NEC conduit fill calculator

Conduit fill is the total cross-section of the conductors divided by the internal area of the raceway. NEC Chapter 9 Table 1 allows 53% for one conductor, 31% for two and 40% for three or more, grounds included¹. Four 10 AWG THHN take 0.0844 in² of the 0.533 in² inside 3/4 in EMT, a 15.8% fill, well under 40%.

By Agustín Policano, founder of AmpCount. Values from the 2023 NEC (NFPA 70), checked November 1, 2026. Check which edition your jurisdiction enforces. A calculator doesn't replace the engineer of record or the inspector.

Raceway

Trade size

Nipple, 24 in or shorter (60%)

Conductors, including grounds

Qty

Size

Insulation

Qty

Size

Insulation

Fill15.8%

Allowed40%

Within the limit for 3/4 in EMT.

Conductors	4
Conductor area	0.0844 in²
Raceway area (100%)	0.533 in²
Allowed area (40%)	0.2132 in²

Smallest EMT that works: 1/2 in.

The rules behind the number

Fill limits by number of conductors: 53% for one, 31% for two, 40% for more than two.¹²
Equipment grounding and bonding conductors count toward fill, at their actual size (Chapter 9, Note 3).¹
Nipples 24 in or shorter between boxes or enclosures may be filled to 60% (Chapter 9, Note 4).¹
When every conductor is the same size, a calculated maximum of x.8 or more may be rounded up to the next whole conductor (Chapter 9, Note 7). Annex C tables already include that rounding; this calculator checks the fill percentage directly.¹
Fill isn't ampacity. More than three current-carrying conductors in a raceway need the adjustment factors in 310.15(C)(1), which may push you to a larger wire before fill does.²

Fill % = (sum of conductor areas, Table 5) ÷ (raceway area, Table 4) × 100

Allowed area at 40%, by raceway

In² available for three or more conductors, from NEC Chapter 9 Table 4²³
Trade size	EMT	PVC Schedule 40	RMC (rigid metal conduit)	IMC	PVC Schedule 80
1/2 in	0.122	0.114	0.126	0.137	0.087
3/4 in	0.213	0.203	0.220	0.234	0.164
1 in	0.346	0.333	0.355	0.384	0.275
1-1/4 in	0.598	0.581	0.610	0.659	0.495
1-1/2 in	0.814	0.794	0.828	0.890	0.684
2 in	1.342	1.316	1.363	1.452	1.150
2-1/2 in	2.343	1.878	1.946	2.054	1.648
3 in	3.538	2.907	3.000	3.169	2.577
3-1/2 in	4.618	3.895	4.004	4.234	3.475
4 in	5.901	5.022	5.153	5.452	4.503

Conductor areas

Approximate area in in², NEC Chapter 9 Table 5²⁴
Size	THHN / THWN-2	XHHW / XHHW-2
1 AWG	0.1562	0.1534
2 AWG	0.1158	0.1146
3 AWG	0.0973	0.0962
4 AWG	0.0824	0.0814
6 AWG	0.0507	0.0590
8 AWG	0.0366	0.0437
10 AWG	0.0211	0.0243
12 AWG	0.0133	0.0181
14 AWG	0.0097	0.0139
250 kcmil	0.3970	0.3904
300 kcmil	0.4608	0.4536
350 kcmil	0.5242	0.5166
400 kcmil	0.5863	0.5782
500 kcmil	0.7073	0.6984
1/0 AWG	0.1855	0.1825
2/0 AWG	0.2223	0.2190
3/0 AWG	0.2679	0.2642
4/0 AWG	0.3237	0.3197

From the takeoff to the conduit schedule

Conduit sizing usually happens after the count: once you know how many circuits leave a panel and how they group into home runs, fill decides the trade size and the trade size drives material and labor. The device and fixture count is the input; that part AmpCount does from the legend, and the electrical takeoff guide explains how the counted and measured halves fit together. For long runs, check the voltage drop before settling the wire size.

Feeder conduits follow from the service and feeder sizes, so on design-build work the electrical load calculation worksheet comes first. At the other end of the run, the conductors you put in the pipe also land in a box, and the NEC box fill check tells you whether that box needs to grow too.

A larger trade size changes more than the pipe: fittings, straps, supports and the labor per 100 ft all step up with it. The Excel estimate template prices conduit per 100 or per 1,000 ft, with a separate labor unit for each size. On jobs with hundreds of runs, that is the work estimating suites for commercial work handle with conduit assemblies.

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Sources

NFPA 70 and National Electrical Code are registered trademarks of the National Fire Protection Association. AmpCount is not affiliated with NFPA.