Phase Academy Interactive Power Systems Lab

Voltage Drop & Conductor Sizing

NEC 2023 · Ch.9 T8/T9 · T310.16
Scope: Educational voltage-drop target check (Chapter 9 Table 8 resistance-only or Table 9 AC-impedance) with an optional simplified ampacity screen. Table data per NEC 2023 (these values are unchanged since 2011). Verify every result against your adopted NEC edition, the AHJ, project specs, equipment instructions, and licensed engineering review.
Voltage Drop
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volts
Drop %
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vs 3% target
Receiving V
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at the load
VD Score
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out of 100
⚡ Voltage-Drop ResultTable 8 · resistance-only—
Score and pass / caution / fail here reflect the voltage-drop target only. Ampacity and parallel-conductor rules are evaluated separately in Design Status.
✅ Design Statusmulti-criteria
Educational estimate only
Overall is a study aid, not a compliance determination. “Ready for engineering review” means the modeled checks passed — it still requires verification per the scope banner above.
🔌 Voltage Flow
Source voltage minus line loss equals what actually arrives at the load.
SourceDelivered to Load
240 V −0 V lost 240 V
🎯 Drop GaugeVD target only
Where does your drop land relative to the target?
—drop
● Target met ● Near target ● Over target

💡 Insight

Enter values to see analysis.

📡 Simplified Circuit View
Animated current flow. Red dashes represent losses in the conductors.
📏 RecommendationVD target only
Min Size for Target
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vs Current Choice
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Smallest conductor that meets the voltage-drop target only — it does not confirm ampacity. Turn on the Ampacity Check to screen that separately. Where parallel conductors are selected, the 1/0 AWG minimum of 310.10(G) is enforced.
📊 What Drives This Result?
Relative influence of each input on your current voltage drop.
📈 Live AnalysisTable 8 · resistance-only—
Full breakdown of your current design.
Drop (V)
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Drop %
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Receiving V
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VD Score
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Min Size (VD)
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Eff. R Ω/1000 ft
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✅ Design Status
Educational estimate only
🌡 Ampacity Screen310.16 · simplified
Turn on the Ampacity Check in the sidebar to evaluate conductor ampacity against the load and OCP.
📏 Voltage Drop vs. Length
Your current design with the target threshold (red dashed). The vertical line marks your current length.
⚡ Voltage Drop vs. Load Current
Linear relationship: more current means more drop.
🌐 Voltage Drop vs. Power Factor
In Table 8 mode PF is not used, so this is flat. Switch to Table 9 to see reactance effects.
📑 Every Conductor Size at a Glance
Drop % for each AWG / kcmil size at your current length, current, and material. The red dashed line is your target.
📋 Conductor Sizing Table
The selected size is highlighted. The Ampacity column appears when the Ampacity Check is on.
SizeR (Ω/1000 ft)Drop (V)Drop %VD TargetAmpacity
⚖ Your Design vs. Alternatives
Your sidebar design compared live against three alternatives. Values are voltage-drop % (target check only).
Your Design
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Other Material
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One Size Larger
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Half the Length
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hypothetical
📉 Drop Over Distance — All Four Options
How each variation performs as length changes.
🪙 Copper vs. Aluminum at Every Size
At your current length and load. Aluminum generally needs 1–2 sizes larger to match copper.
🏆 Material Winner
Same size, same length — just material changed.
Copper
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Aluminum
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💡 Read-out

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📏 Length Impact — Short vs Long Run
Same feeder, different lengths. Distance changes everything.

🔍 Conductor Explorer

Every visual here updates in real time from your sidebar inputs. The goal is to see the shape of each relationship — which inputs move voltage drop a lot, and which barely matter.

⭕ Conductor Cross-Section Visualization
Relative sizes. Your current selection is outlined. Note how rapidly the area grows above 2/0.
📏 Drop vs. Length
Linear. Double the distance, double the drop.
💎 Drop vs. Conductor Size
Non-linear. Returns diminish for larger wire.
⚡ Drop vs. Load Current
Also linear. Same wire, more current, more drop.
🌡 Drop vs. Temperature
Hotter conductors mean higher resistance and more drop. Spans 40–100°C.
🔗 Parallel Conductors Effect
Paralleling splits current and cuts effective impedance. Sizes below 1/0 AWG are not permitted in parallel for normal power conductors — 310.10(G) [310.10(H) in NEC 2017].
👆 Interactive Wire Picker
Click (or focus and press Enter) on any size below to set it as your current design.

🧪 Scenario Lab

Pick a scenario, then tweak any sidebar input and watch it respond. Each one highlights a specific lesson. Loading a scenario overwrites the sidebar circuit inputs.

📋 Choose a Scenario
Select a scenario—
Each scenario loads a starting configuration you can then explore.
📈 Scenario Performance Curve
Drop over length for the loaded scenario, compared to your live sidebar settings.
💡 Scenario Takeaways

📏 Length

Short runs forgive a lot of mistakes. Long runs punish every one.

🪙 Material

Aluminum at the same size has roughly 60% more resistance than copper.

⚙ Motor Loads

Low power factor and starting current both widen the voltage margin you need.

🔄 Three-Phase

The √3 factor replaces 2× — three-phase gives a meaningful edge on the same run.

🎯 Design Challenge
You get a brief with fixed parameters. Tune the conductor (size and, where allowed, parallel sets) to pass voltage drop AND ampacity AND the parallel-conductor rules.
Challenge1/6 Difficulty— Solved0
Challenge 1

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Note: Opening this brief loaded its fixed parameters (voltage, current, length, material, target, ampacity basis) into the sidebar, replacing what was there. Only the conductor size and parallel count are yours to tune.
✅ Your Design — Live Verdict
Drop %
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Receiving V
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VD Margin
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Allowable Ampacity
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Adjust your sidebar inputs to match the brief

A challenge counts as solved only when you press Submit Design and every check passes — the brief parameters, voltage drop, ampacity, and parallel-conductor rules. Passing voltage drop alone is not enough.
📈 Design Quality Curve
See where your design sits on the drop curve for the required load.
📚 Learn Mode
Concepts, tips, and a running quiz that builds intuition piece by piece.

🔌 What is Voltage Drop?

The reduction in voltage as current travels through a conductor. Longer runs and smaller wires mean more drop — and equipment that does not get the voltage it expects.

“Voltage drop is what happens when distance and resistance team up.”

📏 Why Length Matters

Voltage drop is directly proportional to length. Double the wire, double the drop. This is why a conductor that is fine at 50 ft can be a problem at 400 ft.

🔧 Why Conductor Size Matters

Resistance per foot falls as cross-sectional area grows — but not linearly. Upsizing from #12 to #10 helps more than going from 4/0 to 250 kcmil. The curve flattens.

🪙 Copper vs. Aluminum

Aluminum has roughly 1.6× the resistance of copper. It is lighter and cheaper by weight, but you typically need 1–2 sizes larger to match copper performance.

“Sometimes the cheapest wire becomes expensive later.”

🔄 Single vs. Three-Phase

Single-phase uses 2× in the formula. Three-phase uses √3 (about 1.732×). Same conductor and current, three-phase drops about 13% less — and three-phase usually runs at higher voltage too.

⚡ Current & Power Factor

Current is linear — more amps means more drop. Power factor only enters the AC-impedance method (Table 9), where it re-weights resistance against reactance. In the resistance-only method (Table 8), PF is not used at all.

🌡 Temperature Effects

Conductors get hotter when loaded, and hotter metal has more resistance. Table 8 and Table 9 values are at 75°C; this tool corrects resistance with R₂ = R₁[1 + α(T − 75)].

🎯 Design Targets

NEC informational notes suggest about 3% for branch circuits and about 5% combined. These are guidance, not mandatory Code limits. Motors and sensitive electronics may want tighter targets. Ampacity, OCP, and terminations are separate, mandatory checks.

🔢 The Voltage-Drop Formulas

Table 8 — Resistance-only (no PF)

1φ: Vd = 2 · I · L · Reff / 1000
3φ: Vd = √3 · I · L · Reff / 1000

Table 9 — AC impedance

Ze = R · cosθ + X · sinθ
1φ: Vd = 2 · I · L · Ze,eff / 1000
3φ: Vd = √3 · I · L · Ze,eff / 1000

L = one-way length (ft); I = current (A); R = resistance per 1000 ft (Ω); X = reactance per 1000 ft (Ω). PF = cosθ and sinθ = √(1 − PF²). Reff and Ze,eff are per-1000-ft values corrected for conductor temperature and divided by the number of parallel conductors. The 2× (or √3×) accounts for the complete circuit path.

Lower PF does not universally increase voltage drop for every conductor. The reactance term (X·sinθ) grows in importance for larger conductors and in steel raceway; for small conductors the resistance term dominates and PF has little effect. In resistance-only (Table 8) mode PF is ignored entirely. The Table 9 model assumes a lagging power factor.
⚠ Common Mistakes
  • Sizing wire only for ampacity and ignoring drop on long runs — or the reverse: passing voltage drop but never checking ampacity.
  • Using one-way distance without the 2× or √3× factor.
  • Applying power factor to a DC / resistance-only calculation, where it does not belong.
  • Substituting aluminum at the same gauge as copper — it will not match.
  • Paralleling conductors smaller than 1/0 AWG, or ignoring terminal-temperature and ambient / grouping derating.
  • Reading the 3% / 5% figures as mandatory Code limits. They are informational notes.
🧠 Knowledge Check
🔮 Predict Then Peek
Each prompt asks you to predict, then reveal to check. Good for building gut instinct.
🧪 Engine Self-Tests
These assertions exercise the pure calculation engine against primary-source NEC table values and the formula results computed by hand. They run in your browser — nothing is claimed that you cannot re-run here.
Assertions
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Passed
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Failed
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📑 Data Sources & Assumptions
  • DC resistance — NEC Chapter 9, Table 8, uncoated copper / aluminum, Class B stranded column, 75°C.
  • AC resistance and reactance — NEC Chapter 9, Table 9, 75°C, 60 Hz, three single conductors in conduit. Copper rows: 14 AWG–1000 kcmil. Aluminum rows: 12 AWG–1000 kcmil. Table 9 has no 700, 800, or 900 kcmil rows and no 14 AWG aluminum row — the app reports “no Table 9 data” for those rather than interpolating.
  • Ampacity — NEC Table 310.16, 60/75/90°C columns, through 2000 kcmil.
  • Ambient correction — Table 310.15(B)(1), 30°C base. Conductor-count adjustment — Table 310.15(C)(1).
  • OCP — 240.6(A) standard ratings, 240.4(B) next-higher-size allowance, 240.4(D) small-conductor caps, 210.20(A) sufficiency.
  • Continuous load — two-part test of 210.19(A)(1) / 215.2(A)(1): termination-based ampacity before derating must carry 125% of the continuous load, and the derated ampacity must carry 100% of the load.
  • Parallel conductors — 310.10(G) in the 2020 and 2023 NEC; the same rule was 310.10(H) in the 2017 NEC.
  • Temperature correction — R₂ = R₁[1 + α(T − 75)], α(Cu) = 0.00323, α(Al) = 0.00330 per °C.
Values were checked against NEC table reproductions. NEC text is copyrighted and not reproduced here. Confirm against your adopted edition before using any result for design.
⛔ What This Tool Does Not Do
  • Size the equipment grounding conductor (250.122) or the neutral.
  • Model 240.4(E) tap rules or 240.4(G) motor / specific-load OCP allowances.
  • Apply the 110.14(C)(1)(a) steer toward 60°C terminations on circuits rated 100 A or less — you choose the terminal rating.
  • Handle leading (capacitive) power factor, harmonics, or motor starting inrush.
  • Couple the current-carrying-conductor count to the parallel count — parallel sets are assumed to be in separate raceways. If they share one raceway, raise the count yourself.
  • Model conductor behavior once voltage drop is a large fraction of the source voltage; the constant-current linear model is flagged as out of range above 15%.
Phase Academy · Interactive Power Systems Lab. This voltage-drop calculator is an educational estimating tool, not a complete NEC conductor-sizing tool and not a substitute for licensed engineering design or Code-compliance review. Voltage-drop targets are informational-note guidance, not mandatory Code limits. Verify every result against your adopted NEC edition, the authority having jurisdiction, project specifications, and equipment instructions.