Conduit Fill Calculator

This conduit fill calculator checks whether a group of wires fits a raceway under the National Electrical Code. Enter the raceway type and trade size, then the number, insulation and size of each conductor. You can mix sizes, add the equipment grounding conductor, or enter a cable’s area. It applies the Chapter 9 fill limits and shows the fill percent and the room left. It also gives the smallest trade size that works for each raceway type.

How conduit fill works

NEC Chapter 9 limits how much of a raceway’s cross-section conductors may occupy. Space around the conductors lets heat escape, and it keeps insulation from being damaged when the wires are pulled in. The limits in Chapter 9, Table 1 depend only on how many conductors share the raceway:

• One conductor: 53% of the raceway’s total area.
• Two conductors: 31%. Two round conductors side by side take up the full diameter, so the limit is lower.
• Three or more: 40%, the figure most people mean by “conduit fill.”
• Nipples: a raceway 24 in. (600 mm) long or shorter between enclosures may be filled to 60% (Table 1, Note 4).

Table 4 gives the total internal area of each raceway type and trade size. Table 5 gives the approximate area of each insulated conductor, and Table 8 the area of bare conductors.

How to calculate conduit fill

1. List every conductor in the raceway, including equipment grounding and bonding conductors (Table 1, Note 3).
2. Look up each conductor’s area in Table 5 for its insulation type and size, and add them up.
3. Pick the fill limit from Table 1 for the number of conductors.
4. Compare the total with that percent of the raceway’s area from Table 4. If the total is no more than the allowed area, the conductors fit.

Worked example

A three-phase feeder has three 1/0 AWG THHN conductors and a 6 AWG THHN equipment grounding conductor. From Table 5, 1/0 THHN is 0.1855 sq in. and 6 THHN is 0.0507 sq in., so the total is 3 × 0.1855 + 0.0507 = 0.6072 sq in. Four conductors are allowed 40%.

1-1/4 EMT has a total area of 1.496 sq in., and 40% of that is 0.598 sq in. 0.6072 is more than 0.598, so the conductors don’t fit: they would fill 40.6%. 1-1/2 EMT allows 0.814 sq in., and the same conductors fill 29.8%.

Raceway type matters: 1-1/4 rigid metal conduit, with 1.526 sq in., just fits at 39.8%. Load the example in the calculator above to see each step.

Same-size conductors and Annex C

When every conductor is the same size and insulation, Annex C of the NEC gives the maximum number for each raceway type and trade size. The calculation behind it is the 40% area divided by the area of one conductor. Note 7 to Table 1 rounds up to the next whole number when the decimal is 0.8 or more.

• 3/4 EMT holds 0.213 sq in. at 40%. 12 AWG THHN is 0.0133 sq in.: 0.213 ÷ 0.0133 = 16.0, so 16 fit.
• For 14 AWG THHN (0.0097 sq in.): 0.213 ÷ 0.0097 = 21.96. The decimal is more than 0.8, so 22 fit, not 21.
• For 12 AWG XHHW-2 (0.0181 sq in.), the thicker insulation means only 11 fit in the same raceway.

The calculator shows this maximum whenever all the conductors you enter are the same.

Rules that are easy to miss

• Grounding conductors count. Equipment grounding and bonding conductors are included in the fill (Note 3).
• A cable counts as one conductor. A multiconductor cable is treated as a single conductor at its overall area. An oval cable uses its major diameter as the diameter of a circle (Note 9). Use the “Other” option with the cable’s area from the manufacturer.
• Insulation type changes the answer. In small sizes, XHHW-2 and THW-type insulation are thicker than THHN, so fewer fit.
• Compact conductors have their own areas in Table 5A. Enter them with the “Other” option.
• Three conductors can jam. When three conductors of one size are pulled in, a raceway-to-conductor diameter ratio between 2.8 and 3.2 can let them wedge in the pipe. The calculator reports the ratio.
• Schedule 80 PVC is smaller inside. Its thicker wall gives a 1 in. raceway 0.688 sq in. of area, against 0.832 sq in. for Schedule 40.

Fill is not the only limit

A raceway can pass the fill check and still be the wrong choice:

• Ampacity adjustment. More than three current-carrying conductors in one raceway reduces their ampacity under NEC 310.15(C)(1): to 80% for 4 to 6 conductors, 70% for 7 to 9, and 50% for 10 to 20. A full raceway can force larger conductors, which then need a larger raceway.
• Bends and pulls. Each raceway article allows no more than 360° of bends between pull points. Long runs with many bends need pull boxes or a pulling-tension check, even when the fill is legal.
• Room to grow. Many designers leave spare capacity for future circuits rather than filling every raceway to the limit.
• Other raceways. Wireways (Article 376) and cable trays (Article 392) have their own fill rules; this calculator covers conduit and tubing.

The voltage drop calculator checks the same conductors for voltage drop and ampacity.

Frequently asked questions

What is the 40% conduit fill rule?

When three or more conductors are in a conduit or tubing, their total cross-sectional area may not exceed 40% of the raceway’s internal area (NEC Chapter 9, Table 1). One conductor may fill 53% and two conductors 31%.

Does the ground wire count in conduit fill?

Yes. Equipment grounding conductors and bonding conductors are included in the fill calculation (Chapter 9, Table 1, Note 3). Count them in the number of conductors too.

How many 12 AWG wires fit in 1/2 EMT?

Nine 12 AWG THHN conductors fit in 1/2 EMT: 40% of its 0.304 sq in. area is 0.122 sq in., and 0.122 ÷ 0.0133 = 9.2. In 3/4 EMT, 16 fit.

Can I run NM cable (Romex) in conduit?

NM cable can be run in a raceway where it needs protection from physical damage. For fill, each cable counts as one conductor at its overall area. A flat cable uses its larger dimension as the diameter of a circle (Note 9). Three or more cables are held to 40%, which fills a raceway quickly.

Is conduit fill different for PVC Schedule 80?

The percentages are the same, but Schedule 80 has a thicker wall and less internal area than Schedule 40 of the same trade size. Fewer conductors fit, so check Schedule 80 runs, often used where the conduit is exposed to damage, separately.

Can a short conduit nipple be filled more?

Yes. A nipple 24 in. (600 mm) or shorter between boxes, cabinets or similar enclosures may be filled to 60% of its area (Note 4). The ampacity adjustment factors for more than three current-carrying conductors do not apply to such nipples under 310.15(C)(1)(b).

When to bring in an engineer

Conduit fill is one line in a raceway and conductor schedule. Ampacity, voltage drop, derating, pulling tension and coordination with the overcurrent devices all have to agree. Dreiym’s licensed Professional Engineers design feeders and raceway systems, PE-stamped where required. Related tools: the voltage drop calculator and the available fault current calculator. For a free quote, contact us. We typically respond within one business day.