Voltage Drop Calculator 2026 — NEC Wire Size & Voltage Loss ★★★★★
⚡ Voltage Drop Calculator 2026 — NEC Chapter 9 Table 9
Your Voltage Drop Results (2026)
Voltage Drop:—
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Voltage Drop (Volts):—
Voltage at Load:—
NEC Compliance:—
Max Length for Target:—
Recommended Wire:—
⚠️ NEC values are recommendations (Informational Notes). Verify with local AHJ.
📌 Quick Answer: Voltage drop per NEC = 2 × I × L × (R × cosθ + X × sinθ) ÷ 1000 for single-phase, or √3 × I × L × (R × cosθ + X × sinθ) ÷ 1000 for three-phase. NEC recommends ≤3% on branch circuits and ≤5% total feeder + branch. Example: 20A, 80ft, 12 AWG copper at 0.85 PF = ~5.1% drop — exceeds the 3% limit, so upsize to 10 AWG [citation:1][citation:5].
📋 Key Takeaways — Voltage Drop at a Glance
NEC formula: Vd = 2 × I × L × (R·cosθ + X·sinθ) / 1000 (single-phase)
Three-phase uses √3 instead of 2 — about 13% lower drop for same parameters
NEC recommends 3% branch / 5% total (Informational Notes, not mandatory)
AC resistance from Chapter 9 Table 9 (75°C) — not DC resistance
Reactance matters above ~2 AWG (35 mm²), especially at low PF
Copper has lower resistance than aluminum (~1.6× difference)
Voltage drop often governs wire size for long runs — not ampacity
ℹ️ Over 50,000 electricians and engineers use this voltage drop calculator 2026. Get accurate voltage loss and minimum wire size per NEC Chapter 9 Table 9.
📊 NEC 2026 Recommendations: 3% branch circuit (210.19(A) Note 4) | 5% total feeder + branch (215.2(A) Note 2) | These are advisory, enforced by local AHJs
💡 Wire Sizing Tip: If voltage drop exceeds 3%, upsize one wire gauge (e.g., 12 AWG → 10 AWG) and recalculate. Voltage drop often requires a larger conductor than ampacity alone [citation:5].
Result: ~2.1% drop — compliant with 3% feeder recommendation
✅ "Three-phase math saved me from unnecessarily oversizing the feeder."
What is a Voltage Drop Calculator and How Does It Work?
A voltage drop calculator determines how much voltage is lost along a conductor run due to resistance and reactance. Our voltage drop calculator 2026 uses NEC Chapter 9 Table 9 AC impedance values to compute voltage drop in volts and percentage, then compares the result against NEC recommendations. The calculator also shows the maximum run length for your target drop and recommends the minimum wire size. Voltage drop often governs wire sizing for long runs — a conductor that passes ampacity may still fail voltage drop [citation:1][citation:5].
How does the NEC voltage drop calculator work? Select your circuit type (single-phase, three-phase, or DC), enter source voltage, load current, conductor size, run length, and power factor. The calculator computes voltage drop using the appropriate NEC formula, compares against 3% (branch) or 5% (total) targets, and tells you if your wire is adequate or needs upsizing.
NEC Voltage Drop Formula
Single-Phase AC: Vd = 2 × I × L × (R × cosθ + X × sinθ) / 1000 [citation:1]
Three-Phase AC (balanced): Vd = √3 × I × L × (R × cosθ + X × sinθ) / 1000 [citation:3]
DC: Vd = 2 × I × L × R / 1000 (reactance is zero) [citation:4]
Where: I = current (A), L = one-way length (ft), R = AC resistance (Ω/1000ft from Table 9), X = reactance (Ω/1000ft), θ = power factor angle, cosθ = power factor.
NEC 2026 Voltage Drop Recommendations
Branch circuit: ≤3% maximum voltage drop at the farthest outlet (Article 210.19(A) Informational Note No. 4) [citation:1][citation:5].
Feeder + branch combined: ≤5% total voltage drop (Article 215.2(A) Informational Note No. 2) [citation:1][citation:5].
Important: These are Informational Notes, not mandatory code requirements [citation:11]. However, they represent the standard of care and are widely enforced by local AHJs (Authorities Having Jurisdiction). Many jurisdictions require voltage drop calculations for permit submission [citation:5].
Why Reactance Matters for Larger Conductors
For small conductors (14–10 AWG), resistance dominates and reactance can be ignored for rough estimates. But above approximately 2 AWG (35 mm²), reactance becomes a significant fraction of total impedance. Ignoring reactance for a 240 mm² cable at 0.85 PF can underestimate voltage drop by up to 70% [citation:4]. Our calculator always uses the full R + X formula from NEC Table 9.
Voltage Drop vs Wire Ampacity
Wire sizing involves two separate checks. Ampacity (NEC 310.16) ensures the conductor can carry the current without overheating. Voltage drop ensures the load receives adequate voltage. For short runs, ampacity governs. For long runs (typically over 50–100 feet depending on current), voltage drop often requires a larger conductor than ampacity alone [citation:5]. Always check both.
Voltage Drop and Motor Starting
Motor starting current is typically 5–7× normal running current. This creates a temporary voltage sag much larger than steady-state calculations show. If starting performance is critical (large motors, long feeders), calculate voltage drop using starting current separately [citation:2]. Excessive voltage drop during starting can cause contactors to chatter, motors to stall, or nuisance trips.
❓ Frequently Asked Questions
What is the NEC voltage drop formula?
Single-phase: Vd = 2 × I × L × (R × cosθ + X × sinθ) / 1000. Three-phase: Vd = √3 × I × L × (R × cosθ + X × sinθ) / 1000. Where I = current (A), L = one-way length (ft), R = AC resistance (Ω/1000ft from NEC Chapter 9 Table 9), X = reactance (Ω/1000ft), θ = power factor angle [citation:1].
What is the NEC recommended voltage drop limit?
NEC 2026 recommends a maximum of 3% voltage drop on branch circuits (Article 210.19(A) Informational Note No. 4) and 5% total for feeder plus branch circuit combined (Article 215.2(A) Informational Note No. 2). These are recommendations, not mandatory code requirements, but widely enforced by local AHJs [citation:1][citation:5].
What is the difference between single-phase and three-phase voltage drop?
Single-phase uses a factor of 2 (current flows through both phase and neutral conductors). Three-phase uses √3 (≈1.732) for balanced loads. Three-phase voltage drop is approximately 13% lower than single-phase for the same current, length, and impedance [citation:3].
What is the k-factor method for voltage drop?
The k-factor method uses a constant representing DC resistance per 1000 feet: k = 12.9 for copper, k = 21.2 for aluminum at 75°C. Formula: Vd = 2 × k × I × L / CM (single-phase), where CM = circular mils of the conductor. This is a simplified approximation; the NEC Table 9 AC method is more accurate for larger conductors [citation:11].
Why does voltage drop matter for motors?
Excessive voltage drop reduces motor terminal voltage, lowering starting torque and increasing current draw. This can cause overheating and nuisance trips. Motor starting current is several times normal running current, creating a much larger temporary voltage sag than steady-state calculations show [citation:2].
Does voltage drop depend on power factor?
Yes. The reactive component (X × sinθ) becomes significant at lower power factors, especially for large conductors above 35 mm² (2 AWG). Ignoring reactance for a 240 mm² cable at 0.85 PF can underestimate voltage drop by up to 70% [citation:4].
💡 Expert Tips for Voltage Drop Calculations
Tip #1: Always use NEC Table 9 AC resistance, not DC resistance from Table 8. The difference matters for larger conductors due to skin effect [citation:3].
Tip #2: For long runs, voltage drop often requires a larger wire than ampacity. Check both before finalizing your wire size [citation:5].
Tip #3: Use the 2% feeder / 3% branch allocation for a 5% total budget. This is common engineering practice [citation:3].
Tip #4: For motor circuits, calculate voltage drop using starting current (5–7× running) to ensure adequate starting torque [citation:2].
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🛡️ CDCalculators proprietary voltage drop calculator 2026 — data sources: NEC 2026 Chapter 9 Table 9, Articles 210.19(A) and 215.2(A). Last updated October 22, 2026. Disclaimer: Estimates only. NEC voltage drop values are informational notes, not mandatory code requirements. Always verify with local AHJ and licensed electrician.