Voltage Drop Calculator
Estimate voltage drop by wire size, distance, and load current for 1-phase and 3-phase circuits.
Your Voltage Drop Result
- Enter voltage — Use the circuit voltage (e.g. 120, 240, 480).
- Set amps — Enter the expected load current.
- Use one-way distance — For single-phase the formula internally accounts for the round trip.
- Choose wire size — Select the AWG you are considering.
- Check percent drop — Compare to a planning target (commonly 3% branch circuits).
What Is Voltage Drop?
Voltage drop is the reduction in voltage that occurs as current flows through a conductor's resistance. Longer runs and higher current increase drop, and smaller wire sizes increase it even more.
This calculator estimates voltage drop for planning: enter voltage, amps, wire size, and run length to see the approximate drop and percent. For single-phase circuits the return path is part of the total loop length, which is why many formulas use a "2×" length factor.
Example: a 120V circuit carrying a heavy load over a long run can lose several volts on smaller wire. Upsizing the conductor or reducing the run length are the most direct ways to lower voltage drop.
Planning Targets People Use
These targets are commonly referenced in planning discussions:
| Circuit Type | Typical Target | Why It Matters |
|---|---|---|
| Branch circuit | ≈ 3% | Helps motors start cleanly and reduces dimming |
| Feeder + branch total | ≈ 5% | Keeps delivered voltage closer to equipment rating |
| Sensitive electronics | Lower preferred | Reduces nuisance issues in long runs |
Targets are planning guidelines. Your local rules and equipment tolerances may differ.
Formulas Used
This calculator uses standard resistance-based estimates:
3-phase = VD = √3 × I × R × L / 1000
Percent = %VD = (VD ÷ V) × 100
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FAQ
Why does single-phase use a "2 ×" multiplier?
Single-phase circuits have an outgoing and return path, so the effective conductor length is doubled in the resistance calculation.
If voltage drop is high, what is the simplest fix?
The simplest fix is to increase wire size. Going up one AWG size (e.g., from 12 AWG to 10 AWG) reduces resistance by about 25% and cuts voltage drop proportionally. For very long runs (100+ feet), you may need to go up two or three AWG sizes. Running a higher voltage (240V instead of 120V for the same load) also cuts voltage drop in half because current is halved.
What is acceptable voltage drop for a circuit?
The NEC recommends maximum 3% voltage drop for branch circuits and 5% total for the combined feeder and branch circuit. At 120V, 3% is 3.6V — so the endpoint should receive at least 116.4V. For sensitive equipment like computers and motors, keep drop under 2%. LED lighting and most modern electronics tolerate up to 5% without issues.
How far can you run 12 AWG wire on a 20 amp circuit?
For a 20A circuit at 120V with a 3% maximum voltage drop (3.6V), 12 AWG copper can run approximately 50–60 feet one-way before voltage drop becomes significant. For runs up to 100 feet, use 10 AWG. For 150 feet, use 8 AWG. The calculator above handles this math precisely — enter your load, voltage, wire size, and run length for an exact result.
Should I use load amps or breaker size for voltage drop?
Use the expected operating current (load amps) for a more accurate voltage drop estimate. Breaker size can be higher than the actual load, so using breaker amps can overstate the drop.