Flexible Conduit Fill (FMC, LFMC, LFNC)

An apprentice pulls three 12 AWG THHN conductors through 20 feet of 3/8-inch flexible metal conduit and fails the rough-in inspection. The inspector points to NEC Table 348.22, where the limit is two conductors. That is the moment when the difference between flexible conduit fill and rigid conduit fill becomes real money.

Flexible conduit fill is not a single percentage. The NEC 2023 treats flexible metal conduit (FMC), liquidtight flexible metal conduit (LFMC), and liquidtight flexible nonmetallic conduit (LFNC) under Chapter 9 Table 4, with a separate rule for 3/8-inch FMC in 348.22. Use the exact code sections and numbers to defend a pull or pass an exam.

Percentages Stay the Same, Areas Do Not

Flexible conduit fill uses the same Chapter 9 Table 1 percentages as rigid conduit: 40 percent for three or more conductors, 31 for two, 53 for one, and 60 for a nipple 24 inches or less. The difference is in Table 4. FMC, LFMC, and LFNC each have their own inside diameters and cross-sectional areas, and those areas are smaller than EMT or RMC of the same trade size. A 3/4-inch FMC has a smaller 40 percent area than a 3/4-inch EMT because the flexible wall takes up interior space. Do not substitute the EMT area for the flex area in a fill calculation. The NEC 2023 Chapter 9 Table 4 is the only source for those numbers.

Liquidtight Types Follow the Same Rule

LFMC and LFNC follow the same Table 4 structure. Article 350 and Article 356 direct you to Chapter 9 for fill. The 40/31/53/60 percentages apply identically. What changes is the raceway area you plug into the math. An engineer who uses a generic 1-inch area for a 1-inch LFMC will overfill the raceway because the liquidtight jacket reduces the inside diameter compared to EMT.

The failure case is using the wrong Table 4 column. A 1-inch LFMC has a different 40 percent area than a 1-inch PVC Schedule 40. Check the exact row for the material and trade size before you calculate. The NEC 2023 Informative Annex C gives pre-calculated maximum conductor counts for most flex types, which avoids the error entirely.

3/8-inch flexible metal conduit is the exception that catches most electricians. NEC 2023 348.22 states that 3/8-inch FMC is permitted only for motor leads not exceeding 4 feet in length and for electric-discharge or LED luminaires. It is not a general-purpose raceway. The fill limit comes from Table 348.22, not from Chapter 9 Table 4. The maximum number of 18 AWG conductors is 4, 16 AWG is 3, 14 AWG is 3, 12 AWG is 2, and 10 AWG is 1. The total area of the 3/8-inch FMC from Table 4 is 0.191 square inches, but the 40 percent area of 0.076 square inches does not apply. Table 348.22 overrides it.

An apprentice who asks "3/8 flex how many wires" needs the Table 348.22 numbers. Two 12 AWG THHN conductors are the limit. Three 12 AWG conductors fail. The Table 4 40 percent area would allow three 12 AWG THHN conductors based on the 0.076-square-inch 40 percent area and the 0.0133-square-inch area of a 12 AWG THHN from Table 5, but Table 348.22 says no. The code is explicit: the table is the limit, not the calculation.

The practical consequence is that 3/8-inch flex is rarely the right choice for more than a motor whip or a luminaire connection. If you need three 12 AWG conductors, step up to 1/2-inch FMC and use the Chapter 9 Table 4 areas. The 3/8-inch size is a niche product that serves a narrow set of permitted uses.

Liquidtight conduit fill follows the same percentages as FMC, but the raceway area in Table 4 is smaller. A 1/2-inch LFMC has an internal diameter of approximately 0.630 inches, giving a total area of about 0.312 square inches. The 40 percent area is 0.125 square inches. That same trade size in EMT has a 40 percent area of 0.122 square inches, which is close but not identical. The difference becomes significant at larger sizes. A 1-inch LFMC has a 40 percent area of 0.238 square inches. A 1-inch EMT has 0.203 square inches. The liquidtight jacket changes the internal diameter, and you must use the LFMC row in Table 4, not the EMT row.

LFNC follows the same rule. NEC Article 356 sends you to Chapter 9 Table 4. The 40 percent area for a 1-inch LFNC is not the same as for a 1-inch LFMC because the nonmetallic material has a different wall thickness. Check the exact row. The NEC 2023 Informative Annex C includes pre-calculated counts for both LFMC and LFNC. For a 1-inch LFMC, Annex C shows 15 14 AWG THHN conductors, 11 12 AWG THHN, and 8 10 AWG THHN. Those numbers already account for the smaller area, so they are faster and more reliable than recalculating from Table 4.

For 1/2-inch FMC, the 40 percent area is 0.083 square inches. A 12 AWG THHN has an area of 0.0133 square inches from Table 5. Dividing 0.083 by 0.0133 gives 6.2, so the maximum is six conductors. Annex C confirms six 12 AWG THHN conductors in 1/2-inch FMC. For 3/4-inch FMC, the 40 percent area is 0.129 square inches, allowing nine 12 AWG THHN conductors. Annex C shows nine. For 1-inch FMC, the 40 percent area is 0.203 square inches, allowing 15 12 AWG THHN conductors. Annex C shows 15.

LFMC numbers are slightly different because the areas are smaller. For 1/2-inch LFMC, the 40 percent area is 0.125 square inches. A 12 AWG THHN has 0.0133 square inches. The calculation gives 9.4, so nine conductors. Annex C for 1/2-inch LFMC shows nine 12 AWG THHN. For 3/4-inch LFMC, the 40 percent area is 0.180 square inches, allowing 13 12 AWG THHN. Annex C shows 13. For 1-inch LFMC, the 40 percent area is 0.238 square inches, allowing 17 12 AWG THHN. Annex C shows 17.

LFNC numbers follow the same method. For 1/2-inch LFNC, the 40 percent area is 0.119 square inches, allowing eight 12 AWG THHN. Annex C shows eight. For 3/4-inch LFNC, 0.175 square inches allows 13 12 AWG THHN. For 1-inch LFNC, 0.225 square inches allows 16 12 AWG THHN. The Annex C values are the authoritative answer. A hand calculation that rounds differently will produce a number one higher or lower, and the inspector will use Annex C.

The 60 percent fill for a nipple 24 inches or less applies to flexible conduit whips. A factory-made flex whip that is 18 inches long qualifies as a nipple. The 60 percent fill allows more conductors than 40 percent. For a 3/4-inch FMC, the 60 percent area is 0.193 square inches, compared to the 40 percent area of 0.129 square inches. That is 14 12 AWG THHN conductors at 60 percent versus 9 at 40 percent.

The nipple rule does not apply to a flexible whip that is longer than 24 inches. A 30-inch whip uses the 40 percent limit. It does not apply to a whip that is 24 inches or less but part of a longer raceway. The code defines the nipple as the raceway itself, not the gap between boxes. If you have a 24-inch flex whip between a disconnect switch and a motor, it qualifies. If you have a 24-inch flex whip in the middle of a 50-foot run of EMT, the EMT does not get the 60 percent fill, and the flex whip does not extend the 60 percent to the rest of the run.

The most common mistake is applying the 60 percent rule to a short flex whip that is longer than 24 inches. Measure the whip. If it is 25 inches, the limit is 40 percent. The NEC 2023 Chapter 9 Table 1 note is explicit: the 60 percent applies to a nipple not exceeding 24 inches. No rounding, no estimating. Measure the actual length.

XHHW conductors have a larger diameter than THHN of the same AWG size. A 12 AWG XHHW has an area of 0.0170 square inches from Table 5, compared to 0.0133 for THHN. In a 3/4-inch LFNC with a 40 percent area of 0.175 square inches, the calculation gives 10.2, so 10 conductors. Annex C shows 10 12 AWG XHHW for 3/4-inch LFNC. For 1/2-inch LFNC, the 40 percent area of 0.119 square inches allows 7 12 AWG XHHW. Annex C shows 7.

Bare ground conductors use Table 8. A 12 AWG bare conductor has an area of 0.0097 square inches. In a 1/2-inch FMC whip at 60 percent fill with a 60 percent area of 0.124 square inches, the calculation allows 12 bare 12 AWG grounds. The same whip at 40 percent fill with 0.083 square inches allows 8. The bare ground counts toward fill, and it uses its own area from Table 8, not the insulated conductor area from Table 5.

A mix of THHN and XHHW in the same raceway requires summing the individual areas. A 3/4-inch LFMC with a 40 percent area of 0.180 square inches can hold, for example, three 14 AWG THHN at 0.0097 square inches each and two 12 AWG XHHW at 0.0170 square inches each. The total is 0.0291 plus 0.0340 equals 0.0631 square inches, well under the 0.180 limit. Annex C does not cover mixed sizes, so you must calculate. The sum of the areas from Table 5 for each conductor type is the only correct method.

The 60 percent nipple rule applies to a raceway 24 inches or less in length. That includes flexible metal conduit, liquidtight flexible metal conduit, and liquidtight flexible nonmetallic conduit. It does not apply to a stub-up from a panelboard that is 12 inches long if the stub-up is part of a longer raceway. The stub-up is not a separate nipple. The rule applies only when the entire raceway between two boxes or fittings is 24 inches or less.

A 24-inch flex whip between a junction box and a motor qualifies. A 24-inch flex whip between a junction box and a lighting fixture qualifies. A 24-inch flex whip between two junction boxes that are 24 inches apart qualifies. A 24-inch flex whip between a panelboard and a device where the panelboard has a 12-inch stub-up and the whip is 12 inches does not qualify as a 24-inch nipple because the stub-up is part of the panelboard's raceway system, not a separate nipple.

The NEC 2023 Chapter 9 Table 1 note is the sole authority. Read the note. It says "nipple" and it says "24 inches." A whip that is 24 inches and 1/16 inch is not a nipple. A whip that is 24 inches but contains a splice is still a nipple if the splice is in a fitting. The length is the raceway, not the wire. Measure the raceway.

The NEC 2023 Chapter 9 Table 4 areas for FMC, LFMC, and LFNC are minimums. A manufacturer's actual inside diameter may be larger or smaller. The code requires the area from Table 4 for design, not the actual area. An inspector will use the Table 4 area, not a manufacturer spec sheet. If the actual area is smaller than Table 4, the installation is overfilled. If the actual area is larger, the Table 4 limit still applies. The Table 4 values are the legal limit. Use them.

The single thing that most often goes wrong is assuming that a 3/8-inch flex whip uses the Chapter 9 Table 4 40 percent fill. It does not. Table 348.22 is the master. Check it before you pull, not after the inspector arrives.