Voltage Drop Calculator

Calculate AC/DC voltage drop and select conductor size per NEC §210.19 and §215.2 recommendations (3% branch, 5% overall). Uses Chapter 9 Table 9 resistance.

NEC edition

Inputs

A

Full-load amps of the circuit

ft

Distance from panel to load (one way)

Aluminium is not manufactured in #14 — selecting it with aluminium is rejected rather than silently substituted

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⚡ Test your knowledge
What is the maximum recommended voltage drop for a branch circuit under NEC Informational Notes 210.19(A) and 215.2(A)?
  • A. 1%
  • B. 3% on branch / 5% total overall
  • C. 5% on branch / 10% total
  • D. 8%
💡 NEC Informational Notes recommend a maximum 3% drop on branch circuits and 5% total combined drop from the service to the furthest outlet for optimal efficiency.

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How it works & the NEC rules behind it

Voltage drop occurs naturally due to the internal electrical resistance of conductive materials (copper or aluminum) as current flows over long circuit distances. Excessive voltage drop results in flickering lights, motor overheating, tripping breakers, and inefficient equipment operation.

NEC Code Recommendations on Voltage Drop

  • Branch Circuits (§210.19(A) Informational Note No. 4): Conductors should be sized to prevent a voltage drop exceeding 3% at the farthest outlet of power, heating, or lighting loads.
  • Feeders & Combined Total (§215.2(A)(1) Informational Note No. 2): The maximum total voltage drop across both the feeder and branch circuit combined should not exceed 5% for reasonable electrical efficiency.
  • Sensitive Electronic Loads (§647.4(D)): Where technical power systems are utilized, total voltage drop must not exceed 1.5% for branch circuits and 2.5% total.
  • Fire Pump Circuits (§695.7): Voltage drop is strictly limited to 15% during motor starting and 5% under 100% full-load operating conditions.

Formulas & equations

Single-Phase: VD = 2 × (R / 1000) × I × L Three-Phase: VD = √3 × (R / 1000) × I × L (√3 ≈ 1.732) % Drop = (VD / Nominal Voltage) × 100%
Where:
• VD = Total voltage drop in Volts (V)
• R = Conductor AC resistance in Ohms per 1,000 feet from NEC Chapter 9, Table 9 (@ 75°C)
• I = Load current in Amperes (A)
• L = One-way circuit length in feet (ft)
• 2 = Round-trip conductor multiplier for single-phase systems (hot + neutral/return)
• √3 (1.732) = Multiplier for balanced 3-phase systems

NEC reference table

Wire Size (AWG/kcmil) Copper (Ω / 1,000 ft @ 75°C) Aluminum (Ω / 1,000 ft @ 75°C) Max Amps @ 75°C (THHN/XHHW)
#14 AWG3.14 Ω—15 A
#12 AWG1.98 Ω3.18 Ω20 A
#10 AWG1.24 Ω2.00 Ω30 A
#8 AWG0.778 Ω1.26 Ω50 A
#6 AWG0.491 Ω0.808 Ω65 A
#4 AWG0.308 Ω0.508 Ω85 A
#2 AWG0.194 Ω0.319 Ω115 A
#1/0 AWG0.122 Ω0.201 Ω150 A
#2/0 AWG0.0967 Ω0.159 Ω175 A
#3/0 AWG0.0766 Ω0.126 Ω200 A
#4/0 AWG0.0608 Ω0.100 Ω230 A
250 kcmil0.0515 Ω0.0847 Ω255 A

*Values source: NEC Chapter 9, Table 9 (AC resistance for conductors in PVC/steel conduit at 75°C, 60 Hz).

Worked example, step by step

Example Calculation: 100A Subpanel Feeder at 150 Feet (240V 1-Phase)

Scenario: You are feeding a 100 Amp subpanel in a detached workshop located 150 feet away from the main service panel using 240V single-phase power with #4 AWG Copper THHN conductors.

  • Step 1: Lookup resistance of #4 Copper in Table 9: R = 0.308 Ω / 1000 ft.
  • Step 2: Calculate Voltage Drop: VD = 2 × (0.308 / 1000) × 100A × 150 ft = 9.24 Volts.
  • Step 3: Calculate percentage loss: % Drop = (9.24V / 240V) × 100 = 3.85%.
  • Step 4: Evaluation: At 3.85%, this is well within the 5% feeder recommendation. If you wanted to keep it under 3%, upsize to #2 AWG Cu (1.94% drop).

Frequently asked questions

In most general residential and commercial installations, the 3% branch and 5% total voltage drop limits appear in NEC Informational Notes (e.g. §210.19 and §215.2) and are recommendations rather than mandatory requirements. However, in certain specific articles (such as Fire Pumps §695.7, Sensitive Electronic Equipment §647.4, and Solar Photovoltaic Systems), voltage drop limits are strictly mandatory and enforceable by the electrical inspector (AHJ).

As conductor operating temperature increases, electrical resistance increases proportionally. The NEC Chapter 9 Table 9 provides standard resistance ratings calculated at 75°C, which matches standard commercial terminal ratings (such as modern breaker lugs and distribution blocks).

Yes! Under NEC §250.122(B), if phase conductors are increased in size (circular mil area) for voltage drop or any other reason other than temperature/bundling derating, the equipment grounding conductor (EGC) must be proportionately increased in circular mil area.

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