What Is the Voltage Drop for 2/0 AWG at 172A and 125 Feet?

2/0 AWG copper carrying 172 amps over 125 feet on a single-phase / DC circuit drops 4.16 volts (3.47% on a 120V source). This sits past the 3% target NEC 210.19(A) Informational Note 4 cites for branch circuits, but within the 5% target for feeder+branch total. Which one applies depends on whether this run is a branch circuit, a feeder, or a feeder+branch combined: if it's a branch circuit, it's past target; if it's a feeder alone or part of a feeder+branch combined system, the 5% total is the figure to check against whatever the upstream drop adds. Both are planning targets, not code requirements.

2/0 AWG, 172A, 125ft · single-phase / DC

4.16 V drop (3.47% on 120V)

On 120V circuit3.47%

On 240V circuit1.73%

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 →

Gauge

2/0 AWG

Amps

Feet

Drop @ 120V

4.16V (3.47%)

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)

V_drop = (2 × L × I × R) ÷ 1000

(2 × 125 × 172 × 0.0967) ÷ 1000 = 4.16 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 = (V_drop ÷ V_source) × 100

On 120V: (4.16 ÷ 120) × 100 = 3.47%

On 240V: (4.16 ÷ 240) × 100 = 1.73%

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 172A over 125ft on 120V is 3/0 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 2/0 AWG at 172A at different distances:

Distance	Drop (V)	% on 120V	% on 240V	NEC (120V)
25ft	0.8316V	0.693%	0.3465%	OK
50ft	1.66V	1.39%	0.693%	OK
75ft	2.49V	2.08%	1.04%	OK
100ft	3.33V	2.77%	1.39%	OK
150ft	4.99V	4.16%	2.08%	Caution
200ft	6.65V	5.54%	2.77%	Past 5%
300ft	9.98V	8.32%	4.16%	Past 5%

Same Run, Different Wire Gauges

How does wire gauge affect voltage drop for 172A at 125 feet on 120V single-phase / DC? Only gauges whose branch-circuit OCP cap is at or above the 172A load are listed, since thinner gauges would fail the ampacity check before drop even matters.

Gauge	Drop (V)	% on 120V	% on 240V	3% Target (120V)
2/0 AWG	4.16V	3.47%	1.73%	Caution
3/0 AWG	3.29V	2.74%	1.37%	OK
4/0 AWG	2.61V	2.18%	1.09%	OK
250 kcmil	2.21V	1.85%	0.9227%	OK
300 kcmil	1.84V	1.54%	0.7686%	OK
350 kcmil	1.58V	1.32%	0.6575%	OK

Related Calculations

cable Wire size: 172A, 125ft bolt Watts to amps calculator functions Voltage drop formula

Frequently Asked Questions

What is the voltage drop for 2/0 AWG at 172A and 125ft?expand_more

2/0 AWG carrying 172A over 125ft has a 4.16V drop (3.47% on 120V). Reference: 1.73% on 240V.

What wire gauge should I use instead of 2/0 AWG for this run?expand_more

This run is at 3.47% on 120V, past the 3% branch-circuit drop target. If you want to land under 3% at 172A over 125ft on 120V, the smallest gauge in our table that clears it is 3/0 AWG at 2.74%. Going up one size from 2/0 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.

Does wire material affect voltage drop?expand_more

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.

What happens if voltage drop is too high?expand_more

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.

Is voltage drop worse on 120V or 240V?expand_more

Same wire, same amps, same distance: the volts dropped are identical. But the percentage is worse on 120V because the drop is a larger fraction of the source voltage. This run would be 1.73% on 240V versus 3.47% on 120V.

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.