3 × #12 THHN/THWN-2 in 3/4" EMT fills it 7.5 % of 0.533 in², against a 40 % limit — room for 13 more #12 THHN/THWN-2, 16 in all.
Calculator
Conduit Fill
Within the fill limit
7.5%
Wire Area
0.040 in²
Allowable
0.213 in² (40%)
Remaining
Room for 13 more #12 THHN/THWN-2
Limit 40 % of the raceway area, for more than two conductors.
Per NEC 2023 Chapter 9, Table 1, NEC 2023 Chapter 9, Table 4, NEC 2023 Chapter 9, Table 5
How it’s calculated
Awire = Σ (n × area of one conductor)
Aallowed = Araceway × limit
Fill % = Awire ÷ Araceway × 100
n
how many conductors of that size and insulation
Awire
the conductors' total cross-sectional area, insulation included, in square inches
Araceway
the raceway's internal area, in square inches
limit
53 % for one conductor, 31 % for two, 40 % for more than two, and 60 % for a raceway of 24 in. or less
Conductors of different sizes are simply added together: every one is counted at the area for its own size and insulation.
Where every conductor is the same size, the count may round up once the remainder reaches 0.8 of a conductor.
A size and insulation the area table does not list is refused rather than guessed, and the raceway is not sized while one is in the list.
Assumptions
The conductors are the table's figures for their insulation type. Where the real dimensions are known they may be used instead, and compact stranded conductors are smaller.
Conductors, not cables. A cable assembly is treated as a single conductor at the diameter of its outer jacket, which this calculator does not hold.
Every conductor in the raceway is counted, grounded and grounding conductors included.
The insulation names cover the types that share a column: the THHN column also carries THWN and THWN-2, and the XHHW column carries XHHW-2. RHH/RHW is the column for conductors without an outer covering; a covered one is larger.
USE-2 has no row of its own in the table. It is sized like RHH/RHW without an outer covering, which measured USE-2 conductors match within about 5 %.
The area table does not distinguish copper from aluminum, so the metal does not change the result here; compact conductors are smaller than the table's figures.
This is room in the raceway only. It does not check ampacity, the adjustment for more than three current-carrying conductors, the temperature limit, or the pulling tension and the jamming a three-conductor pull can suffer.
Percentages are rounded to 0.1 and areas to 0.001 in².
Worked example
A 100 A feeder in 1-1/4" EMT
Three #1 THHN/THWN-2 and one #6 equipment grounding conductor — four conductors, so the limit is 40 %.
Conductor areas, from the insulated-conductor table:3 × 0.1562 + 1 × 0.0507= 0.519 in²
The raceway's own area, and 40 % of it:1.496 × 0.40= 0.598 in² allowed
Fill percent0.519 ÷ 1.496 × 100= 34.7 %, which is within the limit
One size down, 1" of the same raceway:0.519 ÷ 0.864 × 100= 60.1 %, over the 40 % limit
Answer: 34.7 % — it fits, and 1-1/4" is the smallest size of this raceway that does.
The same 1/0 conductor takes 0.1855 in² as THHN/THWN-2 and 0.2223 in² as RHH/RHW. A tool that assumes one insulation type will disagree with one that asks.
Check: which insulation the other tool assumed.
The 40 % limit used for one or two conductors
One conductor is allowed 53 % and two are allowed 31 %, not 40 %. A tool that applies 40 % everywhere is generous at two conductors and strict at one.
Check: how many conductors the other tool thought were in the raceway.
Rounding the count
Some tools truncate the count, which is one conductor short whenever the remainder has reached 0.8 of a conductor and every conductor is the same size.
Check: whether the count rounds up near a whole conductor.
Table or actual dimensions
Compact stranded conductors, and cables measured over the jacket, are not the table's figures. A manufacturer's calculator using its own dimensions can fit more.
Check: whose dimensions the other tool used.
A nipple's 60 % used for a whole run
A raceway of 24 in. or less between enclosures is allowed 60 %. Applied to a whole run, that answer is far too generous.
Check: whether the other tool assumed a nipple.
Questions
How many #12 THHN/THWN-2 conductors fit in 3/4" EMT?
16, where all of them are that one size. In 1/2" EMT it is 9. Both counts are for conductors alone: a cable assembly is measured by its own outside diameter instead.
What is the 40 % rule?
40 % is the share of the raceway's cross-section that conductors may occupy when there are more than two of them. Two conductors are held to 31 % and a single conductor to 53 %, because of how a smaller number of round conductors packs. A raceway of 24 in. or less between two enclosures may go to 60 %.
Does the equipment grounding conductor count?
Yes. Every conductor in the raceway takes up room, grounded and grounding conductors included, and each one is counted at the area for its own size and insulation. Whether those conductors also force an ampacity adjustment is a separate question, and only the current-carrying ones count for that.
Why does a count sometimes round up?
Where every conductor in the conduit or tubing is the same size, the permitted number is the allowable area divided by one conductor's area, and a remainder of 0.8 or more of a conductor rounds up to the next whole one. #8 THHN/THWN-2 in 3/4" EMT works out at 5.83, so 6 are allowed, not 5.
Do I have to use the table's dimensions?
No. The table's figures are the largest a round stranded conductor of that type is likely to be; where the actual dimensions are known they may be used instead, and compact stranded conductors have their own, smaller figures. A cable counts at its own outside dimensions.
The same question for the box at each end of the run.
About this calculator
Written and maintained by Yusuf Can Samiloglu: MEP/HVAC design background, mechanical-engineering degree, not a licensed electrician.
How it’s checked: the calculation is the method of the Electric Master app, ported to the web and tested case by case against the app’s source code. Every table value was checked against the NEC 2023 values in at least two independent published sources.
The App Store version of Electric Master (2.2.0) predates the NEC corrections this page follows; until its next update, its result can differ from this page for some inputs. Version 2.2.0 sizes USE-2 with the THHN areas, about half the right figure, gives 1" to 2" FMC the EMT areas, never rounds a same-size count up, and lets a conductor it has no area for pass without counting it, so for USE-2 and for FMC it can pass a raceway this page calls full.
: The room left is counted one conductor at a time at the limit for the new total, so a single conductor no longer borrows the 53 % limit.
: First version, NEC 2023. Mixed conductors in one raceway, the fill limits for one, two and over two conductors, the 60 % nipple, Chapter 9 Note 7 for same-size counts, and the smallest raceway of the same type that passes. A size and insulation the area table does not list is refused rather than guessed.