Voltage Drop
Enter the current, the one-way length and the source voltage and get the voltage drop (V and %), power loss and the minimum gauge that meets your target. Supports DC, single-phase and three-phase, in copper or aluminum. Resistive model: ignores reactance and temperature.
Recommended gauge
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Drop by gauge
| Gauge | Ω/km | Drop (V) | Drop (%) |
|---|
meets the target exceeds the target
How it is calculated · formulas and assumptions
1. DC and single-phase (1φ), accounting for
the round trip: Vdrop = 2 · I · (L/1000) · R, with
R in Ω/km and L in meters.
2. Balanced three-phase (3φ):
Vdrop = √3 · I · (L/1000) · R.
3. Percentage:
%drop = Vdrop / Vsource · 100. Voltage at the load
= Vsource − Vdrop.
4. Power loss:
P = I² · Rtotal, with Rtotal = 2·(L/1000)·R
for DC/1φ and ≈ √3·(L/1000)·R for 3φ.
5. Aluminum is modeled as copper
× 1.64. Resistive model: it ignores the
cable's reactance and the variation of R with
temperature. For long runs or reactive loads, check your code.
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How it works
The calculator estimates voltage drop in a conductor from the load current, the one-way length (source → load) and the source voltage. It uses the classic resistive model with copper resistances at 20 °C: for DC and single-phase it applies Vdrop = 2 · I · (L/1000) · R (the factor 2 covers the round trip of the conductor), and for balanced three-phase Vdrop = √3 · I · (L/1000) · R, with R in Ω/km. Aluminum is modeled as copper × 1.64.
With that it reports the drop in volts and percent (%drop = Vdrop / Vsource · 100), the voltage reaching the load, the power loss P = I² · Rtotal and, comparing against your target percentage (typical NEC criterion: 3 % on branch circuits, 5 % total), it recommends the minimum gauge in AWG or mm² that complies. The model ignores reactance and temperature, so for long runs or highly reactive loads you should verify against your local code.
Example: 16 A over 40 m at 230 V with 10 AWG copper
- 10 AWG copper resistance:
3.277 Ω/km. - DC/1φ:
Vdrop = 2 · 16 · 0.040 · 3.277 = 4.19 V. - Percentage:
4.19 / 230 · 100 = 1.82 %, leaving225.8 Vat the load. - Since 1.82 % ≤ the 3 % target, the gauge complies; the table also shows which smaller gauges would still comply.