Don't let long cable runs steal your power—calculate residential voltage drop and ensure your appliances get the voltage they need for safe, efficient operation.
Branch circuits look simple on paper: breaker, wire, outlet, load. In practice, every meter of copper adds resistance, and resistance converts part of your supply voltage into heat in the cable—not at the motor, charger, or compressor where you need it.
Why we calculate voltage drop
Electrical codes limit how far voltage may fall between the panel and the point of use:
- ~3% on many branch-circuit guides (motors, lighting, outlets)
- ~5% as a broader planning ceiling when feeder and branch are combined
Drop is not academic. A 120 V circuit at 6% drop delivers ~113 V—compressors lug, EVSE software derates current, and dimmers buzz.
What drives the numbers
- Current (A) — continuous load, not breaker rating alone
- Length (m) — one-way path from panel to load
- Conductor size — AWG or mm² copper area
- Supply voltage (V) — 120 V line-to-neutral, 230 V line, etc.
Formula in plain language:
Drop (V) = amps × (Ω per meter × 2) × meters
Drop (%) = 100 × drop ÷ supply voltage
The calculator uses typical copper resistance at 20 °C and flags whether you are inside 3% or 5% bands.
Common mistakes
| Mistake | Effect |
|---|---|
| Using breaker amps instead of load amps | Underestimates drop |
| Ignoring return conductor | Drop looks half as bad as reality |
| Mixing metric length with AWG from a US chart | Wrong resistance |
| Forgetting continuous EV/HVAC loads | Wire too small on paper |
Green-home loads that need the math
- Heat pumps — long outdoor runs from subpanels
- Level 2 EVSE — sustained 32–48 A
- Detached garage / barn — 30–60 m feeds
- Pool heat pumps — buried conduit length adds up
Pair with Watts to Amps to convert nameplate kW to current before you run drop.
Planning checklist
- Measure one-way cable path (not round-trip guess)
- Use actual load amps (plate or clamp meter)
- Run Residential Voltage Drop at planned gauge
- If >3%, step up one wire size or shorten route
- Verify local code and permit requirements—this tool plans; AHJ approves
Go deeper
- Why Home Devices Struggle: Voltage Drop Explained — resistance, heat, motor life
- Wire Size for Green Home Projects — HVAC, EV, solar subpanels
Voltage is delivered—not guaranteed. Size copper once, sleep through startup surges later.