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.
- Pick a target - Compare against 3% (branch) or 5% (feeder + branch) - both are NEC recommendations, not hard limits.
A 20 amp load on 12 AWG copper at 120 volts loses 7.9 volts over a 100 foot one-way run - that is a 6.6 percent drop, more than double the 3 percent target, so you would step up to 8 AWG (about 2.6 percent). Put another way, 12 AWG is good for roughly 45 feet one-way at 20 amps and 120 volts before it reaches 3 percent. One thing worth knowing: the 3 percent figure is an NEC recommendation, not a hard rule - it lives in an Informational Note, not the enforceable text. Enter your voltage, load, wire, and run length above.
Where the Voltage Goes
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:
Basis = Resistance from NEC Chapter 9 Table 8 at 75°C (K = 12.9 copper, 21.2 aluminum) - the standard for sizing under load.
3-phase = VD = √3 × I × R × L / 1000
Percent = %VD = (VD ÷ V) × 100
Code Notes & Sources
- The 3 percent branch-circuit and 5 percent total voltage-drop figures are recommendations in an NEC Informational Note, not enforceable code. Many tools call them a code limit - they are guidance for good design, and the calculator treats them as selectable planning targets.NFPA 70 (NEC) 210.19(A) Informational Note - Voltage Drop
- Conductor resistances used here come from the standard K-value field method (K = 12.9 for copper, 21.2 for aluminum), consistent with the direct-current resistance values in NEC Chapter 9 Table 8 at 75 degrees C.NFPA 70 (NEC) Chapter 9 Table 8 - Conductor Properties
Next Steps
If the drop is too high, the fix is usually a bigger wire. Then re-check the rest:
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