The Physics of Resistance
When electricity flows through a copper wire, it is not a perfect, frictionless highway. The copper metal itself resists the flow of electrons. This resistance creates heat, and more importantly, it causes the electrical "pressure" (Voltage) to slowly drop as the electricity travels further down the wire.
This phenomenon is called Voltage Drop.
If you are wiring a light switch 10 feet away from the breaker panel, voltage drop is negligible. However, if you are burying a massive 300-foot underground cable to power a detached garage or a remote well pump, voltage drop becomes the most critical mathematical factor in the entire electrical design.
If a 120-Volt appliance (like a heavy-duty air compressor) is located 300 feet away, the wire resistance might cause the voltage to drop to 105 Volts by the time it reaches the motor. Running an electric motor on "low voltage" causes it to draw excessive amperage, overheat violently, and permanently burn out the motor coils in a matter of minutes.
The National Electrical Code (NEC) Standard
To prevent appliances from burning out and wires from overheating, the National Electrical Code (NEC) highly recommends strict limits on allowable voltage drop:
- Branch Circuits: Maximum of 3% voltage drop from the breaker panel to the final outlet.
- Total System: Maximum of 5% voltage drop from the main utility meter all the way to the final outlet.
If your calculated voltage drop exceeds 3%, you cannot use standard-sized wire. You must "Upsize" the wire (e.g., jump from a 12-gauge wire to a much thicker 10-gauge wire). A thicker wire has more physical copper, which reduces the friction, allowing the electricity to travel the long distance without losing its voltage pressure.
How to Calculate Voltage Drop
Calculating single-phase AC voltage drop requires four distinct variables: the length of the run, the amperage load, the physical cross-sectional area of the wire, and a constant value representing the electrical resistance of copper.
The Variables
- Length (L): The one-way distance of the wire run in feet.
- Current (I): The maximum Amperage the appliance will draw (e.g., 20 Amps).
- Constant (K): The specific resistivity of the metal. For standard Copper wire, this is universally accepted as 12.9 ohms-cmil/ft. (For Aluminum wire, it is 21.2).
- Circular Mils (CM): The exact cross-sectional area of the wire. You must look this up on an AWG chart. (e.g., 12 AWG wire = 6,530 CM; 10 AWG wire = 10,380 CM).
The Formula
The universal engineering formula for single-phase voltage drop is:
Voltage Drop = (2 × K × I × L) ÷ CM
(Note: We multiply by 2 because the electricity has to travel down the hot wire to the appliance, and then travel all the way back along the neutral wire to complete the circuit).
Once you calculate the absolute raw voltage drop, you divide it by the starting System Voltage (e.g., 120V) to find the Percentage of Drop.
Example Calculation
You want to run a heavy 20 Amp power saw at the end of your driveway, which is 100 feet away from the 120V breaker panel. You plan to use standard 12 AWG copper wire (which has an area of 6,530 CM).
- Constant:
2 × 12.9 = 25.8 - Multiply by Amps and Length:
25.8 × 20 Amps × 100 feet = 51,600 - Divide by Circular Mils:
51,600 ÷ 6,530 = 7.9 Volts dropped - Calculate Percentage:
7.9V ÷ 120V = 6.58% Voltage Drop
Result: A 6.58% drop violently violates the 3% NEC limit. The voltage reaching the saw will only be 112 Volts, and the saw will likely overheat.
The Fix: You must upsize the wire to thick 8 AWG (16,510 CM). Doing the math again (51,600 ÷ 16,510) yields a drop of only 3.1 Volts, or 2.5%, which is perfectly safe.