What Is a Volt? The Practical Guide to Electrical Potential Difference

The Physics of the Push (And What It Is Not)

To understand voltage on the bench or the jobsite, you have to separate the concept of "pressure" from the concept of "flow." According to the National Institute of Standards and Technology (NIST), the volt is the SI derived unit for electric potential. It is not the energy itself, but the difference in electrical charge between two specific points that creates the potential for energy to move.

The most reliable way to visualize this is the water hose analogy: voltage is the water pressure at the spigot, current (amps) is the volume of water flowing through the hose, and resistance (ohms) is the diameter of the hose itself. If the spigot is turned off, the pressure (voltage) is still present, but the flow (current) is zero.

What people commonly confuse it with: Beginners often conflate volts with amps or watts, assuming higher voltage always means higher danger or more power. This is false. A static shock from a winter doorknob can generate 20,000 volts, but it delivers only micro-amps of current, which startles you but causes no harm. Conversely, a standard 12V car battery operates at a "low" voltage but can deliver 600 amps to a starter motor—enough current to instantly melt a steel wrench if dropped across the terminals. Voltage is just the push; current is what does the physical damage.

Worked Example: Calculating Voltage Drop in a 12V DC Circuit

What does voltage actually change in a real installation? It dictates wire sizing. In any real-world circuit, voltage is not perfectly conserved from the source to the load; it drops due to the inherent resistance of the copper wire. If the voltage at the load drops too low, equipment will malfunction.

Let us look at a practical scenario: You are wiring a 12V, 5A LED light bar on a work truck, powered by a battery located 20 feet away. You decide to use 14 AWG copper wire.

• Wire Resistance: 14 AWG copper has a resistance of roughly 2.525 ohms per 1,000 feet.
• Total Wire Length: 20 feet out, 20 feet back to ground = 40 feet total.
• Circuit Resistance: (40 / 1000) × 2.525 = 0.101 ohms.
• Voltage Drop (V = I × R): 5A × 0.101Ω = 0.505V Drop.

At the light bar, you will measure 11.495V instead of the battery's 12V. Most 12V LED drivers can handle this easily. But what if you tried to save money and used 18 AWG wire (6.385 ohms/1,000ft)? Your voltage drop would spike to 1.27V, leaving only 10.73V at the load. This marginal voltage will cause the LED driver to flicker, draw more current to compensate for the lower voltage, and potentially overheat the 18 AWG wire. As Fluke's electrical testing guidelines emphasize, measuring voltage at the source is useless if you do not account for the drop across the conductors.

Where You Meet Voltage in Practice

Every electrical system has a "nominal" voltage (the name on the label) and an "actual" measured voltage. Knowing the acceptable tolerance ranges prevents you from chasing ghosts with your multimeter or unnecessarily replacing perfectly good power supplies.

Measuring Voltage: Open Circuit vs. Under Load

One of the most common traps for DIYers is measuring a battery or power supply with nothing connected to it (an open circuit) and assuming the system is healthy. A 12V lead-acid battery might read a perfect 12.6V on your multimeter while sitting on the bench. But the moment you connect a 50A winch motor, the voltage might plunge to 9V.

This happens because every power source has internal resistance. When high current flows, voltage drops across that internal resistance before it ever reaches your external terminals. Always measure voltage under load to see what the equipment is actually experiencing.