What Is the Voltage Drop for 3 AWG at 66A and 125 Feet?
3 AWG at 66A and 125 feet: 4.04V drop (3.37% on 120V), computed on the single-phase / DC basis. Every conductor has resistance, and longer runs at higher currents drop more voltage. Use this calculation to check whether your run clears the 3% branch-circuit drop target before pulling wire.
3 AWG, 66A, 125ft · single-phase / DC
4.04 V drop (3.37% on 120V)
On 120V circuit3.37%
On 240V circuit1.68%
Use this citation when referencing this page.
Circuit basis: This uses the single-phase / DC round-trip formula (factor of 2) for the voltage drop across the two circuit conductors. For a three-phase line-to-line run use the three-phase version of the page (append ?type=3ph). Switch to the three-phase version →
Assumes a 120V source on a single-phase / DC circuit. Use the circuit-basis link above to switch between single-phase/DC and three-phase.
Voltage Drop Formula (single-phase / DC)
Vdrop = (2 × L × I × R) ÷ 1000
(2 × 125 × 66 × 0.245) ÷ 1000 = 4.04 V
DC and single-phase AC use the round-trip factor of 2. Current travels out to the load on one conductor and returns on another.
For a three-phase circuit at the same amps and distance, see the three-phase version (uses √3 instead of 2, so the drop is about 13.4% lower).
Percentage
%VD = (Vdrop ÷ Vsource) × 100
On 120V: (4.04 ÷ 120) × 100 = 3.37%
On 240V: (4.04 ÷ 240) × 100 = 1.68%
How This Estimate Changes with Run Length and Gauge
Gauge That Meets the 3% Target
The smallest gauge in our table that clears the 3% drop target at 66A over 125ft on 120V is 2 AWG. Shorter runs, higher source voltage, or a higher drop tolerance (feeder-only applications often accept up to 5%) can change the pick. Run the full wire-size calculator with your actual variables.
Impact of Distance
Voltage drop is proportional to distance. Here is 3 AWG at 66A at different distances:
Distance
Drop (V)
% on 120V
% on 240V
NEC (120V)
25ft
0.8085V
0.6738%
0.3369%
OK
50ft
1.62V
1.35%
0.6738%
OK
75ft
2.43V
2.02%
1.01%
OK
100ft
3.23V
2.7%
1.35%
OK
150ft
4.85V
4.04%
2.02%
Caution
200ft
6.47V
5.39%
2.7%
Past 5%
300ft
9.7V
8.09%
4.04%
Past 5%
Same Run, Different Wire Gauges
How does wire gauge affect voltage drop for 66A at 125 feet on 120V single-phase / DC? Only gauges whose branch-circuit OCP cap is at or above the 66A load are listed, since thinner gauges would fail the ampacity check before drop even matters.
3 AWG carrying 66A over 125ft has a 4.04V drop (3.37% on 120V). Reference: 1.68% on 240V.
On 120V, this run sits at 3.37%, which is past the 3% branch target; within the 5% feeder+branch total. NEC 210.19(A) Informational Note 4 cites 3% for branch circuits and 5% for total feeder+branch drop as performance recommendations, not hard code requirements.
Motors run hotter and can have trouble starting under load. Incandescent and halogen lighting dims. Some electronics misbehave at the low end of their input tolerance. Energy is wasted as I²R heating in the conductor. These are performance issues; high drop is not itself a code violation unless the specific installation cites a hard limit.
Yes. Aluminum has roughly 1.3 to 1.4 times the resistance of copper at the NEC Chapter 9 Table 8 75°C reference temperature, so for the same voltage drop an aluminum conductor is typically one to two gauges larger than copper. The exact gap depends on whether ampacity or voltage drop is binding, and the install still needs anti-oxidant compound and aluminum-rated lugs.
This run is at 3.37% on 120V, past the 3% branch-circuit drop target. If you want to land under 3% at 66A over 125ft on 120V, the smallest gauge in our table that clears it is 2 AWG at 2.67%. Going up one size from 3 AWG is not always enough, each AWG step only drops the resistance by roughly 20-25%, so on long runs or high currents you often have to skip one or two sizes to meet the target. NEC 210.19(A) Informational Note 4 frames 3% as a recommendation, not a code requirement, so the right answer for you also depends on the load (motor startup, sensitive electronics) and how much drop is tolerable.
This calculator provides estimates for reference purposes only. Always consult a licensed electrician and verify compliance with the National Electrical Code (NEC) and local electrical codes before performing any electrical work.
