Voltage Drop Calculator
Find the voltage drop in single-phase and 3-phase circuits — absolute volts, percentage, final load voltage and heat loss. Uses NEC Chapter 9 Table 8 conductor resistances.
Open calculatorLast updated: 2026-09-15
How the calculation works
- The calculator looks up the DC resistance of the selected conductor at 75 °C from NEC Chapter 9 Table 8 — aluminum applies the standard 1.64 ratio to the copper table values.
- 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.
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 conductors at 75 °C operating temperature (aluminum modeled as the Table 8 copper ratio ×1.64).
- 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 resistance uses a constant 1.64 copper ratio; exact Table 8 aluminum values differ slightly by size.
- 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.
Why 3% for branch circuits and 5% total
The numbers this calculator flags against come from NEC Informational Notes (not mandatory rules by themselves): 3% drop on a branch circuit, 5% combined feeder + branch. The reasoning behind them is practical, not arbitrary — at 5% drop a 120 V motor sees 114 V at startup, when it draws six times rated current and the extra drop is at its worst. Motors, compressors and LED drivers are the loads that complain first: brownout symptoms (slow starting, driver flicker, contactor chatter) usually show up on long runs before any code inspection does.
The mistake the calculator can't catch: the wrong table
The aluminum mode applies a constant 1.64 ratio to the copper Table 8 values — a good approximation across sizes, but real aluminum tables differ slightly and aluminum terminations demand proper preparation (oxidation inhibitor, correct torque). The second classic error is sizing from DC resistance when the run is long and the load is harmonic-rich (VFDs, LED banks): AC skin effect and reactance add drop that Table 8 alone doesn't show. For those loads, verify against the manufacturer's voltage-drop guidance, not just the conductor table.
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.