Conduit Fill Calculator (NEC) with CSA, IEC, BS 7671 and AS/NZS Modes
Use this conduit fill calculator to check whether your wires fit in EMT, RMC or PVC conduit and to find the smallest conduit size that passes. For NEC work the limits are 53% of the conduit area for one conductor, 31% for two and 40% for three or more. Short nipples of 24 inches or less may be filled to 60%.
Quick answer: Conduit fill % = total conductor area ÷ conduit internal area × 100. Take conductor areas from NEC Chapter 9 Table 5, conduit areas from Table 4, then compare with 53%, 31% or 40%. Count every conductor, including the equipment grounding conductor.
Conduit Fill Calculator (NEC, IEC, BS 7671, CSA and AS/NZS)
Work out how many conductors or cables can safely occupy a conduit, and get a recommended trade size, under the code convention you select. Switch between the North American percentage-fill method and the metric space-factor method used across IEC, BS 7671, and AS/NZS jurisdictions.
Step 1: Select Code Standard
Step 2: Add Conductors (AWG / kcmil, THHN-THWN-2 insulation)
Step 2: Add Cables (Outer Diameter, mm)
Results
| Trade / Nominal Size | Internal Area | Allowed Area at Max Fill | Fits? |
|---|
How to Use This Calculator
- Select the code standard you are designing under. NEC and CSA use the North American percentage-fill method with AWG and kcmil conductor sizing; IEC, BS 7671, and AS/NZS use the metric space-factor method based on the actual outer diameter of each cable.
- If you selected NEC or CSA, choose the conduit material (EMT, RMC, or PVC Schedule 40/80), since wall thickness changes the internal usable area for the same trade size.
- Add a row for every distinct conductor or cable size in the run, and enter how many of that size will be pulled together.
- Use “Add Conductor Size” or “Add Cable Size” to include additional sizes, or the remove button on a row to take one out.
- Select Calculate Conduit Fill. The tool totals the cross-sectional area of everything you entered, applies the correct maximum fill percentage for the total item count, and checks it against each standard trade or nominal conduit size.
- Review the recommended size, the fill percentage it results in, and the full size-by-size breakdown table to see how close the next size down would have been.
What Conduit Fill Means
Conduit fill is the proportion of a conduit’s internal cross-section that is taken up by the conductors or cables pulled through it. Every wiring code caps this proportion well below 100 percent, because conductors need clearance to be pulled in without damaging the insulation, and because a fully packed conduit traps heat that the conductors generate under load. Staying within the allowed fill also keeps future re-pulls or additions realistic.
Why the Allowed Percentage Changes With Conductor Count
A single conductor in a conduit is allowed a higher fill percentage than a bundle of three or more, because one conductor can move freely to dissipate heat and does not need room to shift past neighboring conductors during the pull. Two conductors sit in between. Three or more conductors are treated the most conservatively, since bundling multiple current-carrying conductors together concentrates heat and makes the pull itself more physically demanding. This is why every major standard uses a tiered allowance rather than one flat number.
Imperial vs. Metric Approach
NEC and CSA reference a fixed table of conductor areas by AWG or kcmil size and insulation type, cross-checked against a fixed table of conduit internal areas by trade size and material. IEC, BS 7671, and AS/NZS jurisdictions more commonly size conduit against the actual measured outer diameter of the cable being installed, since cable constructions and manufacturers vary more widely outside North America. Both approaches are trying to answer the same underlying question in a way that fits how conductors are specified in that region.
Common Mistakes to Avoid
Mixing conductor insulation types without re-checking the area table, forgetting to count a grounding or earth conductor as part of the total fill, choosing conduit size based on outside diameter alone rather than internal usable area, and reusing a fill calculation from a different conduit material are among the most frequent errors seen in the field. Re-verify the calculation whenever the conductor mix, insulation type, or conduit material changes.
How to use the calculator
- Choose the code standard. For US work select NEC. The calculator is set up for NEC first, with CSA, IEC, BS 7671 and AS/NZS as alternative modes.
- For NEC and CSA, choose the conduit material (EMT, RMC, PVC Schedule 40 or Schedule 80). Wall thickness changes the usable area for the same trade size.
- Add a row for each conductor size and enter the quantity. Include the equipment grounding conductor.
- For metric standards, enter each cable's outer diameter in millimetres.
- Press Calculate. You get the total area, the maximum fill allowed, the recommended size and a size-by-size pass or fail table.
- Check the result against your adopted code edition, then check ampacity, voltage drop and bends separately.
Jump to your standard: NEC | CSA (Canada) | IEC | BS 7671 (UK) | AS/NZS 3000 (Australia and New Zealand)
NEC Conduit Fill Limits
NEC Chapter 9, Table 1 sets the maximum share of a conduit's internal cross-sectional area that conductors may occupy.
| Number of conductors | Maximum fill | Guide |
|---|---|---|
| 1 | 53% | Conduit fill for 1 conductor: the 53% rule |
| 2 | 31% | Conduit fill for 2 conductors: the 31% rule |
| 3 or more | 40% | NEC conduit fill for 3+ conductors: the 40% rule |
| Nipple of 24 in. or less between boxes | 60% | Conduit nipple fill rule under 24 inches |
For the full table, its notes and the area tables it relies on, read our guide to NEC Chapter 9 Table 1 conduit fill percentages.
Notes that change your answer:
- Count the grounding conductor. Equipment grounding or bonding conductors are included in the fill, using their actual dimensions (Table 1, Note 3).
- A multiconductor cable counts as one conductor, and an elliptical cable is measured at its major diameter (Note 9). This matters for Romex in conduit.
- Nipples of 24 in. or less may be filled to 60%, and the ampacity adjustment factors do not apply to that short section (Note 4).
- Table 1 applies to complete conduit systems, not to short sections used only to protect exposed wiring from physical damage (Note 2).
- Rounding rule: when you calculate the maximum number of same-size conductors and the decimal is 0.8 or more, round up to the next whole number (Note 7).
These limits have been stable for years, but table numbering and notes can move between NEC editions, so confirm against the edition your jurisdiction has adopted. You can view the NEC free of charge on the NFPA website.
How to Calculate Conduit Fill
The formula is:
Conduit fill (%) = total conductor area ÷ conduit internal area × 100
Follow four steps:
- Find each conductor's cross-sectional area, including insulation (NEC Table 5, or Table 5A for compact conductors).
- Add the areas of all conductors, including the equipment grounding conductor.
- Find the conduit's total internal area for its type and trade size (NEC Table 4).
- Multiply that area by 53%, 31% or 40% (or 60% for a qualifying nipple) and check that your total is no larger.
For a longer walkthrough with more examples, read how to calculate conduit fill step by step.
Common conductor and conduit areas
These are the NEC Chapter 9 values used in the examples on this page.
| Wire size (THHN) | Area (in²) | EMT trade size | Total area (in²) | 40% limit (in²) |
|---|---|---|---|---|
| 14 AWG | 0.0097 | 1/2" | 0.304 | 0.122 |
| 12 AWG | 0.0133 | 3/4" | 0.533 | 0.213 |
| 10 AWG | 0.0211 | 1" | 0.864 | 0.346 |
| 8 AWG | 0.0366 | 1-1/4" | 1.496 | 0.598 |
| 6 AWG | 0.0507 | 1-1/2" | 2.036 | 0.814 |
| 4 AWG | 0.0824 | 2" | 3.356 | 1.342 |
| 3 AWG | 0.0973 | |||
| 2 AWG | 0.1158 | |||
| 1 AWG | 0.1562 | |||
| 1/0 AWG | 0.1855 | |||
| 2/0 AWG | 0.2223 | |||
| 3/0 AWG | 0.2679 | |||
| 4/0 AWG | 0.3237 |
If you need more detail, see our wire fill chart for conduit and the PVC electrical pipe fill chart. PVC Schedule 80 has thicker walls and a smaller bore than Schedule 40, so never reuse an EMT or Schedule 40 result for Schedule 80.
Conduit Fill Chart: Maximum THHN Conductors in EMT
| Wire size (THHN) | EMT 1/2" | EMT 3/4" | EMT 1" | EMT 1-1/4" | EMT 1-1/2" | EMT 2" |
|---|---|---|---|---|---|---|
| 14 AWG | 12 | 22 | 35 | 61 | 84 | 138 |
| 12 AWG | 9 | 16 | 26 | 45 | 61 | 101 |
| 10 AWG | 5 | 10 | 16 | 28 | 38 | 63 |
| 8 AWG | 3 | 6 | 9 | 16 | 22 | 36 |
| 6 AWG | 1 | 4 | 7 | 12 | 16 | 26 |
| 4 AWG | 1 | 2 | 4 | 7 | 10 | 16 |
| 3 AWG | 1 | 1 | 3 | 6 | 8 | 13 |
| 2 AWG | 1 | 1 | 3 | 5 | 7 | 11 |
| 1 AWG | 1 | 1 | 1 | 4 | 5 | 8 |
| 1/0 AWG | 0 | 1 | 1 | 3 | 4 | 7 |
| 2/0 AWG | 0 | 1 | 1 | 2 | 3 | 6 |
| 3/0 AWG | 0 | 1 | 1 | 1 | 3 | 5 |
| 4/0 AWG | 0 | 0 | 1 | 1 | 1 | 4 |
These counts assume all conductors are the same size and none are compact conductors. For mixed sizes, add each conductor's own area and compare with the allowed area, as in mixed conductor sizes in conduit fill. For compact conductors, see compact conductor conduit fill. Counts for RMC, PVC and other raceways differ, so use the calculator or NEC Annex C for those.
Conduit Fill Worked Examples
Example 1: EV charger circuit. A 48A charger on a 60A circuit uses two 6 AWG THHN conductors and a 10 AWG THHN ground. Total area is 2 × 0.0507 + 0.0211 = 0.1225 in². Half-inch EMT allows 0.1216 in² at 40%, so it fails. Three-quarter inch EMT allows 0.2132 in², so it passes at 23.0% fill. Size the conductors first with our EV charger cable size calculator, and see our guide to the best Level 2 EV charger for circuit sizing.
Example 2: the rounding rule. How many 14 AWG THHN fit in 3/4" EMT? 0.2132 ÷ 0.0097 = 21.98. Because the decimal is 0.8 or more, Note 7 allows rounding up to 22.
Example 3: a short nipple. Twelve 12 AWG THHN conductors total 0.1596 in². In a normal 1/2" EMT run the 40% limit is 0.1216 in², so they do not fit. In a nipple of 24 in. or less the 60% limit is 0.1824 in², so they do. See conduit nipple fill rule.
Example 4: conduit size for a 100 amp service. Many US dwellings use three 4 AWG copper THHN conductors for a 100A service, but the conductor size depends on NEC 310.12, your utility and the local inspector. Three 4 AWG conductors total 0.2472 in². Three-quarter inch EMT allows 0.2132 in² so it fails. One-inch EMT allows 0.346 in², so it passes at 28.6% fill. With three 3 AWG conductors (0.2919 in²) one-inch EMT still passes, at 33.8%.
Example 5: how many 350 kcmil conductors fit in a 4-inch conduit? In 4-inch EMT the 40% area is 5.901 in². A 350 kcmil THHN conductor is about 0.5242 in², so about 11 fit (5.901 ÷ 0.5242 = 11.26). Confirm in Table 5 and Annex C, and recalculate for RMC or PVC.
Conduit Fill vs Ampacity Derating
Conduit fill answers one question: do the wires fit? Ampacity answers another: can they carry the current? A conduit can pass the fill calculation while the conductors still need derating because several current-carrying conductors share the raceway.
Under NEC Table 310.15(C)(1) in recent editions, the adjustment factor is 80% for 4 to 6 current-carrying conductors, 70% for 7 to 9, and 50% for 10 to 20. Check the table in your adopted edition. As a rule of thumb, six 10 AWG conductors plus a ground fit comfortably in 3/4" EMT (about 28% fill), yet their ampacity is still reduced to 80%.
For a quick check of grouping effects use our conduit fill derating calculator, and size the conductors with the AC cable size calculator, the DC cable size calculator or the motor cable size calculator. For loads, use the full load amperage calculator and the residential electrical load calculator. For solar strings sharing a raceway, first size the system with our solar panel size calculator by load.
Special Cases: Mixed Sizes, Cables, Flexible Conduit and Bends
- Mixed conductor sizes. Add each conductor's own area. Do not average sizes. See mixed conductor sizes in conduit fill.
- Romex and other multiconductor cables. A cable is treated as one conductor, measured by its major diameter if it is oval. See Romex in conduit calculation. Many inspectors discourage running NM cable in long conduit because of heat, so check local rules.
- Compact conductors. Use the actual dimensions from Table 5A or the manufacturer. See compact conductor conduit fill.
- Specialty cable. For instrumentation and control cable by outer diameter, see the Belden cable conduit fill calculator.
- Flexible and liquidtight conduit. Internal areas differ from EMT. See the flexible conduit fill chart and the liquidtight flexible nonmetallic conduit fill chart.
- PVC. Schedule 40 and Schedule 80 have different bores. See the PVC electrical pipe fill chart.
- Bends. Fill is not the only limit on pulling. NEC limits a run of EMT to the equivalent of four quarter bends (360 degrees total) between pull points (Article 358, section 358.26), and similar limits apply to other conduit types. For cable bends, use the cable bending radius calculator.
- Cable trays and trenches. Raceway rules do not apply. Use the cable tray fill calculator (NEC 392) or the cable trench fill calculator.
CSA Conduit Fill (Canada)
In Canada, conduit fill falls under the Canadian Electrical Code, CSA C22.1. It uses a percentage and table approach that is similar in spirit to the NEC but is not identical, and the details change between editions. Use the CSA mode of the calculator for planning and confirm against the CEC tables for your edition and your provincial amendments. If you are doing a house, see our guide to [home wiring under the Canadian Electrical Code]([ADD LINK TO YOUR CEC HOME WIRING GUIDE]).
IEC Conduit Fill
IEC 60364-5-52 does not give one universal fill percentage. Selection depends on the installation method, the cable construction, grouping and thermal conditions. The metric mode of the calculator compares the total cable cross-section with the conduit's internal area as a preliminary check only. Do not present an NEC 40% result as an IEC compliance result. See also our earthing cable size as per IEC and cable bending radius as per IEC guides.
BS 7671 Conduit Fill
In the UK, conduit sizing is usually done with the cable factor and conduit factor tables in the IET guidance rather than a single percentage. Use the calculator for a first check, then confirm with the guidance and the conduit manufacturer's data. For house wiring context, read our complete guide to home wiring under BS 7671.
AS/NZS 3000 Conduit Fill
In Australia and New Zealand, conduit selection follows AS/NZS 3000 and the conduit product standards, often supported by manufacturer capacity tables for each cable and conduit combination. The calculator's AS/NZS mode compares cable outer diameters against conduit internal area for preliminary sizing, so always check the manufacturer's table before you order. See our home wiring as per AS/NZS 3000 guide and the AS/NZS 3000 cable sizing table explained.
Common Conduit Fill Mistakes
- Using the conduit's outside diameter. The calculation needs the internal area from Table 4.
- Forgetting the grounding conductor. It counts.
- Using EMT numbers for PVC Schedule 80, which has a smaller bore.
- Using 31% for nipples. Nipples of 24 in. or less may use 60%.
- Using the nominal wire gauge instead of the insulated area, especially with different insulation types such as THHN and THW.
- Mixing up fill and ampacity. Passing one does not pass the other.
- Reusing an NEC answer for another standard.
- Ignoring bends and pull length, which can make a legal fill impossible to pull.
Tools for the Job
Once the size is settled, the right tools make the install cleaner. See our [conduit bender reviews]([ADD LINK TO YOUR CONDUIT BENDER REVIEWS CATEGORY]) and the best wire strippers for electricians. If the same circuit feeds an EV charger, see our guide to the best Level 2 EV charger.
Final Conduit Sizing Checklist
- Standard and edition confirmed (NEC, CEC, IEC, BS 7671, AS/NZS)
- Conduit type and trade size, using the correct internal area
- Every conductor counted, including the ground
- Correct fill limit (53%, 31%, 40% or 60% for a nipple)
- Conductor ampacity and adjustment factors checked
- Voltage drop checked
- Bends and pull length checked
- Local amendments and the inspector's requirements checked
Frequently Asked Questions
What is the maximum conduit fill under the NEC?
For three or more conductors it is 40% of the conduit's internal area. It is 53% for one conductor and 31% for two. Nipples of 24 in. or less may be filled to 60%.
How do I calculate conduit fill?
Add the cross-sectional areas of all conductors, divide by the conduit's internal area and multiply by 100. Compare the result with the applicable limit.
How many 12 AWG wires fit in 3/4 inch EMT?
Sixteen THHN conductors at the 40% limit, using the NEC Table 4 and Table 5 areas. The same calculation gives 9 in 1/2" EMT and 26 in 1" EMT.
What size conduit do I need for a 100 amp service?
Three 4 AWG copper THHN conductors need 1" EMT: they total 0.2472 in², which exceeds the 0.2132 in² allowed in 3/4" EMT. The conductor size depends on NEC 310.12, the utility and the local inspector.
Does the ground wire count toward conduit fill?
Yes. Equipment grounding and bonding conductors are included, using their actual dimensions.
Is conduit fill different for EMT, PVC and RMC?
Yes. The percentage limits are the same, but each conduit type has different internal areas, so the number of conductors that fit differs.
Can I use the 40% rule for IEC, BS 7671 or AS/NZS 3000?
No. The 40% value is an NEC rule. Other standards use their own methods.
Is there a conduit fill calculator for Australia, Canada or the UK?
Yes. Select AS/NZS 3000, CSA C22.1 or BS 7671 in the calculator. Treat the output as preliminary and confirm with your local standard and manufacturer data.
Does conduit fill affect ampacity?
Not directly, but more than three current-carrying conductors in a raceway require ampacity adjustment under the applicable code.
Is there a limit on the number of bends?
For EMT, NEC allows no more than the equivalent of four quarter bends (360 degrees) between pull points.
About This Page
Conductor and conduit areas on this page are taken from NEC Chapter 9 Tables 4 and 5 for THHN conductors and EMT. Table values and notes can change between editions, so verify against the NEC edition your jurisdiction has adopted. This tool and page are design aids, not a substitute for the code or a licensed electrician. Last reviewed 23 October 2026. Spotted an error? Tell us through our contact page.
Update log: 23 Oct 2026: corrected the nipple rule (qualifying nipples may be filled to 60%, not 31%); added the THHN in EMT chart, five worked examples, derating guidance and a comparison of standards; reworked the calculator instructions.
Sources
- NFPA 70, National Electrical Code, Chapter 9 Tables 1, 4, 5 and 5A, Informative Annex C, Article 358 and Table 310.15(C)(1) (view free on NFPA)
- CSA C22.1, Canadian Electrical Code
- IEC 60364-5-52
- BS 7671 and the IET On-Site Guide
- AS/NZS 3000 (Wiring Rules)
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Conduit Fill Calculator: Free NEC Tool + Fill Charts (2026)
Free conduit fill calculator for EMT, PVC and RMC with NEC 53/31/40% limits, nipple rule, THHN chart and worked examples. Also CSA, IEC, BS 7671, AS/NZS.
Price Currency: USD
Operating System: All
Application Category: UtilitiesApplication
