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Conduit fill calculations

Conduit fill compares two areas: the total cross-sectional area of the conductors, and the share of the raceway the Code lets you use. Chapter 9, Table 1 sets that share at 53% for one conductor, 31% for two, and 40% for more than two.

The method

  1. Look up the area of one conductor, for its size and insulation type, in Chapter 9, Table 5.
  2. Multiply by the number of conductors to get the area they occupy.
  3. Look up the total area of the raceway, for its type and trade size, in Chapter 9, Table 4.
  4. Apply the Table 1 percentage for the number of conductors to get the usable area.
  5. If the conductors need more than the usable area, go up one trade size and check again.

The common mistake is using 40% for one or two conductors. One conductor is allowed 53% and two are allowed only 31%.

Worked problems

Problem 1. Five 4 AWG THHN conductors are installed in 1-inch EMT. What total cross-sectional area do the conductors occupy?

Answer: 0.4865 sq in

Look up the area of one 4 AWG THHN in Table 5 and multiply by 5: 0.0973 x 5 = 0.4865 sq in. With 5 conductors, Table 1 permits 40% of the raceway area. 1-inch EMT has 0.866 sq in total, so 0.3464 sq in is usable. The conductors occupy 56.2%: it does NOT fit; go up one trade size.

Reference: NEC Chapter 9, Table 1 (fill percentages), Table 4 (EMT) and Table 5 (areas)

Problem 2. Twelve 6 AWG THHN conductors are installed in 1-1/4-inch EMT. What total cross-sectional area do the conductors occupy?

Answer: 0.8712 sq in

Look up the area of one 6 AWG THHN in Table 5 and multiply by 12: 0.0726 x 12 = 0.8712 sq in. With 12 conductors, Table 1 permits 40% of the raceway area. 1-1/4-inch EMT has 1.495 sq in total, so 0.5980 sq in is usable. The conductors occupy 58.3%: it does NOT fit; go up one trade size.

Reference: NEC Chapter 9, Table 1 (fill percentages), Table 4 (EMT) and Table 5 (areas)

Problem 3. Six 4 AWG THHN conductors are installed in 1-inch EMT. What total cross-sectional area do the conductors occupy?

Answer: 0.5838 sq in

Look up the area of one 4 AWG THHN in Table 5 and multiply by 6: 0.0973 x 6 = 0.5838 sq in. With 6 conductors, Table 1 permits 40% of the raceway area. 1-inch EMT has 0.866 sq in total, so 0.3464 sq in is usable. The conductors occupy 67.4%: it does NOT fit; go up one trade size.

Reference: NEC Chapter 9, Table 1 (fill percentages), Table 4 (EMT) and Table 5 (areas)

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Common questions

What is the maximum conduit fill for more than two conductors?

40% of the cross-sectional area of the raceway, under Chapter 9, Table 1.

Why is the fill for two conductors lower than for three?

Chapter 9, Table 1 allows 31% for two conductors, 40% for more than two and 53% for one.