What Is the Voltage Drop for 12 AWG at 20A and 100 Feet?

12 AWG copper carrying 20 amps over 100 feet on a single-phase / DC circuit drops 7.92 volts (6.6% on a 120V source). This sits past both the 3% branch target and the 5% feeder+branch total target NEC 210.19(A) / 215.2 Informational Notes cite, planning targets rather than code requirements. The run alone clears the 5% figure before any upstream feeder drop is counted, so a larger gauge, shorter run, or higher source voltage would be the typical way to bring the percentage down.

12 AWG, 20A, 100ft · single-phase / DC

7.92 V drop (6.6% on 120V)

On 120V circuit6.6%

On 240V circuit3.3%

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

12 AWG

Amps

Feet

Drop @ 120V

7.92V (6.60%)

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 × 100 × 20 × 1.98) ÷ 1000 = 7.92 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: (7.92 ÷ 120) × 100 = 6.6%

On 240V: (7.92 ÷ 240) × 100 = 3.3%

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 20A over 100ft on 120V is 8 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 12 AWG at 20A at different distances:

Distance	Drop (V)	% on 120V	% on 240V	NEC (120V)
25ft	1.98V	1.65%	0.825%	OK
50ft	3.96V	3.3%	1.65%	Caution
75ft	5.94V	4.95%	2.48%	Caution
100ft	7.92V	6.6%	3.3%	Past 5%
150ft	11.88V	9.9%	4.95%	Past 5%
200ft	15.84V	13.2%	6.6%	Past 5%
300ft	23.76V	19.8%	9.9%	Past 5%

Same Run, Different Wire Gauges

How does wire gauge affect voltage drop for 20A at 100 feet on 120V single-phase / DC? Only gauges whose branch-circuit OCP cap is at or above the 20A 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)
12 AWG	7.92V	6.6%	3.3%	Past 5%
10 AWG	4.96V	4.13%	2.07%	Caution
8 AWG	3.11V	2.59%	1.3%	OK
6 AWG	1.96V	1.64%	0.8183%	OK
4 AWG	1.23V	1.03%	0.5133%	OK
3 AWG	0.98V	0.8167%	0.4083%	OK

Related Calculations

cable Wire size: 20A, 100ft bolt Watts to amps calculator functions Voltage drop formula

Frequently Asked Questions

What is the voltage drop for 12 AWG at 20A and 100ft?expand_more

12 AWG carrying 20A over 100ft has a 7.92V drop (6.6% on 120V). Reference: 3.3% on 240V.

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.

Why does distance affect voltage drop?expand_more

Voltage drop is proportional to distance. The formula multiplies by 2 × the distance (out and back). Doubling the run doubles the drop.

What wire gauge should I use instead of 12 AWG for this run?expand_more

This run is at 6.6% on 120V, past the 3% branch-circuit drop target. If you want to land under 3% at 20A over 100ft on 120V, the smallest gauge in our table that clears it is 8 AWG at 2.59%. Going up one size from 12 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.

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.

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.