12 AWG at 25A and 100 Feet: Ampacity-Invalid Reference Calculation

Reference voltage-drop calculation only. 12 AWG is NEC-capped at 20A branch-circuit OCP per NEC 240.4(D) (75°C ampacity 25A), so 25A on this gauge fails the ampacity check before voltage drop ever enters the conversation. Do not use the number below as an install spec. The reference math on the single-phase / DC basis gives a 9.9-volt drop (8.25% on 120V). For an install at 25A, start with a conductor whose NEC 240.4(D) branch-circuit OCP cap is at or above 25A, then check voltage drop against that gauge.

12 AWG, 25A, 100ft · single-phase / DC · reference only, ampacity-invalid

9.9 V drop (8.25% on 120V)

On 120V circuit8.25%

On 240V circuit4.13%

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

9.90V (8.25%)

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 × 25 × 1.98) ÷ 1000 = 9.9 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: (9.9 ÷ 120) × 100 = 8.25%

On 240V: (9.9 ÷ 240) × 100 = 4.13%

How This Estimate Changes with Run Length and Gauge

Gauge That Meets the 3% Target

12 AWG can't carry the 25A load in the first place, its branch-circuit OCP cap is 20A under typical conditions. The smallest gauge in our table that clears both the ampacity cap and the 3% drop target at these inputs is 6 AWG. Run the full wire-size calculator for run length, material, and drop-target variations.

Impact of Distance

Voltage drop is proportional to distance. Here is 12 AWG at 25A at different distances:

Distance	Drop (V)	% on 120V	% on 240V	NEC (120V)
25ft	2.48V	2.06%	1.03%	OK
50ft	4.95V	4.13%	2.06%	Caution
75ft	7.43V	6.19%	3.09%	Past 5%
100ft	9.9V	8.25%	4.13%	Past 5%
150ft	14.85V	12.38%	6.19%	Past 5%
200ft	19.8V	16.5%	8.25%	Past 5%
300ft	29.7V	24.75%	12.38%	Past 5%

Same Run, Different Wire Gauges

How does wire gauge affect voltage drop for 25A at 100 feet on 120V single-phase / DC? Only gauges whose branch-circuit OCP cap is at or above the 25A 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)
10 AWG	6.2V	5.17%	2.58%	Past 5%
8 AWG	3.89V	3.24%	1.62%	Caution
6 AWG	2.46V	2.05%	1.02%	OK
4 AWG	1.54V	1.28%	0.6417%	OK
3 AWG	1.23V	1.02%	0.5104%	OK
2 AWG	0.97V	0.8083%	0.4042%	OK
1 AWG	0.77V	0.6417%	0.3208%	OK
1/0 AWG	0.61V	0.5083%	0.2542%	OK
2/0 AWG	0.4835V	0.4029%	0.2015%	OK
3/0 AWG	0.383V	0.3192%	0.1596%	OK
4/0 AWG	0.304V	0.2533%	0.1267%	OK
250 kcmil	0.2575V	0.2146%	0.1073%	OK
300 kcmil	0.2145V	0.1788%	0.0894%	OK
350 kcmil	0.1835V	0.1529%	0.0765%	OK
500 kcmil	0.129V	0.1075%	0.0538%	OK
750 kcmil	0.0855V	0.0713%	0.0356%	OK

Related Calculations

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Frequently Asked Questions

What is the voltage drop for 12 AWG at 25A and 100ft?expand_more

12 AWG carrying 25A over 100ft has a 9.9V drop (8.25% on 120V), but 12 AWG is NEC-capped at 20A branch-circuit OCP per NEC 240.4(D), so 25A on 12 AWG is ampacity-invalid and the drop figure above is a reference calculation only, not an install spec. Reference: 4.13% on 240V.

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.

How do I reduce voltage drop?expand_more

Use a larger wire gauge (lower AWG number), shorten the run, or increase the source voltage. Each option reduces the percentage drop, and higher source voltage is usually the most effective change for long runs because the drop is a smaller fraction of a larger reference.

Does 12 AWG meet the 3% drop target at 25A and 100ft on 120V?expand_more

On 120V, this run sits at 8.25%, which is past both the 3% branch and 5% feeder+branch total drop targets. 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. However, 12 AWG is ampacity-invalid for 25A (NEC 240.4(D) caps 12 AWG at 20A branch-circuit OCP), so this is a reference drop calculation only and the 3% check does not make this a valid install.

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.

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.