Voltage Drop
Calculator
Calculate voltage drop and voltage at the load from your current, wire size and one-way cable length. Compare wire sizes and parallel runs against a chosen voltage-drop limit.
System Logs
> Calculating load: 20.00 A (Derived)
> Phase configuration: 1-Phase
> Correction factor: 1 PF
> Parallel sets: 1
AC: conduit sets R and X from NEC Chapter 9 Table 9 (values from the 2017 edition); Zeff = R × PF + X × sin(acos PF).
Based on NEC-style conductor tables; verify against the adopted code and the project engineer. The ampacity check uses the 60°C column for circuits of 100 A or less and the 75°C column above 100 A (NEC 110.14(C)) unless 75°C terminals are selected. When an ambient other than 30°C or more than three current-carrying conductors is entered, the 90°C column is corrected (310.15(B)(1)(1)) and adjusted (310.15(C)(1)) and the lower of that value and the terminal column is used. Sizing recommendations limit 14, 12 and 10 AWG copper (and 12 and 10 AWG aluminum) to the NEC 240.4(D) small-conductor overcurrent limits, which are lower than the table values shown for those sizes.
Understanding Voltage Drop
Master the essential concepts behind voltage drop calculations and why they matter for every electrical installation.
What is Voltage Drop?
Voltage drop is the reduction in electrical potential along a conductor carrying current. As electrons flow through wire, they encounter resistance, converting electrical energy to heat and reducing the voltage available at the load end.
Learn MoreThe Core Formula
For single-phase circuits: Vd = 2 × I × L × R / 1000. For three-phase: Vd = √3 × I × L × R / 1000. Understanding these formulas is fundamental to proper wire sizing.
NEC Requirements
The National Electrical Code recommends maximum 3% voltage drop for branch circuits and 5% total (feeder + branch). These informational design targets do not establish equipment performance or installation approval.
NEC StandardsFactors Affecting Drop
Multiple factors influence voltage drop: conductor material (copper vs aluminum), wire gauge (AWG/kcmil), circuit length, load current, temperature, and whether it's AC or DC. Each must be considered for accurate calculations.
Explore FactorsWhy It Matters
Excessive voltage drop causes motor overheating, dim lighting, equipment malfunction, and energy waste. In critical systems like data centers or medical facilities, proper voltage is essential for reliable operation.
Read MoreAC vs DC Systems
DC systems use pure resistance calculations, while AC systems must account for impedance including reactance. Three-phase AC uses √3 multiplier instead of 2. Power factor also affects AC voltage drop in inductive loads.
Compare SystemsHow the Calculator Works: Formulas and NEC Data
DC results use NEC Chapter 9 Table 8 conductor resistance. AC results use the Chapter 9 Table 9 resistance and reactance for the selected conduit (values from the 2017 edition), so PVC and steel conduit give different drops. Size mode checks ampacity against the 60°C or 75°C terminal column per NEC 110.14(C), with optional 125% continuous load, ambient correction and conductor-count adjustment.
NEC Chapter 9 Table 8
Table 8 DC resistance (75°C) for DC circuits; Table 9 AC resistance and reactance by conduit type for AC circuits.
Fixed Reference Resistance
Resistance and reactance are fixed table values at 75°C conductor temperature; ambient correction is applied only to the ampacity check in size mode.
AC Power-to-Current Conversion
Power factor converts AC watts or horsepower (treated as electrical input, 746 W/hp) to current and weights R and X in Zeff = R × PF + X × sin(acos PF).
Parallel Conductor Support
Calculate effective resistance for parallel conductor sets, essential for high-current applications.
Calculation Reference
NEC Design References
NEC informational notes provide voltage-drop design targets. Use the referenced table values as estimates and verify equipment requirements and locally adopted installation rules.
Maximum recommended drop from panel to outlet (NEC 210.19 Informational Note, NEC 2023)
Combined feeder and branch circuit drop from service to final outlet
Important Notes
- •Voltage drop limits are informational notes, not code requirements
- •Some jurisdictions adopt stricter requirements
- •Motor circuits may require lower drop for starting
- •Sensitive electronics often need tighter tolerances
- •Always verify with local codes and AHJ requirements
Common Use Cases
From residential installations to industrial power distribution, voltage drop calculations are essential across all electrical applications.
Residential
Long runs to detached garages, workshops, and outdoor lighting. Sub-panel feeders and EV charger installations.
View Cases →Industrial
Motor feeder circuits, MCC installations, long factory runs, and high-current distribution systems.
View Cases →Solar & Renewable
PV string wiring, DC combiner to inverter runs, and battery storage system connections.
View Cases →Commercial
HVAC equipment feeders, lighting circuits, data center power, and tenant distribution.
View Cases →Frequently Asked Questions
Get answers to common questions about voltage drop calculations, wire sizing, and electrical design best practices.
Full Knowledge BaseWhat causes voltage drop in electrical circuits?
How do I calculate voltage drop for three-phase systems?
What's the difference between copper and aluminum for voltage drop?
When should I use parallel conductors?
How does power factor affect voltage drop?
Maintenance and corrections
This calculator uses a steady-state model: Vd = 2 × I × L × Z / (1000 × N), replacing 2 with √3 for balanced three-phase circuits. L is one-way length in feet, N is the number of parallel sets and Z, in ohms per thousand feet, is NEC Chapter 9 Table 8 resistance for DC or, for AC, Ze = R × PF + XL × sin(acos PF) with R and XL from Table 9 (2017-edition values) for the selected conduit. The /wire-size/ scenario pages use the same model with fixed steel conduit and PF 0.85.
Wire sizing includes a basic ampacity screen using the site tables with NEC references. Examples are calculated estimates, not installation measurements. Ambient correction and conductor-count adjustment apply only when those size-mode inputs are entered; results exclude other installation adjustments, motor starting and local code review.
Reference dataset version: 2026-10-23 — formulas, resistance inputs and screening rules documented in the maximum run length table; this is not a new NEC edition.
Recent corrections
- · Extension cord wire size: the 120 V, 20 A, 100 ft, copper 12 AWG example changed from 6.3% to 6.6%, using R = 1.98 Ω/kft at 75°C.
- · IEC cable sizing: recalculated the workshop example; the table and explanation now share the stated resistance, temperature and one-way length.
- · NEC reference page: removed the automatic compliance-check claim; results are limited to the calculation checks described.
- · Portuguese wire size calculator: clarified AWG/feet, meter conversion and the need to verify Brazilian designs against NBR 5410.
Content maintenance: Hommer Zhao · Founder & General Manager · WIRINGO.
Report errors through the Contact page. Include the URL, dataset version, inputs and units, displayed result, and your recalculation or supporting reference.
Check Voltage Drop on Your Circuit
Use the calculator above to estimate voltage drop with NEC table references, then verify equipment requirements and applicable local installation rules.
Methodology
DC results use NEC Chapter 9 Table 8 resistance. AC results use the Table 9 resistance and reactance for the selected conduit (values from the 2017 edition) with Zeff = R × PF + X × sin(acos PF) for 14 AWG to 1000 kcmil; sizes not listed in Table 9 are not estimated from resistance alone.
Limits
- Branch 3%
- Feeders 3%
- Total 5%