Avila Solar Drafting handles the electrical and structural engineering behind a Houston solar submittal — wind load determination for a coastal metro, attachment and load path, conductor sizing against real summer conditions, and the AC disconnect configuration CenterPoint expects. Where the scope adds structural load, the sealed engineering is handled in-house rather than sent out to find an engineer.
Key takeaways
- Houston’s binding design constraint is heat, not cold. Most metros size the string against winter; here the ampacity and derating side of the calculation is what gets tight.
- Wind load is an address-level determination. A coastal Baytown roof and an inland Cypress roof are not the same design problem, and the difference is not cosmetic.
- Houston enforces the 2023 NEC with either the 2021 IRC or 2021 IBC, which set the wind determination method for the structural design.
- Sealed structural documentation is required wherever the array adds load — a Texas PE seal, not a generic manufacturer letter.
- The CenterPoint AC disconnect is a placement and device decision made on the drawing, not something a field crew resolves.
Why Houston’s constraint is heat, not cold
String sizing is usually taught cold-first, because open-circuit voltage rises as temperature falls and Article 690.7 requires maximum PV system voltage to be calculated from the module’s temperature coefficient against the lowest expected ambient — with a 600-volt ceiling on one- and two-family dwellings. That calculation still has to be run in Houston, and it still governs the upper bound on string length.
What is different here is where the design actually gets squeezed. Using the 1991–2020 climate normals published by the National Weather Service for Houston Intercontinental, January’s average low is 43.7°F and the average high is 63.8°F. Compare that to July at 94.5°F average high and 75.7°F average low, and August at 94.9°F and 75.4°F, with an annual mean of 70.5°F. A mild winter means the cold-voltage margin is generous; a long, humid summer with overnight lows in the mid-seventies means the conductors, the equipment ratings and the inverter’s temperature behaviour are where the real work is.
Practically, that shifts the effort in three places. Ampacity is corrected for the summer ambient rather than a national default. Conductors in raceway run close to a roof surface in direct sun take Article 310’s ambient correction plus the rooftop adder, which in Houston is a meaningful reduction and not a rounding error. And equipment locations get chosen with heat in mind — an inverter on a west-facing wall in Houston is a different proposition from the same inverter in Portland.
Houston’s all-time record low is 5°F, and it is worth saying plainly that nobody should be sizing against it. Designing a modern array to a once-in-a-century reading strands capacity on every string for the whole life of the system. The cold-side calculation belongs on the site’s expected extreme minimum, not on the coldest thing that ever happened. The full method, with worked examples, is on our solar string sizing page.
Three things manufacturer sizing tools reliably do not do: they do not apply the 600-volt dwelling ceiling, they do not choose a design temperature for your address, and they do not run the Article 690.8 and 690.9 conductor and overcurrent checks a plan reviewer will. A tool returns a string length. A plan set has to return a defensible calculation.
Wind load in a coastal metro, determined per address
Houston enforces the 2021 IRC or the 2021 IBC depending on the structure, and both route the wind question the same way: to the basic design wind speed for the project’s coordinates, taken from ASCE 7 through Table R301.2(1) on the residential side or Section 1609.3 on the commercial side. The City of Houston does not publish a wind map of its own. It defers to the national ASCE 7 Hazard Tool, and its solar guide requires a printout from that tool inside the plan set.
This is not a formality. The metro spans roughly fifty miles from the bay inland, and the wind speed contour runs through it rather than around it. Addresses at or above 140 mph fall inside the windborne debris region, which changes the component and cladding pressures the attachment has to resist, tightens the spacing at roof edges and corners, and — on the permitting side — removes the project from the SolarAPP+ instant-approval track entirely. The permitting consequence is covered on our Houston permit design page; the design consequence is here.
We pull the value per project rather than carrying a metro-wide number, because a figure that is correct for one Houston address can be wrong for another twenty miles away. Publishing a single citywide wind speed would be convenient and it would also be misleading, so we do not. The load taxonomy and how the code edition maps to the ASCE edition are covered in solar structural load calculations.
Attachment and load path on a Gulf Coast roof
Uplift, not gravity, is what governs most Houston residential arrays. What that means in the drawing set: attachment type and spacing selected against the calculated uplift for that roof zone rather than a default pattern, the load path documented from module through rail and standoff into framing, and edge and corner zones treated differently from the field of the roof because the pressures there are not the same.
Two Houston-specific conditions come up often enough to scope at intake. Existing roof penetrations have to appear on the roof plan, which means an accurate survey matters more here than in jurisdictions that do not ask. And older housing stock across the inner loop frequently carries framing that was never designed for a rooftop array, which turns the analysis from a confirmation into an actual evaluation.
When Houston work needs a sealed engineering package
The City of Houston’s solar guide is direct about it: structural letters, calculations, details and manufacturer installation instructions must be sealed by a Texas Professional Engineer wherever the installation adds structural load. In practice that catches far more residential retrofits than installers expect.
The scopes worth flagging at intake are the familiar ones — ground mounts and carports, low-slope roofs, roofs already carrying more than one covering layer, dead load beyond what the existing framing was designed for, structures never engineered for an array, and commercial work. Rather than publish a threshold table that would be wrong for some addresses, the honest answer is to have the scope looked at: call and we will tell you whether a seal is triggered before you quote the job.
We hold seals in-house through our engineering stamps service, so a project that turns out to need one does not go back into a queue. PE stamp triggers covers the general pattern, and ground mount plan sets the structural scope specific to ground arrays.
The CenterPoint disconnect, decided on the drawing
CenterPoint Energy’s distributed generation documentation calls for a lockable disconnect switch with an external handle, visible and readily accessible, typically installed within ten feet of the CenterPoint meter. Where it sits beyond ten feet, or out of line of sight, a weather-resistant and easily read placard must be installed within ten feet of the meter identifying where the disconnect actually is.
That is a placement decision with a documentation consequence, and both belong in the design rather than on a punch list. We locate the disconnect on the site plan and the single-line diagram, specify a device that satisfies the visible-and-lockable requirement, and call out the placard where the geometry of the property forces the longer run. Worth noting on currency: the rule reads identically across several CenterPoint documents and its live guidance, but the newest dated revision publicly available is some years old — so we treat it as settled practice and confirm the current requirement on projects where the placement is marginal.
What you get
Electrical and structural design delivered as a coordinated package — single-line diagram, sizing calculations, structural details, attachment schedule and the wind determination — as submittal-ready PDFs plus source files. The full sheet list and the turnaround tiers are on Houston solar plan sets, and the statewide picture on Texas solar plan sets.
We guarantee accurate solar plan sets, drawn to the electrical and building code requirements of your specific jurisdiction and designed to pass first-time review.
Houston Solar Design FAQs
What temperature do you size a Houston array to?
Both ends, but the summer end is where Houston gets tight. The cold-voltage calculation under Article 690.7 uses the site’s expected extreme minimum against the module’s temperature coefficient, with a 600-volt ceiling on one- and two-family dwellings. Ampacity is then corrected against real summer conditions — NWS 1991–2020 normals for Houston Intercontinental put August at a 94.9°F average high and a 75.4°F average low — plus the rooftop adder for raceway in direct sun.
Do you use Houston’s record low of 5°F for string sizing?
No. Designing to a once-in-a-century reading strands capacity on every string for the life of the system. The cold-side calculation uses the site’s expected extreme minimum temperature, which is the basis Article 690.7 intends.
What wind speed governs a Houston array?
An address-specific basic design wind speed from ASCE 7, reached through Table R301.2(1) of the 2021 IRC or Section 1609.3 of the 2021 IBC. Houston publishes no wind map of its own and requires an ASCE 7 Hazard Tool printout in the plan set, so we pull the value per project rather than applying a metro-wide figure.
What changes if my address is in the windborne debris region?
At 140 mph or above, the component and cladding pressures the attachment must resist go up, edge and corner zone spacing tightens, and the project loses SolarAPP+ eligibility and goes to traditional plan review. It is a design change and a permitting change at the same time.
When does a Houston project need a PE seal?
The city requires sealed structural letters, calculations and details from a Texas Professional Engineer wherever the installation adds structural load. Common triggers are ground mounts and carports, low-slope roofs, roofs with multiple covering layers, added dead load, un-engineered structures and commercial scopes. Call with the scope and we will confirm before you quote.
What AC disconnect does CenterPoint require?
A lockable disconnect switch with an external handle, visible and readily accessible, typically within ten feet of the CenterPoint meter. Beyond ten feet or out of line of sight, a weather-resistant, easily read placard must be installed within ten feet of the meter identifying the disconnect’s location.
Do you handle both the electrical and the structural design?
Yes — conductor and overcurrent sizing, string sizing, grounding and interconnection on the electrical side; wind determination, attachment, load path and calculations on the structural side, with the PE seal where the scope requires one.
Scope a Houston project with a designer
Engineering scope is quoted rather than ordered off a menu — a 1960s inner-loop roof near the debris line and a new-build slab in Katy are not the same job, and pricing them the same way helps nobody.
Or start a standard set at my.avilasolar.com. Avila Solar Drafting LLC, 11995 SW Walker Rd, Beaverton, Oregon · 971-410-0655 · info@avilasolar.com.