Conduit Fill Calculator

A conduit fill calculator checks whether a bundle of conductors fits inside a raceway under the National Electrical Code. It divides the total conductor area by the internal area of the conduit and compares the result with the NEC limit. Example: three 12 AWG THHN conductors in 1/2 inch EMT fill 13.1% of the raceway, far below the 40% cap, and that same conduit holds a maximum of nine 12 AWG conductors.

Two neighboring tools handle the other half of a circuit design: the Voltage Drop Calculator tells you whether a long run needs a heavier conductor, and the Ohm's Law Calculator converts between volts, amps, ohms and watts. Use this page when the wire sizes are already decided and the question is which conduit they physically fit in.

How conduit fill is calculated

The calculation is a single ratio. Every conductor has a cross-sectional area (insulation included) listed in NEC Chapter 9, Table 5, and every raceway has a total internal area listed in Chapter 9, Table 4:

where n is the number of conductors of each size, A is the area of one conductor of that size in square inches, and A_conduit is the 100% internal area of the trade size you selected. The result is compared with the limit from Chapter 9, Table 1: 53% when there is one conductor, 31% for two, and 40% for three or more. Chapter 9, Note 4 allows 60% for a nipple no longer than 24 inches. The calculator applies whichever limit matches the count you enter and reports the exact areas so you can trace the arithmetic.

Worked example: a 60 amp subpanel feed in EMT

Say you are feeding a garage subpanel with four 6 AWG THHN conductors (two hots, a neutral and a spare) plus one 10 AWG equipment grounding conductor, and you want to know whether 3/4 inch EMT is big enough. From Table 5, 6 AWG THHN is 0.0507 sq in and 10 AWG is 0.0211 sq in:

• Conductor area: 4 × 0.0507 + 1 × 0.0211 = 0.2239 sq in
• 3/4 inch EMT internal area (Table 4): 0.533 sq in
• Fill: 0.2239 ÷ 0.533 = 42.0%, which exceeds the 40% limit for five conductors
• Move up to 1 inch EMT (0.864 sq in): 0.2239 ÷ 0.864 = 25.9%, a pass with 0.122 sq in to spare below the 0.346 sq in allowed

The bundle misses 3/4 inch by a whisker, which is exactly the kind of case a quick mental estimate gets wrong. If the run between the two panels were a nipple of 24 inches or less, the 60% rule would allow 0.320 sq in in 3/4 inch EMT and the 42.0% bundle would pass. The calculator shows both answers when you toggle the raceway length.

NEC Chapter 9, Table 4: internal conduit areas

Trade size is a nominal label, not a measurement, so the same size holds different amounts of wire in different raceway types. The table below lists the 100% internal area from Table 4 for the three types built into the calculator, plus the 40% fill area for EMT (0.40 × total area):

Notice the jump at 2-1/2 inch: EMT steps up to a noticeably larger tube than PVC or RMC at that size and above, which is why a bundle that fits 2-1/2 inch EMT can fail in 2-1/2 inch PVC. Below 2 inch the three types are within a few percent of each other, with PVC Schedule 40 always the tightest.

NEC Chapter 9, Table 5: THHN/THWN conductor areas and how many fit in EMT

The conductor areas below are the Table 5 values for THHN, THWN and THWN-2, which share one row. The three right-hand columns give the maximum number of conductors of that single size in 1/2, 3/4 and 1 inch EMT at 40% fill, applying the Note 7 rounding rule:

A zero in the table means that even a single conductor of that size exceeds 40% of the raceway; the calculator still checks the single-conductor case at 53%. For instance, one 4/0 AWG in 1 inch EMT is 0.3237 ÷ 0.864 = 37.5% and passes on its own, while in 3/4 inch EMT it would be 60.7% and fail even against the 53% limit.

Conduit fill chart: maximum THHN conductors in EMT, 1/2 to 2 inch

For branch-circuit sizes this chart answers the everyday question directly. All conductors are the same size, the limit is 40%, and the Note 7 rounding rule is applied:

These counts are the maximum the fill rule allows, not a recommendation. Nine 12 AWG conductors in 1/2 inch EMT sit at 39.4% fill and will be a hard pull through any bend; most electricians treat roughly 30% as a comfortable working limit and size up when a run has several 90s. Remember also that more than three current-carrying conductors triggers ampacity derating under 310.15(C), a separate calculation this tool does not perform.

A second worked example: a 200 amp feeder with mixed sizes

Feeders rarely use one conductor size, which is where the mixed-size math matters. Take a copper feeder of three 4/0 AWG THHN (two hots and a neutral), one 1/0 AWG for a reduced neutral on a second circuit, and a 4 AWG equipment ground:

• Area: 3 × 0.3237 + 0.1855 + 0.0824 = 1.239 sq in
• 1-1/2 inch EMT (2.036 sq in): 1.239 ÷ 2.036 = 60.9%, a clear fail
• 2 inch EMT (3.356 sq in): 1.239 ÷ 3.356 = 36.9%, a pass under the 1.342 sq in allowed
• 2 inch PVC Schedule 40 (3.291 sq in): 37.6%, also a pass; 2 inch RMC (3.408 sq in): 36.4%

Five conductors means the 40% column applies, and because the sizes differ, the Note 7 rounding rule does not come into play. The "smallest trade size that passes" panel in the calculator shows all three raceway answers at once so you can pick the material without re-entering the bundle.

How to use this conduit fill calculator

1. Pick the raceway type: EMT, PVC Schedule 40 or rigid metal conduit. The internal area shown under the size buttons updates from Table 4.
2. Choose the trade size you plan to install, from 1/2 to 4 inch.
3. Enter the conductors: select a THHN/THWN size and type the count. Click "Add another conductor size" for neutrals, grounds or a second circuit of a different gauge; up to eight size groups can be mixed.
4. Set the raceway length: leave "Full run" for a normal conduit, or choose "Nipple" if the piece is 24 inches or shorter to apply the 60% allowance.
5. Read the result: the headline card gives the fill percentage, the limit that applies and how much area is left. Below it you get the smallest passing size in all three raceway types, a pass/fail table for every size, and the maximum count of each conductor size in the conduit you selected.

Who this calculator is for

• Electricians confirming a conduit size before bending and cutting, or checking whether an existing raceway can take one more circuit.
• Apprentices and exam candidates practicing Chapter 9 problems, where showing the Table 4 and Table 5 arithmetic matters as much as the answer.
• Estimators and designers converting a conductor schedule into raceway sizes for a takeoff.
• Homeowners and DIYers planning a permitted subpanel, workshop or EV charger run who want to arrive at the supply house with the right size.
• Inspectors and plan reviewers who want a fast independent check of a submitted fill calculation.

Key terms explained

• Trade size: the nominal designation of a raceway (1/2, 3/4, 1 inch and so on). It is not the inside diameter; the true internal area comes from Chapter 9, Table 4 for each raceway type.
• EMT: electrical metallic tubing (Article 358), the thin-wall steel raceway used for most commercial interior work.
• PVC Schedule 40: rigid polyvinyl chloride conduit (Article 352), common for underground and wet locations. Its thicker wall means less inside area than EMT.
• RMC: rigid metal conduit (Article 344), the heavy-wall threaded steel raceway; slightly more inside area than EMT.
• THHN / THWN / THWN-2: the most common building-wire insulations. They share one line in Table 5, so their areas are identical for fill purposes.
• Nipple: a raceway 24 inches or shorter between enclosures. Chapter 9, Note 4 allows 60% fill and exempts it from conductor-count ampacity adjustment.
• Note 7 rounding: when all conductors are the same size, a calculated maximum with a decimal of 0.8 or more rounds up to the next whole conductor.

What changes the result the most

• Conductor size: area grows faster than the AWG number suggests. Stepping from 12 AWG (0.0133 sq in) to 6 AWG (0.0507 sq in) almost quadruples the area per conductor.
• Conductor count: going from two to three conductors raises the limit from 31% to 40%, so adding a ground can sometimes make a marginal two-wire bundle pass. Two 12 AWG in 1/2 inch EMT is 8.8% against 31%; three is 13.1% against 40%.
• Raceway type: at 1/2 inch, PVC Schedule 40 has 6.3% less area than EMT, enough to drop the 12 AWG maximum from 9 to 8 and the 3/4 inch maximum from 16 to 15.
• Insulation type: XHHW or RHW conductors are larger than THHN of the same gauge and are not covered here; using THHN areas for them would understate the fill.
• Nipple allowance: switching a short piece to the 60% rule adds half again as much usable area, from 0.213 to 0.320 sq in in 3/4 inch EMT.

Tips for sizing conduit in practice

• Leave pulling room. A bundle at 39% is legal but miserable to pull through four 90-degree bends. Many contractors design to about 30% and reserve the last 10% for a future circuit.
• Count every conductor. Grounds, neutrals, travelers and spares all take space. A "3-wire" circuit with a ground is four conductors for fill.
• Check derating next. Once more than three current-carrying conductors share the raceway, 310.15(C) reduces ampacity, which may push you to a larger wire and therefore a larger conduit.
• Check voltage drop on long runs. A 200 foot run to a detached building often needs an upsized conductor for the 3% recommendation before fill even becomes a question; use the Voltage Drop Calculator first, then come back here with the final gauge.
• Use the right Table 4 column. ENT, flexible metal conduit, LFMC and Schedule 80 PVC have their own columns with different areas; do not borrow the EMT numbers for them.

Limitations and assumptions

• Conductor areas are for THHN/THWN/THWN-2 only. Other insulation types need their own Table 5 rows.
• Only EMT, PVC Schedule 40 and RMC from 1/2 to 4 inch are built in. Larger sizes and other raceway types are not included.
• Bare equipment grounding conductors are treated as insulated THHN, which slightly overstates their area (a conservative error).
• Multiconductor cables pulled in conduit are counted differently under Chapter 9, Note 9 (as a single round conductor of the cable diameter) and are not modeled.
• The tool does not check ampacity, derating, box fill, bend limits or voltage drop, and it does not know your local amendments or which NEC edition your jurisdiction has adopted.

Related electrical calculators

Conduit fill is the last of three checks on a circuit. Start with the Ohm's Law Calculator to turn a load in watts into amps, use the Voltage Drop Calculator to see whether the run length forces a heavier conductor, and finish here to confirm the raceway size. To estimate what the circuit will cost to run once it is energized, the Electricity Cost Calculator converts watts and hours into dollars.

Sources

• NFPA 70, National Electrical Code (NEC), Chapter 9, Table 1 - Percent of cross section of conduit and tubing for conductors and cables.
• NFPA 70, National Electrical Code (NEC), Chapter 9, Table 4 - Dimensions and percent area of conduit and tubing (Articles 344, 352 and 358).
• NFPA 70, National Electrical Code (NEC), Chapter 9, Table 5 - Dimensions of insulated conductors and fixture wires (THHN, THWN, THWN-2).
• NFPA 70, National Electrical Code (NEC), Chapter 9, Notes to Tables (Notes 4, 7 and 9) and Informative Annex C.