AC vs DC Voltage Drop: Key Differences Explained
Understand the critical differences between AC and DC voltage drop calculations, including impedance effects, skin effect, and when to use each formula.
Understanding AC and DC Systems
While the fundamental principle of voltage drop applies to both AC (Alternating Current) and DC (Direct Current) systems, the calculations and considerations differ significantly. DC systems use pure resistance for calculations, while AC systems must account for impedance—the combination of resistance and reactance. Understanding these differences is crucial for accurate voltage drop calculations in modern electrical installations, especially with the growing prevalence of solar PV, battery storage, and EV charging systems.
This guide explores the key differences between AC and DC voltage drop, explains when each type of calculation applies, and provides practical guidance for electrical professionals working with both system types.
DC Voltage Drop: Pure Resistance
Direct current flows continuously in one direction, making DC voltage drop calculations straightforward. The only opposition to current flow is the conductor's resistance. There is no frequency-dependent reactance, skin effect, or proximity effect to consider. This simplicity makes DC calculations more predictable and easier to verify.
DC Voltage Drop Formula
Where R is the DC resistance from NEC Chapter 9, Table 8
Common DC applications include solar PV string wiring, battery systems, automotive circuits, telecommunications power, and low-voltage control systems. In these applications, conductor selection is based purely on DC resistance and ampacity requirements.
AC Voltage Drop: Impedance Matters
Alternating current introduces additional factors that affect voltage drop. AC creates magnetic fields around conductors, and these fields induce reactance—an additional opposition to current flow that varies with frequency. The total opposition to AC flow is called impedance (Z), which combines resistance (R) and reactance (X).
Skin Effect
AC current tends to flow near the conductor surface rather than uniformly through the cross-section. This effectively reduces the usable conductor area and increases resistance. More pronounced at higher frequencies and larger conductor sizes.
Proximity Effect
Adjacent conductors carrying AC create magnetic fields that interact, further distorting current distribution within each conductor. Significant in parallel conductors and large cable assemblies.
AC Voltage Drop Formula
Where Z = √(R² + X²) is the impedance, or for simplified calculations: Z = R×cos(θ) + X×sin(θ)
When to Use Each Calculation
Use DC Calculations For:
- • Solar PV DC circuits (panel strings, DC home runs)
- • Battery storage systems
- • Automotive and marine 12V/24V/48V systems
- • Telecom power (-48VDC)
- • Low-voltage control circuits
- • LED lighting with DC drivers
Use AC Calculations For:
- • Standard 120/240V residential circuits
- • Commercial 208V/480V three-phase systems
- • Motor feeders (especially with low power factor)
- • Long feeder runs where reactance is significant
- • Large conductors (500 kcmil and above)
- • Conductors in steel conduit
Calculate Both AC and DC
Our voltage drop calculator supports both AC and DC calculations with appropriate formulas for each. Select your system type and get accurate results with proper consideration of impedance effects for AC or pure resistance for DC circuits.
AC/DC Calculator