Conductor Sizing in Solar is one of those things that seems simple—until you fail an inspection or your system underperforms.
Many installers focus on racking and modules, but the wire you choose determines how much power actually makes it to the inverter and, ultimately, the grid.
In this article, we’ll break down how to size conductors properly, avoid voltage drop issues, and stay compliant with NEC requirements — without overcomplicating your design or overspending on copper.
Why Conductor Sizing Matters
Think of your conductors as the arteries of your solar system. If they’re too small, energy loss increases; too large, and you waste money.
The goal is to strike the perfect balance between safety, efficiency, and cost.
Undersized conductors cause:
- Voltage drop, leading to reduced system performance.
- Overheating, creating safety risks.
- Failed inspections, especially under NEC 690 and 310.
Proper conductor sizing ensures:
- Full power delivery from array to inverter.
- Compliance with NEC codes.
- Longer lifespan for both cables and connected equipment.
Understanding the Key Factors in Conductor Sizing
Before you grab your wire chart, here are the core factors that affect conductor size:
Current (Amperage):
Based on module output, inverter input, and total circuit current. NEC requires conductors to handle 125% of the maximum current.
Voltage Drop:
NEC recommends keeping voltage drop under 3% for efficiency, though it’s not a mandatory code. For long runs, this is critical.
Ambient Temperature:
High rooftop temperatures reduce ampacity. Always apply temperature correction factors when sizing for exposed conditions.
Conduit Fill and Derating:
Too many wires in one conduit reduce current-carrying capacity. You must apply conduit fill and derating factors.
Conductor Type (Copper vs. Aluminum):
Copper has higher conductivity but costs more. Aluminum works well for feeders but requires careful terminations.
Distance Between Components:
Longer runs = higher resistance = higher voltage drop. Always calculate voltage drop over the total circuit length.
Common Mistakes Installers Make
Even experienced installers sometimes overlook these NEC details:
- Using module STC current instead of Isc × 1.25 × 1.25 (for continuous load and safety factor).
- Forgetting to account for temperature derating on rooftop circuits.
- Running long homeruns with 10 AWG when they actually need 8 AWG or 6 AWG.
- Mixing THHN and USE-2 conductors incorrectly across transitions.
- Failing to update wire sizes when system voltage or string count changes.
These small oversights can lead to voltage losses up to 5–8% — which directly impacts system yield and customer satisfaction.
How to Calculate Voltage Drop (Simplified Formula)
Here’s a simplified approach you can use during design or pre-install planning:
Voltage Drop (V) = (2 × K × I × D) / CM
Where:
- K = 12.9 (for copper) or 21.2 (for aluminum)
- I = Current (amps)
- D = One-way distance (feet)
- CM = Circular mil area of the wire
Keep your total voltage drop below 3% for DC runs and 2% for AC feeders to stay efficient.
NEC Code References You Should Know
To keep your design compliant, focus on these key NEC articles:
- NEC 690.8 – Calculating maximum current and conductor ampacity.
- NEC 690.31 – Wiring methods for PV systems.
- NEC 310.15(B) – Ampacity adjustment factors.
- NEC 310.16 – Ampacity tables for conductors.
- NEC 705.60 – Interconnection conductor requirements.
Understanding these sections helps you justify your wire sizing decisions during AHJ plan reviews and field inspections.
Pro Tips from Field Experience
- Always oversize conductors for long DC homeruns.
The copper cost increase is minor compared to long-term production loss. - Pre-label conductor sizes on your plan set.
Inspectors love when your drawings include AWG and voltage drop values. - Use MC4-compatible crimp tools and ferrules.
Poor terminations create resistance points and hot spots. - Document everything.
Include calculations in your submittal packet—it shows professionalism and reduces correction rounds.
Example: How Small Changes Affect Performance
Let’s say you have a 10 kW system with a 75-foot DC run using 10 AWG wire.
Your voltage drop is roughly 3.8%.
If you switch to 8 AWG, it drops to 2.3%, improving efficiency and reducing thermal loss.
That’s a difference of around 150–200 kWh/year — which adds up over the system’s 25-year life.
How Vishtik Can Help
Designing with proper conductor sizing requires more than guesswork — it takes code knowledge, math, and real-world experience.
At Vishtik, we help contractors and EPCs across the U.S. create accurate, NEC-compliant permit designs that pass inspections the first time.
Our designs include:
- Correct conductor sizing for DC, AC, and interconnection circuits.
- Voltage drop calculations clearly labeled on plan sets.
- AHJ-specific code compliance for NEC 2020/2023.
- Fully engineer-stamped drawings ready for submittal.
Eliminate Voltage Drop Failures Before They Happen
Don’t lose time or money fixing failed inspections.
Let Vishtik’s expert design team size your conductors right — for maximum performance and full NEC compliance.
Get your solar permit design today at www.vishtik.com
