IRC Tables E3904.6(1) to (10) and E3905.12, the NEC's Annex C and 314.16 for dwellings
The residential code reprints the NEC's conduit fill tables for ten raceways, and checked cell by cell five entries cannot be right. The IMC table prints 2/0 THHN twice and 4/0 not at all; flexible metal conduit lets 14 10 AWG conductors into a half inch where 12 AWG gets three. The box fill table, by contrast, is exact arithmetic on every row. This calculator sizes a conduit from the tables and a box from the volumes.
Conduit size, or box fill
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Count every conductor in the conduit, including neutrals and grounds, for conduit fill. For box fill, count each conductor that enters and ends or splices in the box once and each that passes through once; grounds, clamps, fixture studs and device yokes are entered separately.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
Fill against what the code allows
Worked example
Nine 12 AWG THHN conductors in EMT; four 4/0 THHN in IMC; then a switch box with two 12/2 cables, clamps and one device.
EMT, 9 x 12 AWG THHN. Table E3904.6(1) allows 9 in 1/2 in: 1/2 in. The same 1/2 in of Schedule 80 PVC allows 6.
IMC, 4 x 4/0 THHN. The published table has no 4/0 row; its second 2/0 row, which matches every other table's 4/0, allows 4 in 2 in.
Box conductors. Two cables bring 4 conductors at 12 AWG: 4 x 2.25 = 9.00 cu in.
Allowances. Clamps 2.25, one yoke double 4.50, grounds once 2.25: 18.00 cu in.
Box. A 4 x 1 1/4 in square box is 18.0 cu in: exactly enough.
Nine conductors in one raceway also derate each to 70 per cent of its ampacity, which matters more than the fill does for a 20 amp circuit. The wire size calculator applies that.
The formula
Conduit fill is a table lookup. Box fill is a sum:
box volume ≥ conductors × a + clamps a + fitting types × a + yokes × 2a + grounds (a, plus a/4 each beyond four)
a
cubic inches per conductor from Table E3905.12.2.1: 2.00 for 14 AWG, 2.25 for 12, 2.50 for 10, 3.00 for 8, 5.00 for 6
clamps
one allowance for all internal clamps, at the largest conductor
yokes
two allowances per device strap, at the largest conductor on that device
grounds
one allowance for up to four, a quarter more for each additional one
Table E3904.6
most conductors of one type and size per trade size, by raceway
The conduit tables come from conductor and raceway areas that the IRC does not print, so they cannot be recomputed here; the box table can be, and it checks out.
Why this page checks the tables
Conduit fill calculators reproduce a table and trust it. The NEC version is behind a login; the IRC reprints it for dwellings.
A fill table has to fall as the wire grows and rise as the conduit does. Checking that across all ten tables, as published for two states, found five cells that break it, including a missing 4/0 row in IMC.
The box table was checked the other way, against its own arithmetic, and every row is exact.
Five cells in the conduit tables that cannot be right
The code's rule is one sentence:
The maximum number of conductors installed in conduit or tubing shall be in accordance with Tables E3904.6(1) through E3904.6(10).
A fill table has two properties you can check without the conductor areas behind it: counts fall as the wire gets larger and rise as the conduit does. Checked across all ten tables as published for Utah and Virginia, these break it:
Table E3904.6(3): RHH, RHW, RHW-2 without outer covering, 10 AWG in 1/2 in FMC printed as 14, where 12 AWG is 3 and 8 AWG is 1.
Table E3904.6(3): 4/0 printed twice in the combined RHH, RHW, RHW-2, TW, THW, THHW, THW-2 group, allowing 3 and 2 in 2 in FMC.
Table E3904.6(4): THHN, THWN, THWN-2 has a second 2/0 row after the page break and no 4/0 row; the second row matches the 4/0 counts of the other tables.
Table E3904.6(5): RHH, RHW, RHW-2 8 AWG in 2 in FNMC-B printed as 1, where 6 AWG is 12.
Table E3904.6(5): Type letters printed RGW-2, which is not a conductor type, where the other tables print RHW-2.
The IMC one is the one that matters in a house. Someone looking up 4/0 THHN in IMC finds no row, and two rows for 2/0 that disagree. The calculator treats the second row as 4/0, because its counts match the 4/0 row of every other metal conduit table, and says so when you select it.
Where these errors were introduced could not be settled. Both states' texts come through one publisher, and the International Code Council's own print is not readable here. They may be transcription slips in that copy. They are still the text a reader of either adoption is given.
The same trade size holds different counts in different raceways
Trade size is a name, not a bore. The inside of 3/4 inch EMT, rigid PVC Schedule 80 and flexible metal conduit are different, and the tables say how different:
Maximum 12 AWG THHN conductors, IRC 2021 Tables E3904.6(1) to (10) as adopted in Utah
Pick the table for the raceway you are installing. Schedule 80 PVC has the thickest wall and the smallest bore for its trade size, so it holds the fewest; a pull sized from the EMT table can be over-full in Schedule 80.
Fill is not ampacity. A conduit that may hold 16 conductors still derates them: more than three current-carrying conductors take 80 per cent of their ampacity, and 7 to 9 take 70 per cent, under Table E3705.3.
Common residential answers
From Table E3904.6(1), EMT with THHN: 1/2 in holds 12 conductors of 14 AWG or 9 of 12 AWG; 3/4 in holds 22 of 14 AWG, 16 of 12 AWG or 10 of 10 AWG.
Schedule 80 PVC holds fewest at each trade size: 12 conductors of 12 AWG in 3/4 in, where EMT holds 16.
A single-gang switch box with two 12/2 cables, clamps and one device needs 18.00 cubic inches, which is the whole of a 4 by 1 1/4 in square box, and equal to the deepest device box in the table, 3 by 2 by 3 1/2 in.
Box fill is arithmetic, and the table agrees with it
A box must have at least as many cubic inches as the conductors and fittings in it need. Each conductor counts its size's allowance: 2.00 cubic inches for 14 AWG, 2.25 for 12, 2.50 for 10. Devices count double:
For each yoke or strap containing one or more devices or equipment, a double volume allowance in accordance with Table E3905.12.2.1 shall be made for each yoke or strap based on the largest conductor connected to a device(s) or equipment supported by that yoke or strap.
Internal clamps count once for the whole box, at the largest conductor:
Where one or more internal cable clamps, whether factory or field supplied, are present in the box, a single volume allowance in accordance with Table E3905.12.2.1 shall be made based on the largest conductor present in the box.
And grounds count once, for up to four:
Where up to four equipment grounding conductors or equipment bonding jumpers enter a box, a single volume allowance in accordance with Table E3905.12.2.1 shall be made based on the largest equipment grounding conductor or equipment bonding jumper entering the box. A one-quarter volume allowance shall be made for each additional equipment grounding conductor or equipment bonding jumper that enters the box,
The standard box table is that arithmetic. Every one of its 20 metal boxes lists, for each wire size, exactly its volume divided by the allowance, rounded down. A 4 by 1 1/2 inch square box is 21.0 cubic inches: 10 conductors of 14 AWG at 2.00, 9 of 12 AWG at 2.25. Its conductor counts assume nothing else in the box, so a device, a clamp and the grounds come off before you use them.
The sources, and what this does not cover
Read 2026-09-13.
Utah Residential Code 2021, Chapter 39 Power and Lighting Distribution, International Residential Code 2021 as adopted with amendments in Utah, read through UpCodes with the jurisdiction and edition stated on the page. Section E3904.6 and Tables E3904.6(1) to (10), bracketed to NEC Annex C; Section E3905.12 and Tables E3905.12.1 and E3905.12.2.1, bracketed to NEC 314.16.
Virginia Residential Code 2021, Chapter 39, Read through UpCodes to compare the tables with Utah's. All ten conduit tables and the metal box table are identical to Utah's, including the five anomalies.
Covered: THHN, THWN and THWN-2; XHH, XHHW and XHHW-2; TW, THHW, THW and THW-2, from 14 AWG to 4/0, in the ten raceways the IRC tabulates. Not covered: mixed sizes in one conduit, which need conductor areas the IRC does not print; data and communications cable; nonmetallic boxes, which are marked with their volume and entered as such. Where your jurisdiction adopts the NEC directly, its Chapter 9 and Annex C govern.
Frequently asked questions
How many 12 AWG THHN wires fit in 3/4 inch EMT?
16, from IRC Table E3904.6(1). In 1/2 inch EMT, 9. Other raceways differ at the same trade size: 3/4 inch Schedule 80 PVC holds 12 and Schedule 40 PVC 15.
How many 14 AWG wires in 1/2 inch conduit?
In EMT with THHN, 12. In Schedule 80 PVC, 9, because its wall is thicker at the same trade size.
What size conduit for four 4/0 wires?
For THHN in EMT or IMC, 2 inch, which holds 4. In Schedule 80 PVC, 2 inch holds 3, so it needs larger.
How do you calculate box fill?
Add 2.00 cubic inches per 14 AWG conductor, 2.25 per 12 AWG or 2.50 per 10 AWG, one allowance for internal clamps, two per device yoke, one per fixture stud type, and one for up to four grounds, all at the largest conductor. The box must hold at least that total.
Do ground wires count in box fill?
Yes, but only once for up to four equipment grounding conductors, at the largest one's allowance, with a quarter allowance for each beyond four.
Is there an error in the conduit fill tables?
As published for the Utah and Virginia adoptions of the 2021 IRC, five cells cannot be right, including a 4/0 THHN row in the IMC table printed as a second 2/0 row. They may be transcription errors in that publisher's copy; the ICC print could not be checked.
Does conduit fill include derating?
No. Fill limits how many conductors fit; ampacity adjustment for more than three current-carrying conductors is a separate rule, Table E3705.3.
Can I mix wire sizes in one conduit?
Yes, but the tables only cover conductors all of one size and type. Mixed fills need conductor areas and the 40 per cent fill rule from the NEC, which the IRC does not reproduce.
Check these numbers yourself
Utah Residential Code 2021, Chapter 39IRC 2021 as adopted in Utah, read 2026-09-13 through UpCodes with the jurisdiction and edition stated on the page. Section E3904.6, Tables E3904.6(1) to (10); Section E3905.12, Tables E3905.12.1 and E3905.12.2.1.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-13. Codes are amended locally; confirm against the edition your jurisdiction enforces.
Fill limits follow the 2021 International Residential Code as adopted in Utah, read on 13 September 2026, for conductors of one size and type. Your jurisdiction may adopt the NEC directly, a different edition or amendments. Five published cells are noted as anomalies and one is corrected in the calculator as described. Electrical work is commonly permit work.