How to figure conduit fill?

Three steps. (1) Look up each conductor's area in NEC Chapter 9 Table 5 — for #12 THHN it is 0.0133 in², for #6 THHN it is 0.0507 in². (2) Sum the area × number of conductors of each type. (3) Divide by the maximum-fill percentage from NEC Chapter 9 Table 1: 53 % for one conductor, 31 % for two, 40 % for three or more. The result is the minimum required conduit cross-section in in². Match this against the available cross-sections in NEC Chapter 9 Table 4 for the conduit type you are using and pick the smallest size whose available area meets or exceeds the requirement.

How to install conduit underground?

For PVC Schedule 40 in residential use under driveways, NEC §300.5 Table 300.5 requires 18 in cover for direct burial below grade or 6 in below a 4 in concrete slab. Slope 1/4 in per foot to drain condensate, install long-radius sweeps (no LB elbows below grade), use NEC-listed PVC cement on every joint, and install red identification tape 12 in above the conduit. For RMC underground, the cover requirement drops to 6 in. Pull the conductors after the conduit is glued, capped, and cured — do not push wires into wet PVC.

What is the maximum conductor fill of a conduit?

For 3 or more current-carrying conductors of any type, the NEC Chapter 9 Table 1 limit is 40 % of the conduit cross-section. For 2 conductors it drops to 31 %; for a single conductor, it is 53 %. The lower 31 % limit on two conductors exists because a pair of conductors interlocks during the pull, jamming up against the conduit wall, which would damage insulation if the fill were higher. The 40 % limit on 3+ conductors is the most common case in practice.

What size conduit do I need for 4 #6 THHN?

Four #6 THHN conductors total 4 × 0.0507 = 0.2028 in². At the 40 % fill limit, required conduit area = 0.2028 / 0.40 = 0.507 in². Per NEC Chapter 9 Table 4: 1 in EMT has 0.864 in² available (well above), 3/4 in EMT has 0.533 in² (just above). Use 3/4 in EMT, with 1 in EMT a safer choice if the future may add a fourth or fifth conductor. For PVC Sched 40 the available cross-sections are slightly different — verify against Table 4 column for PVC.

What is the OD of common electrical conduit?

EMT 1/2 in trade = 0.706 in OD; 3/4 in = 0.922 in; 1 in = 1.163 in; 1-1/4 in = 1.510 in; 2 in = 2.197 in. RMC and IMC have larger OD because of the thicker wall. PVC Sched 40 1/2 in = 0.840 in OD (matches the steel-pipe NPS dimensions); 1 in = 1.315 in. Always check the conduit type — "1/2 in trade size" produces different OD across EMT, RMC, and PVC because trade size is approximate, not literal.

How to figure conduit fill?

Three steps. (1) Look up each conductor's area in NEC Chapter 9 Table 5 — for #12 THHN it is 0.0133 in², for #6 THHN it is 0.0507 in². (2) Sum the area × number of conductors of each type. (3) Divide by the maximum-fill percentage from NEC Chapter 9 Table 1: 53 % for one conductor, 31 % for two, 40 % for three or more. The result is the minimum required conduit cross-section in in². Match this against the available cross-sections in NEC Chapter 9 Table 4 for the conduit type you are using and pick the smallest size whose available area meets or exceeds the requirement.

How to install conduit underground?

For PVC Schedule 40 in residential use under driveways, NEC §300.5 Table 300.5 requires 18 in cover for direct burial below grade or 6 in below a 4 in concrete slab. Slope 1/4 in per foot to drain condensate, install long-radius sweeps (no LB elbows below grade), use NEC-listed PVC cement on every joint, and install red identification tape 12 in above the conduit. For RMC underground, the cover requirement drops to 6 in. Pull the conductors after the conduit is glued, capped, and cured — do not push wires into wet PVC.

What is the maximum conductor fill of a conduit?

For 3 or more current-carrying conductors of any type, the NEC Chapter 9 Table 1 limit is 40 % of the conduit cross-section. For 2 conductors it drops to 31 %; for a single conductor, it is 53 %. The lower 31 % limit on two conductors exists because a pair of conductors interlocks during the pull, jamming up against the conduit wall, which would damage insulation if the fill were higher. The 40 % limit on 3+ conductors is the most common case in practice.

What size conduit do I need for 4 #6 THHN?

Four #6 THHN conductors total 4 × 0.0507 = 0.2028 in². At the 40 % fill limit, required conduit area = 0.2028 / 0.40 = 0.507 in². Per NEC Chapter 9 Table 4: 1 in EMT has 0.864 in² available (well above), 3/4 in EMT has 0.533 in² (just above). Use 3/4 in EMT, with 1 in EMT a safer choice if the future may add a fourth or fifth conductor. For PVC Sched 40 the available cross-sections are slightly different — verify against Table 4 column for PVC.

What is the OD of common electrical conduit?

EMT 1/2 in trade = 0.706 in OD; 3/4 in = 0.922 in; 1 in = 1.163 in; 1-1/4 in = 1.510 in; 2 in = 2.197 in. RMC and IMC have larger OD because of the thicker wall. PVC Sched 40 1/2 in = 0.840 in OD (matches the steel-pipe NPS dimensions); 1 in = 1.315 in. Always check the conduit type — "1/2 in trade size" produces different OD across EMT, RMC, and PVC because trade size is approximate, not literal.

Reference · Lookup · NEC Chapter 9 · UL 797 / 651

Electrical Conduit Sizes — EMT, PVC, RMC & IMC Reference

Electrical conduit sizes for the four common types — EMT, PVC, RMC, IMC — span from 1/2 in trade size up to 6 in. This page tabulates outside diameter, internal diameter, and conductor cross-section for each, lists the NEC Chapter 9 Table 1 fill limits (53 % / 31 % / 40 %), and walks through a worked sizing example for 4 #6 THHN conductors. Reviewed by a licensed PE.

Electrical conduit size calculator

Conduit fill is a substitution exercise — pick the right table column, sum the conductor areas, divide by the fill limit. The wire-size calculator returns conductor cross-sections in in² and mm² automatically; cross-reference the result against the conduit-fill table below for the smallest stocked size that meets NEC Chapter 9 Table 1.

→ Wire size calculator (conductor area output) · → Amp to wire size chart · → Voltage drop calculator

Conduit fill formulas

Eq. 01 — Total conductor area SI

A_{conductors} = \sum_i n_i \cdot a_i

·: n_i = number of conductors of type i (count grounding conductor too)
·: a_i = area per conductor from NEC Chapter 9 Table 5 (in² or mm²)
·: Sum across all conductor types in the same conduit

Eq. 02 — Required conduit cross-section SI

A_{conduit} \geq \frac{A_{conductors}}{f}

·: f = 0.53 for 1 conductor, 0.31 for 2, 0.40 for 3 or more (NEC Ch 9 Table 1)
·: A_conduit = minimum cross-section to meet fill limit
·: Match to NEC Chapter 9 Table 4 for the conduit type

Eq. 03 — Pull-length / bend limit (NEC §358.26 EMT) SI

\sum \theta_{bends} \leq 360°

·: 4 quarter turns max between pull points
·: Add a pull box if the run exceeds 360° of cumulative bend
·: Long-radius sweeps preferred underground to reduce sidewall pressure

Standards governing electrical conduit

Document	Scope
NFPA 70 (NEC) Article 358	EMT — Electrical Metallic Tubing
NFPA 70 (NEC) Article 344	RMC — Rigid Metal Conduit
NFPA 70 (NEC) Article 342	IMC — Intermediate Metal Conduit
NFPA 70 (NEC) Article 352	PVC Schedule 40 / 80 conduit
NFPA 70 (NEC) Chapter 9 Tables	Conduit fill — Tables 1, 4, 5 (governing fill calculations)
UL 797	EMT product standard
UL 651	PVC Schedule 40 / 80 conduit standard
UL 6 / UL 1242	RMC and IMC product standards

Reference: conduit dimensions and available cross-section

Trade size	EMT OD (in)	EMT ID (in)	EMT 40 % fill (in²)	PVC Sched 40 ID (in)	PVC 40 % fill (in²)
1/2	0.706	0.622	0.122	0.602	0.114
3/4	0.922	0.824	0.213	0.804	0.203
1	1.163	1.049	0.346	1.029	0.333
1-1/4	1.510	1.380	0.598	1.360	0.581
1-1/2	1.740	1.610	0.814	1.590	0.794
2	2.197	2.067	1.342	2.047	1.316
2-1/2	2.875	2.731	2.343	2.445	1.878
3	3.500	3.356	3.538	3.042	2.907
3-1/2	4.000	3.834	4.618	3.521	3.895
4	4.500	4.334	5.901	3.998	5.022

40 % fill values are the maximum allowable conductor cross-section per NEC Chapter 9 Table 1 for ≥ 3 conductors. PVC Sched 40 ID matches NPS pipe ID for 1/2 to 2 in trade sizes.

Reference: typical conductor cross-sections (NEC Chapter 9 Table 5)

Conductor	THHN/THWN-2 (in²)	XHHW-2 (in²)	RHH/RHW (in²)
14 AWG	0.0097	0.0139	0.0209
12 AWG	0.0133	0.0181	0.0260
10 AWG	0.0211	0.0243	0.0333
8 AWG	0.0366	0.0437	0.0556
6 AWG	0.0507	0.0590	0.0726
4 AWG	0.0824	0.0814	0.0973
3 AWG	0.0973	0.0962	0.1134
2 AWG	0.1158	0.1146	0.1333
1 AWG	0.1562	0.1534	0.1901
1/0 AWG	0.1855	0.1825	0.2223
2/0 AWG	0.2223	0.2190	0.2624
4/0 AWG	0.3237	0.3197	0.3718

Pick the conduit type EMT (Electrical Metallic Tubing) — thin-wall steel, indoor dry/damp; RMC (Rigid Metal Conduit) — thick-wall steel for outdoor or damaged-prone areas; IMC (Intermediate Metal Conduit) — between EMT and RMC; PVC (Sched 40 / 80) — underground burial, corrosive environments; Liquidtight Flexible (LFNC) — final connection to motors and vibrating equipment. Each type has its own dimension table.
List the conductors Count conductors and read the OD or area of each from NEC Chapter 9 Table 5. Three #6 THHN: each 0.0507 in² area. Always include the equipment grounding conductor (EGC) — even though NEC §250.122 lets you size it small, it counts toward conduit fill.
Apply the fill limit NEC Chapter 9 Table 1: 1 conductor → 53 % fill of conduit cross-section, 2 conductors → 31 %, 3 or more → 40 %. The limits prevent overheating and allow conductors to be pulled without insulation damage.
Pick the conduit size Multiply each conductor area × count, sum, divide by 0.40 (≥ 3 CCCs). Match the result to NEC Chapter 9 Table 4 for the conduit type. Always round up to the next stocked trade size.
Verify pull length and bends NEC §358.26 (EMT) and §344.26 (RMC) limit the run between pull points to 360° of total bends — typically 4 quarter-turns. For long runs, install a pull box every 100 ft to reduce sidewall pressure during the pull.

Worked example — 6 #10 THHN feeding a sub-panel

A 30 A 240 V split-phase sub-panel feeder: two hots, one neutral, one equipment grounding conductor — all #10 THHN copper.

Conductor count: 4 conductors total (2 hots + 1 N + 1 EGC).
Area per conductor: 0.0211 in² for #10 THHN.
Total area: 4 × 0.0211 = 0.0844 in².
Required conduit area (40 % fill, 3+ conductors): 0.0844 / 0.40 = 0.211 in².
Look up smallest fitting EMT: 3/4 in EMT has 0.213 in² at 40 % — exactly meets demand.
Practical choice: Use 1 in EMT (0.346 in² available) for some future expansion margin and easier pull.

Comparison — EMT vs. RMC vs. IMC vs. PVC

Aspect	EMT	RMC	IMC	PVC Sched 40
Wall thickness	Thin (0.04–0.10 in)	Thick (0.08–0.18 in)	Mid	~0.12 in
Cost (per ft)	Lowest	Highest steel	Mid-high	Lowest plastic
Corrosion resistance	Low — galvanised steel	Better with field-paint	Better than EMT	Excellent
Allowed locations	Indoor dry / damp; outdoor with care	Anywhere except where prohibited	Same as RMC	Underground, wet, corrosive
Connection	Set-screw or compression couplings	Threaded	Threaded	Solvent-cement
Best for	Indoor commercial branch / feeder	Industrial, hazardous, exposed	Replacing RMC where weight matters	Burial, corrosive, wash-down

Variants and related queries

Conduit — the umbrella term

"Conduit" refers generically to any tubular protection housing for electrical conductors. In NEC parlance, conduit specifically excludes cable trays (NEC Article 392) and surface raceways (Article 386); it covers EMT, RMC, IMC, PVC, ENT, FMC, LFNC, and LFMC. Each Article in NEC Chapter 3 covers one conduit family with its own dimension table, fill rules, support spacing, and minimum bending radius.

Flexible PVC electrical conduit (ENT and LFNC)

"Flexible PVC electrical conduit" usually means ENT (Electrical Nonmetallic Tubing, NEC Article 362) — corrugated blue plastic for in-wall residential and light-commercial use up to 600 V. For wet locations or motor connections, LFNC (Liquidtight Flexible Nonmetallic Conduit, Article 356) is the wet-rated equivalent. Both follow the same Chapter 9 fill rules as rigid PVC.

Conduit for 6/3 wire

"6/3 wire" is a Romex-type cable with three #6 THHN insulated conductors plus a bare ground — typically four total countable conductors. Per NEC Chapter 9 fill rules: 3 × 0.0507 + 1 × 0.0507 (counting the EGC) = 0.203 in² → divide by 0.40 → 0.508 in² required → smallest match is 3/4 in EMT (0.213 in² at 40 %; not enough for that calculation actually). Re-check: at 0.508 in², use 1 in EMT (0.346 in² is too small either) — actually 1-1/4 in EMT (0.598 in²) is the right choice. 3/4 in works only for stranded #6 in two-conductor configurations where fill drops to 31 %.

Underground cable conduit

Direct-buried PVC Schedule 40 is the default for underground residential service entrances, sub-panel feeders to detached buildings, landscape lighting branch circuits, and EV-charger trenches. NEC Table 300.5 sets minimum cover (18 in residential under driveway, 6 in below 4 in concrete slab). RMC underground requires only 6 in cover but is rarely cost-effective except in industrial sites.

Frequently asked questions

How to figure conduit fill?: Three steps. (1) Look up each conductor's area in NEC Chapter 9 Table 5 — for #12 THHN it is 0.0133 in², for #6 THHN it is 0.0507 in². (2) Sum the area × number of conductors of each type. (3) Divide by the maximum-fill percentage from NEC Chapter 9 Table 1: 53 % for one conductor, 31 % for two, 40 % for three or more. The result is the minimum required conduit cross-section in in². Match this against the available cross-sections in NEC Chapter 9 Table 4 for the conduit type you are using and pick the smallest size whose available area meets or exceeds the requirement.
How to install conduit underground?: For PVC Schedule 40 in residential use under driveways, NEC §300.5 Table 300.5 requires 18 in cover for direct burial below grade or 6 in below a 4 in concrete slab. Slope 1/4 in per foot to drain condensate, install long-radius sweeps (no LB elbows below grade), use NEC-listed PVC cement on every joint, and install red identification tape 12 in above the conduit. For RMC underground, the cover requirement drops to 6 in. Pull the conductors after the conduit is glued, capped, and cured — do not push wires into wet PVC.
What is the maximum conductor fill of a conduit?: For 3 or more current-carrying conductors of any type, the NEC Chapter 9 Table 1 limit is 40 % of the conduit cross-section. For 2 conductors it drops to 31 %; for a single conductor, it is 53 %. The lower 31 % limit on two conductors exists because a pair of conductors interlocks during the pull, jamming up against the conduit wall, which would damage insulation if the fill were higher. The 40 % limit on 3+ conductors is the most common case in practice.
What size conduit do I need for 4 #6 THHN?: Four #6 THHN conductors total 4 × 0.0507 = 0.2028 in². At the 40 % fill limit, required conduit area = 0.2028 / 0.40 = 0.507 in². Per NEC Chapter 9 Table 4: 1 in EMT has 0.864 in² available (well above), 3/4 in EMT has 0.533 in² (just above). Use 3/4 in EMT, with 1 in EMT a safer choice if the future may add a fourth or fifth conductor. For PVC Sched 40 the available cross-sections are slightly different — verify against Table 4 column for PVC.
What is the OD of common electrical conduit?: EMT 1/2 in trade = 0.706 in OD; 3/4 in = 0.922 in; 1 in = 1.163 in; 1-1/4 in = 1.510 in; 2 in = 2.197 in. RMC and IMC have larger OD because of the thicker wall. PVC Sched 40 1/2 in = 0.840 in OD (matches the steel-pipe NPS dimensions); 1 in = 1.315 in. Always check the conduit type — "1/2 in trade size" produces different OD across EMT, RMC, and PVC because trade size is approximate, not literal.

Historic source — origin of the modern conduit system

The pull-chain lamp socket and the first separable attachment plug-and-receptacle, both patented by Harvey Hubbell at the turn of the twentieth century, made it practical to wire commercial buildings with permanent conduit-protected feeders feeding portable plug loads. The dimensional standards that became NEC Chapter 9 grew from the trade-size system Hubbell\'s industry helped popularise.

Harvey Hubbell — patent records, 1890s–1900s → Hubbell's pull-chain socket and the first separable plug-and-receptacle system catalysed the adoption of conduit-protected branch circuits in U.S. commercial buildings

Related calculators and references

Sources and further reading

NFPA 70 — NEC, Articles 342, 344, 348, 352, 358, 362, 392; Chapter 9 Tables 1, 4, 5 (2023 edition).
UL 797 — EMT product standard.
UL 651 — PVC Schedule 40 / 80 conduit standard.
UL 6 — Rigid Metal Conduit standard; UL 1242 — IMC standard.
Allied Tube & Conduit — Engineering Reference Manual, current edition.
Cantex / JM Eagle — PVC conduit dimensional and load tables.