Everyone on site quotes the “40% rule,” but few can say when it actually applies - or what the limits are when it does not. Conduit fill matters because cramming too many conductors into a raceway makes them hard to pull (damaging insulation) and traps heat, reducing ampacity. The rules live in NEC Chapter 9 and its companion tables, and they are more nuanced than a single percentage. Here is what the Code really requires.
The fill limits depend on conductor count
NEC Chapter 9, Table 1sets the maximum percentage of a conduit's interior cross-sectional area that conductors may occupy, and it changes with how many conductors you install:
- 1 conductor - maximum fill 53%
- 2 conductors - maximum fill 31%
- 3 or more conductors - maximum fill 40%
So the famous “40% rule” only applies to the most common case: three or more conductors. The lower 31% limit for two conductors exists because two circles pack inefficiently and leave little room to pull; the higher 53% for a single conductor reflects that a lone wire pulls easily.
The two areas you need
A conduit-fill check is simply a comparison of two areas: the total area of all the conductors against the allowed fraction of the conduit's internal area.
- Conductor area - from Chapter 9, Table 5 (insulated conductors, by insulation type and size) or Table 8 (bare conductors). Each insulation type (THHN, XHHW, RHH, etc.) has a different outer diameter, so the same AWG can occupy different areas.
- Conduit area - from Table 4, which lists the internal area of each trade size and conduit type (EMT, rigid, PVC, etc.), plus the pre-computed 53% / 31% / 40% allowable fills.
Note that conduit dimensions differ by type - a trade size 1″ EMT, rigid metal conduit and PVC Schedule 40 each have slightly different internal areas - so always read Table 4 for the specific material you are installing.
The calculation, step by step
- List every conductor, its size, and its insulation type.
- Look up each conductor's area in Table 5 and add them together to get the total fill area.
- Count the conductors to choose the fill limit (53/31/40%).
- Divide total conductor area by the fill percentage to get the minimum required conduit internal area.
- In Table 4, choose the smallest trade size of your conduit type whose allowable-fill area at that percentage meets or exceeds your total conductor area.
Worked example with mixed AWG sizes
Suppose you are pulling the following THHN copper conductors through EMT:
- 3 × 8 AWG THHN - area 0.0366 in² each = 0.1098 in²
- 2 × 10 AWG THHN - area 0.0211 in² each = 0.0422 in²
- 1 × 12 AWG THHN (ground) - area 0.0133 in²
Total conductor area = 0.1098 + 0.0422 + 0.0133 = 0.1653 in². There are 6 conductors, so the three-or-more limit of 40% applies.
- From Chapter 9 Table 4, trade size 3/4″ EMT has an internal area of 0.533 in² and a 40% allowable fill of 0.213 in².
- 0.1653 in² (our conductors) ≤ 0.213 in² (allowed) → 3/4″ EMT is adequate.
- A 1/2″ EMT (40% fill = 0.122 in²) would be over-filled, so it is not permitted.
The mistakes that fail inspections
- Counting the wrong conductors for ampacity vs. fill. For fill, every physical conductor counts, including equipment grounding conductors. For ampacity derating (a separate check), grounding conductors and a balanced neutral usually do not count.
- Using the wrong insulation area. THHN is slimmer than RHH/RHW with an outer covering - substituting insulation types changes the area and can tip you over the limit.
- Forgetting nipples.A raceway not over 24 inches long (a “nipple”) is permitted a 60% fill under Chapter 9 Note 4.
- Ignoring derating. Passing the fill check does not exempt you from the conductor bundling ampacity adjustments of 310.15(C)(1) once you exceed three current-carrying conductors.
Let the calculator handle the tables
Looking up Table 4 and Table 5 by hand is slow and error-prone. The free Voltix conduit fill calculator (NEC) sums the conductor areas, applies the correct 53/31/40% limit and returns the smallest compliant trade size. Because more than three current-carrying conductors also triggers ampacity derating, follow up with the wire size calculator (NEC) and read our NEC wire sizing guide to see how fill and derating interact.