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Voltage Drop Calculator

Voltage drop is the loss of voltage along a conductor between the source and the load. Long runs, small conductors and high currents all increase it; too much drop means dim lights, overheating motors and nuisance tripping. This calculator uses the DC resistance values from NEC Chapter 9, Table 8 to estimate the drop for copper and aluminum conductors.

Circuit
System Most homes and small shops are single-phase
Conductor
Material Standard building wire (NM-B, THHN) is copper
Load & run
Voltage drop 3.22%
In volts 3.86 V
Voltage at load 116.14 V
NEC check Too much drop — use 10 AWG/kcmil copper or larger

Single-phase · 120 V · Copper · 12 · 20 A · 50 ft

How it works

Single-phase: VD = 2 × I × R × L / 1000. Three-phase: VD = √3 × I × R × L / 1000

I is the load current in amperes, R is the conductor DC resistance in ohms per 1000 ft from NEC Chapter 9, Table 8 (75°C, uncoated stranded), and L is the one-way circuit length in feet. The single-phase formula doubles the length to account for the return conductor; the three-phase formula uses √3 because the currents in a balanced system are 120° apart. If the drop pushes you to a larger conductor, check that the new size still meets the NEC conduit fill limits for its raceway.

Code references

FAQ

What is an acceptable voltage drop?

The NEC does not mandate a maximum, but Informational Note No. 4 to 210.19(A) recommends keeping branch-circuit drop within 3% and the combined feeder-plus-branch drop within 5%. Some local codes and energy standards make these limits mandatory.

Should I enter the one-way length or the total wire length?

Enter the one-way distance from the source to the load. The formula already accounts for the return path (factor 2 for single-phase, √3 for three-phase).

Why does the calculator use DC resistance instead of AC impedance?

For conductors up to roughly 4/0 AWG at 60 Hz, AC reactance is negligible and the DC resistance from Table 8 is the standard field method. For very large conductors or long high-current feeders, an impedance-based calculation per NEC Chapter 9, Table 9 gives a more precise figure.

Does temperature affect the result?

Yes. Table 8 values are listed at 75°C conductor temperature. A cooler conductor has slightly lower resistance, so the real-world drop is usually a little lower than calculated — the result is conservative.

This calculator is provided for estimation purposes. Always verify results against the current NEC edition and local amendments with a licensed electrician or electrical engineer before sizing conductors.

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Calculated with Voltage Drop Calculator — https://hardhatmath.com/electrical/voltage-drop-calculator/