RMC (rigid metal conduit)

3-1/2" RMC conduit fill capacity

3-1/2" RMC has an internal area of 10.01 in². At the 40% limit that applies to more than two conductors, it holds up to 301 12 AWG THHN conductors.

Capacity

Maximum conductors by size and insulation

NEC 2023Every count in this table is transcribed from the 2023 NEC (NFPA 70, 2023). The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build.

Maximum number of same-size, same-insulation conductors in 3-1/2" RMC (10.01 in² internal). Counts above two use the 40% limit; a dash means not even one fits, or the published area table does not list that insulation at that size.
Conductor	THHN / THWN-2	XHHW-2	THW
14 AWG	400	288	191
12 AWG	301	221	154
10 AWG	189	164	120
8 AWG	109	91	72
6 AWG	78	67	55
4 AWG	48	49	41
3 AWG	41	41	35
2 AWG	34	34	30
1 AWG	25	26	21
1/0 AWG	21	21	18
2/0 AWG	18	18	15
3/0 AWG	14	15	12
4/0 AWG	12	12	10
250 kcmil	10	10	8
300 kcmil	8	8	7
350 kcmil	7	7	6
400 kcmil	6	6	6
500 kcmil	5	5	5
600 kcmil	4	4	4
700 kcmil	4	4	3
750 kcmil	3	3	3
800 kcmil	3	3	3
900 kcmil	3	3	2
1000 kcmil	2	2	2
1250 kcmil	-	1	1
1500 kcmil	-	1	1
1750 kcmil	-	1	1
2000 kcmil	-	1	1

Fill limits

NEC 2023Every area and limit below is transcribed from the 2023 NEC (NFPA 70, 2023). The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build.

Internal area	10.01 in²
One conductor	53% - 5.3053 in²
Two conductors	31% - 3.1031 in²
More than two	40% - 4.0040 in²

Fitting is not the same as finishing

These counts are the mathematical fill limit, not a promise the pull will go well. A raceway at 39% fill with four 90° bends and a hundred feet between pull points is legal and miserable. Experienced installers routinely size up a trade size on long or bendy runs, and the material cost difference is usually smaller than the labour cost of a fight.

Two other things this table does not model. Nipples 24 inches and shorter between enclosures are permitted a much higher fill. And a mixed set of conductor sizes has to be summed by area rather than looked up by count - the conduit fill calculator handles that.

Remember also that conductor count drives ampacity derating separately. Filling this conduit with current-carrying conductors will reduce what each one can carry - see the bundling adjustment table.

Source: NFPA 70, National Electrical Code, Chapter 9, Table 1; NFPA 70, National Electrical Code, Chapter 9, Table 4; NFPA 70, National Electrical Code, Chapter 9, Table 5. Published by NFPA.NEC 2023Transcribed from the 2023 NEC (NFPA 70, 2023). Confirm against your locally adopted edition and your authority having jurisdiction (AHJ).

Other sizes and types

3" RMC 4" RMC 3-1/2" EMT 3-1/2" PVC Sch 40 Conduit fill calculator

Every figure on this page comes from the 2023 NEC (NFPA 70, 2023). The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build.

Reference estimate - verify before you build. AmpSizer reports reference estimates transcribed from published tables in the 2023 NEC (NFPA 70, 2023), and from no other edition. It is not a substitute for the code adopted in your jurisdiction. The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build. Final conductor sizing, overcurrent protection, and installation must be determined by a licensed electrician and approved by your AHJ.

Jurisdictions adopt editions on their own schedule and frequently amend them, so the edition enforced where you work may not be the one cited here. Confirm which edition and which local amendments apply to your job before relying on any value on this page.