What is a good voltage drop percentage?

NEC informational notes often cite 3% on branch circuits and 5% total from service to load. Critical equipment may need tighter limits.

Is length one-way or round-trip?

Enter one-way (single) length; the calculator applies the 2× factor for single-phase automatically.

Does aluminum use the same formula?

Yes, with aluminum resistance values from Chapter 9 Table 8.

Field electrical

Voltage Drop Calculator

Estimate voltage drop for copper and aluminum runs using resistance per NEC Chapter 9 Table 8.

Circuit parameters

Phase

System voltage (V)

Material

Wire size

Load current (A)

One-way length (ft)

How to use this voltage drop tool

Choose single-phase or three-phase and nominal system voltage.
Select conductor material and wire size.
Enter load current and one-way circuit length in feet.
Compare drop percentage to your branch (3%) and total (5%) targets.

Disclaimer

Voltage drop estimates use DC resistance at 75°C and do not include power factor, harmonics, or parallel runs. Size conductors per NEC and engineering practice.

Sources & references

NEC Chapter 9 Table 8 — Conductor properties

Based on NFPA 70 (NEC) 2023 unless noted. Confirm the edition adopted locally.

Frequently asked questions

What is a good voltage drop percentage?: NEC informational notes often cite 3% on branch circuits and 5% total from service to load. Critical equipment may need tighter limits.
Is length one-way or round-trip?: Enter one-way (single) length; the calculator applies the 2× factor for single-phase automatically.
Does aluminum use the same formula?: Yes, with aluminum resistance values from Chapter 9 Table 8.