Voltage drop is the reduction in electrical potential (voltage) across a conductor caused by the inherent resistance of the wire carrying current. When you push amps through copper or aluminum, the wire acts like a small resistor, burning off a fraction of your source voltage as heat before it ever reaches the load. Think of it like water pressure loss due to friction in a 100-foot garden hose: the pump pushes 60 PSI, but by the time the water reaches the nozzle, friction has dropped it to 45 PSI. If you don't calculate this in your electrical runs, your 120V outlet at the end of a long run might only deliver 112V, causing motors to overheat and LED drivers to fail prematurely.
The Physics of the Drop (and What It Actually Changes)
Voltage drop fundamentally changes how a load operates by starving it of its designed electrical pressure. In a real circuit, this manifests in three specific ways:
- Inductive Loads (Motors, Compressors, Pumps): AC motors are constant-power devices. If voltage drops, they draw more current to maintain their mechanical output. This excess current overheats the windings, degrades insulation, and drastically shortens the motor's lifespan.
- Resistive Loads (Heaters, Incandescent Lights): These simply output less power. A 1500W space heater on a severely dropped circuit might only pull 1200W, taking longer to heat a room and causing the breaker to run cooler than expected while the wire itself runs hot.
- Electronic Loads (LED Drivers, Appliances with Control Boards): Switch-mode power supplies will attempt to draw more current to compensate, but if the drop hits their brownout threshold (often around 104V to 110V for 120V nominal systems), the microcontroller will reset, flicker, or fail to start entirely.
The Math: A Worked 120V Branch Circuit Example
To figure out voltage drop, you need four variables: system voltage, one-way wire length, continuous current draw, and the specific AC resistance of your chosen wire gauge. We will use the AC resistance values from NEC Chapter 9, Table 9 for uncoated copper in PVC conduit at 75°C.
The Scenario: You are wiring a dedicated 120V circuit for a 15A continuous load (like a large window AC unit or a workshop dust collector). The panel is 120 feet away from the receptacle. Your target is to stay under a 3% drop, which equals 3.6V maximum.
The Formula:
VD = 2 × Current (A) × Resistance (Ω/1000ft) × (Length (ft) / 1000)
Testing 12 AWG Copper (1.98 Ω/kft):
VD = 2 × 15A × 1.98 × 0.120 = 7.12V drop (5.9%)
Verdict: Fails. The motor will run hot.
Testing 10 AWG Copper (1.20 Ω/kft):
VD = 2 × 15A × 1.20 × 0.120 = 4.32V drop (3.6%)
Verdict: Fails. Still over the 3% threshold.
Testing 8 AWG Copper (0.77 Ω/kft):
VD = 2 × 15A × 0.77 × 0.120 = 2.77V drop (2.3%)
Verdict: Passes. This is your concrete pick.
Where You Meet Voltage Drop in Practice
You don't need to run these calculations for a 15-foot run to a bedroom outlet. You only need to pull out the calculator when you hit specific physical layouts:
- Subpanel Feeders: Running a 60A or 100A feeder to a detached garage 150 feet away. Even at 240V, pushing 60A through 150 feet of 4 AWG aluminum will result in a massive drop. You often have to upsize feeders to 2 AWG or 1/0 AWG aluminum purely for voltage drop, not ampacity.
- Long Outdoor Branch Circuits: Landscape lighting transformers, well pumps, and remote gate motors frequently sit 200+ feet from the main panel.
- Low-Voltage DC Systems: In 12V or 24V solar and RV systems, voltage drop is brutal. A 2V drop on a 120V AC line is a minor 1.6% loss. A 2V drop on a 12V DC line is a catastrophic 16.6% loss that will prevent your lithium BMS from charging and your inverter from firing.
Decision Tree: Sizing Wire to Beat the 3% Rule
Use this decision path to terminate your wire sizing process with a concrete purchase. This assumes 120V single-phase AC, copper THHN in conduit, and a strict 3% maximum drop.
| IF your Load & Distance is... | THEN calculate with... | Calculated Drop | Concrete Pick (Buy This) |
|---|---|---|---|
| 15A at 50 ft or less | 14 AWG or 12 AWG | 14 AWG: 2.5% | 12 AWG (Standard practice for 20A breakers) |
| 15A at 120 ft | 12, 10, or 8 AWG | 12 AWG: 5.9% | 10 AWG: 3.6% | 8 AWG THHN (Pigtail at receptacle) |
| 20A at 100 ft | 10 or 8 AWG | 10 AWG: 4.8% | 8 AWG: 3.0% | 8 AWG THHN |
| 30A at 150 ft (e.g., RV outlet) | 8, 6, or 4 AWG | 8 AWG: 7.4% | 6 AWG: 4.6% | 4 AWG THHN (Yields 2.9% drop) |
Note: Always verify your final pick against NEC Table 310.16 for ampacity and terminal temperature ratings (60°C vs 75°C) before purchasing.
Common Confusions: Voltage Drop vs. Voltage Sag vs. Bad Connections
When a tool bogs down or lights dim, DIYers often blame "voltage drop" when the actual culprit is something else. Here is how to tell them apart at the workbench:
- Voltage Drop (The Wire's Fault): This is steady-state and proportional. If you measure 118V at the panel and 112V at the receptacle while the load is running, and the voltage instantly recovers to 118V when you turn the load off, you have wire voltage drop. The fix is thicker wire.
- Voltage Sag (The Utility's Fault): Also called a "dip." If your lights dim when the neighbor's AC kicks on, or if your panel itself reads 110V instead of 120V, the drop is happening on the utility's transformer or service drop. Upsizing your interior branch circuit wire will not fix this; you need to call the utility or install a buck-boost transformer.
- High-Resistance Fault (The Termination's Fault): If the wire is sized correctly but you are losing 10V, check your terminations. A loose set-screw on a breaker, a back-stabbed receptacle, or oxidized aluminum wire without antioxidant paste creates a localized high-resistance point. This generates massive, dangerous heat at a single junction rather than a gentle, distributed drop along the wire length. Use a thermal camera or an IR thermometer to find the hot spot.
FAQ: Quick Answers for the Workbench
Does upsizing the breaker fix voltage drop?
No. A breaker only protects the wire from overcurrent; it does not alter the physical resistance of the copper. Putting a 30A breaker on a 12 AWG wire that is suffering from voltage drop is a severe fire hazard, as it allows the wire to overheat without tripping. You must upsize the wire, not the breaker.
Can I use aluminum wire to save money on long runs?
Yes, aluminum is significantly cheaper for long feeder runs, but it has about 61% of the conductivity of copper. To achieve the same voltage drop, you must upsize aluminum by two AWG steps compared to copper (e.g., if the math calls for 6 AWG copper, use 4 AWG aluminum). Always apply Noalox antioxidant paste to aluminum terminations to prevent galvanic corrosion and subsequent high-resistance faults.
Do I need to calculate voltage drop for a 240V circuit?
Yes, but you have an inherent advantage. Because the voltage is doubled (240V instead of 120V), your 3% allowable drop budget is now 7.2V instead of 3.6V. Furthermore, a 240V load draws half the amperage of an equivalent wattage 120V load. Both factors mean 240V circuits can run much further on smaller wire before voltage drop becomes an issue.
When in doubt, default to the 3% rule and size up. The cost of an extra spool of 8 AWG THHN is trivial compared to the cost of replacing a burned-out compressor motor on a well pump sitting 150 feet from your subpanel. For complex or multi-leg installations, verify your final calculations using a trusted utility like the Southwire Voltage Drop Calculator before pulling wire.
