Conduit Fill Calculator

Before sizing a raceway you need to know how much of its inside area the conductors may occupy.

Where to look
Chapter 9, Table 1 and the Notes to Tables that follow it:

• one conductor or cable: 53 percent;
• two: 31 percent;
• more than two: 40 percent.

Note 3: equipment grounding and bonding conductors are counted. Note 4: a nipple not over 24 in. long between boxes or cabinets may be filled to 60 percent, and the ampacity adjustment factors do not apply to it. Note 9: a multiconductor cable counts as one conductor. Table 4 then gives each raceway's area at each percentage, so you rarely multiply by hand.

Procedure

1. Count the conductors and cables, including EGCs (a multiconductor cable is one).
2. Is it a nipple of 24 in. or less? If so, 60 percent.
3. Otherwise read Table 1 for the count.
4. In Table 4, read the column for that percentage and the trade size.

Worked example
Four conductors in a 1-in. EMT run 40 ft long.

• More than two conductors, not a nipple: 40 percent.
• Table 4, EMT, 1 in., 40 percent column: 0.346 in.².

Most common mistake
Using 40 percent for two conductors, and forgetting the 60 percent nipple rule or applying it to a long run.

Conduit fill is worked in square inches, so every conductor's area, including its insulation, comes from one table.

Where to look
Chapter 9, Table 5. It is grouped by insulation type; find the group heading that lists your type, then the size row, and read the approximate area in in.² (the mm² column and the diameter columns sit beside it). Watch the groups: THHN, THWN and THWN-2 share the thinnest-insulation group; THW, THHW, TW and THW-2 are in a thicker group; XHHW has its own. Note 10 says the values are for concentric stranded conductors; compact conductors are in Table 5A. Bare conductors use Table 8.

Procedure

1. Identify the insulation type and size.
2. Find the right group in Table 5.
3. Read the in.² area; multiply by the number of conductors of that kind.
4. Add the groups together for a total.

Worked example
Area of three 6 AWG THWN conductors?

• THWN is in the THHN group; 6 AWG = 0.0507 in.².
• 3 × 0.0507 = 0.1521 in.².

Most common mistake
Reading the THW row for THWN (the names look alike but the THW insulation is thicker), or reading the diameter column instead of the area.

Most real raceways carry different sizes, so you total the areas and then find the smallest trade size big enough.

Where to look
Chapter 9: Table 5 for each conductor's area, Table 1 for the percentage, and Table 4 for the raceway. Table 4 has a separate section for each raceway type (EMT, RMC, PVC and so on), with columns already worked out for 40, 60, 53 and 31 percent plus the total area. Note 6 sends mixed sizes to Tables 5 and 4. Count the EGC (Note 3).

Procedure

1. Area of each conductor type × quantity, from Table 5.
2. Add them for the total.
3. Pick the percentage (normally 40 percent for more than two).
4. In Table 4, for the raceway type, go down that column to the first value at least the total.

Worked example
Three 4 AWG THHN and one 8 AWG THHN EGC in EMT.

• 3 × 0.0824 = 0.2472; plus 0.0366 = 0.2838 in.².
• EMT, 40 percent: ¾ in. = 0.213 (too small); 1 in. = 0.346.
• Answer: 1-in. EMT.

Most common mistake
Reading the wrong raceway section in Table 4 (RMC and EMT differ), reading the total-area column, or leaving the EGC out.