Phase Academy Interactive Power Systems Lab

Service & Feeder Conductor Sizing Calculator

Educational NEC study tool
CODE — NEC 2023
Local adoption varies. NEC edition availability depends on your Authority Having Jurisdiction. This is an educational study tool — not an AHJ-approved design tool. Always verify the adopted code cycle and have designs reviewed by a qualified engineer.
🔍 Calculator Scope

In scope

  • Individual insulated conductors in raceway, cable, or earth, ≤2000 V
  • Table 310.16 ampacity (Cu and Al/CCA), 60/75/90°C columns
  • Ambient correction 310.15(B)(1) & bundling 310.15(C)(1)
  • Terminal limits 110.14(C), OCPD 240.4 / 240.6, continuous 125% (two-part 215.2/230.42 test)

Out of scope (shown as warnings, not passes)

  • Motors, HVAC/AC, welders, fire pumps (Art. 430/440)
  • Taps, transformers, capacitors (Art. 240.21 / 450)
  • Free-air / cable tray / MV, and parallel conductors
  • Full 220.61 neutral load calc & AHJ-specific amendments
⚡ NEC Code Checks
Rule-by-rule result for the implemented mandatory checks. The overall result is FAIL if any mandatory check fails, WARN if checks pass but assumptions/out-of-scope items remain, and PASS only when every implemented mandatory check passes with no unresolved critical assumptions.
📏 Design Margin engineering advisory — not code pass/fail
📊 Live Metrics
Snapshot of the current sizing analysis. Change inputs on the left to watch everything update.
🎯 Utilization Gauge
Utilization against the governing NEC check — the worst of the two 215.2/230.42 sizing legs (or the 83% target under 310.12).
🔄 Sizing Path
From load to selection.
📈 Conductor Size Ladder
Final ampacities of standard sizes from the shared calculation engine. Your selection is highlighted; next-larger is shown in light blue. Click any bar to explore.
Selected
Next Larger
Below Requirement
Above Requirement
🧮 Ampacity Adjustment Flow
How the insulation-column base ampacity is corrected, adjusted, then capped at the terminal-limited column.
🌡️ Temperature Correction Curve
Exact Table 310.15(B)(1) step values vs ambient, for each insulation rating. Curves stop where the column becomes invalid. The dashed marker shows current ambient.
60°C
75°C
90°C
Your Setting
📦 Bundling Adjustment
Table 310.15(C)(1) adjustment factor per number of current-carrying conductors. Your bracket is highlighted.
⚖ Copper vs Al/CCA Comparison
Final ampacity by material across common sizes, from the shared engine at your current settings.
Copper
Al/CCA
🔥 Insulation Column Comparison
For your selected size and material: Table 310.16 base ampacity at each temperature column.
📉 Voltage Drop vs Distance
Estimated voltage drop along the run for several candidate sizes. The 3% line is an informational-note recommendation, not a code limit. R basis: Ch. 9 Table 8 DC @75°C (AC/skin effect neglected — underestimates VD for large sizes); XL for 700–900 kcmil interpolated.
⭕ Cross-Section Visualization
Relative conductor cross-sections for nearby sizes (scaled by circular mil area).

Larger area = more current capacity. Aluminum requires a larger physical size to match copper's ampacity.

🕳️ Raceway Cross-Section
Visual of current-carrying conductors in a raceway. More conductors = less heat dissipation = derating.
📏 Headroom Margin
The governing sizing leg only: the requirement and the ampacity it is actually measured against (termination column for leg (a), corrected/adjusted for leg (b)). Matches the gauge and the Design Margin card exactly.
Ampacity Sensitivity
How final ampacity for your selected size changes across the full ambient range, at your current bundling and terminal cap — from the shared engine.
🧭 Step-by-Step Calculation
Every number traced back to its source, for the selected candidate.
📋 Assumptions & Out-of-Scope Notes
    📝 Sizing Results
    ✅ Code Checks
    ↕ Next-Larger Size Comparison
    ⚡ Voltage Drop Estimate design advisory
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    Design Challenge

    Loading a scenario...

    How It Works

    You'll receive a scenario with specific design constraints (load, environment, code requirements). Use the sidebar inputs to configure a conductor that satisfies all constraints. When you're ready, click Evaluate My Design — the system grades the constraint match plus the same rule-based code checks used on the Dashboard.

    • Constraints: Do your inputs match the scenario (use, phase, voltage, load, environment)?
    • Code checks: Do the implemented mandatory NEC checks pass (ampacity, terminal, OCPD, service minimum)?
    • Efficiency: Are you avoiding unnecessary oversizing? Cost matters.
    📜 Challenge History

    Complete a challenge to start building your history.

    Why Conductor Sizing Matters

    Undersized conductors overheat. Oversized conductors waste money. Conductor sizing sits at the intersection of safety, economics, and code compliance — getting it right means the circuit functions safely under the worst expected conditions, not just on a mild Tuesday.

    Service conductors carry additional weight because they typically lack upstream overcurrent protection. A feeder at least has a breaker watching its back. A service conductor has to fend for itself until the main disconnect trips.

    Common Mistakes

    • Ignoring terminal temperature limits. A 90°C wire may be corrected/adjusted on its 90°C column, but the final ampacity is capped at the terminal column (60 or 75°C) per 110.14(C).
    • Forgetting the 125% continuous factor. Loads running 3+ hours must be sized at non-continuous + 125% of the continuous portion.
    • Confusing ampacity with OCPD. #10 Cu can carry its 90°C ampacity for derating, but 240.4(D) still caps its breaker at 30 A.
    • Skipping ambient correction. A 45°C attic is not the same as a 30°C basement.
    • Bundling without adjusting. >3 CCCs in a raceway requires Table 310.15(C)(1) adjustment.
    🧠 Interactive Quiz
    Test your understanding of NEC conductor sizing concepts.

    Pro Tip: Correct on the high column, cap at the terminal

    You may apply ambient and bundling factors to the conductor's own insulation column (e.g., 90°C), which gives more headroom against derating. But the final ampacity is then limited to the terminal-rated column value (60 or 75°C) per 110.14(C). The insulation sets the derating ceiling; the terminal sets the practical cap.

    Copper vs Aluminum / Copper-Clad

    Aluminum and copper-clad aluminum share one NEC ampacity column and require larger conductors for equivalent ampacity, compatible terminations, and correct torque. Neither material is inherently wrong, but mixing column references is a recipe for trouble.

    🧪 Developer Self-Tests
    Runs the embedded NEC assertion suite against the calculation engine. Also callable from the browser console as runSelfTests().
    Educational Disclaimer: This tool is for learning and study support only. It implements a documented subset of NEC conductor-sizing rules and is not a substitute for licensed engineering judgment, formal plan review, an AHJ-approved design tool, or interpretation by the Authority Having Jurisdiction. It does not claim to be "100% code compliant."
    📚 NEC Reference Panel
    Articles and tables relevant to this calculation, keyed to the selected NEC edition. References paraphrase NEC logic; always consult the code text for exact requirements.
    🔗 Service vs. Feeder Comparison
    AspectFeederService Conductor
    Definition Conductors between service equipment and a downstream OCPD Conductors from service point to service disconnecting means
    Primary Article Art. 215 Art. 230
    Overcurrent Protection Protected by upstream OCPD Generally no OCPD on supply side
    Conductor Ampacity Basis Ampacity ≥ load per Art. 215.2 Ampacity ≥ load per Art. 230.42
    Minimum Size No universal minimum beyond ampacity #8 Cu / #6 Al minimum (230.23(B) overhead / 230.31(B) underground; limited-load exceptions not modeled)
    Key Concern Voltage drop, load growth No upstream OCPD — conductor sizing is critical