Voltage Drop Calculator
Calculate AC/DC circuit voltage drop percentage, end-of-run voltage, and find the minimum wire size to stay within NEC 3% standards.
Recommended Tools & Equipment
Tested hardware and components for high reliability
How Voltage Drop is Calculated
Every conductor exhibits electrical resistance proportional to its length and inversely proportional to its cross-sectional area (circular mils). When electric current flows through this resistance, a voltage drop occurs according to Ohm's Law ($V = I imes R$).
Single Phase & DC
V_drop = (2 × K × I × L) / CM
Three Phase AC
V_drop = (√3 × K × I × L) / CM
Where K is resistivity constant ($12.9,Omegacdot ext{cmil/ft}$ for copper, $21.2,Omegacdot ext{cmil/ft}$ for aluminum), I is current in amperes, L is one-way distance in feet, and CM is wire cross-section in circular mils.
NEC Guidelines on Voltage Drop
The National Electrical Code (NEC 210.19(A) Informational Note 4) recommends that the maximum voltage drop on branch circuits should not exceed 3%, and total combined drop across feeder and branch circuits should not exceed 5% for optimal efficiency and equipment longevity.
Frequently Asked Questions
Why does voltage drop happen over long distances?
Copper and aluminum conductors have intrinsic resistance. Over long runs (e.g. out to a shed or subpanel), this resistance causes energy to dissipate as heat, reducing the usable voltage reaching connected equipment.
What happens if voltage drop is too high?
Excessive voltage drop causes motors and compressors to overheat and fail prematurely, lighting to flicker or dim, sensitive electronics to reset, and electric heaters to produce significantly less heat.
Why does three-phase have less voltage drop than single-phase?
In a balanced three-phase system, return currents cancel each other in the neutral conductor. The formula uses sqrt(3) (1.732) instead of 2, resulting in approximately 13.4% less voltage drop for identical wire gauges and load currents.