Volts, Amps, Watts, and Ohms Made Simple

The water pipe picture

I teach apprentices with a water analogy because it just clicks. Voltage is the pressure pushing the water. Current, measured in amps, is how much water is actually flowing. Resistance, measured in ohms, is how narrow the pipe is. Watts are the work the water does when it comes out the other end.

Raise the pressure and more flows. Narrow the pipe and less flows. That relationship is the whole game.

Ohm's law

The core formula is voltage equals current times resistance, written V equals I times R. Rearrange it and you can find any one value when you know the other two. If a circuit is 120 volts across a 60 ohm load, current is 120 divided by 60, which is 2 amps.

• Find volts: multiply amps by ohms.
• Find amps: divide volts by ohms.
• Find ohms: divide volts by amps.

Watts and the power formula

Power in watts equals volts times amps. A 1500 watt heater on a 120 volt circuit draws 1500 divided by 120, which is 12.5 amps. That is why you cannot run two of them on one 15 amp circuit without tripping the breaker.

This single calculation is how you predict whether a load will overload a circuit before you ever plug it in.

Why this keeps you compliant

Overcurrent protection exists because too many amps in a wire makes heat, and heat starts fires. When you size a breaker and conductor to the calculated load, you are honoring the whole point of the code. Guessing is how people burn houses down.

Get comfortable with these four quantities and load calculations, voltage drop, and wire sizing all become the same simple arithmetic wearing different hats.