Crash course: Voltage, amperage, and resistance basics for apprentices (part 1)

What voltage, amperage, and resistance actually are

Voltage is electrical pressure. It's the potential difference that pushes electrons through a conductor, measured in volts (V). No pressure, no flow.

Amperage is the rate of flow. One amp equals one coulomb of charge passing a point per second. When you read 20A on a clamp meter, that's how much current is moving through the conductor right now.

Resistance opposes flow, measured in ohms. Every conductor, connection, and load has it. Copper has less than aluminum at the same size, which is why NEC 310.16 ampacity tables list separate columns for each.

Ohm's Law: the only formula you need on day one

V = I x R. Voltage equals current times resistance. Rearrange it as needed: I = V/R to find current, R = V/I to find resistance. Memorize this before you touch a single wire.

Power follows: P = V x I. A 120V circuit pulling 12A draws 1440W. That's why a 15A branch circuit at 120V tops out around 1800W theoretical, and why NEC 210.19(A) and 210.20(A) require continuous loads to be sized at 125 percent.

If a calculation gives you a weird number, check your units before you check your math. Milliamps versus amps is the most common rookie error on a meter.

Series vs parallel circuits in the field

In a series circuit, current is the same at every point and voltage drops across each load. Pull one device and the whole string dies. Old-school Christmas lights are the classic example. So is a switch loop: the switch and load are in series.

In parallel, voltage is the same across each branch and current divides. Every receptacle on a 20A small-appliance branch circuit is wired in parallel. That's why NEC 210.11(C)(1) lets you put multiple kitchen counter receptacles on the same circuit without each one starving the next.

• Series: same current, voltages add, one fails and all fail
• Parallel: same voltage, currents add, branches operate independently
• Most building wiring downstream of the panel is parallel
• Switches, fuses, and breakers are always wired in series with the load they protect

Voltage drop: where theory meets a long run

Resistance scales with conductor length. NEC 210.19(A) Informational Note No. 4 recommends keeping branch-circuit voltage drop under 3 percent, and total drop (feeder plus branch) under 5 percent. It's a recommendation, not a hard rule, but inspectors and engineers expect it.

The quick field formula for single-phase: VD = (2 x K x I x L) / CM. K is roughly 12.9 for copper and 21.2 for aluminum, I is current in amps, L is one-way length in feet, CM is conductor circular mils from NEC Chapter 9 Table 8. For three-phase, swap the 2 for 1.732.

Long runs to detached garages, well pumps, or rooftop units are where this bites. A 120V 20A circuit at 150 feet on #12 copper drops about 5.8 percent. Bump it to #10 and you're back inside spec.

Reading a meter without lying to yourself

A digital multimeter reads voltage in parallel and current in series. Stick the leads across a load to see voltage. To read current with the meter itself, you have to break the circuit, which is why most electricians use a clamp meter on the line conductor instead.

Resistance only reads accurately on de-energized circuits. Apply ohms to a live conductor and you'll either get garbage or smoke the meter. Lock out, verify dead per NFPA 70E, then test.

1. Set the meter to the expected range, or use auto-range
2. Test the meter on a known live source first (proves the meter works)
3. Take your reading on the dead circuit
4. Test on the known source again (proves the meter still works)

If your continuity beep doesn't sound when you short the leads, your meter battery is dying or a fuse is blown. Don't trust a single reading from a meter you haven't verified.

Why this matters for code work

Half of NEC Article 210 and Article 215 is voltage and current math wearing different hats. Branch-circuit ratings, feeder sizing, overcurrent protection, GFCI and AFCI thresholds, all of it traces back to V, I, and R.

A loose neutral raises resistance at the connection, which drops voltage to the load and dumps the difference as heat at the splice. That's how backstabbed receptacles start fires. Understanding the three quantities together is what lets you look at a scorched device and know what actually happened.

Part 2 covers AC versus DC, power factor, and how three-phase changes the math for commercial work.