PV system DC circuit voltage at 600 V for one- and two-family dwellings. Mike Holt’s 2023 NEC material describes the method: sum the series modules’ Voc, corrected for the lowest expected ambient temperature with the manufacturer’s coefficient.
| Step | Arithmetic | Result |
|---|---|---|
| Cold correction factor | 1 + (-10 - 25) x (-0.0027) | 1.0945 |
| Corrected Voc per module | 48.9 V x 1.0945 | 53.5 V |
| Maximum modules under 600 V | 600 / 53.5 | 11.2, so 11 |
| Corrected Voc, 10-module string | 10 x 53.5 V | 535 V, under 600 V |
| Inverter maximum DC voltage | SMA data sheet | 600 V |
| Inverter MPPT operating range | SMA data sheet | 100 to 550 V |
| Hot Vmp per module | 40.5 V x (1 - 0.003 x 40) | 35.6 V |
| Hot Vmp, 10-module string | 10 x 35.6 V | 356 V |
| Inverter rated MPP range | SMA data sheet | 220 to 480 V |
| Minimum modules for the rated range | 220 / 35.6 | 6.2, so 7 |
| String current check | Imp 9.88 A against 10 A per MPPT | Within the limit, small margin |
| Short circuit check | Isc 10.45 A against 18 A per MPPT | Within the limit |
A string of 10 passes every check. The Imp margin is only 0.12 A. A module with a slightly higher Imp would fail the MPPT current check even though the voltage math passes. That is the kind of failure a microinverter design never meets.
Two details travel to the SLD and the labels. The SolarAPP+ sample checklist treats the lowest ambient temperature as a project field tied to 690.7(A). The plan set must state the temperature and its source. The corrected string voltage also supports the maximum voltage label required by 690.7(D) in the 2023 edition, per Mike Holt.
If Design B used DC optimizers instead of array disconnect switches, the voltage rule moves. Mike Holt notes that under 690.7(B) the voltage of a DC-to-DC converter circuit comes from the converter’s rated output voltage. It does not come from the module Voc sum. The Midway City string checklist says the same for optimizer systems. The SLD and calculations sheet would then cite the optimizer’s limits instead.
The string design also has the current ratings. Under 690.8(A)(1)(a), circuit current is 125 percent of Isc for systems under 100 kW. Section 690.8(B) adds a second 125 percent for conductor ampacity (Mike Holt, 2023 NEC). For Design B:
- Circuit current: 1.25 x 10.45 A = 13.06 A.
- Minimum conductor ampacity before temperature and fill adjustments: 1.25 x 13.06 A = 16.3 A.
- Inverter output: 25.0 A from the SMA data sheet. At 125 percent that is 31.25 A, so a 35 A breaker, subject to the maximum overcurrent device in the inverter manual.
Design A and Design B both land on a 35 A point of interconnection breaker. That is why the busbar and 120 percent checks in the NEC 705.12 guide do not change. If a battery is in scope, the battery addition guide covers the separate sheets that come with it.
Where do the conductors run in each design?
The wiring path is the biggest visible difference between the two architectures on the layout sheet.
| Run | Design A: microinverters | Design B: string inverter |
|---|---|---|
| On the roof | AC trunk cable and connectors, two branches | PV wire in free air under modules, two strings |
| Roof to building | AC branch conductors in raceway or cable | DC homerun in raceway, two string pairs |
| Combiner | AC combiner with two 20 A breakers | None needed for two strings, each on its own MPPT input |
| Enters building as | AC circuits | DC circuits |
| Inverter location | Behind each module | One unit, typically on a wall |
| Rapid shutdown wiring | None added | Module-level devices and initiator wiring |
Midway City’s microinverter checklist and string inverter checklist, both from the 2020 NEC, ask for different things on each side. The microinverter checklist requires outdoor AC trunk cables to be listed and identified for the application and secured at no more than 6 ft on centre. The string checklist requires exposed DC wire to be USE-2, RHW-2 or listed PV wire. It says DC circuits entering the building must be in metal conduit or MC cable. See the PV wire vs USE-2 entry for the wire types.
An older example handout from Riverton, Utah shows the same pattern. It was written against NEC 2014 era rapid shutdown. Its microinverter system uses trunk cable, a junction box and an AC combiner. Its string system uses #10 PV wire to a junction box, then conductors in conduit to the inverter. Use it only as a layout illustration, not as a code reference.
The conductor count matters for both. The SolarAPP+ sample limits current carrying conductors in a raceway to nine. It sets 12 AWG as the minimum PV wire gauge for a single series string. A conduit schedule that is correct on one design is not automatically correct on the other. The current carrying conductor count and the temperature adjustment differ. A combiner box appears in Design A as an AC element and does not appear in Design B at all.
Which labels change between the two designs?
A separate guide covers labels in depth. This section is limited to what changes with the architecture.
- DC side labels. Design B has DC conductors that run to the inverter. The DC circuit marking and the maximum voltage label tied to 690.7(D) apply there. Design A has no DC circuit leaving the array.
- Rapid shutdown building label. Both need a permanent label showing the initiation device location. Mike Holt reports this as 690.12(D) in the 2023 edition. Midway City’s checklist, written for 2020, cites 690.56(C). The wording and diagram format depend on the edition.
- Initiator label. Both need the label at the initiation device. Midway City’s checklist specifies the wording “Rapid Shutdown Switch for Solar PV System.”
- Mixed systems. If a house has an older array with a different shutdown type, the Midway checklist requires a service equipment sign showing which part of the array stays energized. This matters when Design A is added to an older string system.
Labels can fail plan check when they describe the other architecture. Our AHJ rejection reasons guide lists wrong label wording for the equipment as a common trigger.
What do the utility and the AHJ ask for differently?
Utilities ask the same questions of both, but the counting differs. The Colorado Springs Utilities application (V2.2024) asks for the number of inverters or microinverters and the model numbers. It also asks that the equipment appear on the California Energy Commission eligible lists. For Design A that is a count of 20 and one model. For Design B it is a count of one.
The AHJ differences are about evidence, not preference:
- Design A evidence. The datasheet showing units per branch, the rapid shutdown statement, and the grid-interactive listing.
- Design B evidence. The datasheet with inverter voltage and current limits, the string sizing calculation and its lowest temperature source, the shutdown device datasheets, and the manual page on enabling the function.
The SolarAPP+ sample checklist also shows the AHJ side. It caps a project at two string inverters or one microinverter type. It also excludes the “no exposed wiring” method of rapid shutdown. These are program eligibility limits, so a set built from an old template can fall outside them.
Every item here can differ by jurisdiction. The adopted code edition, local amendments and utility rules decide what a specific reviewer asks for. Confirm with the AHJ and utility for the project address, and do not assume a design that cleared one AHJ will clear another.
Microinverter vs string inverter permit: which sheets change?
This is the table to keep next to the plan set template.
| Sheet | Design A: microinverters | Design B: string inverter |
|---|---|---|
| Cover and system data | 20 microinverters, 5,900 VA AC | 1 inverter, 6,000 W AC |
| Site plan | AC combiner location, AC disconnect, initiator | Inverter, DC disconnect, AC disconnect, initiator |
| Array layout | Branch grouping, trunk cable route | String assignment, junction box, DC homerun route |
| Single-line diagram | AC branches, combiner, no DC | DC strings, inverter, shutdown devices, boundary |
| Calculations | Units per branch, branch OCPD, AC output sum | Cold Voc, MPPT window, Isc and Imp, AC output |
| Rapid shutdown | Datasheet statement and initiator | Device count per string, listing, enable setting, initiator |
| Conductor schedule | AC branch conductors, trunk cable | PV wire, DC raceway, AC output |
| Labels | AC side and shutdown labels | DC marking, max voltage label, shutdown labels |
| Datasheets | Microinverter, module, combiner | Inverter, module, shutdown device, listing evidence |
| Structural, fire access, 705 math | No change | No change |
How should a team scope a platform switch?
Use the five changing items as the scope. A switch from string to microinverter touches the SLD, calculations, rapid shutdown evidence, conductor schedule and labels, and nothing structural. A switch the other way adds the cold-temperature voltage work and the shutdown device documentation.
The practical effect on cost and schedule is in the rework. A set that is drawn right once passes review on its own merits. A set with one sheet from each architecture is the kind a reviewer returns with a comment list. If your team lacks drafting capacity for a platform change, the US solar permit design service at Heaven Designs covers AHJ-ready permit packages. Scope and turnaround are agreed at scoping. Stamping requirements are confirmed at quote stage.
FAQ
Is a microinverter permit set simpler than a string inverter permit set?
It has fewer calculations, because there is no cold-temperature Voc or inverter window check. It is not exempt from review. The branch circuit count, the rapid shutdown statement and the label wording are all checked, and a mismatch with the equipment is the most common correction.
Does a string inverter always need module-level electronics?
On a pitched residential roof, that is the usual outcome. Under NEC 2023 690.12(B)(2) the alternatives are a listed PVHCS or the 80 V limit inside the array boundary. The public PVHCS listings we found for string inverters were commercial flat roof products. Check the listing for the exact equipment and your AHJ’s adopted edition.
Where do I find the maximum microinverters per branch?
On the manufacturer data sheet, in the AC output table. Enphase lists the figure per model, for example 13 for the IQ8+ and 16 for the IQ8 per 20 A branch. The data sheet notes the limit may vary with local requirements, so confirm with the AHJ.
Which NEC edition do these examples follow?
NEC 2023 section numbers for 690.12, 690.7 and 690.8 come from IAEI and Mike Holt material. The AHJ checklists we cite were written for the 2020 NEC, and we say so where they appear. Always check the edition your AHJ and utility enforce.
Do I need a different SLD template for each?
Yes. A single template that tries to cover both can produce a diagram showing MLPE shutdown while the datasheets show a string inverter with no shutdown devices. Keep one template per architecture and select by equipment.
Get the right plan set for the equipment
Choose the architecture first, then draw the five changing items from that architecture. For a second pair of eyes on a platform switch, the US solar permit design service is built for installers who need sets that match their equipment. You can also review sample US permit packages before you decide, or contact the team with the equipment list and the AHJ.