NECwire

Conduit fill calculator

Fill · 3/4 in EMT22.5%WITHIN 40 %0.1197 in² of conductors (Table 5) in 0.533 in² of raceway (Table 4); NEC 2017 Chapter 9 Table 1 (identical to 2011) allows 40 % for more than two conductors (0.213 in²)

ID 0.824 in

Nipple ≤ 24 in
Conductors
9 × 12 AWG THHN / THWN / THWN-20.0133 in² each, 0.130 in dia (NEC 2017 Chapter 9 Table 5, identical to 2011)
0.1197 in²
Smallest EMT that holds this bundleat 40 %
1/2 in
Maximum 12 AWG THHN in 3/4 in EMTNote 7: round up when the decimal is 0.8 or more
16
12121212121212121222.5 %FILLEDALLOWED 40 %

Areas from NEC Chapter 9 Table 4 (raceway) and Table 5 / 5A (conductors), 2017 text verified identical to 2011; fill limits from Table 1 with Notes 3, 4, 7 and 9. Equipment grounding conductors count at their actual size (Note 3). More than three current-carrying conductors also derates ampacity (310.15(C)(1)); see the wire size calculator.

Nothing typed leaves the browser; the inputs live only in the page address.

Choose the raceway and trade size, list the conductors by insulation and size, and read the percent of the cross-section they take against what Chapter 9 Table 1 allows: 53 % for one conductor, 31 % for two, 40 % for more than two, 60 % for a nipple no longer than 24 in. The figure draws the conduit at its Table 4 internal diameter with the conductors at their Table 5 diameters. The tally also names the smallest trade size that holds the bundle and, for a single conductor type, the maximum count with the Note 7 rounding rule.

Worked examples

Nine 12 AWG THHN in ¾ in EMT
9 × 0.0133 = 0.1197 in² against 0.213 in² allowed (40 % of 0.533). 22.5 % of the cross-section: fits. Sixteen 12 AWG THHN is the maximum (0.213 ÷ 0.0133 = 16.0). But nine current-carrying conductors also cost 30 % of the ampacity (310.15(C)(1)). Open this run
Three 8 AWG plus one 10 AWG THHN in 1 in EMT
3 × 0.0366 + 0.0211 = 0.1309 in² against 0.346 allowed: 15.2 %, and ¾ in would hold it too. Open this run
200 A feeder: three 3/0 and one 6 AWG THHN in 2 in PVC 40
3 × 0.2679 + 0.0507 = 0.8544 in² against 1.316 allowed (40 % of 3.291): 26.0 %. 2 in is also the smallest Schedule 40 size that takes the bundle. Open this run
One 12 AWG THHN in ½ in EMT
A single conductor may fill 53 %: 0.0133 in² against 0.161 allowed, 4.4 %. The one- and two-conductor percentages exist because a single conductor jams differently from a bundle. Open this run

How it is worked out

Total conductor area = Σ (count × Table 5 approximate area) for each insulation and size; compact conductors use Table 5A. Bare equipment grounding conductors are counted at their Table 8 area (Note 3); the calculator does not list bare wire, so enter it as the nearest insulated type or add its area by hand.

Allowed area = Table 4 total internal area for the raceway and trade size × the Table 1 percentage for the number of conductors (53 / 31 / 40 %), or 60 % for a nipple of 24 in or less (Note 4).

Percent fill = total conductor area ÷ Table 4 total area × 100. The minimum trade size is the first row of the raceway’s Table 4 whose allowed area is at least the total. Maximum count of one conductor type = allowed area ÷ one conductor’s area, rounded up when the decimal is 0.8 or more (Note 7).

The figures behind it

NEC Chapter 9 Table 1 and Notes 3, 4, 7, 9
53 % one conductor, 31 % two, 40 % over two; nipples ≤ 24 in at 60 %; 0.8 rounding rule; grounding conductors counted at actual size; multiconductor cables as one conductor. 2017 text, identical to 2011. Source
NEC Chapter 9 Table 4, Dimensions and Percent Area of Conduit and Tubing
Internal diameter and 100 / 53 / 31 / 40 / 60 % areas for EMT, ENT, FMC, IMC, LFNC-A, LFNC-B, LFMC, RMC, PVC 80, PVC 40 and HDPE, PVC Type A and EB. Every row passed π/4·ID² and percentage arithmetic checks. 2017 text, cross-checked with the 2011 NECA-IBEW reproduction. Source
NEC Chapter 9 Table 5 and 5A, Dimensions of Insulated Conductors
Approximate area and diameter for THHN/THWN/THWN-2, XHHW/XHHW-2/XHH/ZW, TW/THW/THHW/THW-2, RHH/RHW/RHW-2 with and without outer covering, TFN/TFFN, and compact THHN and XHHW. USE/USE-2 appear only in the compact table. 2017 text, identical to 2011. Source
Edition note
No open 2020 or 2023 copy of Chapter 9 was reachable; the 2011 and 2017 values match cell for cell, and “unchanged in 2023” is inferred from that. If your jurisdiction is on 2023 and a cell looks wrong, verify against your code book. Source

Questions people ask

How many 12 AWG THHN fit in ¾ in EMT?
Sixteen. 40 % of the 0.533 in² internal area is 0.213 in²; each 12 AWG THHN is 0.0133 in²; 0.213 ÷ 0.0133 = 16.0. In ½ in EMT the answer is nine (0.122 ÷ 0.0133 = 9.2).
Why 40 % and not more?
Chapter 9 Table 1 sets 40 % for more than two conductors so the bundle can be pulled without damaging the insulation and so heat can escape. One conductor may fill 53 % and two 31 %, because those cases jam and cool differently.
Does the ground count?
Yes. Note 3 to the tables: equipment grounding or bonding conductors, insulated or bare, are included at their actual dimensions.
What about derating for many conductors?
Fill and ampacity are separate rules. Nine current-carrying conductors may fit in the conduit, but Table 310.15(C)(1) cuts their ampacity to 70 %. The wire size calculator applies that factor.

Embed this calculator

Paste this anywhere HTML is allowed. Free; keep the credit link.

<iframe src="https://necwire.com/embed/conduit-fill-calculator" width="100%" height="760" loading="lazy" title="Conduit fill calculator"></iframe>
<p><a href="https://necwire.com/conduit-fill-calculator">Conduit fill calculator</a> by NECwire</p>