Wire Size and Voltage Drop Calculator
Check a branch circuit or feeder for ampacity and voltage drop. The diagram is a one-line from the panel, through the conduit, to the load.
Formula SSC-2026.1
K method · SSC-2026.1
Minimum size
6 AWG · 13.3 mm²
Single conductor set
Voltage drop
3.69 V
1.54% · end 236.3 V
Power loss
184 W
I²R at the operating current
Required ampacity
62.5 A
Each conductor 65 A
Material estimate
$597.00
3 conductors + EMT
Conductor weight
22.1 lb
10.0 kg · approximate insulated weight
Circle diameters track circular mils. The drawing is a schematic, not a conduit-fill layout.
| Item | Net | Waste | Order | Unit price | Line total |
|---|---|---|---|---|---|
| copper 6 AWG · 13.3 mm² | 75 ft one way | cuts excluded | 3 conductors | USD | $458.25 |
| EMT conduit | 75 ft | — | 75 ft | USD | $138.75 |
| Material | Minimum size | mm² | Drop | End voltage | Loss |
|---|---|---|---|---|---|
| copper | 6 AWG · 13.3 mm² | 13.3 | 1.54% | 236.3 V | 184 W |
| aluminum | 4 AWG · 21.2 mm² | 21.2 | 1.59% | 236.2 V | 190 W |
| Distance | Minimum size | Drop | Percent | End voltage |
|---|---|---|---|---|
| 50 ft | 6 AWG · 13.3 mm² | 2.46 V | 1.02% | 237.5 V |
| 75 ft | 6 AWG · 13.3 mm² | 3.69 V | 1.54% | 236.3 V |
| 100 ft | 6 AWG · 13.3 mm² | 4.92 V | 2.05% | 235.1 V |
Show the deterministic formula
Single phase: Vdrop = 2 × K × I × D / cmil. Three phase: Vdrop = 1.732 × K × I × D / cmil. K is 12.9 for copper and 21.2 for aluminum. D is one-way feet.
Percent drop = Vdrop ÷ source voltage × 100. Green is 3% or less, amber is 3 to 5%, and red is over 5%. Power loss is I²R on the current-carrying conductors.
The smallest conductor that meets both the ampacity (125% when continuous) and the drop limit is selected. Ampacity does not include bundling or temperature correction.
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Project: Wire Size and Voltage Drop Calculator
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This sheet is an estimate for ordering and layout discussion. It is not a sealed design. Confirm the quantities with a licensed local professional and the authority having jurisdiction before you buy or build.
Engineering notes
Voltage drop uses the approximate K method: 2 × K × I × D / cmil for single phase and 1.732 × K × I × D / cmil for three phase. K is 12.9 for copper and 21.2 for aluminum. D is the one-way run in feet. Ampacity is the 75°C column, with the small-conductor limits of 15 A, 20 A, and 30 A on 14, 12, and 10 AWG copper.
A continuous load is sized at 125 percent for ampacity. Voltage drop is calculated at the operating current. The tool does not apply ambient, bundling, or rooftop derating, and it does not replace the utility or the authority having jurisdiction for service conductors.
Code reference
NEC 210.19(A) Informational Note for 3 percent branch-circuit drop, NEC 215.2(A) Informational Note for feeder drop, Chapter 9 Table 8 conductor properties, and Table 310.16 ampacity. Confirm the adopted NEC edition.
Field questions
- Why do some runs need a larger wire than the breaker size?
- Ampacity only keeps the conductor within its temperature limit. A long run can still lose more than 3 or 5 percent of the source voltage, so the calculator steps up until both checks pass.
- What does the color on the voltage-drop result mean?
- Green is 3 percent or less, amber is between 3 and 5 percent, and red is over 5 percent. Those bands follow the common NEC informational notes for branch circuits and feeders.
- Does the cost include labor or conduit fill?
- No. The estimate is material only: current-carrying conductors, a smaller equipment ground allowance, and the selected conduit at an editable price per foot.