Voltage Drop Calculator

How to use the voltage drop calculator

1. Pick the circuit type. House circuits are single-phase at 120 or 240 V. Use DC for batteries, solar, RVs and 12 V landscape lights.
2. Enter the load current, not the breaker size. Watts divided by volts: a 1,800 W heater on 120 V draws 15 A. For motors and chargers, use the nameplate amps.
3. Measure the run one way along the cable route, including the climb up walls. Do not double it; the formula already counts the return wire.
4. Choose copper or aluminum and a size to test, and add the breaker rating if you want the ampacity check too.
5. Read the results. You see the drop in volts and percent, the voltage left at the load, the smallest size that meets your target, the longest run for the size you picked, the size the breaker needs and the larger of the two as the recommendation.

The voltage drop formula

For single-phase and DC circuits, VD = 2 × K × I × L ÷ CM; for balanced three-phase, replace 2 with 1.732. I is the current in amps, L the one-way length in feet, CM the conductor area in circular mils from NEC Chapter 9, Table 8, and K the resistance of a one-circular-mil, one-foot conductor at 75 °C: 12.9 ohms for copper and 21.2 for aluminum.

Mike Holt’s worked example is a 44 A, 240 V load 160 ft away on 6 AWG copper (26,240 circular mils): 2 × 12.9 × 44 × 160 ÷ 26,240 = 6.9 V, or 2.9%. K × 1,000 ÷ CM gives ohms per 1,000 ft; for 12 AWG copper that is 1.98, the figure Table 8 prints for stranded wire. Rearranged, the formula gives the area a target needs and the longest run a size allows. The lost volts become heat in the wire, current times drop: in Holt’s example about 305 W.

How far can you run each wire size at 120 V?

Longest one-way copper run at 120 V that stays within 3% (3.6 V). At 240 V the same current goes twice as far, so a 30 A load on 10 AWG can reach 97 ft; at 12 V, a tenth as far. Dashes mark currents above the 240.4(D) limit for that size. A 20 A circuit on 12 AWG reaches 3% at only 46 ft, which is why runs to a shed usually go up a size or two.

Where the 3% and 5% figures come from

The informational notes in NEC 210.19 (branch circuits) and 215.2(A) (feeders) suggest at most 3% drop on a branch circuit or feeder and 5% for both together. IAEI points out that informational notes are not enforceable under 90.5(C), and Holt adds that more than 3% is no violation unless equipment ends up below its rated voltage. Two rules still matter: 110.3(B) makes the equipment’s installation instructions binding, including any minimum voltage, and 250.122(B) requires a larger grounding conductor when you upsize the circuit wires. Your jurisdiction may set its own limits.

Section numbers on this page follow the 2023 NEC (NFPA 70). The 2026 edition moved two of them: the feeder note is now in 215.4(A)(2) and the larger grounding conductor rule is 250.122(D). The 3% and 5% figures, the branch-circuit note in 210.19 and 110.14(C) are unchanged. 240.4(D) gained entries for 16 AWG copper, but its limits for 14, 12 and 10 AWG are the same, and the Table 310.16 ampacities used here (14 AWG and larger) are the same in both editions. Check which edition your state enforces.

Voltage drop and ampacity are separate checks

Ampacity is about heat, and Holt notes the Table 310.16 values ignore voltage drop entirely. For circuits of 100 A or less, 110.14(C)(1)(a) usually limits you to the 60 °C column, the rating of NM-B and UF-B cable, and 240.4(D) caps small wire at 15 A on 14 AWG copper, 20 A on 12 and 30 A on 10 (15 A and 25 A on 12 and 10 AWG aluminum). The calculator reads Table 310.16 in the column you pick, for copper and aluminum alike (8 AWG aluminum is 35 A at 60 °C but 40 A at 75 °C), applies those caps and reports the larger of the two sizes. Use 75 °C only when the wire and every termination are rated 75 °C. It applies no temperature correction, bundling adjustment or 125% continuous-load sizing, so treat its answer as a minimum.

Worked examples for common long runs

• Detached garage: 240 V, 40 A, 120 ft. 8 AWG copper drops 7.5 V (3.1%); 6 AWG drops 4.7 V (2.0%). Ampacity alone would have stopped at 8 AWG.
• Well pump: 240 V, 10 A, 250 ft. 12 AWG loses 4.1%, 10 AWG 2.6%. Check the pump’s instructions for a minimum voltage.
• 12 V landscape lights: 5 A over 50 ft of 12 AWG loses 0.99 V, 8.2%, and the far fixtures dim; 8 AWG brings it to 3.3%.

Aluminum has about 1.64 times copper’s resistance, so it needs roughly two AWG sizes more for the same drop. Running power along a fence line or out to a deck? The fence calculator and deck calculator cover the structure.

What this calculator leaves out

The K method uses DC resistance at 75 °C, which Holt says suits AC circuits up to 1/0; on larger AC conductors skin effect adds a little (Table 9). Wire running cooler than 75 °C has less resistance, so the result usually errs safe. Motor starting current and power factor are not modeled. Use the answer to plan, and let a licensed electrician or your inspector confirm the final design.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• VD = M × K × I × L ÷ CM, M = 2 (single-phase, DC) or 1.732 (three-phase), L one-way feet, CM from NEC Chapter 9 Table 8.
• K = 12.9 copper, 21.2 aluminum (75 °C); matches Table 8 stranded resistance within 0.5% for every size offered.
• Code edition: section numbers follow the 2023 NEC (NFPA 70-2023). In the 2026 NEC the feeder note is 215.4(A)(2) and the grounding-conductor upsizing rule is 250.122(D); Table 310.16 values for 14 AWG and larger and the 240.4(D) limits for 14–10 AWG did not change (checked 2026-10-29).
• Default 3% target from the informational notes in 210.19 and 215.2(A) (2026 NEC: 215.4(A)(2)) (3% branch or feeder, 5% combined); a recommendation, not a requirement.
• Ampacity: Table 310.16 at 60 or 75 °C for copper and aluminum, capped by 240.4(D); no correction, adjustment or 125% sizing.
• Heat lost = current × volts dropped (× 1.732 for three-phase).
• Not modeled: skin effect on 2/0 and larger, other temperatures, power factor, motor starting.

Sources

1. Mike Holt Enterprises, Voltage Drop Calculations, Part One. Single- and three-phase formulas; K = 12.9 copper, 21.2 aluminum, suitable for AC up to 1/0; worked examples (6.9 V, 2.9%); 3%/5% notes; 250.122(B).
2. IAEI Magazine, Voltage Drop Formulas. How K comes from Table 8; formulas for drop, circular mils and length; informational notes not mandatory (90.5(C)); 110.3(B).
3. Cerrowire, Ampacity Charts | Wire Gauge Chart. Table 310.16 ampacities by size and temperature column (2017 NEC), no corrections.
4. NFPA, Using the National Electrical Code (NEC) Ampacity Charts. Reprint of Table 310.16, copper and aluminum 60/75/90 °C columns (used for aluminum 60 °C and copper 60 °C above 2 AWG), and the termination-temperature workflow of 110.14(C).
5. EC&M, Code Requirement for Sizing the Overcurrent Protection of Conductors. 240.4(D) small-conductor limits; 110.14(C)(1)(a) 60 °C column; 125% continuous-load example.
6. Southwire, Re³ Voltage Drop Calculator. Manufacturer calculator; residential mode is single-phase at 60 °C with a 3% default.
7. National Fire Protection Association, NFPA 70, National Electrical Code (code development page). The NEC itself (Table 8, Table 310.16, voltage drop notes). Your state adopts a specific edition.
8. National Fire Protection Association, NEC Code-Making Panel 10, First Draft working output (A2025 cycle). First Revision 8995: 215.2 relocated to 215.4 for the 2026 NEC; the feeder voltage drop note becomes Informational Note No. 2 under 215.4(A)(2).
9. NEC Mastery, 2026 NEC Change to 250.122: What Electricians Need to Know About EGC Sizing. In the 2026 NEC the rule to upsize the equipment grounding conductor with the circuit conductors is 250.122(D) (was 250.122(B)).

Last reviewed October 29, 2026. Product figures vary by brand; the label on what you buy wins.