This voltage drop calculator shows how many volts a wire loses between the source and the load, what percentage of your supply that represents, and the smallest copper or aluminum conductor that keeps the loss within your limit. It works for single-phase AC, DC and three-phase circuits.
Estimate only, using the NEC K-factor method at 75 °C and ampacities from NEC Table 310.16 (2023). Local codes vary. This is not engineering, permit or inspection approval.
Choose the system type, then enter the supply voltage (line-to-line for three-phase), the load current in amps and the one-way distance from the source to the load. Pick a wire size and material, set the maximum drop you will accept (3% is the usual branch-circuit target), and press Calculate. You get the volts lost, percent drop, voltage at the load, power wasted as heat, a recommended minimum wire size and a table comparing every standard size.
Every wire has resistance, and resistance eats voltage whenever current flows. On a short run you never notice. On a long one, lights dim, motors run hot, electronics reset and chargers fail to start. Take a 240 V, 30 A well pump 250 feet from the panel on 10 AWG copper. The drop is about 18.6 V, or 7.8%. The pump sees roughly 221 V, draws more current to compensate, and its windings run hot and wear out sooner.
Low-voltage systems are hit hardest, because a volt or two is a large share of a small supply. A 12 V LED lighting run of 30 feet at 10 A on 14 AWG loses almost 1.9 V, nearly 16%. Electricians, solar installers, RV and marine owners, irrigation contractors and DIYers wiring a detached garage, shed or EV charger all run into this. Checking the numbers before you buy wire is far cheaper than pulling it out afterwards, and the extra copper often costs less than the callback.
The calculator uses the standard K-factor (circular mil) method:
K is the resistivity constant: 12.9 for copper and 21.2 for aluminum, in ohm-circular mils per foot at 75 °C. I is the current in amps. L is the one-way length in feet (meters are multiplied by 3.28084). CM is the conductor's area in circular mils, for example 6,530 for 12 AWG and 10,380 for 10 AWG. The factor of 2 covers the current going out and back. Percent drop is VD ÷ source voltage × 100, and voltage at the load is the source voltage minus VD. Heat loss is VD × I for single-phase and DC, and 1.732 × VD × I for three-phase.
The 3% branch-circuit and 5% total (feeder plus branch) targets come from the informational notes to NEC 210.19(A) and 215.2(A). They are recommendations rather than hard rules for most circuits, though some systems have mandatory limits. For the minimum size, the tool picks the smallest standard conductor that meets both your drop limit and ampacity. Ampacity uses the 75 °C columns of NEC Table 310.16, capped by NEC 240.4(D) for small conductors (copper 14/12/10 AWG at 15/20/30 A, aluminum 12/10 AWG at 15/25 A). If you tick "continuous load," the required ampacity is 125% of the current. Ambient temperature, conduit fill and bundling are not derated here, so apply those separately. Results are always rounded to the next larger standard size, never smaller.
A 240 V single-phase subpanel feeds a 20 A continuous load 100 feet away, wired with 12 AWG copper and a 3% limit.
The NEC recommends no more than 3% on a branch circuit and 5% combined for feeder and branch. For sensitive electronics or low-voltage lighting, aim for 2% or less. Some special systems, such as fire pumps, have mandatory limits, so check your local code.
Multiply 2 (or 1.732 for three-phase) by the K constant, the current and the one-way length in feet, then divide by the wire's circular mils. Divide the result by the supply voltage for percent drop. This calculator does all of that and compares every standard size.
Current flows out on one conductor and returns on another, so the effective path is twice the distance. The formula's factor of 2 already covers that, so you only measure the straight run from source to load.
Yes. Aluminum's resistivity is about 64% higher (K of 21.2 versus 12.9), so the same gauge drops noticeably more voltage. Upsizing aluminum by one or two sizes usually compensates.
Use a larger wire, shorten the run, lower the current, or raise the system voltage. For long runs, moving from 120 V to 240 V cuts the percent drop to a quarter for the same power, because current halves and the voltage doubles.
No. It is a planning estimate based on common NEC methods and does not account for every installation condition. Have a licensed electrician or engineer confirm the design, and follow your local code and inspector.
Calculator by Toolwick