Calculate voltage drop in electrical wiring by AWG, distance, current, and conductor material. NEC compliance check, AWG comparison table, power loss, and transmission efficiency.
Every wire has resistance, and when current flows through it, a portion of the supply voltage is lost as heat in the wire itself — this is voltage drop. For residential and commercial electrical installations, the National Electrical Code (NEC) recommends that total voltage drop not exceed 3% for branch circuits and 5% for combined feeder-plus-branch circuits. Excessive voltage drop causes lights to dim, motors to run hot, and sensitive electronics to malfunction.
Voltage drop depends on four factors: wire gauge (cross-sectional area), conductor material (copper vs. aluminum), one-way distance from the source to the load, and the current flowing through the wire. For a single-phase circuit, the total wire length is twice the one-way distance (supply and return paths). For three-phase circuits, the multiplier is √3 instead of 2.
This calculator computes the voltage drop, power loss, and NEC compliance status for any combination of wire gauge, distance, and current. An AWG comparison table shows all standard wire sizes so you can quickly upsize if needed, and the visual indicator clearly marks whether your installation meets code requirements.
Correctly sizing wire for voltage drop requires looking up wire resistance tables, converting between AWG and metric, and accounting for conductor material and circuit type. This calculator handles all the math and provides an instant NEC compliance check, plus a full AWG comparison table so you can select the most economical wire size that meets code.
Voltage Drop: V_drop = I × R_wire Wire Resistance (single-phase): R_wire = 2 × ρ × L / A Wire Resistance (three-phase): R_wire = √3 × ρ × L / A AWG Cross-section: A = 0.012668 mm² × 92^((36−AWG)/39) Drop Percentage: %drop = (V_drop / V_source) × 100 Power Loss: P_loss = I² × R_wire Where: ρ = resistivity (Ω·m) L = one-way distance (m) A = wire cross-section (m²)
Result: V_drop = 4.89 V (4.1%)
With AWG 14 copper wire (2.08 mm², 8.286 Ω/1000ft) carrying 15 A over 50 ft single-phase: total wire length = 100 ft, R = 0.829 Ω, V_drop = 15 × 0.829 = 4.89 V (4.1%). This exceeds the 3% NEC guideline — consider upgrading to AWG 12 which drops to 3.07 V (2.6%).
The NEC (NFPA 70) Informational Note in Article 210.19 recommends that branch circuit conductors be sized to prevent a voltage drop exceeding 3% at the farthest outlet. Combined with feeder voltage drop, the total should not exceed 5% from the service entrance to the final outlet. While these are recommendations rather than mandatory requirements, most jurisdictions enforce them as part of good electrical practice.
Voltage drop is the fundamental reason that electrical power is transmitted at high voltages. A 100-mile transmission line carrying 100 MW at 345 kV has a current of only 290 A and manageable losses. The same power at 12 kV would require 8,300 A and enormous conductors. Step-up transformers at the generating station and step-down transformers at the distribution level are the solution that makes long-distance electrification practical.
Solar panel arrays, battery banks, RV electrical systems, and marine wiring all operate at 12-48 VDC where every tenth of a volt matters. A 20-foot run from a 12V battery bank to an inverter carrying 100A can drop several volts in undersized wire, wasting power as heat and potentially damaging the inverter. For DC systems, use the largest practical wire size and keep runs as short as possible.
NEC requires minimum AWG 14 for 15A circuits based on ampacity. But for long runs (over ~50 ft), you may need AWG 12 or even AWG 10 to keep voltage drop under 3%. Always check both ampacity and voltage drop requirements.
A 3.6V drop in a 120V system is only 3%. The same 3.6V drop in a 12V system is 30% — devastating for the equipment. This is why 12V and 24V DC systems require careful wire sizing and short runs.
The calculator uses standard 20°C resistivity values. Wire resistance increases approximately 0.4% per °C for copper. At 60°C (typical for loaded wire in conduit), add about 16% to the calculated resistance.
Ampacity is the maximum current a wire can safely carry without overheating — it is a safety limit. Voltage drop is the voltage lost along the wire — it is a performance limit. A wire can meet ampacity requirements but still have excessive voltage drop on a long run.
Copper has lower resistance and is standard for branch circuits. Aluminum is cheaper, lighter, and common for larger feeders and service entrances. When using aluminum, go up two AWG sizes (e.g., AWG 2 aluminum instead of AWG 4 copper) for equivalent resistance.
For balanced three-phase circuits, the multiplier is √3 instead of 2 for the wire length. This is because the neutral carries no current in a balanced system, so only the phase conductor resistance matters (multiplied by √3 for line-to-line voltage).