How many wires fit in 3-1/2-inch Rigid (RMC) conduit?

A 3-1/2-inch Rigid (RMC) conduit has 10.010 square inches of internal area. With more than two conductors the NEC limits fill to 40%, or 4.004 in². That works out to 301 #12 THHN, 412 #14 THHN, 189 #10 THHN, or 109 #8 THHN conductors.

How many 12 AWG THHN wires fit in 3-1/2-inch Rigid (RMC)?

301 conductors. Each 12 AWG THHN wire is 0.0133 in² per NEC Chapter 9, Table 5, and the 40% fill area of 3-1/2-inch Rigid (RMC) is 4.004 in².

What is the maximum fill percentage for conduit?

NEC Chapter 9, Table 1 sets three limits: 53% for a single conductor, 31% for exactly two conductors, and 40% for three or more. The lower two-conductor figure exists because two round conductors pack poorly inside a round raceway.

Does the equipment grounding conductor count toward conduit fill?

Yes. Every conductor in the raceway counts toward the cross-sectional fill calculation, including equipment grounding conductors and neutrals. Grounding conductors do not count as current-carrying conductors for the separate NEC 310.15(C)(1) derating calculation, but they still occupy space.

3-1/2" Rigid (RMC) Conduit Fill Chart

14 AWG THHN

412

conductors max

12 AWG THHN

301

conductors max

10 AWG THHN

189

conductors max

Usable area at 40% fill: 4.004 in² · NEC Chapter 9, Tables 1, 4 and 5

About Rigid Metal Conduit (RMC)

Heavy-wall threaded steel conduit — the thickest and strongest raceway in the NEC. Used for service masts, hazardous locations, and anywhere physical damage is likely. Trade size 3-1/2" is covered by NEC Article 344, and its internal dimensions come from NEC Chapter 9, Table 4.

3-1/2" Rigid (RMC) — Internal Area & Fill Limits

Trade size	3-1/2"
Total internal area (100%)	10.010 in²
1 conductor (53%)	5.305 in²
2 conductors (31%)	3.103 in²
Over 2 conductors (40%)	4.004 in²

3-1/2" Rigid (RMC) — Max THHN / THWN-2 Conductors

Wire size	Area (in²)	Max conductors
14	0.0097	412
12	0.0133	301
10	0.0211	189
8	0.0366	109
6	0.0507	78
4	0.0824	48
3	0.0973	41
2	0.1158	34
1	0.1562	25
1/0	0.1855	21
2/0	0.2223	18
3/0	0.2679	14
4/0	0.3237	12
250 kcmil	0.3970	10
300 kcmil	0.4608	8
350 kcmil	0.5242	7
400 kcmil	0.5863	6
500 kcmil	0.7073	5

Conductor areas from NEC Chapter 9, Table 5 (THHN, THWN, THWN-2). Counts assume all conductors are the same size and insulation type, which is the condition NEC Annex C tables are built on. Mix sizes or insulations and you must add the individual areas by hand against the fill area above.

3-1/2" Rigid (RMC) — Max XHHW / XHHW-2 Conductors

Wire size	Area (in²)	Max conductors
14	0.0139	288
12	0.0181	221
10	0.0243	164
8	0.0437	91
6	0.0590	67
4	0.0814	49
3	0.0962	41
2	0.1146	34
1	0.1534	26
1/0	0.1825	21
2/0	0.2190	18
3/0	0.2642	15
4/0	0.3197	12
250 kcmil	0.3904	10
350 kcmil	0.5153	7
500 kcmil	0.6984	5

XHHW and XHHW-2 have no nylon jacket, so below 4 AWG they are physically larger than THHN and fewer fit. From 4 AWG up they are slightly smaller. Areas from NEC Chapter 9, Table 5.

How the 40% conduit fill rule works

Conduit fill is a pure area calculation. Take the internal area of the raceway from NEC Chapter 9, Table 4, multiply by the allowable percentage from Table 1, then divide by the area of one conductor from Table 5. Round down — you cannot install a fraction of a wire.

Max conductors = floor( conduit area × fill % ÷ conductor area )

For 3-1/2" Rigid (RMC) with 12 AWG THHN: floor( 10.010 × 0.40 ÷ 0.0133 ) = 301 conductors

The three percentages in NEC Chapter 9, Table 1 are not arbitrary. A single conductor gets 53% because it can be pulled straight down the middle. Exactly two conductors get only 31% because two circles inside a circle waste the most space. Three or more conductors nest efficiently again, so the limit rises to 40%.

Fill is only half the job. Once you have more than three current-carrying conductors in the same raceway, NEC 310.15(C)(1) also derates their ampacity — 80% for 4–6 conductors, 70% for 7–9, 50% for 10–20. A conduit can be legally filled and still leave your conductors undersized for the load. See NEC Table 310.15 for the full derating factors.

Nipples are the exception: a conduit or tubing length not exceeding 24 inches between boxes or enclosures may be filled to 60% per NEC Chapter 9, Table 1, Note 4 — and no ampacity derating applies to it. For 3-1/2" Rigid (RMC) that raises the usable area from 4.004 in² to 6.006 in².