Voltage Drop Calculator

What Is Voltage Drop?

Voltage drop is the reduction in electrical voltage that occurs as current flows through a wire. Every conductor has resistance, and that resistance converts a small portion of the electrical energy into heat rather than delivering it to the load. A 120-volt circuit with 5% voltage drop delivers only 114 volts at the outlet. Excessive voltage drop causes lights to dim, motors to overheat and underperform, and sensitive electronics to malfunction. The calculator above determines the voltage drop for your wire gauge, distance, load, and voltage, checking the result against NEC recommended limits.

What Are the NEC Voltage Drop Limits?

The NEC recommends (but does not mandate as a hard requirement) a maximum voltage drop of 3% for branch circuits (from the panel to the outlet) and 5% total for the combined feeder and branch circuit. On a 120-volt circuit, 3% equals 3.6 volts, meaning the outlet should receive at least 116.4 volts. On a 240-volt circuit, 3% equals 7.2 volts. These are recommendations for optimal performance, not code violations if exceeded. However, most inspectors flag circuits exceeding 3% on branch runs, and equipment warranties may not cover damage from chronic low voltage. Designing within the 3% limit is standard professional practice.

How to Calculate Voltage Drop?

The formula for single-phase circuits is: VD = (2 x K x I x D) / CM. K is the resistivity constant (12.9 for copper, 21.2 for aluminum). I is the current in amps. D is the one-way distance in feet. CM is the circular mil area of the wire (listed in NEC Chapter 9, Table 8). The factor of 2 accounts for the round-trip distance (current flows out on one conductor and returns on the other). For three-phase circuits, replace 2 with 1.732. The calculator performs this computation automatically from your inputs and compares the result to the 3% NEC recommendation.

How Does Wire Gauge Affect Voltage Drop?

Larger wire (lower gauge number) has a larger cross-sectional area and lower resistance per foot, reducing voltage drop. 14 AWG wire has 4,107 circular mils. 12 AWG has 6,530. 10 AWG has 10,380. 8 AWG has 16,510. 6 AWG has 26,240. Doubling the circular mil area roughly halves the voltage drop for the same current and distance. When a circuit exceeds the 3% drop limit, the solution is to use a larger wire gauge. Going from 12 AWG to 10 AWG reduces voltage drop by about 37%. Going to 8 AWG reduces it by about 60% compared to 12 AWG.

When Is Voltage Drop a Problem?

Long wire runs are the primary cause of excessive voltage drop. A 15-amp circuit on 14 AWG wire stays within 3% for runs up to about 50 feet. Beyond 50 feet, the drop exceeds 3% and a larger wire is needed. A 20-amp circuit on 12 AWG wire hits the limit at roughly 55 feet. Detached garages, barns, outbuildings, well pumps, and landscape lighting circuits commonly involve long runs of 100-300 feet where voltage drop is a primary design concern. Interior circuits in a typical house rarely exceed 50 feet from the panel and usually stay within limits with standard wire sizing.

Common Scenarios and Recommended Wire Sizes

15-amp, 120V at 50 feet: 14 AWG is adequate (2.9% drop). At 100 feet: use 12 AWG (2.9%) or 10 AWG (1.8%). 20-amp, 120V at 75 feet: use 10 AWG (2.9%). At 150 feet: use 6 AWG (2.7%). 30-amp, 240V at 100 feet: 10 AWG (2.2%). 50-amp, 240V at 100 feet: 6 AWG (2.4%). These are guidelines for copper conductors. Aluminum wire has higher resistance and requires one or two sizes larger than copper for the same voltage drop performance. The calculator shows the exact drop percentage for your specific combination of variables.

Voltage Drop for Detached Structures

Feeding a detached garage, workshop, or pool house involves a long underground feeder from the main panel. A 60-amp subpanel 100 feet from the house on 240V needs at least 6 AWG copper (2.4% drop) or 4 AWG aluminum (2.6% drop). At 200 feet, upgrade to 4 AWG copper (2.4%) or 2 AWG aluminum (2.5%). The NEC allows the 5% total drop (feeder plus branch circuits), so a 2.5% drop on the feeder leaves room for 2.5% on the branch circuits within the building. Planning the wire size for both the feeder and the branch circuits together ensures the total stays under 5%.

Copper vs Aluminum Wire for Long Runs

Copper has lower resistance and carries more current per gauge size. Aluminum costs less per foot but requires a wire one to two sizes larger to match copper performance. For a 100-foot, 50-amp, 240V run: 6 AWG copper has 2.4% drop; 4 AWG aluminum has a similar drop at lower material cost. Aluminum requires anti-oxidant compound at all connections and is only compatible with terminals rated for aluminum (marked AL-CU). For underground feeder runs to outbuildings, aluminum is the cost-effective choice because the wire cost savings on long runs outweigh the slightly larger conduit needed for the bigger wire. For branch circuits inside buildings, copper is the standard because of its smaller size and easier termination.

Frequently asked questions