Understanding Electrical Voltage Drop and Wire Gauge
Running a wire out to a detached garage or a distant solar array? If you don't calculate voltage drop, the equipment at the end of that line is going to starve for power, overheat, and eventually burn out.
What Causes Voltage Drop?
Think of electricity like water flowing through a hose. The longer the hose, the more friction the water encounters, reducing the pressure by the time it reaches the nozzle. In electrical terms, every wire has internal resistance. As current travels over long distances, some voltage is lost as heat. If a wire is too thin (too high a gauge) for the distance, the voltage drops drastically.
The Danger of Under-Voltage
When an appliance (like an air compressor or table saw) receives too little voltage, it tries to compensate by drawing more amperage. This excess current creates massive heat, which can melt insulation, trip breakers, or permanently destroy the motor.
The NEC 3% and 5% Rules
The National Electrical Code (NEC) provides strict recommendations to keep electrical systems safe and efficient:
- Branch Circuits: The voltage drop from the breaker panel to the final outlet should not exceed 3%.
- Total System: The combined drop from the main service entrance (feeders) through the branch circuit should not exceed 5%.
For a standard 120V US circuit, a 3% drop means the outlet must deliver at least 116.4 Volts under a full load.
How to Fix Voltage Drop
You cannot change the distance to your outbuilding, and you usually cannot change the required amperage of your tools. The only variable you can control is the Wire Gauge (AWG). By using a thicker wire (a lower AWG number), you reduce the resistance and allow the voltage to carry over long distances.
Don't guess your wire size.
Calculate exact voltage loss percentages and find out if you need to upsize your AWG wire using our free tool.
Open Voltage Drop Calculator