10 Common Calculation Mistakes on the Journeyman Exam

Most wrong answers on calculation questions are not math errors. They are setup errors: the right formula with the wrong input, or the right table with the wrong column. The good news is that these mistakes repeat, so you can learn to catch them. Here are ten of them, each with a short example showing the wrong answer and the right one.

1. Doubling the length twice in voltage drop

The single-phase formula VD = 2 × K × I × L / CM already includes the 2 for the return conductor. L is the one-way length.

120 V, 20 A, 12 AWG copper, 100 ft one way Right: 2 × 12.9 × 20 × 100 / 6,530 = 7.90 V Wrong (L = 200 ft): 2 × 12.9 × 20 × 200 / 6,530 = 15.80 V

If the question says "total conductor length" or "round trip", divide by 2 before you use the formula. Full walkthrough: how to calculate voltage drop.

2. Using 2 instead of 1.732 for three-phase

480 V three-phase, 100 A, 1/0 copper, 250 ft Right (1.732): 1.732 × 12.9 × 100 × 250 / 105,600 = 5.29 V Wrong (2): 2 × 12.9 × 100 × 250 / 105,600 = 6.11 V

The wrong answer is about 15.5% too high (2 / 1.732 = 1.155). The same mix-up happens in power and current formulas: three-phase current is P / (1.732 × E), single-phase is P / E.

3. Sizing motor conductors from the nameplate

Article 430 wants the table full-load current (Tables 430.248 and 430.250) for conductors and the short-circuit device, and the nameplate current for overloads.

10 hp, 460 V, three-phase: table FLC = 14 A, nameplate FLA = 13 A Right conductor ampacity: 14 × 1.25 = 17.5 A Wrong (nameplate): 13 × 1.25 = 16.25 A

Here both happen to lead to the same wire size, but on a test with tight answer choices the difference can be one size. More in motor circuit calculations.

4. Forgetting 125% for continuous loads

A continuous load runs for three hours or more. Branch circuits for continuous loads are sized at 125% of the continuous part (210.19(A) and 210.20(A)), and feeders follow the same idea in 215.2 and 215.3.

Lighting load of 32 A, continuous Right: 32 × 1.25 = 40 A minimum Wrong: 32 A

5. Leaving the ground out of conduit fill

Equipment grounding conductors count in conduit fill.

Three 8 AWG THHN + one 10 AWG THHN ground in EMT Right: 3 × 0.0366 + 0.0211 = 0.1309 sq in → 3/4 in EMT (0.213 at 40%) Wrong: 3 × 0.0366 = 0.1098 sq in → 1/2 in EMT (0.122 at 40%)

See conduit fill step by step for the full method.

6. Box fill: grounds, yokes and clamps

Three counting errors show up again and again: counting every ground wire separately (all together count as 1), counting a device yoke as 1 (it counts as 2), and forgetting internal clamps (1, based on the largest conductor).

4 in. square × 1-1/2 in. box (21.0 cu in), three 12/2 NM cables, one receptacle, internal clamps Right: (6 + 1 + 1 + 2) × 2.25 = 22.5 cu in → box too small Wrong (yoke as 1, no clamps): (6 + 1 + 1) × 2.25 = 18.0 cu in → looks fine

7. Ignoring the terminal temperature rating

THHN has a 90 °C rating, but most terminations are rated 60 °C or 75 °C. Under 110.14(C), the conductor cannot be used above the ampacity of the column that matches the terminals.

6 AWG THHN copper, no derating, 75 °C terminals Right: 65 A (75 °C column) Wrong: 75 A (90 °C column)

Use the 90 °C column only as the starting point for derating, then compare with the terminal column. See ampacity and derating.

8. Mixing up the derating order or the column

Temperature correction and bundling adjustment both multiply the same base value, and both must use the factors for the same insulation column.

Nine 10 AWG THHN, 40 °C ambient Right: 40 A × 0.91 × 0.70 = 25.48 A (90 °C base, 90 °C factor) Wrong: 35 A × 0.91 × 0.70 = 22.30 A (75 °C base with a 90 °C factor)

The order of the two multiplications does not change the result, but mixing columns does.

9. Applying the lighting demand factor to the whole load

In the dwelling standard calculation, the first 3,000 VA stays at 100%. Only the portion above it gets the 35% factor.

General lighting + small appliance + laundry = 10,500 VA Right: 3,000 + (7,500 × 0.35) = 5,625 VA Wrong: 10,500 × 0.35 = 3,675 VA

That error alone takes almost 2,000 VA out of the service calculation. Walkthrough: residential load calculation.

10. Rounding too early

Rounding in the middle of a problem can push a borderline answer across a line.

Find the minimum CM for 240 V, 50 A, 200 ft copper, 3% max Right: 2 × 12.9 × 50 × 200 / 7.2 = 35,833 cmil → 4 AWG (41,740) Rounding K to 13 first: 2 × 13 × 50 × 200 / 7.2 = 36,111 cmil → still 4 AWG, but on a closer problem the rounded value can change the answer

Keep full values on your calculator until the last step, then round once, in the direction the code requires: up for minimum sizes, down for maximum protection like the multi-motor feeder rule in 430.62.

A two-minute check before you pick an answer

Single-phase or three-phase? Is 1.732 in the formula only where it belongs?
Right current? Table FLC or nameplate, continuous or not.
Right length? One-way for voltage drop.
Right column? Insulation for derating, terminals for the final limit.
Everything counted? Grounds in conduit fill, yokes and clamps in box fill.
Rounded once, in the right direction?

The fastest way to fix these mistakes is to work problems where you can see every step. The voltage drop calculator shows each line of the calculation, and you can get 5 worked problems in your inbox.

Frequently asked questions

What is the most common voltage drop mistake?

Using round-trip length in the single-phase formula, which already includes a 2 for the return conductor. That doubles the answer.

Should I round during a calculation?

Keep full values until the last step, then round once in the direction the code requires: up for minimum sizes, down for maximum protection limits.

Which column of Table 310.16 should I use?

Start derating from the insulation temperature column, often 90 degrees C for THHN, then compare the result with the column that matches the terminal rating, usually 60 or 75 degrees C.

Why does my box fill answer come out too small?

Usually because the device yoke was counted as one instead of two, internal clamps were left out, or grounding conductors were counted separately instead of as one.

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Single-phase or three-phase, copper or aluminum, 14 AWG to 500 kcmil.