Voltage Drop Calculator

How the Voltage Drop Calculator Works

This voltage drop calculator estimates how many volts a wire run loses between the panel and the load, and what share of the system voltage that loss represents. It multiplies the load current by the resistance of the conductors, using the direct current resistance values in NEC Chapter 9, Table 8 for AWG and kcmil sizes, or the standard resistivity of copper and aluminum for metric mm² sizes. The result is compared with the 3 percent and 5 percent guidelines in the National Electrical Code informational notes.

Use it when planning a long branch circuit, a feeder to a garage or subpanel, an EV charger, a well pump, or a 12 V DC run in an RV or solar system. It also reports the voltage at the load, the power lost as heat, the longest run and largest load under 3 percent, and the smallest wire for each guideline.

The factor of 2 covers current flowing out on one conductor and back on the other, so you always enter the one way distance. A balanced three phase circuit uses the square root of 3, about 1.732, because the phase currents return through the other phase conductors. A DC circuit is calculated exactly like a single phase circuit.

Voltage Drop Formula With Circular Mils (K Factor Method)

The K factor form replaces the table resistance with a constant divided by the conductor area in circular mils. A circular mil is the area of a circle 0.001 inch across (1 mm² is about 1,973.5 cmil). Multiplying a Table 8 resistance per foot by its area gives about 12.9 for uncoated copper and 21.2 for aluminum at 75 °C, the K values IAEI gives for voltage drop work.

The K method is approximate because the product of resistance and area changes slightly from size to size. Across the sizes in this calculator it runs from 12.84 to 12.93 for stranded copper and from 21.13 to 21.25 for aluminum. The calculator uses the exact Table 8 resistance for the size you pick and shows the K factor result in the steps as a cross check.

AWG Wire Resistance Chart (Ohms per 1,000 Feet)

These direct current resistance values come from NEC Chapter 9, Table 8, at a conductor temperature of 75 °C, for Class B stranded uncoated copper and aluminum. They are the values the calculator uses for AWG and kcmil sizes.

Solid wire, often used for 14 to 10 AWG in nonmetallic sheathed cable, has slightly lower resistance: Table 8 lists solid copper at 3.07 Ω for 14 AWG, 1.93 Ω for 12 AWG and 1.21 Ω for 10 AWG per 1,000 ft. Choose Solid under Stranding to use those values. Tinned or coated copper has higher resistance and is not covered here.

Metric cable sizes

For mm² sizes the calculator uses resistivity: 0.017241 Ω·mm²/m for annealed copper (100 percent IACS) and 0.028264 Ω·mm²/m for 61 percent IACS aluminum, both at 20 °C, corrected to the conductor temperature. At 75 °C copper becomes 0.017241 × [1 + 0.00393 × 55] = 0.020968 Ω·mm²/m and aluminum 0.034529 Ω·mm²/m. Dividing by the area gives 8.387 Ω per km for 2.5 mm² copper, 5.242 Ω for 4 mm² and 3.495 Ω for 6 mm².

Acceptable Voltage Drop: The NEC 3 Percent and 5 Percent Guidelines

The National Electrical Code (NFPA 70) does not set a general voltage drop limit. Informational Note No. 4 under Section 210.19(A) says branch circuit conductors sized to prevent a drop over 3 percent at the farthest outlet, with no more than 5 percent total on the feeder and branch circuit together, provide reasonable efficiency of operation. Informational Note No. 2 under Section 215.2(A) repeats the same guidance for feeders.

Section 90.5(C) states that informational notes are explanatory material, not enforceable requirements. Voltage drop is mandatory in a few places, such as Section 647.4(D) for sensitive electronic equipment, and it can become mandatory through local amendments, project specifications, or equipment instructions that Section 110.3(B) requires installers to follow. Subsection numbering shifts slightly between NEC editions, so check the edition your area enforces.

Because the 5 percent figure covers the whole path from the service to the farthest outlet, a feeder that drops 2 percent leaves 3 percent for the branch circuit, while a short feeder at 1 percent leaves 4 percent.

Wire Size for Voltage Drop Over 100, 150 and 200 Feet

Voltage drop grows in direct proportion to length and current, so doubling the run doubles the drop. The table shows a 20 A load at 120 V on stranded copper, single phase, at several one way lengths.

At 100 feet, 8 AWG is the smallest copper size under 3 percent for this load, and at 200 feet it takes 4 AWG. The same 20 A at 240 V drops the same number of volts, but each volt is half the percentage, so 10 AWG at 100 feet comes in at 2.07 percent.

Maximum length for a 3 percent drop

These lengths assume the full current flows continuously, which gives a worst case. A circuit that rarely runs at full load drops less in practice. The pairs are examples, not ampacity ratings.

Voltage Drop for EV Chargers, Subpanels, Well Pumps and DC Systems

EV charger: 50 A at 240 V, 80 ft

An EV charger can draw full current for hours, so the circuit must also meet the charger instructions and the NEC sizing rules for the charging circuit.

Subpanel feeder: 100 A at 240 V, 150 ft

Feeder drop adds to every branch circuit fed from the subpanel. With 2 AWG copper at 2.43 percent, branch circuits have about 2.6 percent left under the 5 percent total. The full 100 A is a worst case; actual demand is usually lower.

Well pump: 10 A at 240 V, 250 ft

Measure the full cable length from the panel or pump control box to the motor, including the depth down the well. Submersible pump installation manuals list maximum cable lengths for each motor, so follow them where they are stricter.

12 V and 24 V DC: solar, RV and low voltage lighting

A low voltage run loses the same volts as a 120 V circuit carrying the same current, but each volt is a far larger share of the supply. Raising the system voltage lowers the current for the same power, which is why 24 V and 48 V battery systems allow longer runs on the same wire.

Copper vs Aluminum Wire and the Effect of Temperature

Table 8 is based on aluminum with 61 percent of the conductivity of annealed copper, so aluminum wire has about 1.64 times the resistance of copper of the same size (3.25 against 1.98 Ω per 1,000 ft for 12 AWG). Installers make up for it with a larger size. On a 50 A, 240 V circuit with a 100 ft run, 4 AWG aluminum drops 5.08 V (2.12 percent), close to the 4.91 V (2.05 percent) of 6 AWG copper, while 6 AWG aluminum drops 8.08 V (3.37 percent).

Resistance rises with temperature. NIST Handbook 100 gives the coefficient of standard annealed copper as 0.00393 per °C at 20 °C, and NIST Handbook 109 gives 0.00403 per °C for 61 percent conductivity aluminum. Referred to 75 °C those become 0.00323 and 0.00330, the values in note 2 of Table 8.

IAEI notes that most terminals in new equipment are rated 75 °C, which is why Table 8 uses that temperature. A wire carrying much less than its rated current runs cooler, so the 75 °C result is slightly conservative for lightly loaded circuits.

Why Voltage Drop Matters

• Heaters and lights. A resistive heater's output follows the square of the voltage, so a 6.6 percent drop cuts its output by about 12.8 percent. Incandescent and some LED lamps dim at the end of a long run.
• Motors. A motor running below its rated voltage needs more current to carry the same load, which raises winding temperature. Starting current is several times running current, so a long run also causes deeper dips at startup.
• Electronics and chargers. Many devices list a minimum input voltage in their instructions, and IAEI points out that listed equipment is meant to run within that range.
• Wasted energy. The default circuit turns 158.4 W into heat in the wire at full load, about 1.27 kWh over 8 hours, energy you pay for without using.

Limitations of This Estimate

The calculator uses direct current resistance. In AC circuits, large conductors also have skin effect and inductive reactance, covered by NEC Chapter 9, Table 9. IAEI notes that Table 8 values are close to the Table 9 resistance values, but long feeders with large conductors and a low power factor may be designed with the Table 9 impedance method instead.

Metric results use nominal cable area; a data sheet gives the measured resistance of a specific cable. Motor starting current, harmonics and unbalanced loads are not modeled. Electrical work must follow your local code and be done or inspected by a licensed electrician.

Frequently Asked Questions

Sources & References

• National Fire Protection Association. NFPA 70, National Electrical Code, document information page (current edition and free access to the code text). nfpa.org
• NFPA 70, Chapter 9, Table 8 Conductor Properties (DC resistance at 75 °C, areas, temperature note), 2011 edition reproduction hosted by NECA IBEW Electricians; the 2017 values are identical. necaibewelectricians.com
• IAEI Magazine. Voltage Drop Formulas, April 5, 2017 (K factors, formulas, informational note sections, 90.5(C), 647.4(D)). iaeimagazine.org
• Lamendola, M. Tip of the Week: Insights from NEC Article 90, Part 4, EC&M, August 28, 2017 (voltage drop notes are explanatory, not requirements). ecmweb.com
• National Bureau of Standards. Copper Wire Tables, Handbook 100, 1966 (International Annealed Copper Standard, 1.7241 μΩ·cm and 0.017241 Ω·mm²/m at 20 °C, coefficient 0.00393). nist.gov
• National Bureau of Standards. Aluminum Wire Tables, Handbook 109, 1972 (61 percent IACS aluminum, 0.028264 Ω·mm²/m at 20 °C, coefficient 0.00403). nist.gov

Results are estimates from conductor resistance and do not replace a design by a qualified person or review by your local inspector. Every number on this page was recomputed with the same formulas the calculator uses. Methodology reviewed: 2026.