Voltage Drop Calculator
Calculate voltage drop and minimum conductor size for AC/DC circuits per NEC/CEC requirements. Supports copper and aluminum conductors from 14 AWG to 500 kcmil.
Input Parameters
Results
VD = (K × I × R × L) / 1000 where K = 2 for single-phase/DC, K = 1.732 for three-phase. R is DC resistance in Ω/km at 75°C. NEC recommends max 3% for branch circuits and 5% for feeder + branch combined.
Understanding Voltage Drop in Electrical Circuits
Voltage drop is the reduction in electrical potential along a conductor carrying current. Every wire has inherent resistance, and as current flows through it, some energy is lost as heat. This loss manifests as a lower voltage at the load end compared to the source end. While some voltage drop is unavoidable, excessive drop can lead to equipment malfunction, reduced efficiency, and code violations.
Why Voltage Drop Matters
The National Electrical Code (NEC) Section 210.19(A) Informational Note No. 4 recommends that voltage drop on branch circuits should not exceed 3%, and the total voltage drop for both feeder and branch circuit combined should not exceed 5%. The Canadian Electrical Code (CEC) Rule 8-102 contains similar provisions. These are recommendations, not mandatory requirements, but they represent sound engineering practice.
For sensitive electronic equipment, motors, and LED lighting systems, keeping voltage drop below these thresholds is critical. Motors operating at reduced voltage draw more current, run hotter, and have shorter lifespans. LED drivers may flicker or fail to start if supply voltage is too low.
How This Calculator Works
This tool uses DC resistance values from NEC Chapter 9, Table 8 at 75°C conductor temperature. It iterates through standard AWG and kcmil conductor sizes from 14 AWG to 500 kcmil, calculating the voltage drop for each size until it finds the smallest conductor that keeps voltage drop within your specified limit. The calculator supports both copper and aluminum conductors and handles single-phase (K=2) and three-phase (K=1.732) circuits.
Practical Considerations
This calculator addresses voltage drop only. In practice, conductor sizing must also consider ampacity per NEC 310 or CEC Table 2, short-circuit withstand capability, and physical protection requirements. The selected conductor size should satisfy all applicable requirements. Always verify your design with a licensed professional engineer before installation.
Frequently Asked Questions
What is an acceptable voltage drop per NEC?
NEC 210.19(A) Informational Note No. 4 recommends a maximum voltage drop of 3% for branch circuits and 5% total for the combination of feeder and branch circuit. These are recommendations (not mandatory code requirements), but most engineers and inspectors consider them best practice. Some jurisdictions adopt them as requirements through local amendments.
How do I calculate voltage drop for three-phase circuits?
For three-phase circuits, the voltage drop formula uses K = 1.732 (the square root of 3): VD = 1.732 × I × R × L / 1000. This accounts for the phase relationship between conductors. Single-phase and DC circuits use K = 2 because current travels through both the line and neutral/return conductor.
What is the difference between NEC and CEC voltage drop limits?
Both codes recommend similar limits. The NEC (used in the United States) recommends 3% for branch circuits and 5% total. The CEC (Canadian Electrical Code) Rule 8-102 recommends 3% for feeders and 5% total from service entrance to point of utilization. The calculations are essentially the same; only the code reference numbers differ.
Should I use copper or aluminum conductors?
Copper has lower resistance (about 61% of aluminum for the same size), making it better for voltage drop. However, aluminum is lighter and less expensive per foot. For long runs where voltage drop is critical, copper may allow a smaller (and more practical) conductor size. For large feeders (4/0 and above), aluminum is commonly used due to cost savings, with the larger size offset by lower weight and cost.
Does this calculator account for conduit fill and temperature derating?
This calculator uses DC resistance values at 75°C, which is the standard temperature rating for most building wire. It does not account for AC reactance (which is significant for larger conductors in steel conduit), conduit fill limits per NEC 310, or ambient temperature corrections. For conductors larger than 250 kcmil in magnetic conduit, consider using AC impedance values for more accurate results.