Crash course: Voltage, amperage, and resistance basics exam prep (part 2)

Why Part 2 Matters

Part 1 covered the definitions. Part 2 is where the math meets the meter. If you can't move fluently between volts, amps, and ohms under exam pressure, you'll lose points you should own. Every journeyman and master exam leans on these three.

The trick is not memorizing formulas. The trick is recognizing which formula the question is asking for before you reach for your calculator. Read the prompt, identify the two known values, then write the equation.

Ohm's Law, Rewritten for Speed

You know E = I x R. On the exam, you'll see it disguised. A question gives you wattage and voltage, then asks for current. Another gives resistance and current, asks for voltage drop. Train your eye to spot the two givens.

The Ohm's Law wheel covers twelve variations. Don't memorize all twelve. Memorize the three core forms and derive the rest:

• E = I x R (voltage from current and resistance)
• I = E / R (current from voltage and resistance)
• R = E / I (resistance from voltage and current)

Power formulas plug in the same way: P = E x I, P = I squared x R, P = E squared / R. Six equations, total. That's the whole toolbox for resistive-load problems.

Voltage Drop Problems

Voltage drop questions are exam favorites because they combine Ohm's Law with conductor properties. The NEC recommends a maximum 3% drop on branch circuits and 5% combined feeder plus branch (informational note in NEC 210.19(A) and 215.2(A)). Recommended, not required... but expect questions that treat it as a hard limit.

The standard single-phase formula: VD = (2 x K x I x D) / CM. K is 12.9 for copper, 21.2 for aluminum at 75 degrees C. D is one-way distance in feet. CM is the conductor circular mils from Chapter 9, Table 8.

Three-phase swaps the 2 for 1.732 (square root of 3). Memorize both. Examiners love to slip a three-phase scenario into a stack of single-phase problems to see if you're paying attention.

Field tip: when a question gives you distance in feet and asks for the minimum conductor size, solve for CM, then look up the next size up in Chapter 9, Table 8. Don't round down.

Series and Parallel Circuits

Series circuits: current is the same everywhere, voltages add, resistances add. Parallel circuits: voltage is the same across each branch, currents add, resistances combine reciprocally.

For two parallel resistors, use the product-over-sum shortcut: R total = (R1 x R2) / (R1 + R2). For three or more, use 1/Rt = 1/R1 + 1/R2 + 1/R3. Most exam questions stay at two or three resistors, so the shortcut covers you 80% of the time.

1. Identify whether the circuit is series, parallel, or combination.
2. Reduce parallel sections to a single equivalent resistance first.
3. Apply Ohm's Law to the simplified circuit.
4. Work back to find branch currents or voltage drops as needed.

Combination circuits trip people up. Slow down, redraw if you have time, and reduce one section at a time.

Power, Wattage, and Load Calculations

Resistance heating, incandescent lighting, and resistive loads follow P = E x I cleanly. Motors, transformers, and discharge lighting introduce power factor and efficiency, which the exam will flag explicitly. If the question doesn't mention power factor, assume 1.0.

For service and feeder calcs, NEC Article 220 governs. You'll convert between VA and amps constantly: I = VA / E for single-phase, I = VA / (E x 1.732) for three-phase. Keep those two conversions automatic.

Field tip: nameplate data on motors gives horsepower, not watts. Use NEC Table 430.248 for single-phase and 430.250 for three-phase motor FLA. Never calculate motor current from horsepower on an exam... use the table.

Common Exam Traps

Watch for unit mismatches. A question might give you milliamps, kilowatts, or megaohms. Convert to base units before plugging in. One missed decimal flips the answer.

Watch the question stem for "minimum" versus "maximum." A voltage drop question asking for minimum conductor size gets a different answer than one asking for maximum allowable distance, even with identical numbers.

• Convert all units to base (volts, amps, ohms, watts) before solving.
• Check whether the circuit is single-phase or three-phase.
• Read the last sentence twice. That's where the actual question lives.
• If two answers are close, recheck your decimal placement.

Part 3 will cover transformer calculations, motor circuit sizing, and the conductor ampacity tables that anchor most service calc questions. Drill the six core equations from this post until they're reflex. Everything else builds on them.