Conduit Size Calculator

What size conduit do you need? Enter wire count, gauge, and type — get NEC-compliant minimum conduit size and fill percentage table.

Wire configuration

Number of conductors?

wires

Wire gauge (AWG)?

Wire type?

Conduit type?

Minimum conduit size

1/2" conduit

Total wire area0.0633 in²

NEC fill limit (40%)0.1582 in² required

Conduit internal area0.3040 in²

Actual fill percentage20.8%

Size	Internal Area	Fill %	Status
1/2"	0.304 in²	20.8%	✓ Recommended
3/4"	0.533 in²	11.9%	Fits
1"	0.864 in²	7.3%	Fits
1-1/4"	1.496 in²	4.2%	Fits
1-1/2"	2.036 in²	3.1%	Fits
2"	3.356 in²	1.9%	Fits
2-1/2"	5.858 in²	1.1%	Fits
3"	8.846 in²	0.7%	Fits
4"	15.904 in²	0.4%	Fits

Link copied to clipboard

How to Use This Calculator

Enter wire count and gauge

Enter the total number of conductors you're running in the conduit. For a standard 120V circuit: 2 wires. For 240V: 3 wires (2 hots + ground or neutral). For a solar PV string to a combiner box: typically 2 wires. Ground wires count toward conduit fill but NOT toward the ampacity derating that applies when more than 3 current-carrying conductors share a conduit.

Select wire type and conduit type

Choose your wire insulation type — THWN-2 is standard for most conduit runs; USE-2 for outdoor solar PV runs. The wire type affects insulation thickness and thus cross-sectional area. For conduit, select EMT (indoor), PVC (underground), or rigid metal (outdoor exposed).

Read the fill table

The results show the recommended minimum conduit size and a full table of all standard sizes with fill percentages. The highlighted row is the recommended size — use the next size up if you want room for future wiring additions.

The Formula

Total wire area = Conductor count × Per-conductor area (NEC Chapter 9 Table 5) Fill limit = 53% (1 conductor), 31% (2), 40% (3+) — NEC Chapter 9 Table 1 Required conduit area = Total wire area ÷ Fill limit Select smallest conduit where internal area ≥ Required conduit area Fill % = Total wire area ÷ Conduit internal area × 100

The NEC fill limits protect wires from heat buildup (high fill restricts airflow) and prevent physical damage during installation (pulling wires through an overfilled conduit can damage insulation). The 40% limit for 3+ conductors is the most common case in solar installations.

Example

Solar inverter AC output — 3 conductors of 10 AWG THWN-2 in EMT

Running 3 conductors of 10 AWG THWN-2 from a solar inverter to the main panel in EMT conduit. What size conduit is needed?

Conductors3 × 10 AWG THWN-2

Per-conductor area0.0211 in²

Total wire area0.0633 in²

Fill limit (3+ conductors)40%

Required conduit area0.158 in²

Result

1/2" EMT internal area0.304 in²

Fill percentage20.8% — fits in 1/2" EMT

A 1/2" EMT conduit is sufficient for 3 conductors of 10 AWG THWN-2 at only 20.8% fill — well under the 40% limit. If you need to add a 4th conductor later, you'd still fit — 4 × 0.0211 = 0.0844 in², which is 27.8% of 0.304 = still under 40%. To future-proof, installing 3/4" EMT gives you room for 6-8 conductors of this size.

FAQ

What NEC rule governs conduit fill? ▼

NEC Chapter 9, Table 1 specifies maximum conduit fill percentages: 53% for 1 conductor, 31% for 2 conductors, and 40% for 3 or more conductors. These are based on the ratio of conductor cross-sectional area to conduit internal cross-sectional area. Conduit internal dimensions are in NEC Chapter 9, Table 4. Wire cross-sectional areas are in Table 5. This calculator handles all that table lookups for you.

Does the ground wire count toward conduit fill? ▼

Yes — all conductors including equipment grounding conductors count toward conduit fill calculations per NEC 310.15(B)(3). However, grounding conductors do NOT count as current-carrying conductors for the ampacity derating that applies when more than 3 current-carrying conductors share a conduit. This is an important distinction: fill calculation includes ground wire; ampacity derating does not count it.

What conduit type should I use for underground solar runs? ▼

PVC Schedule 40 is the most common choice for underground conduit runs — it's cost-effective, corrosion-resistant, and easy to work with. For burial depth: PVC minimum 24 inches, 18 inches with GFCI and warning tape, 12 inches under a concrete slab. Use schedule 80 PVC where mechanical damage risk exists (shallow burial or conduit entering above grade). For exposed above-ground runs (surface mounting), use EMT or rigid metal conduit — UV-resistant PVC conduit is available but less common.

How many solar PV conductors can fit in 3/4" conduit? ▼

For 10 AWG USE-2 (area 0.0254 in²) in 3/4" EMT (0.533 in²) at 40% fill: maximum area = 0.213 in²; conductors = 0.213 ÷ 0.0254 = 8.4 → maximum 8 conductors. This means you can run 4 PV string pairs (8 conductors) in a single 3/4" conduit — common for solar runs from roof to inverter. Enter your specific wire gauge and type above to get exact numbers.