Crash course: Voltage, amperage, and resistance basics step by step (part 1)

The three quantities you actually use

Voltage, amperage, and resistance run every circuit you touch. Voltage (V) is electrical pressure, measured in volts. Amperage (I) is current flow, measured in amps. Resistance (R) is opposition to flow, measured in ohms. Get these straight and the rest of the trade gets easier.

Ohm's Law ties them together: V = I x R. Rearrange as needed. If you know any two, you can solve for the third. This is not academic. You use it to size conductors, troubleshoot voltage drop, and verify that what you measured matches what the print says.

Power (P), measured in watts, comes next: P = V x I. On a 120V circuit drawing 12A, you are pushing 1,440 watts. That number drives breaker sizing, conductor ampacity per NEC 310.16, and load calculations under NEC Article 220.

Voltage: pressure, not flow

Voltage is potential difference between two points. No difference, no current, even if the conductor is energized to ground. That is why a bird sits on a 13.8kV line and walks away. One point of reference, no path, no flow.

Standard service voltages you will meet in the field: 120/240V single phase residential, 120/208V wye and 277/480V wye commercial, 120/240V high-leg delta on older industrial. NEC 220.5(A) sets the nominal voltages used for load calcs. Always verify with a meter before you trust a label.

Test your meter on a known live source before and after every measurement. A meter that fails silent will get you killed. NFPA 70E calls this a live-dead-live test, and it is not optional.

Amperage: what actually does the work and the damage

Current is the flow of electrons through the conductor. It is what heats the wire, trips the breaker, and stops a heart. As little as 100 mA across the chest can cause ventricular fibrillation. That is why GFCIs in NEC 210.8 trip at 4 to 6 mA, well below the danger threshold.

Amperage in a circuit is set by the load, not the source. A 200A panel does not push 200A into a 60W lamp. The lamp draws what it draws. The breaker only matters when the load tries to pull more than the conductor can safely carry, or when a fault drops resistance to near zero.

Common conductor ampacities at 75 degrees C, copper, per NEC 310.16:

• 14 AWG: 20A (limited to 15A by 240.4(D))
• 12 AWG: 25A (limited to 20A by 240.4(D))
• 10 AWG: 35A (limited to 30A by 240.4(D))
• 8 AWG: 50A
• 6 AWG: 65A

Resistance: the conductor is not a perfect path

Every conductor has resistance. Copper is low, aluminum higher, steel higher still. Resistance increases with length and decreases with cross-sectional area. That is why a long run on undersized wire drops voltage and overheats, and why NEC 210.19(A) Informational Note 4 recommends keeping branch circuit voltage drop under 3 percent.

Resistance also lives in connections. A loose wire nut, a corroded lug, a backstabbed receptacle. Any of these add ohms where you do not want them. That added resistance turns into heat under load, which is exactly how a connection becomes a fire.

If a device is warm and it should not be, kill the circuit and torque check every termination. NEC 110.14(D) requires terminations be tightened to the manufacturer's torque spec. A 4 dollar torque screwdriver pays for itself the first time it saves a callback.

Putting Ohm's Law to work in the field

You walk up to a 240V baseboard heater pulling 12.5A. What is the resistance of the element? R = V / I = 240 / 12.5 = 19.2 ohms. Megger reads 19 ohms cold, you are good. Megger reads 2 ohms, the element is shorted. Megger reads infinite, the element is open.

Voltage drop on a 120V, 20A circuit, 100 feet one way, 12 AWG copper: roughly 3.2V drop, or 2.7 percent. Bump to 10 AWG and you are at 1.7 percent. This is why long runs to detached garages and well pumps usually get upsized one or two gauges.

1. Identify the two known quantities (measure or read the nameplate).
2. Pick the right form of Ohm's Law: V=IR, I=V/R, or R=V/I.
3. Solve, then sanity check against the equipment rating.
4. If the math does not match the field reading, trust the meter and find out why.

What carries into part 2

Once these three quantities make sense, series and parallel circuits stop being a textbook exercise and start telling you why a shared neutral on a multiwire branch circuit behaves the way it does, why parallel feeders share load only when terminations are clean, and how AC adds reactance on top of resistance to give you impedance.

Part 2 will cover series versus parallel behavior, how AC differs from DC in practice, and how to read a voltage drop calculation without reaching for a calculator every time. Keep your meter on you and keep testing.