Crash course: Voltage, amperage, and resistance basics exam prep (part 5)

Voltage, amperage, resistance: the working trio

Every circuit you touch comes down to three values. Voltage pushes, amperage flows, resistance restricts. Get these wrong on the exam and you fail. Get them wrong in the field and someone gets hurt or something burns.

Voltage (E or V) is electrical pressure, measured in volts. Amperage (I) is the rate of current flow, measured in amps. Resistance (R) is the opposition to current, measured in ohms. The NEC does not define these terms in detail, but Article 100 sets the language you will see referenced throughout the code.

Memorize the units, memorize the symbols. Inspectors and exam writers use both interchangeably, and you need to read either without stopping to think.

Ohm's Law: the formula you cannot skip

E = I x R. Voltage equals current times resistance. Rearrange it: I = E / R, and R = E / I. That is the entire foundation. Every load calculation, every voltage drop check, every troubleshooting decision starts here.

On the exam you will get a problem like: a 240V circuit feeds a load with 12 ohms of resistance, what is the current? I = 240 / 12 = 20 amps. Done. Now size the conductor and overcurrent device per NEC 210.19 and 240.4.

• E = I x R (find voltage)
• I = E / R (find current)
• R = E / I (find resistance)
• P = E x I (find power, watts)

The power wheel and watts

Power (P) in watts ties the trio together. P = E x I is the basic form, but the power wheel gives you twelve combinations so you can solve for any unknown when two values are known. If you know watts and volts, I = P / E. If you know watts and amps, E = P / I.

This matters for sizing. A 1500W heater on 120V draws 12.5 amps. On 240V the same heater pulls 6.25 amps. Same wattage, half the current, smaller conductor allowed under NEC 310.16 ampacity tables. That is why commercial and industrial loads run higher voltages.

Field tip: when a customer asks why their new 240V appliance "uses less electricity," it does not. It uses less current. The watt-hours billed are the same. Explain it once, save the callback.

Voltage drop: where theory meets the jobsite

NEC 210.19(A) Informational Note No. 4 recommends branch circuit voltage drop not exceed 3 percent, with total drop on feeders and branch circuits combined not exceeding 5 percent. It is a recommendation, not a rule, but inspectors flag it and engineers spec it.

The formula for single-phase: VD = (2 x K x I x D) / CM. K is 12.9 for copper, 21.2 for aluminum. D is one-way distance in feet. CM is circular mils from Chapter 9, Table 8. Three-phase swaps the 2 for 1.732.

Practical example: a 20A load at 120V running 150 feet on #12 copper (6530 CM). VD = (2 x 12.9 x 20 x 150) / 6530 = 11.85 volts. That is just under 10 percent. Way too much. Bump to #10 or #8, or move the panel.

Series and parallel: read the circuit before you measure

In a series circuit, current is the same at every point, voltage divides across loads, and total resistance is the sum: Rt = R1 + R2 + R3. In a parallel circuit, voltage is the same across every branch, current divides, and total resistance is less than the smallest branch: 1/Rt = 1/R1 + 1/R2 + 1/R3.

Most building wiring is parallel. Receptacles on a 20A circuit are wired in parallel so each one sees full 120V. Series wiring shows up in switch loops, control circuits, and the old Christmas-tree-light scenarios the exam loves.

1. Identify if the circuit is series, parallel, or combination
2. Solve the simplest section first (often the parallel branch)
3. Reduce to an equivalent resistance
4. Apply Ohm's Law to find total current
5. Work back to find branch values

Troubleshooting with the trio

A meter reads voltage at the panel but not at the device. Resistance somewhere in between is too high, usually a loose terminal, corroded splice, or broken conductor. Voltage drop under load confirms it: no-load voltage looks fine, loaded voltage tanks.

Current readings tell you load. If a 15A breaker keeps tripping and your clamp meter shows 14.2A steady, the load is legitimate and you need a larger circuit per NEC 210.20(A). If it shows 22A, you have an overload, a short developing, or a miswired neutral sharing.

Field tip: always check voltage under load, not open circuit. A bad connection can read 120V with nothing drawing, then collapse to 90V the moment a motor starts. Open-circuit readings lie.

Master the trio, master the formulas, and the exam questions stop looking like math problems and start looking like jobsite calls you have already solved a hundred times.