What This Calculator Does
This tool calculates voltage drop across a cable run using conductor material (copper or aluminum), AWG wire size, one-way run length, current, system voltage (120–480 V), and phase (single or three-phase). The result shows drop in volts, drop as a percentage of supply voltage, voltage delivered at the load, and a pass/fail check against NEC 3% and 5% thresholds.
This is different from the Ohm's Law Calculator, which solves the general V = IR relationship between any three variables. This page is scoped to conductor runs: it uses tabulated AWG resistivity data, accounts for round-trip conductor length, and cross-references the result against NEC recommendations.
What Voltage Drop Means in a Real Circuit
Voltage drop is the reduction in electrical potential between two points caused by conductor resistance. In a zero-resistance conductor there would be no drop — every volt at the source would arrive at the load. In practice, all conductors resist current, and that resistance produces a proportional voltage loss.
How significant the drop is depends entirely on the load. A 2% drop on a long lighting branch is inconsequential. A 4% drop supplying a variable-frequency drive, PLC, or low-voltage control circuit may trigger mis-operation, reduced motor torque, or equipment fault.
How Each Input Affects the Result
- Current (A): Drop scales linearly with current. Doubling the load current doubles the voltage drop across the same conductor.
- One-way run length (ft): The formula uses 2× run length for the round-trip conductor path. A 100 ft run uses 200 ft of wire resistance. Run length is the highest-leverage variable on long distribution circuits.
- Wire size (AWG): Lower AWG = thicker wire = lower resistance per foot. Moving from 14 AWG to 10 AWG copper cuts resistance by more than half (3.07 → 1.21 Ω/1000 ft).
- Conductor material: Aluminum has approximately 1.6× the resistivity of copper. For the same AWG, aluminum drops roughly 1.6× more than copper — which is why aluminum feeders are upsized by one or two gauges.
- System voltage: A fixed voltage drop in volts is a smaller percentage on a higher supply voltage. 5 V on 120 V is 4.2%; on 240 V it is 2.1%. This is why 240 V circuits are preferred for long runs.
- Phase: Three-phase circuits use the √3 (≈1.732) factor. For the same current and resistance, three-phase drop is slightly higher in absolute volts but the same as single-phase as a percentage of line-to-line voltage.
NEC Limits and When Drop Becomes a Problem
The National Electrical Code (NEC) includes informational notes recommending:
- 3% maximum for branch circuits (NEC 210.19 informational note)
- 5% maximum combined for feeder plus branch circuit (NEC 215.2 informational note)
These are recommendations, not enforceable violations — but most engineers treat them as design targets. Exceeding 5% combined drop is a signal to upsize wire, shorten the run, or split the load. Voltage-sensitive equipment (VFDs, PLCs, solenoid valves, low-voltage systems) often has tighter internal tolerances (±5–10% of rated voltage), so staying well within the 3% limit is the practical goal on those circuits.
Worked Examples
Example 1 — Short residential branch circuit:
12 AWG copper • 25 ft run • 15 A • 120 V single-phase
Round-trip: 50 ft → R = 1.93 × 50 ÷ 1000 = 0.0965 Ω → V_drop = 15 × 0.0965 = 1.45 V (1.2%) → PASS
Short residential branch circuits at typical loads stay well within limits on 12 AWG.
Example 2 — Long 240 V feeder:
2 AWG copper • 150 ft run • 60 A • 240 V single-phase
Round-trip: 300 ft → R = 0.191 × 300 ÷ 1000 = 0.0573 Ω → V_drop = 60 × 0.0573 = 3.44 V (1.4%) → PASS
Higher system voltage absorbs the drop effectively. 2 AWG handles 60 A over 150 ft on 240 V with margin.
Example 3 — Long 120 V circuit where drop is too high:
14 AWG copper • 100 ft run • 20 A • 120 V single-phase
Round-trip: 200 ft → R = 3.07 × 200 ÷ 1000 = 0.614 Ω → V_drop = 20 × 0.614 = 12.28 V (10.2%) → FAIL
Fix: upsize to 8 AWG (0.764 Ω/1000 ft). R = 0.153 Ω → V_drop = 3.06 V (2.5%) → PASS.
How to Reduce Voltage Drop
- Upsize the wire. Each AWG step up roughly halves resistance. Calculate both options and compare — the tool makes this fast.
- Shorten the run. Moving a sub-panel closer to the load is the most direct fix; drop scales directly with length.
- Use copper instead of aluminum. Copper resistivity is ~1.6× lower than aluminum for the same AWG. Critical when aluminum at the required AWG would still fail and further upsizing is impractical.
- Use a higher system voltage. A fixed-watt load draws half the current at 240 V vs. 120 V, cutting drop by half. Three-phase distribution achieves further reduction on long industrial runs.
- Split the load. Two separate circuits from a closer sub-panel may have lower combined drop than one circuit from a distant panel.
Important Limitations
This calculator uses tabulated DC resistivity values from NEC tables at 20°C. Actual measured drop may differ because:
- Temperature: Conductor resistance increases with temperature. A conductor running hot under full load will have higher actual resistance than the 20°C table value.
- Connector and termination losses: Loose, corroded, or undersized terminations add resistance not accounted for in the conductor calculation.
- AC impedance: At 50/60 Hz, skin effect and inductive reactance add to effective impedance — particularly relevant for large conductors (≥2/0 AWG) at high currents.
- Bundled conductors: Conductors in conduit or bundled together may require ampacity derating; this affects allowable current but also influences real-world drop.
- Regulatory compliance: NEC voltage drop notes are recommendations. Designs requiring code compliance should be reviewed by a licensed electrical engineer.
