Journeyman electrician exam calculations

Updated November 8, 2026 · Written for the 2023 NEC

Examples below follow the 2023 NEC. Section numbers can move between editions, so check the edition your exam uses.

Ohm's law and power

Ohm's law ties voltage (E), current (I) and resistance (R) together. Power (P) in watts is voltage times current.

Example: a 120-volt circuit feeds a 24-ohm resistive load. I = 120 ÷ 24 = 5 A. The load uses P = 120 × 5 = 600 W.

Try it with our free Ohm's law calculator.

Single-phase and three-phase current

For a single-phase load, current is watts divided by volts. For a balanced three-phase load, divide by volts times the square root of 3 (1.732).

Example: a 15 kW, 208-volt, three-phase heater draws I = 15,000 ÷ (208 × 1.732) = 41.6 A.

Transformer current

Transformer full-load current uses the same formula with kVA instead of watts.

Example (single-phase): a 50 kVA transformer with a 240-volt secondary: I = 50,000 ÷ 240 = 208.3 A.

Example (three-phase): a 75 kVA, 480 to 208Y/120-volt transformer. Primary: 75,000 ÷ (480 × 1.732) = 90.2 A. Secondary: 75,000 ÷ (208 × 1.732) = 208.2 A.

Voltage drop

Voltage drop uses the conductor's resistivity constant (K), the current, the one-way length in feet and the conductor's area in circular mils (CM), from NEC Chapter 9, Table 8.

Use K = 12.9 for copper and K = 21.2 for aluminum unless the question gives you a value. The NEC recommends, in informational notes, keeping branch-circuit voltage drop to 3% and the combined feeder and branch circuit to 5%.

Example (single-phase): 120 volts, 20 A, 100 ft one way, 12 AWG copper (6,530 CM). VD = 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.9 V, which is 6.6% of 120 V. Too much. 10 AWG gives 4.1%, still over 3%. 8 AWG (16,510 CM) gives 3.1 V, or 2.6%.

Example (three-phase): 208 volts, 40 A, 150 ft, 8 AWG copper. VD = 1.732 × 12.9 × 40 × 150 ÷ 16,510 = 8.1 V, or 3.9%. Upsizing to 6 AWG (26,240 CM) gives 5.1 V, or 2.5%.

Check your own numbers with our free voltage drop calculator.

Continuous loads at 125%

A continuous load runs for three hours or more. Overcurrent devices and conductors for continuous loads are sized at 125% of the continuous load plus 100% of the noncontinuous load (210.19(A)(1), 210.20(A), 215.2(A)(1) and 215.3).

Example: a branch circuit supplies 16 A of continuous lighting. 16 × 1.25 = 20 A minimum overcurrent device.

Dwelling general lighting load

For dwelling units, the general lighting load is 3 volt-amperes per square foot of livable floor area (220.41). Open porches, garages and unused spaces not adaptable for future use are left out.

Example: a 2,400 sq ft house: 2,400 × 3 = 7,200 VA general lighting load, before demand factors.

Box fill

Each conductor and item in a box uses a volume allowance from Table 314.16(B)(1), based on conductor size. Count them using 314.16(B), then multiply.

How to count:

• Each conductor that enters and ends or splices in the box: 1.
• Internal cable clamps, all together: 1, based on the largest conductor.
• Each device yoke (receptacle or switch): 2, based on the largest conductor connected to it.
• Equipment grounding conductors: 1 for up to four, plus 1/4 for each one beyond four.

Example: a box has four 12 AWG insulated conductors, two 12 AWG equipment grounding conductors, internal clamps and one receptacle. Count: 4 + 1 + 1 + 2 = 8. Volume: 8 × 2.25 = 18.0 in³ minimum.

Motor branch-circuit conductors

Size motor conductors from the full-load current in the NEC motor tables (Tables 430.247 to 430.250), not the nameplate (430.6(A)(1)). Branch-circuit conductors for a single motor must be rated at least 125% of that full-load current (430.22).

Example: a three-phase motor has a table full-load current of 28 A. 28 × 1.25 = 35 A minimum conductor ampacity.

Parallel resistance

Example: 12, 12 and 6 ohms in parallel: 1 ÷ (1/12 + 1/12 + 1/6) = 3 ohms. For two resistors, the shortcut is (R1 × R2) ÷ (R1 + R2).

Conduit offsets

Distance between bends equals the offset depth times the multiplier for the bend angle. Shrink is how much the offset shortens the run.

Example: a 6 in offset with 30° bends: 6 × 2 = 12 in between bends, and 6 × 1/4 = 1.5 in of shrink.

Frequently asked questions