Voltage Drop Calculator: Wire Gauge, Length, Amps, and NEC Limits

The two-wire voltage drop formula is V_drop = (2 x L x R x I) / 1,000, where L = one-way wire run length in feet, R = conductor resistance in ohms per 1,000 feet (from NEC Table 9), and I = current in amps. The result is in volts. Percent voltage drop = (V_drop / V_source) x 100. For copper AWG 12 wire: R = 1.98 ohm/kft. Running 15 amps through 50 feet of AWG 12 at 120V: V_drop = (2 x 50 x 1.98 x 15) / 1,000 = 2.97V. Percent drop = 2.97 / 120 x 100 = 2.48%. This is within the NEC 3% branch circuit guideline. For aluminum conductors, use the aluminum resistance values from NEC Table 9 (approximately 1.6 times higher than copper for the same AWG).

The NEC provides voltage drop limits as informational guidance rather than mandatory minimums in most cases, but they are widely treated as de facto standards for residential and commercial wiring. The NEC guidance: branch circuits (from outlet to device): maximum 3% voltage drop. Feeders (from main panel to subpanel): maximum 2% voltage drop. Combined (feeder plus branch circuit from main panel to final device): maximum 5% voltage drop. Some applications have stricter requirements: the NEC requires voltage drop verification for certain sensitive equipment, and some state codes adopt stricter limits. The International Building Code and some state building codes adopt the NEC informational notes as mandatory through code language. For practical purposes, designing to the 3% branch circuit and 2% feeder limits ensures reliable equipment operation, protects motors from overheating (which occurs when operating voltage is too low), and provides the energy efficiency benefit of lower conductor losses.

Electric motors are particularly sensitive to low voltage for two interconnected reasons. First, motor torque is proportional to the square of the applied voltage: if voltage drops 10%, available torque drops 19% (0.9 squared = 0.81, or 19% less than full voltage). A motor running a well pump or HVAC compressor that suddenly loses torque capacity may not be able to maintain the required load, leading to it stalling or operating with excessive slip in induction motors. Second, when a motor operates below its rated voltage, it draws more current (not less) to try to maintain the same power output: a motor operating at 10% below rated voltage draws approximately 10 to 15% more current than at rated voltage. This excess current causes I-squared-R heating in the motor windings, accelerating insulation degradation and dramatically reducing motor service life. NEMA standards specify that motors must be able to operate at plus or minus 10% of their nameplate voltage, but optimum efficiency and life occur at rated voltage. A motor on a circuit with 8% voltage drop (5% of which is in the feeder) is operating in the marginal zone where premature failure becomes significantly more likely. The 3% and 5% NEC voltage drop guidelines are explicitly designed to protect motor loads.

The voltage drop formula is the same for DC and single-phase AC circuits: V_drop = (2 x L x R x I) / 1,000. However, the resistance values differ slightly between AC and DC for large conductors because of the skin effect (at 60 Hz, AC current preferentially flows near the surface of the conductor, effectively reducing the usable cross-section). For conductors AWG 2 and smaller, the AC and DC resistance values are essentially identical; for larger conductors (1/0 AWG and above), AC resistance is slightly higher than DC resistance. The NEC Table 9 values used in this calculator are AC resistance values, which slightly overestimate DC voltage drop for large gauges but provide a conservative (safe) result. For DC systems (12V solar, 24V battery banks, 48V off-grid systems), using the AC resistance values from NEC Table 9 gives a result that is accurate to within 2 to 5% for conductors AWG 2 and smaller, which is more than adequate for sizing purposes. The 12V and 24V source voltage options in this calculator use the same NEC Table 9 resistance values and the same formula, producing voltage drop calculations appropriate for low-voltage DC wiring in solar, RV, and marine installations.

The run length where voltage drop exceeds 3% (branch circuit limit) depends on the load amps and wire gauge. For AWG 12 wire at full load (20A), the maximum run for 3% drop at 120V is: 3% x 120V = 3.6V drop allowed. L = (V_drop x 1,000) / (2 x R x I) = (3.6 x 1,000) / (2 x 1.98 x 20) = 3,600 / 79.2 = 45.5 feet one-way. So AWG 12 at full 20A load must upsize beyond 45 feet one-way to stay within 3%. At half load (10A), the same AWG 12 can extend to 91 feet. For AWG 10 at full load (30A): L = 3.6 x 1,000 / (2 x 1.24 x 30) = 48.4 feet. Even the larger AWG 10 at full load is limited to about 48 feet for a 3% voltage drop on a 120V circuit. For 240V circuits at 3% limit: V_drop allowed = 7.2V. AWG 12 at 20A: L = 7.2 x 1,000 / (2 x 1.98 x 20) = 90.9 feet. So 240V circuits can extend roughly twice as far as 120V for the same gauge and current before hitting the 3% limit – another practical advantage of 240V for long runs.

Yes, the NEC applies different limits: feeders (conductors between panels) have a 2% limit, while branch circuits (from the final overcurrent device to the load) have a 3% limit, for a combined maximum of 5%. The tighter feeder limit reflects that feeder losses affect all loads downstream; a 4% feeder voltage drop would leave no room for any branch circuit drop within the 5% combined limit. In practice, feeder sizing for long runs often drives the wire selection: a subpanel 200 feet from the main panel serving a 100-amp load requires a very large conductor to stay within 2% drop. For a 100A load at 240V over 200 feet, 2% drop = 4.8V. Using the formula: L = 4.8 x 1,000 / (2 x R x 100). Solving for R: R = 4.8 x 1,000 / (2 x 200 x 100) = 0.12 ohm/kft. Looking at NEC Table 9: AWG 1/0 (0.122 ohm/kft) just meets this requirement, making AWG 1/0 the minimum for this feeder application. Using the combined 5% limit instead (which permits 3% for the feeder in this context): R = 12/40,000 = 0.30 ohm/kft, pointing to AWG 4. The difference between designing to the strict 2% feeder limit versus the combined 5% limit can be several wire sizes, with significant cost implications for long feeder runs.

Because current is inversely proportional to voltage for the same power (I = W/V), a 12V system must carry 10 times more current than a 120V system for the same wattage. A 1,200W load at 120V draws 10A; the same 1,200W at 12V draws 100A. Since voltage drop equals 2 x L x R x I, and current is 10x higher at 12V, the voltage drop is 10x worse on the same wire. Furthermore, the 3% NEC limit at 12V allows only 0.36V of drop versus 3.6V at 120V. The combined effect: a 12V system must use 10 to 20 times the wire cross-section as a 120V system for equivalent voltage drop performance. This is why RV and marine wiring uses heavy gauge cables (AWG 4/0 to AWG 2 is common for 12V inverters) while the equivalent 120V circuit would use only AWG 12 or AWG 10. Off-grid system designers address this by using 24V or 48V battery banks instead of 12V for higher-power systems: every doubling of voltage halves the required current and reduces wire sizing by two to four AWG sizes. A 48V system carries the same 1,200W at only 25A instead of 100A, making AWG 10 appropriate where 12V would require AWG 4/0.

The voltage drop formula uses the total current flowing through the circuit at the point of maximum load. When multiple loads share a branch circuit and are distributed along its length, the voltage drop calculation becomes more complex because different sections of wire carry different currents. The simple formula (using total circuit current for the full one-way run length) overestimates voltage drop when loads are distributed. For most residential wiring, the worst-case scenario (all loads at the end of the run) is the conservative approach and appropriate for design purposes. For commercial lighting circuits with evenly distributed loads across a long run, the effective voltage drop can be calculated as approximately half that of an equivalent concentrated load at the end, because the average load is at the midpoint of the run. For residential purposes, always calculate as if the full load is at the far end of the run; this ensures that even the worst-case connection will meet NEC limits. For a basement workshop with multiple outlets distributed across 60 feet of AWG 12, use the full 60-foot length and the anticipated total coincident load (all tools that might run simultaneously) for the calculation.

Yes, excessive voltage drop can damage or reduce the life of several appliance types, and the risk varies by appliance type. Motors (refrigerators, AC compressors, well pumps, pool pumps, garbage disposals, power tools): the most voltage-sensitive category. Low voltage causes motors to draw excess current, overheat, and fail prematurely. A sustained 10% voltage drop can reduce motor life by 30 to 50 percent. Incandescent and halogen lighting: dims perceptibly but is not typically damaged by low voltage; in fact, it lasts longer at lower voltage (at the cost of efficiency). LED lighting with electronic drivers: generally tolerates voltage variation well within a +/- 10% range due to their voltage-regulating power supplies. Resistive heating (space heaters, electric ovens, water heaters): output power decreases proportionally to the square of voltage (a 10% drop reduces output to 0.81 of rated, or 19% less heat). The appliance is not typically damaged, but a water heater or space heater will take longer to reach temperature and cycle more frequently. Electronic equipment (computers, TVs, audio equipment): most modern electronics with switching power supplies tolerate 85V to 265V AC input and are not sensitive to the 5 to 15V variation caused by typical residential voltage drop. Sensitive medical equipment and industrial control systems may have much tighter voltage requirements specified in their documentation.

In most cases, yes. Outdoor circuits for detached garages, sheds, workshops, outdoor receptacles at the property line, pool pumps, and landscape lighting are among the most common applications where voltage drop mandates a larger wire than the NEC ampacity table would indicate. A 100-foot run to a detached garage for a 20A circuit: AWG 12 at 20A over 100 feet at 120V has a 4.95% drop, failing the 3% guideline. AWG 8 (1.95% drop) easily passes. When running any outdoor circuit more than 50 feet at 120V or more than 100 feet at 240V, calculate voltage drop before committing to a wire gauge; the cost difference between AWG 12 and AWG 10 or AWG 8 for a 100-foot run is typically $20 to $80 in materials, a trivial amount compared to the labor cost of replacing undersized wire later. If the run is underground (direct burial or conduit), the NEC requires THWN-2, XHHW-2, or direct burial rated cable; the same AWG sizing applies. For outdoor subpanel feeders at 240V, always calculate voltage drop and size the feeder to stay within the 2% feeder limit to preserve headroom for branch circuit drop within the 5% combined maximum.

To measure actual voltage drop on an installed circuit: (1) Load the circuit to the level you want to test (plug in a resistive load of known wattage for a reliable, stable current draw). (2) Measure voltage at the source (at the circuit breaker terminals or immediately at the panel) with a digital multimeter; record this as V_source. (3) Measure voltage at the far end of the run (at the outlet or load terminals) while the load is running; record this as V_load. (4) Voltage drop = V_source minus V_load. Percent drop = (V_source minus V_load) / V_source x 100. Note that measuring at the outlet and comparing to the nominal 120V or 240V listed voltage is not accurate, because the utility voltage itself may not be exactly 120V or 240V (typical utility voltage tolerance is plus or minus 5%, so your service may be at 114V to 126V nominal). Always measure at the source and at the load under load simultaneously for an accurate voltage drop measurement. A differential measurement can be taken with a second multimeter at the same moment, or with a single meter if you can quickly switch between points while the load remains constant. Clamp meters that display true watts can also indirectly confirm circuit loading to verify you are testing at the intended current level.

This calculator uses the AC resistance values for uncoated copper conductors in PVC conduit from NEC Table 9 (2020 NEC edition). The values in ohms per 1,000 feet: AWG 14 = 3.14, AWG 12 = 1.98, AWG 10 = 1.24, AWG 8 = 0.778, AWG 6 = 0.491, AWG 4 = 0.308, AWG 3 = 0.245, AWG 2 = 0.194, AWG 1 = 0.154, AWG 1/0 = 0.122, AWG 2/0 = 0.097, AWG 3/0 = 0.0766, AWG 4/0 = 0.0608. These values are slightly different from DC resistance due to skin effect and proximity effect in AC systems at 60 Hz. For aluminum conductors, the resistance is approximately 1.6 times higher for the same AWG (aluminum conducts electricity less efficiently than copper). The NEC Table 9 values in PVC conduit are marginally lower than in steel conduit (which has slightly higher effective resistance due to magnetically induced eddy currents in the conduit walls). Using NEC Table 9 values for direct burial or free air installations is slightly conservative (actual DC or PVC-conduit resistance is lower), providing a safe margin in the calculation. For the most accurate results on specific installations, consult the exact NEC Table 9 values for your conductor material, conduit type, and installation method. Find the complete NEC Table 9 and installation specifications at NFPA.org.

Aluminum conductors have about 1.61 times the resistance of copper for the same AWG size. To achieve the same voltage drop as AWG 10 copper, aluminum requires approximately AWG 8 (one size larger). Aluminum wiring is common for large feeder circuits (100A and above) because the cost savings over copper are significant at large conductor sizes, despite requiring a larger AWG. Aluminum conductors for US residential service entrance wiring are essentially universal; the aluminum conductors from the utility connection to your main panel are standard practice regardless of whether your in-house wiring is copper. Aluminum branch circuit wiring (AWG 12 and AWG 10) was common in US homes built between approximately 1965 and 1973, during a period of high copper prices. This wiring has known connection problems (aluminum expands and contracts more than copper with temperature cycling, and it oxidizes at connections, increasing resistance and fire risk) but is not inherently more dangerous than copper if proper aluminum-rated devices and connectors (CO/ALR rated receptacles and switches, AlumiConn or Purple wire connectors, Ideal 65 connectors) are used. When calculating voltage drop for aluminum circuits, apply the formula with aluminum resistance values from NEC Table 9 (approximately 1.6x the copper values at the same AWG).

Extension cords add additional conductor resistance to a circuit and can cause meaningful voltage drop, especially for long, thin extension cords. Heavy-duty 12-gauge extension cords (orange, typically 25 to 100 feet) have resistance approximately matching AWG 12 building wire (1.98 ohm/kft). A 100-foot, 12-gauge extension cord under a 15-amp load: V_drop = (2 x 100 x 1.98 x 15) / 1,000 = 5.94V at 120V = 4.95% additional drop. Added to a circuit that already has 2% building wire drop from the panel, total drop at the appliance is nearly 7%, significantly exceeding the 5% combined NEC guideline and potentially damaging motor-driven tools. Common extension cord AWG ratings: light-duty (16 AWG, not suitable for power tools, 2.58 ohm/kft), standard (14 AWG, 3.14 ohm/kft), heavy duty (12 AWG, 1.98 ohm/kft), extra-heavy-duty (10 AWG, 1.24 ohm/kft). For power tools with motors, use the shortest, heaviest extension cord available. For runs over 50 feet with tools drawing 10+ amps, use 10 AWG extension cord or install a permanent outlet closer to the work area. The OSHA 1926.403(b)(1)(i) construction standard limits extension cord length and requires cord gauges appropriate for the load, consistent with NEC voltage drop guidance.

LED fixtures with integrated electronic driver circuits are significantly more tolerant of input voltage variation than older incandescent or halogen lighting. Most LED drivers accept an input voltage range of 100V to 277V (sometimes 90V to 305V) and regulate the output to the LED chips regardless of input voltage within that range. A 5% voltage drop from 120V to 114V falls well within the input range of virtually all modern LED drivers; the output light level and color temperature remain unchanged. The LED driver does draw slightly more current at lower voltage to maintain the same power output, but the effect is minimal within normal voltage drop ranges. For long LED landscape lighting circuits or low-voltage (12V DC) LED tape lighting, voltage drop matters more because the lower operating voltage means percentage effects are larger and LED drivers designed for tight DC voltage ranges may dim perceptibly below their minimum input voltage. For low-voltage LED landscape lighting systems (typically 12V AC from a transformer), keeping the circuit drop below 1V (8.3% of 12V) is recommended by most manufacturer installation guides to avoid visible dimming at fixtures far from the transformer. Use 12 AWG or 10 AWG low-voltage cable for landscape runs exceeding 100 feet to limit voltage drop.

The voltage drop formula is the same regardless of installation method (free air, in conduit, direct burial, or cable tray) because it depends only on conductor resistance, length, and current. However, the resistance values from NEC Table 9 vary slightly between installation methods. NEC Table 9 provides AC resistance values for conductors in both steel conduit and PVC (non-metallic) conduit. Steel conduit values are slightly higher (by approximately 3 to 6% for large conductors) due to eddy current losses in the steel conduit walls created by the alternating magnetic field of the current. PVC conduit and direct burial values are essentially the same as the DC resistance for smaller conductors, and slightly higher for large conductors due to skin effect alone. This calculator uses PVC conduit values from NEC Table 9, which provides accurate results for PVC conduit installations and conservative (slightly overstated) results for free air and direct burial, giving a safe margin. For steel conduit installations, the actual voltage drop will be marginally higher than this calculator shows for conductors AWG 1/0 and larger. For most residential applications (AWG 14 through AWG 4), the difference between steel conduit and PVC conduit resistance is less than 2%, well within calculation error margins.