Free Home Wiring Tools
These tools help you plan and understand home wiring jobs. Each one shows its formula, the sources behind its numbers and a worked example, and runs in your browser. They give general information: they are not a design, and they do not replace your local code or a licensed electrician.
- Voltage drop calculator: volts lost and percent drop for a circuit of a given current, length and wire size, compared with the NEC informational-note recommendations of 3% and 5%.
- Wire size calculator: the minimum copper or aluminum wire size from NEC Table 310.16 for a given current and temperature column, with the 240.4(D) small-conductor limits.
- Interactive 3-way switch wiring diagram: choose power at the switch, power at the light or a 4-way extension, flip the switches and see which conductors carry power.
Before any electrical work, turn off the breaker, verify with a tester that the circuit is dead and follow your local code. See our editorial policy for how we check electrical content, and the 3-way switch wiring guide and the panel labeling guide for the background.
All tools
Voltage Drop Calculator
Volts lost and percent drop against the NEC 3% and 5% recommendations.
Wire Size Calculator
Minimum AWG from NEC Table 310.16 and the 240.4(D) limits.
3-Way Wiring Diagram
Interactive diagram: power at the switch, at the light, or with a 4-way.