Voltage Drop Calculator
Why Voltage Drop Matters
Voltage drop is the reduction in voltage as electricity travels through a wire. Excessive drop can cause flickering lights, overheating motors, and inefficient operation. The National Electrical Code (NEC) recommends keeping voltage drop under 3% for branch circuits and under 5% total (feeder + branch).
How This Calculator Works
Our tool uses the DC resistance values from NEC Table 8 for copper and aluminum conductors. The formula applied is:
Vdrop = I × R × L × K
- I = current (amps)
- R = resistance per foot (Ω/ft) = ohms per 1000 ft ÷ 1000
- L = one‑way cable length (feet)
- K = 2 for single‑phase (2‑wire) or √3 (≈1.732) for three‑phase
The result shows both the absolute voltage drop (in volts) and the percentage of the system voltage.
Tips to Reduce Voltage Drop
- Increase wire size – moving to a thicker AWG lowers resistance.
- Shorten the run – place the load closer to the source if possible.
- Raise system voltage – for the same power, a higher voltage means less current.
- Use copper instead of aluminum when feasible, as copper has lower resistance.
- Balance three‑phase loads to minimise neutral current.
Frequently Asked Questions
What is an acceptable voltage drop?
For most residential and commercial circuits, the NEC suggests no more than 3% on a branch circuit and 5% total from the service entrance to the farthest outlet.
Does voltage drop depend on temperature?
Yes. The resistance values used here are for 75°C (167°F). In colder environments resistance decreases slightly, while in high‑temperature conduits it increases. For critical designs, always refer to the latest NEC tables and consider ambient temperature corrections.
Can I use this for DC systems?
Absolutely. For DC circuits, select “Single‑phase (2‑wire)”. The multiplier K=2 is valid for two‑wire DC circuits where the current goes out and returns on the second wire.
Disclaimer: This calculator provides estimates for educational purposes. Always consult a qualified electrician and adhere to local electrical codes for actual installations.
