Voltage Drop Calculator
Calculate voltage drop in volts and percent for any AWG, length, and conduit type, with a full gauge comparison against NEC 3% and 5% guidance.
100% client-side. Inputs stay in your browser (ons-voltage-drop-inputs).
Inputs
Results
Voltage drop
7.72 V (6.43%)
Receiving voltage
112.28 V
Selected wire
12 AWG
| AWG | VD % | VD (V) | Status |
|---|---|---|---|
| 14 | 10.23% | 12.28 | Fail |
| 12(selected) | 6.43% | 7.72 | Fail |
| 10 | 4.03% | 4.84 | Marginal |
| 8 | 2.55% | 3.06 | Pass |
| 6 | 1.64% | 1.96 | Pass |
| 4 | 1.03% | 1.23 | Pass |
| 2 | 0.65% | 0.78 | Pass |
| 1/0 | 0.41% | 0.49 | Pass |
| 2/0 | 0.32% | 0.39 | Pass |
| 3/0 | 0.26% | 0.31 | Pass |
| 4/0 | 0.20% | 0.24 | Pass |
Based on NEC 2023, Chapter 9 Table 9 (AC resistance) with Table 8 baseline. Verified 2026-05-25.
Enter the system voltage (120V, 240V, 480V, or other). Input the current load in amperes. Specify the one-way wire length in feet (the distance from the source to the load; total circuit length is double this for the round trip through the hot and neutral or return conductor). Select the conductor material (copper or aluminum) and the wire size in AWG. The calculator applies the voltage drop formula: Vdrop = (2 × K × I × L) / A, where K is the resistivity constant (12.9 for copper, 21.2 for aluminum), I is current, L is one-way length, and A is the conductor cross-sectional area in circular mils.
Consult a licensed electrician before installation. Local codes may be more stringent than NEC recommendations.
How does the Voltage Drop Calculator work step by step?
Enter the system voltage (120V, 240V, 480V, or other). Input the current load in amperes. Specify the one-way wire length in feet (the distance from the source to the load; total circuit length is double this for the round trip through the hot and neutral or return conductor). Select the conductor material (copper or aluminum) and the wire size in AWG. The calculator applies the voltage drop formula: Vdrop = (2 × K × I × L) / A, where K is the resistivity constant (12.9 for copper, 21.2 for aluminum), I is current, L is one-way length, and A is the conductor cross-sectional area in circular mils.
What does a typical Voltage Drop Calculator result look like?
A 20 amp, 120 volt copper branch on 12 AWG PVC conduit with a 100-foot one-way run drops about 7.7 volts (6.4%). That is above the 3% branch guideline, so the table points you to 8 AWG (about 2.6% on the same run). Shorten the run to 40 feet and 12 AWG lands near 2.6% while 14 AWG is still over 4%.
Frequently asked questions
How much voltage drop is acceptable for different types of loads?
The NEC recommends 3 percent drop for branch circuits and 5 percent combined for feeders and branches. Motor circuits can tolerate drop at the low end of this range; excessive drop causes motors to overheat and draw higher current, worsening the drop. Sensitive electronics (computers, medical equipment, instrumentation) should see drop under 2 percent to avoid glitches or damage. Resistive loads (heaters, incandescent lights) tolerate higher drop but deliver reduced output proportional to voltage reduction.
Can I reduce voltage drop by increasing system voltage?
Yes. Doubling system voltage from 120V to 240V cuts current in half for the same power load, which reduces voltage drop by half (drop is proportional to current). This is why large appliances (dryers, ranges, water heaters) and heavy equipment use 240V circuits instead of 120V. For very long runs or high-power loads, consider 480V three-phase (industrial) or stepping up voltage at the source and stepping down at the load using transformers.
How does wire size affect voltage drop?
Larger wire has lower resistance per foot, reducing drop. Upsizing from 12 AWG to 10 AWG cuts resistance roughly in half, cutting voltage drop in half for the same load and length. Doubling wire diameter (e.g., 10 AWG to 7 AWG) quarters the resistance. The cost of upsizing one or two gauge sizes is often small compared to the efficiency loss and equipment performance degradation from excessive drop. For critical or long runs, calculate drop for several wire sizes to find the optimal balance of cost and performance.
How do I calculate voltage drop for circuits with multiple loads at different distances?
Calculate drop separately for each segment. For a feeder running 100 feet to a subpanel, then branch circuits running an additional 50 feet to loads, calculate feeder drop based on total current and 100 feet, then branch drop based on branch current and 50 feet. Sum the two drops to verify total drop stays under 5 percent. For parallel branch circuits fed from the same feeder, calculate the worst-case branch (longest run, highest current) and add its drop to the feeder drop.
Does voltage drop affect energy consumption or just equipment performance?
Voltage drop wastes energy as heat in the conductors. Power loss in watts equals I² × R, where R is conductor resistance. A circuit carrying 20 amps through 100 feet of 12 AWG copper (resistance approximately 0.16 ohms) loses 20² × 0.16 = 64 watts as heat. Over 8,760 hours per year, that is 560 kWh wasted, costing $50 to $100 annually at typical residential rates. Upsizing to 10 AWG cuts loss by half, paying back the wire cost difference in a few years through energy savings.
How do temperature and conduit fill affect voltage drop?
Higher ambient temperature increases wire resistance, slightly increasing voltage drop. NEC ampacity tables include temperature derating factors, but these primarily affect ampacity, not resistance. Conduit fill affects ampacity through heat buildup but does not change the resistance or voltage drop calculation directly. However, if you must derate and upsize wire due to conduit fill, the larger wire will have lower resistance and less voltage drop as a beneficial side effect.
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