NEC Conduit Fill Practice Questions

Mixed sizes → you can't use Annex C. Look up each conductor's area in Table 5, add them all, then find the raceway whose 40% column (Table 4) fits.

1. Reaching for Annex C. It only works when every conductor is the same size and type.
2. Grabbing the wrong Table 5 row. It's sorted by insulation type first — match THW to THW.
3. Leaving a conductor out. Everything in the raceway counts toward fill.

All the same size and type → 40% fill for 3+ conductors. One area from Table 5 × the count, then find the smallest EMT whose 40% column (Table 4) beats it.

One conductor's area × the number of conductors = total, then step up trade sizes until the 40% column is bigger than your total.

1. Using the 53% / 31% column with more than 2 conductors.
2. Grabbing the wrong Table 5 row — match THHW to THHW.

Circular mils measure a round conductor's cross-section. You look them up in Table 8 — you don't calculate them on the exam.

Find the AWG size in Table 8 and read the circular-mils column. (A bigger AWG number = fewer cmils = smaller wire.)

1. Confusing Table 8 (properties / cmils) with Table 5 (insulated dimensions for fill).

Mixed sizes → look up each area in Table 5, add them all, then find the smallest EMT whose 40% column fits.

Add up every conductor's area (match the insulation type in Table 5), then find the raceway whose 40% column beats the total.

1. Leaving a conductor out — everything counts.
2. Mixing up the Table 5 insulation rows.

To size a raceway you first need the total cross-sectional area. Look up each conductor's area in Table 5, multiply by how many of each, and add.

Match each size to its THHN row in Table 5, multiply by the quantity, and sum. This total is what you compare to a raceway's 40% column to pick the conduit.

1. Reading the wrong insulation column.
2. Forgetting to multiply the six #10s by 6.

All the same size and type → 40% fill for 3+ conductors. One area from Table 5 × the count, then find the raceway whose 40% column (Table 4) fits.

One area × the number of wires = total, then step up trade sizes until the 40% column beats your total. Same size and type? Annex C gives the max count directly.

1. Using the 53% / 31% column with more than 2 conductors.
2. Not matching the insulation type in Table 5.
3. Grabbing the Annex C compact table (Table C.1(A)) — it is only for COMPACT conductors. This question says plain 4/0 THW (it does not say compact), so use the regular Table C.1, or the Table 5 area method shown above.

For more than 2 conductors, the raceway may be filled to 40% of its area (Ch. 9 Table 1).

Always round DOWN for conductor count: 20 conductors fit.

Three or more conductors fill to 40 % of the raceway. Add each conductor's area from Table 5, then find the smallest raceway whose 40 % column (Table 4) beats the total. All one size and type? Annex C gives the count directly — no math.

1. Using the 53 % / 31 % columns with 3 or more conductors.
2. Grabbing the wrong Table 5 insulation row.
3. Forgetting to count every conductor, including the equipment ground.

Ch. 9 Table 1 & Table 4 (40 % fill)

Three or more conductors fill to 40 % of the raceway. Add each conductor's area from Table 5, then find the smallest raceway whose 40 % column (Table 4) beats the total. All one size and type? Annex C gives the count directly — no math.

1. Using the 53 % / 31 % columns with 3 or more conductors.
2. Grabbing the wrong Table 5 insulation row.
3. Forgetting to count every conductor, including the equipment ground.

Read it straight from Annex C.

Annex C, Table C.1

Three or more conductors fill to 40 % of the raceway. Add each conductor's area from Table 5, then find the smallest raceway whose 40 % column (Table 4) beats the total. All one size and type? Annex C gives the count directly — no math.

1. Using the 53 % / 31 % columns with 3 or more conductors.
2. Grabbing the wrong Table 5 insulation row.
3. Forgetting to count every conductor, including the equipment ground.