Resources
Tools for your next project. Start with a calculation.
Use the public calculators without signing in. For a complete solar design, explore the designer and create an account to save your work.
Free Solar Diagram Editor
Build a solar single-line diagram with equipment, conductor sizing and electrical checks. An account is required to use the designer.
Conduit Fill Calculator
Determine proper conduit size based on NEC fill requirements.
Load Calculator
Residential service load calculations to NEC Article 220.
EV Charger Load Calculator
Will an EV charger fit an existing panel? Instant NEC 220.83 verdict with a free PDF report.
Voltage Drop Calculator
Wire sizing and voltage drop across distance, AC and DC.
Fault Current Calculator
Available fault current at the end of a conductor run.
Row Spacing Calculator
Solar panel inter-row spacing to avoid winter-solstice shading. Latitude, tilt, azimuth.
Guides
Reference articles for the rules behind the tools.
The NEC sections, design conventions, and inspection checks that drive what the calculators output. Bookmark for the next plan check or AHJ conversation.
Planset
Solar Planset Requirements: 20 Ingredients Ranked
What goes in a solar plan set, and which parts actually decide approval. Every ingredient scored on approval impact, field impact, effort by hand, and how much a tool can own. Includes a pre-submittal checklist.
Grounding & bonding
Subpanels: Why Neutral and Ground Are Separated
Why the bond happens once at the service and nowhere else, what a second bond does to normal load current, and how a gateway or a new disconnect turns an existing main panel into a subpanel that still has its bond in it.
NEC Compliance
The 83% Rule and Solar Feeder Taps
A dwelling feeder sized at 83% under NEC 310.12 does not stop qualifying when solar taps it. Article 705 adds a second test. What changes, the two resolutions the trade uses, and how to avoid the problem entirely.
NEC Compliance
The 120% Rule vs Power Control Systems
What the solar 120% rule (NEC 705.12) is, how an NEC 705.13 Power Control System differs, when a PCS lets you skip a service upgrade, and where the 120% rule still applies.
NEC Compliance
The Solar 120% Rule (NEC 705.12), Done Right
The 120% busbar rule with the correct citation for every code cycle (2017, 2020, 2023), worked examples, which breaker counts on MLO panels, subpanel propagation, and every fix when a design fails it.
NEC Compliance
PV String Sizing, Explained Properly
The full variable checklist behind NEC 690.7: eight panel inputs, six inverter constraints, ASHRAE temperature corrections, per-MPPT current limits, and the step-by-step math.
NEC Compliance
Service Disconnect Locations
The two-layer problem: NEC 230 and 705 are the floor, but the utility interconnection rules are where a code-perfect planset still gets rejected. Both layers, with the common rejection causes called out.
Solar Design
Solar Wiring Diagram Guide
What every PV (and PV + storage) wiring diagram needs: the single-line and its conductor callouts, conductor schedule, OCPD, EGC, AC disconnects, and the labels every AHJ checks.
Solar Design
Solar Panel Row Spacing
How inter-row spacing is computed at the worst-case sun angle of the year, with a live 3D viewer that rotates as you change azimuth and tilt.
Beyond the calculators
These tools are part of a bigger platform.
Bring equipment, roof layout and electrical connections together in Solar Design Lab. Review the design checks and schedules as you work. See pricing for project access and separately quoted engineering services.