Voltage Drop Calculator
Find the voltage drop in a copper circuit — absolute volts, percentage, final load voltage and heat loss. Uses NEC Chapter 9 Table 8 conductor resistances.
Last updated: 2026-09-15
How the calculation works
- The calculator looks up the DC resistance of the selected copper conductor at 75 °C from NEC Chapter 9 Table 8.
- For single-phase circuits the current path is out and back, so the one-way length is doubled. Three-phase circuits use the √3 factor with the one-way length.
- The drop is multiplied by the load current to get volts lost, then divided by the source voltage for the percentage.
- Power lost as heat is I²R — it grows with the square of the current, which is why undersized cables waste energy.
Formula
Single-phase: Vd = 2 × I × R × L Three-phase: Vd = √3 × I × R × L R = conductor resistance per unit length (NEC Ch. 9 Table 8)
| Symbol | Meaning | Unit |
|---|---|---|
Vd | Voltage drop | V |
I | Load current | A |
R | Conductor resistance of the full current path | Ω |
L | One-way run length | ft or m |
Worked example
Interpreting the result
The NEC recommends (Informational Note, 210.19) keeping branch-circuit drop at or below 3% and total drop (feeder + branch) at or below 5%. Above these values motors lose torque, lights dim and electronics may misbehave. The power-loss figure tells you how much energy is being burned in the cable for the life of the installation.
Assumptions
- Uncoated copper conductors at 75 °C operating temperature.
- DC resistance only — AC reactance is ignored, which is accurate for conductors up to about 1/0 AWG and slightly optimistic for large conductors.
- Balanced three-phase load (no neutral current).
Limitations
- Aluminum conductors are not included (about 61% higher resistance than copper).
- Very long runs at high current in large conduit should be checked with the NEC reactance tables.
- Does not verify ampacity, termination temperature ratings or code compliance — voltage drop is a performance metric, not a substitute for circuit sizing.
Frequently asked questions
What is an acceptable voltage drop?
A common guideline from the NEC (Informational Notes to 210.19 and 215.2) is 3% for a branch circuit and 5% total from the service to the load. These are recommendations, not enforceable limits, but exceeding them noticeably degrades motor and lighting performance.
Does the length include both directions?
No — enter the one-way distance. For single-phase circuits the calculator doubles it because current travels out on one conductor and back on the other. Three-phase uses the one-way length with the √3 factor.
Why is my measured drop higher than calculated?
Loose or corroded terminations add resistance that isn't in the conductor tables, and a heavily loaded or long service entrance adds feeder drop on top of the branch circuit.