Ohm's Law Practice Questions

C. 10 amperes

Divide the voltage by the resistance and 120 over 12 is 10 amperes. Multiplying the two instead gives 1440, which is the number of watts if you then read it as power, and it is the most common slip on a first pass. Check it backward before you move on, since 10 amperes times 12 ohms is 120 volts.

NEC 2026 Ohm's law

A. 12 ohms

Voltage over current, so 240 divided by 20 is 12 ohms. The 4800 figure is the product rather than the quotient and is watts, not ohms. Naming the unit you want before you touch the arithmetic is what keeps the two apart under time pressure.

NEC 2026 Ohm's law

D. 12.5 amperes

Power divided by voltage, so 1500 over 120 is 12.5 amperes. This form only holds because the load is resistive, and the stem tells you so by calling it a heater. Hand the same arithmetic to a motor and you have skipped the power factor that makes the answer wrong.

NEC 2026 Ohm's law, power formula

B. 9.6 ohms

Square the voltage and divide by the power, so 14,400 over 1500 is 9.6 ohms. You can also divide 120 by the 12.5 amperes the heater draws and land in the same place, which is the check worth doing. Two routes agreeing costs about ten seconds and catches a slipped decimal every time.

NEC 2026 Ohm's law, power formula

A. 960 watts

Square the current and multiply by the resistance, so 64 times 15 is 960 watts. Forgetting to square the current gives 120, which is the voltage across the resistance rather than the power in it, and it looks plausible enough to pass. Once you have the 120 volts, multiplying by 8 amperes gets you to 960 as well.

NEC 2026 power formula

C. 1664 watts

Square the voltage and divide by the resistance, so 43,264 over 26 is 1664 watts. The 43,264 answer is what you get by stopping one step early, and the 8 is the current in amperes rather than the power. Multiplying that 8 amperes by 208 volts arrives at 1664 the other way around.

NEC 2026 power formula

B. The power factor, since volts times amperes gives volt-amperes and those equal watts only where the current and voltage stay in step

On a resistive load the two numbers are the same, which is why the distinction feels academic right up to the first motor question. An item that means for you to convert hands you the power factor in the stem. An item that hands you horsepower and a voltage with no power factor is not an arithmetic item at all, it is a full-load current table lookup.

NEC 2026 Informative Annex D

D. The square root of three times line voltage times line current

The factor appears because you are crossing between line quantities and phase quantities, and it appears exactly once. The same power taken per phase is three times phase voltage times phase current, and the two forms agree. Using three where the square root of three belongs puts you high by about 1.73.

NEC 2026 three-phase power formula

C. 35.3 kilowatts

The square root of three times 480 times 50 is 41,568 volt-amperes, and multiplying by 0.85 gives 35,333 watts, or 35.3 kilowatts. The 41.6 figure is the apparent power with the power factor left off, which is the answer written for whoever stops one step early. Volt-amperes and watts differ here by exactly the 15 percent the power factor takes out.

NEC 2026 three-phase power formula

B. 10,808 volt-amperes

The square root of three times 208 times 30 works out to 10,808 volt-amperes. Checking it per phase gives three times 120 times 30, or 10,800, and the small gap is 208 being the rounded name for the square root of three times 120. Using three instead of its square root gives 18,720, and leaving the factor out entirely gives 6240.

NEC 2026 three-phase power formula

A. Line-to-line is the square root of three times line-to-neutral

Take 277 volts line-to-neutral, multiply by the square root of three and you get about 480, which is where the name 480Y/277 comes from. The line current on a wye is the phase current and does not change, so the factor lands on the voltage and nowhere else. Inverting the relationship gives 160 volts, which no system in the book is called.

NEC 2026 wye and delta relationships

D. On the current, since line current is the square root of three times phase current

A delta is the mirror image of a wye. Line voltage is the phase voltage across one winding and does not change, and the line current is the sum of two winding currents that sit 120 degrees apart, which brings the factor in. Naming which connection you have before reaching for the factor is the whole discipline here.

NEC 2026 wye and delta relationships

B. 171 amperes

Divide the volt-amperes by 480 times the square root of three, so 142,000 over 831 is about 171 amperes. Dividing by 480 alone gives 296, which is the answer you get by treating a three-phase feeder as single-phase. Dividing by three times 480 gives 99, which is the other half of the same confusion.

NEC 2026 Informative Annex D, Example D3(a)

A. The nominal system voltages the code lists, such as 120, 120/240, 208Y/120, 240, 480Y/277 and 480

The list is printed so that everyone calculating the same building arrives at the same amperes. Measured voltage varies through the day and would make one building's calculation disagree with the next. The section that carries the list opens Article 120 rather than sitting in the ampacity article, which is where candidates tend to look for it.

NEC 2026 120.5(A)

C. Drop it, since decimal fractions smaller than 0.5 may be dropped when rounding to the nearest whole ampere

The permission runs both ways from the halfway point, so 47.4 becomes 47 and 47.6 becomes 48. It is a permission rather than a command, so carrying the decimal is not itself wrong, it is just extra work. Rounding everything up is what people assume the code says and it is not what it says.

NEC 2026 120.5(B)

D. The two are treated as equivalent, and kilovolt-amperes are treated as equivalent to kilowatts

There is no phase angle on a direct-current circuit, so there is nothing for a power factor to describe and the two quantities coincide. The code says so outright rather than leaving it to be inferred, which is worth knowing because the same words do not hold on the alternating-current side. This is one of the few places the book bothers to state an arithmetic identity.

NEC 2026 120.5(D)

A. No, load calculations are not required to calculate continuous loads at 125 percent, although a 125 percent multiplier can be required for other reasons

The multiplier that everyone remembers belongs to sizing the conductor and the overcurrent device, not to totalling the load. A continuous load of 40 amperes is 40 amperes in the calculation and 50 amperes when you go to pick a breaker. Keeping the two questions apart is what this subsection is for.

NEC 2026 120.5(E)

C. A value assigned to a circuit or system for the purpose of conveniently naming its voltage class

The name is a label for a class, not a measurement, which is why 208 and 480 are used for systems whose actual voltage wanders either side of them. The definition says so and then adds that the operating voltage can differ from the nominal one. That is the whole reason the calculation rules name their own voltages instead of asking you to measure.

NEC 2026 Article 100

B. It is the greatest voltage between the given conductor and any other conductor of the circuit

On a grounded circuit the answer is the voltage between the conductor and the grounded point, which is what most people carry in their heads. The definition then covers the ungrounded case with a completely different rule, and that second half is what an item tests. So a 480 volt ungrounded delta has 480 volts to ground for code purposes even though nothing is tied to earth.

NEC 2026 Article 100

D. The voltage at which the circuit operates

One short section settles it and then adds that equipment has to be rated at least at the nominal voltage of the circuit it is connected to. So a 277 volt lighting circuit taken off a 480Y/277 system is a 277 volt circuit for every rule that asks. The distinction matters most on working space, where the voltage to ground picks the row in the table.

NEC 2026 110.4

C. 6 amperes

Series resistances add, so the total is 20 ohms and 120 over 20 is 6 amperes. There is one path, so that same 6 amperes flows through all three. The 30 ampere answer comes from using the 4 ohm resistance alone, which is the shape of the error when someone reads only the first value in the stem.

NEC 2026 series resistance

D. 6 ohms

Equal resistances in parallel divide by how many there are, so two 12 ohm elements give 6 ohms. Parallel resistance is always smaller than the smallest branch, which is the sanity check that rules out the 24 ohm and 12 ohm answers without any arithmetic. Adding them is what a series circuit does.

NEC 2026 parallel resistance

A. 12 ohms

Multiply the two and divide by their sum, so 600 over 50 is 12 ohms. That is less than the 20 ohm branch, which is what a parallel result always has to be. Averaging the two gives 25 and adding them gives 50, and both of those fail the sanity check before you look any further.

NEC 2026 parallel resistance

B. 6 amperes

Three equal branches give 20 ohms, and 120 over 20 is 6 amperes. You can also work it per branch, since each 60 ohm element draws 2 amperes at 120 volts and three of those add to 6. The parallel branches share the voltage and the currents add, which is the opposite of what a series circuit shares.

NEC 2026 parallel resistance

D. 80 volts

The total is 24 ohms, so the current is 5 amperes, and 5 times 16 is 80 volts. The remaining 40 volts sits across the 8 ohm element, and the two add back to the source, which is the check. A series circuit divides the voltage in proportion to resistance, so the larger element always takes the larger share.

NEC 2026 series resistance