Calculation questions are where a prepared candidate can lose points without knowing anything wrong. The formula is right, the code section is right, and the answer is still off by a factor of two because a one-way length was used as a round trip, or a kilovolt-ampere value went into a formula that wanted volt-amperes. This guide is a five-step checking routine you run on every numeric question, with original worked examples that show what each step catches.
The five steps
- Restate the givens with units. Copy every number from the stem onto scratch paper with its unit and its role: "V = 240 V, single-phase," "L = 150 ft one-way," "load = 24 A continuous." If a number has no unit, find out what it is before you use it.
- Write the formula in symbols before numbers. "I = P / V" or "VD = 2 x K x I x L / CM." Writing symbols first exposes a missing factor (the 2 for a two-wire circuit, the 1.732 for three-phase) before it hides inside arithmetic.
- Check the units before you compute. Cancel units on paper the way you would in a physics class. If the units do not come out to what the question asked for, the setup is wrong; no amount of arithmetic will fix it.
- Compute, keeping precision until the end. Round only the final answer, and round in the direction the code or the question requires (conductor sizes round up to the next size, for example, not to the nearest).
- Test the result for plausibility. Compare it to something you can derive or to the conditions in the stem: a three-phase line current for a given kVA is smaller than the single-phase current for the same kVA by a known factor; a conductor size that is far outside what the stated circuit rating would normally call for deserves a second look. Plausibility is a signal to re-check the givens and the setup, not a substitute for the calculation or for the limits the question states. If the answer looks wrong, go back to Step 1 and find out why, then trust the corrected arithmetic.
Original worked examples
These are original practice examples written for this guide, not official exam questions. Where a constant such as K or a circular-mil area is needed, it is given in the problem, the way a well-written exam question gives it or points you to the table that holds it.
Example A: power, current, and resistance
Problem. A 240-volt, single-phase resistive heater is rated 4,800 watts. What current does it draw, and what is its resistance?
Step 1. V = 240 V, P = 4,800 W, single-phase, resistive (so watts and volt-amperes are the same). Step 2. I = P / V, then R = V / I. Step 3. W / V = A; V / A = ohms. Step 4. I = 4,800 / 240 = 20 A. R = 240 / 20 = 12 ohms. Step 5. Check with a different formula: P = I squared x R = 20 x 20 x 12 = 4,800 W. It matches, so both numbers are consistent.
Example B: voltage drop, and the factor of two
Problem. A two-wire, 120-volt branch circuit carries 16 amperes to a load 150 feet from the panel. The conductors are 12 AWG copper with a circular-mil area of 6,530 (given), and the question says to use K = 12.9 ohm-circular mils per foot for copper. Using VD = 2 x K x I x L / CM, what is the voltage drop and what percentage of the circuit voltage is it?
Step 1. V = 120 V, I = 16 A, L = 150 ft one-way, CM = 6,530, K = 12.9. Step 2. VD = 2 x K x I x L / CM. The 2 is there because current travels out and back on two conductors; L stays one-way. Step 3. (ohm-cmil/ft) x A x ft / cmil = ohm x A = V. Units check. Step 4. 2 x 12.9 = 25.8; 25.8 x 16 = 412.8; 412.8 x 150 = 61,920; 61,920 / 6,530 = 9.482 V, so 9.48 V. Percentage: 9.482 / 120 = 0.0790, so 7.9 percent. Step 5. Compare against the setup rather than a rule of thumb: a 150-foot one-way run of 12 AWG at 16 A is a long run for a small conductor, so a drop of several volts is consistent with the givens. The classic error here is doubling L as well as using the 2, which gives 18.96 V and 15.8 percent; the way to catch it is to re-read the stem ("150 feet from the panel" is one-way) and the formula (the 2 already accounts for the return), not to guess what a typical drop should be.
Note on what the code says: in the 2023 NEC, the general branch-circuit and feeder voltage-drop percentages appear as informational notes that recommend limits, while specific installations (for example, certain sensitive electronic equipment and fire pump circuits) have mandatory voltage-drop requirements in their own articles. A question that asks what the code requires is testing whether you can tell a recommendation from a rule, and whether the stem describes one of the installations that has its own mandatory rule. Confirm both in the edition your exam names.
Example C: continuous load and the 125 percent factor
Problem. A branch circuit supplies a continuous load of 24 amperes and no noncontinuous load. Using the branch-circuit overcurrent rule that the device rating shall be not less than the noncontinuous load plus 125 percent of the continuous load (210.20(A) in the 2023 NEC; confirm the number in your edition) and ignoring its 100 percent-rated assembly exception, what is the minimum standard device rating?
Step 1. 24 A, continuous. Step 2. Minimum OCPD rating = noncontinuous load + 1.25 x continuous load. Step 3. A x (dimensionless) = A. Step 4. 0 + 1.25 x 24 = 30 A. Step 5. 30 A is a standard overcurrent device rating and is larger than the load, as the rule requires. If the stem had said noncontinuous, the minimum would be 25 A (the smallest standard rating not below 24 A). Read the word. As in every example here, the conductors and equipment must also be rated for the device chosen; the stem sets that aside so the arithmetic can be checked on its own.
Example D: kVA to amperes, single-phase versus three-phase
Problem. A balanced three-phase load of 45 kVA is supplied at 208 volts line-to-line. What is the line current?
Step 1. S = 45 kVA = 45,000 VA (convert the prefix now, not later), V = 208 V line-to-line, balanced three-phase. Step 2. I = S / (1.732 x V), which applies to a balanced load with line-to-line voltage. Step 3. VA / V = A. Step 4. 1.732 x 208 = 360.256; 45,000 / 360.256 = 124.91 A, so 125 A when rounded at the end. Step 5. Check by a second route: the single-phase current for the same kVA at 208 V would be 45,000 / 208 = 216.35 A, and the balanced three-phase line current should be smaller by the factor 1.732: 216.35 / 1.732 = 124.9 A. It checks. The two common errors are forgetting the 1.732 (answer 216 A) and forgetting to convert kVA (answer 0.125 A). Note that the check works because both routes were derived from the same givens; it is a consistency check, not a rule of thumb.
Example E: plausibility alone can eliminate choices
Problem. A question asks for the minimum copper conductor size for a 20-ampere, 120-volt general-purpose branch circuit in a dwelling, with no voltage-drop, derating, or special-location conditions stated, and the choices are 4/0 AWG, 14 AWG, 12 AWG, and 6 AWG.
Step 5 does not answer this question; the ampacity table and the small-conductor overcurrent protection rule (240.4(D) in the 2023 NEC; confirm in your edition) do. What Step 5 does is tell you where to spend your time: with no conditions in the stem that would push the size up (long run, high ambient temperature, bundling), 4/0 AWG and 6 AWG are far from what the stated rating calls for, so read the rule and the table carefully for the two small sizes first. If the stem had included such a condition, the larger sizes would be back in play, and the plausibility check would be telling you to re-read the givens, not to cross anything out.
A checking table you can copy
| Check | Question to ask yourself | Catches |
|---|---|---|
| Role of each number | Is this length one-way or total? Is this a rating or a measured value? | Doubling errors, nameplate mix-ups |
| Prefixes | Did I convert kVA, kW, and kcmil before using them? | Answers off by 1,000 |
| Phase factor | Single-phase or three-phase? Did 1.732 appear exactly once? | Answers off by 1.732 |
| Percent factors | Continuous load, demand factor, derating: applied to the right quantity? | Factor applied to the whole instead of the part |
| Rounding direction | Does the code say next standard size up, or not less than? | Rounding down to a size that does not comply |
| Consistency check | Can I reach the same number by a second route from the same givens? Does the result fit the conditions the stem states? | Setup errors the other rows did not name |
Where to practice
The Load Calculations topic explains the load-calculation method (Article 220 in the 2023 NEC, Article 120 in the 2026 NEC) and has checkpoint questions; run this routine on each one. Then take the free electrician practice test with a calculator and whatever note-taking your exam allows. On review, write the step where each miss happened next to the question; if one step keeps showing up, that is the step to drill. Keep those notes in an error log so the pattern is visible after a few sets.
Sources and verification notes
Examples are original practice examples with all constants (K, circular mils, phase factor) supplied in the problem, not official exam questions. The distinction between informational notes and mandatory rules is the editorial team's paraphrase of NEC Section 90.5; the references to Sections 210.20(A) and 240.4(D) and to the voltage-drop informational notes are 2023-edition section numbers from editorial knowledge and were not re-read in the code text for this revision. No code table values are reproduced. Arithmetic was re-checked by calculator during this revision and is covered by the repository's guide-math check script.
- NFPA 70, National Electrical Code, Article 90.5 (mandatory and permissive rules, informational notes)Checked September 18, 2026
- NFPA 70, 2023 NEC, Section 210.20(A) (branch-circuit overcurrent device rating: noncontinuous load plus 125 percent of continuous load; 100 percent-rated assembly exception)Section number and rule as known to the editorial team for the 2023 edition; the code text was not re-read in the free-access viewer for this revision. Confirm in your edition.
Written by the ExamsLib editorial team. Practice examples in this guide are original and are not official exam questions. Exam rules change; the candidate bulletin from your licensing authority is the final word. Found an error? Contact us.
