Voltage Drop Calculator
How many volts you lose on a run — educational DC/AC feeder math, not a panel schedule. Click any i for detail.
Circuit
Voltage
i
Source voltage
Nominal volts (120, 240, 208, 480…). |
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|---|---|
Load (amps)
i
Current
Continuous load in amps. |
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One-way length (ft)
i
Length
Distance one way. The formula already doubles for the return on single-phase. |
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AWG
i
Size
Conductor size. |
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Metal
i
Conductor metal
Copper K ≈ 12.9; aluminum K ≈ 21.2. |
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Phase
i
Phase
Single-phase uses 2×; three-phase uses √3 ≈ 1.732. |
How to use this calculator
- Enter voltage and load amps.
- Enter one-way length in feet.
- Pick AWG, copper vs aluminum, and phase.
- Compare % drop to a 3% branch / 5% feeder+branch sketch.
Results explained
Single-phase VD = 2 × K × I × L ÷ CM. Three-phase substitutes 1.732 for 2. CM is circular mils for that AWG.
Quick reference: drop rules of thumb
Many designers target ≤3% on a branch and ≤5% feeder plus branch combined.
| Item | Detail |
|---|---|
| K copper | 12.9 |
| K aluminum | 21.2 |
| 12 AWG CM | 6,530 |
| 10 AWG CM | 10,380 |
| Example 3% of 120 V | 3.6 V |
Temperature, bundling, and conduit fill are ignored here. See also conduit fill.
How the estimate is built
VD = F × K × I × L / CM, where F is 2 (1φ) or 1.732 (3φ). % = VD / V × 100.
Example scenario
120 V, 12 A, 50 ft, 12 AWG copper, 1φ → about 2.4 V (2.0%).
FAQ
Is 3% a code maximum?
NEC Fine Print Notes discuss it; it is not a universal hard cap. Follow the job spec and a licensed electrician.
Round trip length?
Enter one-way. The 2× in single-phase is the round trip.
Need a size instead?
Use the wire size calculator with a max % drop.