Calculate Voltage Drop
Voltage Drop
Percentage Drop
Voltage at Load
Wire Gauge Reference — Ampacity & Resistance
| AWG | Copper Ampacity | Aluminum Ampacity | Cu Ω/1000ft | Al Ω/1000ft | Common Uses |
|---|---|---|---|---|---|
| 14 | 15A | — | 3.14 | 5.17 | Lighting circuits, 15A outlets |
| 12 | 20A | 15A | 1.98 | 3.25 | General outlets, 20A circuits |
| 10 | 30A | 25A | 1.24 | 2.04 | Dryers, water heaters, 30A circuits |
| 8 | 40A | 30A | 0.778 | 1.28 | Ranges, large appliances |
| 6 | 55A | 40A | 0.491 | 0.808 | Subpanels, A/C units, EV chargers |
| 4 | 70A | 55A | 0.308 | 0.508 | Feeder circuits, large subpanels |
| 3 | 85A | 65A | 0.245 | 0.403 | Service entrance, large feeders |
| 2 | 95A | 75A | 0.194 | 0.319 | Service entrance, large feeders |
| 1 | 110A | 85A | 0.154 | 0.253 | Service entrance, large feeders |
| 1/0 | 125A | 100A | 0.122 | 0.201 | 200A service, large feeders |
| 2/0 | 145A | 115A | 0.0967 | 0.159 | 200A service entrance |
| 3/0 | 165A | 130A | 0.0766 | 0.126 | Large service entrance |
| 4/0 | 195A | 150A | 0.0608 | 0.100 | 200A+ service entrance |
Ampacity ratings based on NEC Table 310.16 for 60°C insulation (NM-B/UF-B). Actual ampacity depends on insulation type, ambient temperature, and conduit fill. Resistance values at 75°C.
When Voltage Drop Matters
Running power to a detached garage, barn, or workshop 100+ feet from the main panel is the most common scenario where voltage drop becomes a problem. A 100-foot run on 12 AWG copper at 20A drops 3.96V (3.3%) — already over the 3% recommendation.
Low-voltage landscape lighting runs can be very long. Even at 12V, a 200-foot run to garden lights can lose significant voltage, causing dim lights at the end of the run. Upsize wire or use a hub-and-spoke layout.
Level 2 EV chargers draw 30-50A continuously for hours. At 40A on a 60-foot run, you need at least 6 AWG copper to stay under 3%. Many installations require 4 AWG for longer distances.
Motors are especially sensitive to low voltage. A well pump 200+ feet from the panel on undersized wire will overheat, lose efficiency, and fail prematurely. Always upsize wire for motor loads on long runs.
Pool pumps and hot tubs are often 50-100+ feet from the panel and draw significant current. The combination of distance and load makes voltage drop a critical consideration.
DC circuits between solar panels, charge controllers, and batteries carry high current at low voltage. Even small resistance causes significant power loss — use the shortest, thickest wire practical.
How to Calculate Voltage Drop
Voltage drop occurs when electrical current flows through a wire's resistance, converting some energy to heat. The longer the wire and the higher the current, the more voltage is lost before reaching the load. The standard formula for single-phase circuits is:
Where VD is voltage drop in volts, L is one-way length in feet, R is wire resistance in ohms per 1,000 feet, and I is current in amps. The factor of 2 accounts for the round-trip path (hot and neutral conductors). For three-phase circuits, replace 2 with 1.732 (√3).
NEC Voltage Drop Recommendations
The National Electrical Code (NEC) Section 210.19(A) recommends limiting voltage drop to 3% for branch circuits and 5% total from the service entrance to the farthest outlet. While these are recommendations rather than hard requirements, exceeding them can cause equipment damage, reduced efficiency, and safety issues.
For a 120V circuit, 3% equals 3.6V — meaning the minimum acceptable voltage at the outlet is 116.4V. For a 240V circuit, 3% equals 7.2V, so the minimum is 232.8V.
Copper vs. Aluminum Conductors
Copper wire has approximately 61% lower resistance than aluminum wire of the same gauge, resulting in significantly less voltage drop. However, aluminum is lighter and cheaper, making it popular for large service entrance cables and long feeder runs. When using aluminum, you typically need to upsize by two gauge sizes (e.g., use 2 AWG aluminum instead of 4 AWG copper) to achieve similar performance.
Frequently Asked Questions
What is an acceptable voltage drop for residential wiring?
The NEC recommends no more than 3% for branch circuits and 5% total from the service entrance. For a 120V circuit, that means keeping the drop under 3.6V on any single branch circuit.
How do I calculate voltage drop in a wire?
Use the formula VD = (2 × L × R × I) / 1000, where L is one-way length in feet, R is resistance in ohms per 1,000 feet, and I is current in amps. This calculator handles the math automatically for any wire gauge and material.
Does wire length affect voltage drop?
Yes, voltage drop increases proportionally with length. This is why long runs to detached buildings, outdoor lighting, and EV chargers often require upsizing the wire gauge beyond the minimum required for the circuit ampacity.
Is copper or aluminum better for reducing voltage drop?
Copper has lower resistance and less voltage drop for the same gauge. But aluminum can match copper's performance by going up two wire sizes, often at lower total cost for large runs.
What happens if voltage drop is too high?
Lights dim, motors overheat, electronics malfunction, and heaters underperform. Motors are especially vulnerable — low voltage increases current draw, which can burn out windings and drastically shorten motor life.
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