Written against the 2026 NEC and checked section by section, September 2026
Series and Parallel Practice Questions
Series shares current and parallel shares voltage, and everything else on this topic follows from those two sentences. These 25 items work the two configurations directly, the way voltage and current split inside each one, the two parallel shortcuts with the exact condition on each, and the check that catches an arithmetic slip in two seconds. The last few reach the places the code itself turns on this arithmetic: a paralleled set of conductors, protective devices joined at both ends, and the fused switch.
Pick an answer and the explanation opens with the section it comes from. These are original questions written for this site, not questions from the exam. Have your own code book open: the Texas exam is open book, and looking the section up is the skill being tested.
Read the series and parallel study page first if the topic is new to you.
1. In a series circuit, which quantity is the same in every element?
Show the answer
C. The current through each element
One path means one current, and it has to be the same at every point along the string. What divides is the voltage, and the drops across the elements add back up to the source. The voltage answer belongs to the other configuration, since equal voltage is what parallel branches share.
NEC 2026 series circuit relationships
2. Resistances of 5, 10 and 25 ohms are connected in series across a 120 volt source. What current flows?
Show the answer
A. 3 amperes
Series resistances add, so the string is 40 ohms and 120 divided by 40 is 3 amperes. Reaching for the 25 ohm element alone gives 4.8 amperes, which is the error of treating one element as though it saw the whole source. Check the answer the other way: the drops are 15, 30 and 75 volts, which add back to 120.
NEC 2026 series resistance, Ohm's law
3. A 6 ohm branch and a 12 ohm branch are connected in parallel. What is the combined resistance?
Show the answer
D. 4 ohms
For exactly two branches, multiply them and divide by their sum: 6 times 12 is 72, over 18, which is 4 ohms. The 9 ohm figure is the average of the two branches, and the check refuses it, since a parallel total always lands below the smallest branch. Adding them as though they were in series produces 18 ohms.
NEC 2026 parallel resistance
4. Three heating elements of 12, 18 and 30 ohms are wired in series across a 240 volt source. What current flows?
Show the answer
B. 4 amperes
The three add to 60 ohms and 240 over 60 is 4 amperes. Dividing 240 by the largest element alone gives 8 amperes and dividing by the smallest gives 20, which is where that option comes from. The 0.25 ampere answer is the reciprocal of the current, which is what happens when the division is written upside down.
NEC 2026 series resistance, Ohm's law
5. An element is added to a series string that is already carrying load. What happens to the elements that were already there?
Show the answer
A. Each one sees less current than before
Adding an element raises the total resistance, and there is only one current in the string, so that current falls and every element in the string feels it. Nothing in a series circuit is independent of anything else, which is the whole difference from a parallel set. That is also why a single open device stops the entire string.
NEC 2026 series circuit relationships
6. The shortcut that takes one branch value and divides it by the number of branches. When does it hold?
Show the answer
C. Only where all the branches are equal to each other
The rule has nothing to work with unless there is a single branch value to divide, so it holds only while the branches match. Two equal branches is the case that turns up most, and the answer there is simply half. Applied to unequal branches it turns into an average, which produces a number larger than the smallest branch and fails the check.
NEC 2026 parallel resistance
7. A contactor coil is wired through a stop button, a float switch and an overload contact in one string. What happens when the float switch opens?
Show the answer
B. The coil drops out, since the one current path has been broken
A control string is a series circuit, so every device in it has to be closed for the coil to pull in and any one of them can drop the load. That behavior is why the arrangement is chosen in the first place. It is also the troubleshooting shape: one open kills everything, and the fault could be any device in the string.
NEC 2026 series circuit relationships
8. A 240 volt circuit runs a long way and the conductors drop 7 volts under load. What voltage appears at the equipment?
Show the answer
D. 233 volts
Conductor resistance sits in series with the load, so the two drops add up to the source and the equipment gets whatever the run did not take. 240 less 7 is 233 volts. Adding the drop instead of subtracting it gives 247, and subtracting it twice gives 226, which double counts a figure that already covers both conductors.
NEC 2026 series circuit relationships
9. On that same series string of 5, 10 and 25 ohms across 120 volts, what voltage appears across the 25 ohm element?
Show the answer
D. 75 volts
The string carries 3 amperes, and 3 times 25 is 75 volts. Voltage divides in proportion to resistance, so the largest element in a string takes the largest share, which is the opposite of how current behaves in a parallel set. Handing the element the full source voltage is the error the 120 volt option collects.
NEC 2026 voltage division
10. Branches of 8, 12 and 24 ohms are connected in parallel. What is the total resistance?
Show the answer
B. 4 ohms
Add the reciprocals over a common denominator of 24: three parts plus two parts plus one part is six parts of 24, which is one quarter, so the total is 4 ohms. The 14.7 ohm option is the average of the three branches, which is the repair people reach for when the fractions look ugly. Every wrong answer here sits at or above the smallest branch, so the check kills them all without any arithmetic.
NEC 2026 parallel resistance
Ten down. If you want these timed and scored by content area, the practice exam runs thirty questions free, and the full bank of 178 plus the calculation ladder is $79 once.
11. A candidate works a parallel set of 10, 15 and 30 ohms and writes down 18 ohms. What tells you it is wrong before you rework it?
Show the answer
A. A parallel total is always smaller than the smallest branch
18 ohms sits above the 10 ohm branch, and a parallel total never gets there, so the answer is wrong before you look at the work. Reworking it over a denominator of 30 gives six parts of 30, which is one fifth, so the real answer is 5 ohms. The average of the three branches is a little over 18, which is where the number came from.
NEC 2026 parallel resistance
12. A 6 ohm element is in series with a pair of 8 ohm elements that are in parallel with each other, all across a 100 volt source. What current does the source deliver?
Show the answer
C. 10 amperes
Reduce the parallel pair first, since two equal 8 ohm branches give 4 ohms. That 4 ohms is in series with the 6 ohm element, so the circuit is 10 ohms and the source delivers 10 amperes. Treating the 8 ohm branch as though it saw the whole source gives 12.5 amperes, and adding all three values as one series string gives the other low figures.
NEC 2026 series-parallel reduction, Ohm's law
13. A branch is added to a set of parallel loads that are already running. What happens to the branches that were there before?
Show the answer
B. Each one carries exactly what it carried before
The voltage across the set has not moved, so nothing about the existing branches has changed and each draws what it drew before. What changes is the total: the combined resistance falls and the source current rises by whatever the new branch takes. That is why plugging in a second tool does not dim the first one.
NEC 2026 parallel circuit relationships
14. Product over sum is applied to three parallel branches of 12, 6 and 4 ohms and returns about 13.1 ohms. What went wrong?
Show the answer
D. That form works on exactly two branches and nothing more
Multiplying all three and dividing by their sum gives 288 over 22, which is larger than every branch in the circuit, so the check refuses it on sight. With three or more branches you either work two at a time and combine the result with the third, or go to reciprocals. Worked properly the set is 2 ohms.
NEC 2026 parallel resistance
15. Two 240 ohm heating elements are connected in parallel across 120 volts. What total power do they draw?
Show the answer
C. 120 watts
Each element takes 120 squared over 240, which is 60 watts, and parallel branches add, so the pair takes 120 watts. Reading 60 watts off one element and stopping is the common miss. Confirming it the other way, the pair combines to 120 ohms and 120 squared over 120 is the same 120 watts.
NEC 2026 parallel resistance, power formula
16. Values of 2, 4 and 6 ohms are wired first as a series string across 24 volts and then as parallel branches across 24 volts. Which value takes the largest share in each case?
Show the answer
A. The 6 ohm value takes the most voltage in series, and the 2 ohm value takes the most current in parallel
Voltage divides in proportion to resistance and current divides inversely with it, so the same three values produce opposite winners. In series the string is 12 ohms, draws 2 amperes and drops 4, 8 and 12 volts, so the 6 ohm element takes the most. In parallel the same three carry 12, 6 and 4 amperes, so the 2 ohm branch takes the most.
NEC 2026 voltage division, current division
17. Two identical loads sit on the same two conductors of a 240 volt circuit, and each one draws 9 amperes. What single resistance would the pair behave like?
Show the answer
A. About 13.3 ohms
Loads on the same two conductors are parallel branches, so the currents add: 9 plus 9 is 18 amperes, and 240 over 18 is about 13.3 ohms. Each load on its own is 240 over 9, about 26.7 ohms, and two equal branches halve that, which is the same answer by the other route. Adding the two branch resistances gives about 53.3 ohms, which the check refuses since it sits above both branches.
NEC 2026 parallel circuit relationships, Ohm's law
18. Why does the code put a list of conditions on a paralleled set of conductors rather than trusting the device ahead of them?
Show the answer
C. Current divides by impedance rather than by intention, so one conductor can carry more than its share while the device ahead reads only the total
Two conductors joined at both ends are parallel branches, and a parallel branch takes current in inverse proportion to its impedance. The overcurrent device sits on the common trunk and reads the sum, so it never learns that one conductor is running hot. That is why 310.10(G) is written as conditions making the paths identical to each other rather than as a single dimension anyone could measure.
NEC 2026 310.10(G)
19. A total of 16 amperes divides between a 4 ohm branch and a 12 ohm branch. How much of it flows in the 4 ohm branch?
Show the answer
D. 12 amperes
Current divides inversely with resistance, so the smaller branch takes the larger share. The pair combines to 3 ohms, 16 amperes through 3 ohms puts 48 volts across the set, and 48 over 4 is 12 amperes. The 4 ampere option is what the 12 ohm branch carries, which is the answer people write down after applying the voltage rule to a current question.
NEC 2026 current division
20. A 15 ohm element is in series with two 20 ohm branches that are in parallel with each other, all across a 125 volt source. What voltage appears across the 15 ohm element?
Show the answer
B. 75 volts
The two equal branches reduce to 10 ohms, so the circuit is 25 ohms and draws 5 amperes. Five amperes through 15 ohms is 75 volts, leaving 50 volts across the parallel section, and the two add back to 125. The 50 volt option is that parallel section rather than the element the question asked about.
NEC 2026 series-parallel reduction, voltage division
21. A parallel set has branches of 12, 6 and 4 ohms. Combining the 12 and the 6 first, then combining that result with the 4, what is the total?
Show the answer
C. 2 ohms
Product over sum on the first pair gives 72 over 18, which is 4 ohms, and that 4 ohms in parallel with the remaining 4 ohm branch is 2 ohms. Reciprocals confirm it, since a twelfth plus a sixth plus a quarter is six twelfths. Stopping after the first pair is what the 4 ohm option collects.
NEC 2026 parallel resistance
22. A 120 volt branch circuit carries 12 amperes, and the two conductors together have a resistance of 0.4 ohms. What voltage appears at the load?
Show the answer
B. 115.2 volts
The conductor resistance is in series with the load, so the drop is 12 times 0.4, which is 4.8 volts, and the load gets 120 less 4.8. The resistance given already covers both conductors, so doubling the drop a second time produces the 110.4 volt option. Adding the drop rather than subtracting it produces 124.8.
NEC 2026 series circuit relationships, Ohm's law
23. In the circuit of a 6 ohm element in series with two parallel 8 ohm branches across 100 volts, what current flows in each 8 ohm branch?
Show the answer
A. 5 amperes
The source delivers 10 amperes through a 10 ohm circuit, and the parallel section is 4 ohms, so 40 volts sits across it and each 8 ohm branch carries 5 amperes. Equal branches split the current evenly, and the two branch currents add back to the 10 amperes coming in. Handing one branch the whole source current is the error the 10 ampere option collects.
NEC 2026 series-parallel reduction, current division
24. What does the code say about connecting fuses or circuit breakers in parallel?
Show the answer
D. Permitted only where they are factory assembled in parallel and listed as a unit
Protective devices joined at both ends share current the way conductors do, so neither one carries the half of the total its rating was chosen against. NEC 240.8 closes the practice except where a manufacturer built and listed the pair as one device, and individual devices or combinations of them may not otherwise be paralleled. Matching ratings is not a permission, because matched ratings do not make matched impedances.
NEC 2026 240.8
25. A fused switch is proposed with two fuses in parallel in each pole. Which rule answers it?
Show the answer
A. The switch article refuses parallel fuses in a fused switch except as the overcurrent article permits
NEC 404.27 is one sentence and it points straight at 240.8, so a fused switch gets the same answer as the devices themselves. What survives is a factory assembled, listed parallel unit, which is not what a field installer is proposing. Either tab gets you there, which is unusual in this book and worth knowing on a clock.
NEC 2026 404.27, 240.8
One question a day keeps the habit, the practice exam runs the clock, and the tools page has the calculators and games.