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
Based on NEC-style conductor tables; verify against the adopted code and the project engineer. 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 SystemsProfessional Design Approach
Our calculation methodology follows IEEE and NEC standards, providing engineers with reliable data for critical decisions. Every calculation considers real-world factors that affect conductor performance.
NEC Chapter 9 Table 8
Resistance values derived from official NEC tables for both copper and aluminum conductors at 75°C.
Fixed Reference Resistance
Uses fixed reference resistance values; no ambient temperature correction is applied.
AC Power-to-Current Conversion
Power factor is used when converting AC watts or horsepower to current. Voltage drop here uses a resistive estimate.
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(A) Informational Note No. 4)
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 resistance model: Vd = 2 × I × L × R / (1000 × N), replacing 2 with √3 for balanced three-phase circuits. L is one-way length in feet, R is resistance in ohms per thousand feet and N is the number of parallel sets. Power factor affects AC power-to-current conversion only; reactance is excluded. Voltage-specific pages in AC mode use the impedance model stated in their reports.
Wire sizing includes a basic ampacity screen using the site tables with NEC references. Examples are calculated estimates, not installation measurements. Results exclude installation adjustments, temperature correction, conductor grouping, 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.
Ready to Calculate?
Use the calculator above to estimate voltage drop with NEC table references, then verify equipment requirements and applicable local installation rules.
Methodology
Calculations derived from NEC Chapter 9 Table 8. This tool uses a resistive voltage-drop estimate; power factor affects only AC power-to-current conversion. For large steel conduit runs (>4/0 AWG), impedance may be higher.
Limits
- Branch 3%
- Feeders 3%
- Total 5%