12 AWG at 20 A, 100 ft — Voltage Drop
Based on copper, single-phase/DC, 120V. Use the calculator below to adjust any of these.
Voltage Drop
7.90 V (6.58%)
Example
A 12 AWG copper conductor carrying 20A over 100 ft drops about 7.9V (6.6%) — leaving 112.1V at the load.
Voltage at the Load
112.1 V
What is Voltage Drop?
Every wire has some resistance, so some voltage is inevitably "lost" as electricity travels from the source to the load — this is voltage drop. Longer runs, higher current, and thinner wire all increase it. Excessive voltage drop can cause dim lighting, reduced motor performance, and unnecessary heat in the wiring itself.
Voltage Drop by Wire Gauge
Using your exact length, current, and voltage — only the wire gauge changes. Your selected gauge is highlighted in blue.
| Gauge | Drop (V) | Drop (%) | Within 3%? |
|---|---|---|---|
| 14 AWG | 12.55 | 10.46% | No |
| 12 AWG | 7.90 | 6.58% | No |
| 10 AWG | 4.97 | 4.14% | No |
| 8 AWG | 3.13 | 2.60% | Yes |
| 6 AWG | 1.97 | 1.64% | Yes |
| 4 AWG | 1.24 | 1.03% | Yes |
| 3 AWG | 0.98 | 0.82% | Yes |
| 2 AWG | 0.78 | 0.65% | Yes |
| 1 AWG | 0.62 | 0.51% | Yes |
| 1/0 AWG | 0.49 | 0.41% | Yes |
| 2/0 AWG | 0.39 | 0.32% | Yes |
| 3/0 AWG | 0.31 | 0.26% | Yes |
| 4/0 AWG | 0.24 | 0.20% | Yes |
How Is Voltage Drop Calculated?
This calculator uses the standard NEC-referenced formula based on the conductor's circular-mil area (a measure of cross-sectional size derived from its AWG gauge) and a material resistivity constant, K — about 12.9 for copper and 21.2 for aluminum at typical operating temperature.
Three-Phase: Vdrop = √3 × K × Length × Current ÷ Circular Mils
Why the Formula Doubles the Length
Current has to travel from the source to the load and then back again to complete the circuit, so the single-phase/DC formula multiplies the one-way length you enter by 2 to account for the full round-trip resistance. Three-phase circuits use a √3 factor instead, reflecting how the three phases share the load differently than a simple two-wire circuit.
Why Copper and Aluminum Differ
Copper is a better electrical conductor than aluminum — aluminum has roughly 64% of copper's conductivity, which shows up directly in the higher K constant used for aluminum in the formula above. Aluminum wiring is still commonly used for larger feeders and service entrances because it's lighter and less expensive, but it requires a larger gauge than copper to carry the same current with the same voltage drop.
The 3% Guideline
The NEC doesn't strictly require a voltage drop limit for most branch circuits, but its informational notes recommend keeping branch-circuit drop to 3% or less, and combined feeder-plus-branch-circuit drop to 5% or less, to maintain reasonably efficient operation. This is a design guideline rather than a hard safety limit — check local code requirements for any circuit where compliance is mandatory.
Example — Your Current Inputs
A 12 AWG copper conductor carrying 20A over 100 ft drops about 7.9V (6.6%) — leaving 112.1V at the load.
Additional Example — A Long Garage Sub-Feed
Running a 20A, 120V circuit 150 feet to a detached garage with 12 AWG copper wire produces about a 7.7V drop — 6.4%, well above the 3% guideline. Upgrading to 8 AWG for that same run brings the drop down to about 3%, illustrating why long outdoor or detached-structure runs often need a noticeably heavier gauge than the same circuit would need indoors.
About These Parameters
- Wire Gauge & Material
- Smaller AWG numbers mean thicker wire with less resistance and therefore less voltage drop. Copper always outperforms the same gauge in aluminum.
- One-Way Length
- The distance from the power source to the load, measured one way — not the total round-trip wire length, which the formula accounts for automatically.
- Load Current & System Voltage
- The actual (or expected) current draw of the load, and the nominal system voltage (commonly 120V or 240V for residential circuits in the US).
Frequently Asked Questions
Is exceeding 3% voltage drop actually dangerous?
Not typically a direct safety hazard by itself, but it can cause dim lighting, reduced motor torque and lifespan, and extra heat dissipated in the wire itself. For safety-critical or code-mandated circuits, always verify actual requirements with a licensed electrician rather than relying solely on this general guideline.
Why does this calculator ask for one-way length?
It's the more intuitive real-world measurement (the distance you'd actually measure with a tape measure or on a site plan) — the calculator applies the correct round-trip or three-phase multiplier internally so you don't have to double it yourself.
Does temperature affect voltage drop?
Yes — a conductor's resistance rises with temperature, so a wire running hot from heavy sustained load will have slightly higher voltage drop than the same wire at a cooler ambient temperature. This calculator uses resistivity constants for a typical 75°C operating temperature, a standard reference point for this kind of estimate.