Journeyman Electrician Exam Calculations: Formulas and Examples
The calculations on a journeyman exam come from a short list of formulas. Learn them, know which NEC table each one needs, and practice until you can do each in under two minutes. The 25 calculation questions in our free practice test cover every type below.
Ohm's law and power
- E = I × R (volts = amps × ohms); I = E ÷ R; R = E ÷ I.
- P = E × I for DC and single-phase resistive loads; P = I² × R; P = E² ÷ R.
- Three-phase: P = E × I × 1.732 (× power factor for real power). Current from power: I = P ÷ (E × 1.732).
Example. A 4,800 W, 240 V single-phase heater draws 4,800 ÷ 240 = 20 A. Its resistance is 240 ÷ 20 = 12 Ω.
Voltage drop
- Single-phase: VD = 2 × K × I × L ÷ CM. Three-phase: VD = 1.732 × K × I × L ÷ CM.
- K ≈ 12.9 for copper and 21.2 for aluminum (ohms per circular-mil foot, a common approximation); L is the one-way length in feet; CM is the conductor's area in circular mils from Chapter 9, Table 8.
- Recommended limits (informational notes, not requirements in most cases): 3% for a branch circuit, 5% for feeder plus branch circuit.
Example. 20 A at 120 V, 100 ft one way, 12 AWG copper (6,530 CM): VD = 2 × 12.9 × 20 × 100 ÷ 6,530 ≈ 7.9 V, about 6.6% — too much; a larger conductor is needed.
Box fill (NEC 314.16)
Count each conductor that enters the box, add allowances for internal clamps, support fittings, each device yoke (double volume) and equipment grounding conductors (one volume for up to four of them, plus a quarter volume for each additional one, since NEC 2020), then multiply by the volume per conductor for the size involved (from Table 314.16(B)). Compare with the box volume from Table 314.16(A) or the volume marked on the box.
Conduit fill (Chapter 9, Table 1)
Maximum fill: 53% for one conductor, 31% for two, 40% for three or more. Add the areas of the conductors (Chapter 9, Table 5) and compare with the allowed area of the raceway (Table 4); for same-size conductors, Annex C tables give the answer directly.
Motors (Article 430)
- Use the table full-load current (430.247–430.250), not the nameplate, to size conductors and short-circuit protection; use the nameplate for overloads.
- Branch-circuit conductors: 125% of the table FLC (430.22).
- Overloads: generally 125% of nameplate current for a service factor of 1.15 or more or a temperature rise of 40 °C or less, otherwise 115% (430.32).
- Short-circuit and ground-fault protection: percentage of FLC from Table 430.52, by device type.
Dwelling load calculations
General lighting at the unit load per square foot, small-appliance and laundry circuits, demand factors, then fixed appliances, dryer, range (Table 220.55 in NEC 2023) and the largest of heating or A/C. NEC 2026 moved these rules from Article 220 to Article 120: if your state has adopted NEC 2026, learn the new numbering.
Continuous loads
Loads expected to run 3 hours or more are continuous: size the overcurrent device and conductors at 125% of the continuous load (plus 100% of the non-continuous load).
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