NEC Chapter 9 · Tables 4 & 5 · Annex C

How to use NEC Chapter 9 conduit and conductor dimensions

Raceway internal area (Table 4) + conductor area (Table 5) + the fill percentage (Table 1) = conduit size. Annex C is the same-size shortcut. Here is how they fit together, with a worked example and a free link to the official tables.

Need the actual tables? Look up Chapter 9 Tables 1, 4, 5 and Annex C in the free NFPA viewer →

Which Chapter 9 / Annex C table answers which question

Question on the job	Where to look	What it gives you
What percentage of the raceway can I fill?	Chapter 9, Table 1 (and its notes)	Allowable fill % based on how many conductors are in the raceway
How much room is inside this conduit?	Chapter 9, Table 4	Internal diameter and total internal area (sq in) by raceway type and trade size — plus pre-computed columns for common fill %
How big is this insulated wire?	Chapter 9, Table 5	Approximate diameter and area (sq in) by insulation type and size
Compact-stranded aluminum or compact copper?	Chapter 9, Table 5A	Areas for compact conductors (smaller than standard Table 5 values)
Bare conductor (e.g. bare EGC)?	Chapter 9, Table 8	Overall diameter/area of bare conductors
All one wire size and type — just give me the count	Annex C (one table per raceway type)	Maximum number of same-size, same-type conductors — pre-calculated from Tables 1, 4, 5

Our own summary, not the code text (2023 NEC). Look up Chapter 9 and Annex C in the free NFPA viewer (nfpa.org/freeaccess) and verify against the edition adopted by your AHJ.

How to run a conduit-fill calc with Tables 4 and 5, step by step

Step	What you do	Watch out for
1	Count every conductor in the raceway, including equipment grounding and bonding conductors	EGCs count toward fill (Chapter 9 notes). Only the notes-listed exceptions drop out
2	Use Table 1 to get the fill % for that conductor count	Short nipples (24 in. or less) get a higher allowance — check the Chapter 9 notes
3	In Table 5, find each conductor’s area by its exact insulation type and size	THHN/THWN-2, XHHW-2 and RHW-2 sit on different lines with different areas — use what is on the reel, not "close enough"
4	Add up the conductor areas	Mixed sizes: add each one; you cannot use Annex C for mixed fills
5	In Table 4, find the section for your raceway type (EMT, RMC, PVC-40, PVC-80, FMC, LFMC…)	The same trade size has different internal areas in different raceway types — never cross columns
6	Pick the smallest trade size whose allowable area (at your fill %) is at least your total	Table 4 often lists the area already multiplied by the common fill % — read the right column
7	All-same-size shortcut: check Annex C for the raceway type	Chapter 9 notes let you round up to the next whole conductor when the calculated fraction is 0.8 or more — Annex C already does this

Our own summary, not the code text (2023 NEC). Look up Chapter 9 and Annex C in the free NFPA viewer (nfpa.org/freeaccess) and verify against the edition adopted by your AHJ.

Worked example — how many 12 AWG THHN fit in 3/4" EMT?

Input / step	Value	Source
Conductor count	3 or more	Design
Allowable fill	40%	Chapter 9, Table 1
3/4" EMT total internal area	0.533 sq in	Chapter 9, Table 4 (EMT)
Allowable area at 40%	0.533 × 0.40 = 0.213 sq in	Our calculation
One 12 AWG THHN	0.0133 sq in	Chapter 9, Table 5
Conductors that fit	0.213 ÷ 0.0133 = 16.0 → 16	Our calculation
Cross-check	16 conductors	Annex C, EMT table

Worked-example values only — not the table. Check your edition in the free NFPA viewer; do the same math for your own raceway type and insulation.

Common field mistakes

Mistake	What to do instead
Leaving the EGC out of the count	Grounding and bonding conductors count toward fill — include them at their actual area (Table 5 if insulated, Table 8 if bare)
Using the EMT column for PVC or flex	Each raceway type has its own Table 4 section; Schedule 80 PVC is noticeably tighter than EMT at the same trade size
Using Annex C for a mixed fill	Annex C only works when every conductor is the same size and insulation — otherwise add areas from Table 5
Assuming THHN and XHHW are the same size	Look up the insulation actually being pulled; the areas differ, especially in larger sizes
Treating the fill limit as the pulling limit	Legal fill is a ceiling. Long runs, many bends, or big conductors may need a larger raceway for a clean pull — and jam ratio matters with three conductors
Forgetting derating	More than three current-carrying conductors in a raceway triggers the 310.15(C)(1) adjustment — fill and ampacity are separate checks

Our own summary, not the code text (2023 NEC). Look up Chapter 9 and Annex C in the free NFPA viewer (nfpa.org/freeaccess) and verify against the edition adopted by your AHJ.

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Three tables, one answer

Look up Chapter 9 and Annex C in the free NFPA viewer. NFPA offers free read-only access to current NEC editions. This page explains how to use the tables (2023 NEC); it does not reproduce them.

Conduit fill is an area problem. Table 5 tells you how much cross-section each insulated conductor takes up. Table 4 tells you how much cross-section is inside each raceway type and trade size. Table 1 tells you how much of that inside area you are allowed to use — the familiar 40% for three or more conductors, with different figures for one or two conductors and a higher allowance for short nipples. Add the wires, compare against the pipe, pick the smallest size that works.

Why the raceway type matters. A 1" EMT, a 1" rigid, and a 1" Schedule 80 PVC do not have the same inside diameter — wall thickness eats into the bore. Table 4 is split into a section per raceway type for exactly that reason. The most common take-off error we see is reading the EMT line and then pulling in Schedule 80.

Annex C is the shortcut, not the rule. When every conductor is the same size and insulation, Annex C has already done the Table 1/4/5 math for each raceway type and applied the Chapter 9 rounding note (a calculated fraction of 0.8 or more rounds up to the next whole conductor). Mixed sizes — say a feeder with a smaller EGC — need the long-hand calc.

Worked example. 3/4" EMT has a total internal area of 0.533 sq in; at 40% that is 0.213 sq in. A 12 AWG THHN is 0.0133 sq in, so 0.213 ÷ 0.0133 = 16 conductors — which matches Annex C. Swap in your own raceway type and insulation from the tables and the method is identical.

Skip the arithmetic with the free conduit fill calculator, or see the conduit fill chart for THHN. Remember fill and ampacity are separate checks: once you have more than three current-carrying conductors in a raceway, apply the adjustment factors covered in the ampacity correction and adjustment guide and size from the NEC 310.16 ampacity chart. Field PM keeps conduit schedules, pull records, and inspection photos with the job.

FAQ

How do NEC Chapter 9 Tables 4 and 5 work together?+

Table 5 gives the area of each insulated conductor, so you add up everything going in the pipe. Table 4 gives the internal area of each raceway type and trade size. Table 1 sets the fill percentage (40% for three or more conductors). The smallest raceway whose allowable area covers your conductor total is the minimum size. Look up the tables free at nfpa.org/freeaccess.

When can I use Annex C instead of doing the math?+

Annex C is a shortcut for fills where every conductor is the same size and insulation type. It has one table per raceway type and already applies the Chapter 9 rounding rule. For mixed sizes or insulation types, go back to Tables 4 and 5.

How many 12 AWG THHN fit in 3/4-inch EMT?+

Sixteen. Table 4 gives 3/4" EMT a total area of 0.533 sq in; 40% of that is 0.213 sq in. Table 5 gives 12 AWG THHN 0.0133 sq in, and 0.213 ÷ 0.0133 = 16. Annex C’s EMT table agrees. Verify against the edition your AHJ enforces.

Do ground wires count toward conduit fill?+

Yes. Equipment grounding and bonding conductors are counted toward fill. Use the Table 5 area for an insulated EGC and the Table 8 dimensions for a bare one.

Is Annex C part of the enforceable code?+

Annex C is informational — it is pre-calculated from the enforceable Chapter 9 tables and notes, which govern. If the two ever seem to disagree, the Chapter 9 calculation is what the inspector will hold you to.

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