3.1 Ohm's Law, Power, and Basic Units

Why the basic units matter on an open-book exam

The ICC journeyman electrician exams are open book for R17, T17, and G17, but open book does not mean slow book. You may be able to look up a table or article rule, but you will not have time to relearn Ohm's Law during the exam. The calculation foundation is the language behind conductor ampacity, voltage drop, appliance loads, motor full-load current, transformer current, and service calculations.

Start every calculation by naming the quantity. Voltage is electrical pressure in volts (V). Current is flow in amperes (A). Resistance is opposition in ohms. Power can be real power in watts (W), apparent power in volt-amperes (VA), or reactive power in volt-amperes reactive (VAR). NEC load calculations commonly use VA because it works cleanly with voltage and current before the load's power factor is known.

Core formulas

Many exam references and field texts use E for voltage because electromotive force is the older term. You may also see V. Treat E and V as the same electrical quantity for these formulas.

Worked example: find current from watts

A 240-volt fixed electric heater is rated 4,800 watts. What current does it draw?

Known values: P = 4,800 W, E = 240 V. Use I = P / E.

I = 4,800 / 240 = 20 A.

That answer is the operating current of the heater, not automatically the overcurrent device size and not automatically the branch-circuit conductor ampacity. A later NEC rule may require a continuous-load adjustment or a specific fixed-heating rule. The arithmetic only gives the base current.

Worked example: find resistance

A test question says a circuit has 120 volts applied and 5 amperes flowing through a resistance load. Find the resistance.

R = E / I = 120 / 5 = 24 ohms.

A common distractor is 600, which comes from multiplying instead of dividing. Another distractor is 0.0417, which comes from I / E. If the circuit has normal building voltage and only a few amperes, a resistance of a few dozen ohms is sensible. A sanity check catches formula reversal.

Watts, VA, and power factor

For a pure resistance load, watts and VA are usually the same for exam arithmetic. For loads with coils, motors, electronic power supplies, or discharge lighting, apparent power and real power can differ. The relationship is:

W = VA x power factor

VA = W / power factor

If a 1,200-watt load has a power factor of 0.80, the apparent load is 1,200 / 0.80 = 1,500 VA. The current on a 120-volt circuit is 1,500 / 120 = 12.5 A, not 10 A. The exam trap is using watts directly when the question gives power factor and asks for current or conductor loading.

Unit conversion before calculation

Convert before you press equals. Kilowatts must become watts when used with volts and amperes. Milliamperes must become amperes. Horsepower questions often require a table rather than a simple watt conversion because NEC motor current rules use table full-load current values in many places. Do not substitute 746 watts per horsepower when the code rule tells you to use a motor table.

Common conversions:

Calculator discipline

Use a four-step pattern on every calculation question. First, copy the given values with units. Second, circle or mentally tag what the question asks for. Third, choose the formula and convert units. Fourth, estimate the expected size before selecting an answer.

For example, a 3,600 VA load on 120 volts must be about 30 amperes because 120 x 30 = 3,600. If your calculator says 0.033, you divided in the wrong direction. If it says 432,000, you multiplied when you should have divided. These errors are not knowledge problems; they are exam-process problems.

NEC navigation connection

The NEC is not organized as a formula book. It gives definitions, installation rules, tables, adjustment factors, demand factors, and special calculations. Electrical theory lets you move between those rules. A branch-circuit question may begin with a VA load, require current in amperes, then require a conductor size from an ampacity table, then require an overcurrent device rule. The formula is only the first link in the chain.

For ICC R17, T17, and G17, know which NEC edition your jurisdiction selected: R17 is based on the 2023 NEC, T17 on the 2020 NEC, and G17 on the 2017 NEC. The basic math does not change, but table numbers, article text, and local amendments may. ICC provides the exam; licensing remains with the state or local jurisdiction.

Exam traps

Watch for nameplate values that are already in amperes. If the question gives 16 A at 120 V and asks for minimum circuit ampacity, do not convert to watts unless a later step needs VA. Watch for two loads at different voltages. Add currents only when they are on the same system part, and add VA when combining unlike branch loads for service or feeder sizing. Watch for rounded answers. Carry at least one extra decimal place through the work, then round according to the question or the NEC rule being applied.

Finally, answer every multiple-choice question. ICC contractor and trades exams use four-option multiple choice with one correct answer and no guessing penalty. If time is short, eliminate answers that have impossible units or unreasonable magnitude.