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NEC Calculations for the Journeyman Exam

Updated 2026-06-23

Five calculation types show up on almost every journeyman and master electrician exam: box fill, conduit fill, voltage drop, conductor ampacity, and conduit bending. Below, each one is worked step-by-step with real NEC 2023 values the way you'll do it in the exam room — then linked to a free Fieldwatt calculator so you can check your work while you study.

Exams are open-book on the NEC and typically allow a basic calculator, but not a phone. Learn the long-hand method here; use the Fieldwatt calculators on the job once you're licensed.

1. Box fill — NEC 314.16

What the exam tests: counting the conductors, grounds, clamps, and devices in a box correctly, then proving the box is large enough by cubic inches.

Problem: A single-gang device box has two 12-2 NM cables (four #12 insulated conductors + two #12 grounds), internal cable clamps, and one duplex receptacle. What box volume is required?

Step by step using the counts in 314.16(B):

  1. Insulated conductors: four #12 → 4 counts.
  2. All equipment grounding conductors together count as one → 1 count.
  3. One or more internal cable clamps count as one → 1.
  4. The receptacle yoke counts as two → 2.
  5. Total = 4 + 1 + 1 + 2 = 8 conductors, all sized on the largest conductor (#12 = 2.25 in³ per Table 314.16(B)).
  6. Required volume = 8 × 2.25 = 18.0 in³. Pick a box rated 18.0 in³ or larger.

Check your count with the free box fill calculator →

2. Conduit fill — NEC Chapter 9

What the exam tests: the 53 / 31 / 40 percent fill limits (Table 1) and looking up conductor and conduit areas (Tables 4 and 5).

Problem: How many #12 THHN conductors fit in ½″ EMT?

  1. #12 THHN area = 0.0133 in² (Chapter 9, Table 5).
  2. ½″ EMT internal area = 0.304 in² (Table 4).
  3. Three or more conductors → 40% fill limit (Table 1): 0.304 × 0.40 = 0.1216 in² usable.
  4. 0.1216 ÷ 0.0133 = 9.14 → 9 conductors (always round down).

So three #12 THHN in ½″ EMT is only 0.0399 in² — about 13% fill, well under the limit, with room for up to nine.

Run any combination with the free conduit fill calculator →

3. Voltage drop — NEC Chapter 9, Table 8

What the exam tests: the single-phase voltage-drop formula and the 3% branch-circuit recommendation, using the K constant and circular-mil areas.

VD = (2 × K × I × L) ÷ CM

Problem: A 120 V, 12 A load is 100 ft away on #12 copper. Is the voltage drop acceptable?

  1. K (copper) ≈ 12.9; CM of #12 = 6,530 cmil (Table 8).
  2. VD = (2 × 12.9 × 12 × 100) ÷ 6,530 = 30,960 ÷ 6,530 = 4.74 V.
  3. % drop = 4.74 ÷ 120 = 3.95% — over the 3% recommendation.
  4. Upsize to #10 (CM = 10,380): VD = 30,960 ÷ 10,380 = 2.98 V = 2.49%. Now it passes.

Voltage-drop limits are NEC recommendations (Informational Notes), not mandatory rules: 3% on a branch circuit, 5% total.

Size the run with the free voltage drop calculator →

4. Conductor ampacity — NEC Table 310.16

What the exam tests: reading Table 310.16 at the right temperature column, and sizing to the termination rating per 110.14(C).

Problem: What is the smallest copper conductor for a 40 A load with 75°C terminations?

  1. Use the 75°C copper column of Table 310.16.
  2. #10 Cu = 35 A → too small for 40 A.
  3. #8 Cu = 50 A → carries 40 A. Answer: #8 copper.
  4. Always size to the lowest-rated termination temperature (110.14(C)), and remember 240.4(D) caps overcurrent for small conductors (#14/#12/#10 Cu at 15/20/30 A).

Look up any load with the free wire ampacity calculator →

5. Conduit bending — offset multipliers

What the exam tests: the offset multiplier and shrink for a given bend angle.

Problem: Lay out a 4″ offset using 30° bends.

  1. 30° multiplier = 2.0; shrink = ¼″ per inch of offset.
  2. Distance between bends = 4 × 2.0 = 8″.
  3. Shrink = 4 × ¼″ = 1″ — add it to your measurement.
  4. Common multipliers: 22.5° = 2.6, 30° = 2.0, 45° = 1.4, 60° = 1.2.

Mark your bends with the free conduit bending calculator →

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Frequently asked questions

What NEC calculations are on the journeyman electrician exam?

Most journeyman exams test box fill (NEC 314.16), conduit/raceway fill (Chapter 9), voltage drop (Chapter 9, Table 8), conductor ampacity (Table 310.16), and conduit bending (offset multipliers and shrink). You are expected to look values up in the code book and run the arithmetic by hand under time pressure.

Can I use a calculator on the electrician exam?

Most journeyman and master exams are open-book (the NEC) and allow a basic, non-programmable calculator, but app- and phone-based calculators are not permitted in the exam room. Practice the long-hand method here, then use the Fieldwatt app on the job once you pass.

How is box fill calculated for the exam?

Add the volume allowances from NEC Table 314.16(B): one count per insulated conductor passing through or terminating, one count for all equipment grounding conductors combined, one count for one or more internal cable clamps, and two counts for each device yoke — all based on the largest conductor connected. Multiply the total count by that conductor's cubic-inch allowance.

What is the maximum conduit fill percentage?

Per NEC Chapter 9, Table 1: 53% for one conductor, 31% for two conductors, and 40% for three or more conductors in a raceway.

What voltage drop is allowed by the NEC?

Voltage drop limits are recommendations, not mandatory rules. NEC Informational Notes recommend a maximum of 3% on a branch circuit and 5% total (feeder plus branch) to the farthest outlet for reasonable efficiency.

Worked examples use NEC 2023 values for study purposes. Always verify against the current National Electrical Code, your local amendments, and your jurisdiction's exam rules. Fieldwatt does not replace an engineer of record.