NEC Conduit Fill: How Many Conductors Fit
Conduit fill is governed by NEC Chapter 9, Table 1: one conductor may occupy 53% of the raceway's internal area, two conductors 31%, and three or more 40%. The two-conductor figure is the one that surprises people — it is lower than the limit for three.
That is not a typo in the code. Two conductors pulled together tend to lie side by side and wedge, so the allowance is tightened to keep them from jamming. Three or more settle into a roughly circular bundle and pull more predictably.
This page covers how to size a raceway correctly, when you can skip the arithmetic entirely, and the mistakes that cause a rework. It is written for electricians, estimators and engineers specifying conduit and raceway for commercial and industrial work.
The three fill percentages
| Number of conductors | Maximum fill |
|---|---|
| One | 53% |
| Two | 31% |
| Over two | 40% |
These percentages are about the physical pull and about heat getting out — not about how much current the conductors may carry. A raceway can sit comfortably inside 40% and still require the conductors to be derated. Those are two separate calculations and the second one catches people out; see conductor ampacity, derating and the termination limit.
The nipple exception: 60%
A short raceway segment between two boxes or enclosures — a nipple, not exceeding 24 inches — may be filled to 60% under the Chapter 9 notes. Ampacity adjustment for more than three current-carrying conductors does not apply over that short a length either.
Both parts of that matter. A 24-inch nipple between a wireway and a panel is one of the few places you are allowed to pack a raceway, and one of the few places a high conductor count does not trigger derating.
Two ways to size the raceway
The short way: Annex C
Annex C is a set of pre-calculated tables, one per raceway type, that tell you directly how many conductors of a given size and insulation fit in a given trade size. No area arithmetic.
You may use it only when every conductor in the raceway is the same size and the same insulation type. Twelve 12 AWG THHN — Annex C. Nine 12 AWG THHN plus three 10 AWG XHHW — not Annex C, even though two of the three groups match. Mixed insulation disqualifies it just as surely as mixed sizes.
Annex C is informational rather than mandatory, which trips up some plan reviewers. It is a convenience built on the same Table 1 percentages, not a separate permission.
The long way: Tables 1, 4 and 5
When conductors differ, you calculate:
- Table 5 gives the cross-sectional area of each insulated conductor, by size and insulation type. (Table 5A covers compact conductors; Table 8 covers bare conductors.)
- Add the areas of every conductor going into the raceway.
- Table 4 gives the internal area of each raceway type and trade size, already worked out at the 53%, 31%, 40% and 60% columns.
- Pick the trade size whose allowable area at the right percentage is equal to or greater than your total.
A worked example: a conductor bundle totalling 1.1880 in² needs a raceway allowing at least that at 40%. Trade size 1½ RMC allows 0.829 in² — too small. Trade size 2 RMC allows 1.363 in² — that is the one.
Does the equipment grounding conductor count?
Yes. Equipment grounding and bonding conductors are included in the fill calculation — use Table 5 if they are insulated, Table 8 if bare. Leaving the EGC out of the total is one of the more common ways a conduit ends up a size too small on paper.
Rounding
Where a calculation produces a fractional number of conductors, the usual practice is to round up to the next whole conductor only when the decimal is 0.8 or greater. Confirm the rounding note in the edition you are working to before relying on it — this is one of the details that moves between editions.
Raceway type changes the answer
The percentages are the same for every raceway type. The areas are not, because wall thickness differs. At trade size 2:
| Raceway | Area available at 40% fill |
|---|---|
| RMC | 1.363 in² |
| EMT | 1.342 in² |
| Schedule 40 PVC | 1.316 in² |
| Schedule 80 PVC | 1.150 in² |
Schedule 80 PVC is the one to watch. Its thicker wall costs roughly 15% of the usable area against EMT at the same trade size, and because Schedule 80 is usually chosen where the raceway is exposed to physical damage, the substitution often happens late — after the conduit size is already on the drawing.
Use the table for the raceway you are actually installing. Sizing with EMT figures and installing Schedule 80 is the single most common fill error.
Before you order
- Count every conductor going in, including the EGC.
- Confirm the insulation type for each — THHN and XHHW of the same gauge are not the same area.
- Decide whether Annex C applies: all the same size, all the same insulation.
- If not, total the Table 5 areas and compare against Table 4 for the raceway type you are installing.
- Check whether the run is a nipple of 24 inches or less, which changes the limit to 60%.
- Separately, check ampacity adjustment for more than three current-carrying conductors. Fill and derating are different calculations.
- Confirm the figures against the NEC edition adopted in your jurisdiction.
Sourcing conduit, fittings and wire
ATEK Distribution is an electrical distributor based in Minneapolis, Minnesota, supplying contractors, industrial facilities and government buyers nationwide. Send a conduit schedule, a drawing set or a bill of materials and we will quote the raceway, fittings and conductors together with lead times you can build a schedule on.
Frequently asked questions
What is the maximum conduit fill allowed by the NEC?
The maximum conduit fill depends on how many conductors are in the raceway: 53% for a single conductor, 31% for two, and 40% for three or more, under NEC Chapter 9, Table 1. A nipple not exceeding 24 inches between enclosures may be filled to 60%.
Why is the two-conductor fill limit lower than the three-conductor limit?
The two-conductor limit is lower because two conductors pulled together tend to lie side by side and wedge against the raceway wall rather than settling into a round bundle. The tighter 31% allowance reduces the risk of jamming during the pull. Three or more conductors arrange themselves more circularly and pull more predictably.
Does the ground wire count toward conduit fill?
Yes, the equipment grounding conductor counts toward conduit fill. Use its area from Chapter 9 Table 5 if it is insulated, or Table 8 if it is bare, and include it in the total before comparing against the allowable area in Table 4.
When can I use Annex C instead of calculating?
You can use Annex C when every conductor in the raceway is the same size and the same insulation type. If the sizes differ, or the insulation types differ, Annex C does not apply and you have to total the conductor areas from Table 5 and compare them with Table 4.
Is conduit fill the same as derating?
No. Conduit fill limits how much of the raceway's internal area the conductors may occupy, and it is about pulling and heat dissipation. Ampacity adjustment, often called derating, reduces how much current each conductor may carry when more than three current-carrying conductors share a raceway. A conduit can satisfy the 40% fill limit and still require derating.
Does conduit fill differ between EMT, PVC and rigid?
The fill percentages are identical for every raceway type, but the internal areas differ because wall thickness differs. At trade size 2, RMC allows 1.363 in² at 40% fill while Schedule 80 PVC allows 1.150 in². Always use the Table 4 row for the raceway you are actually installing.
Related guides
- Conductor ampacity, derating and the termination limit — the calculation that runs alongside this one.
- EMT vs rigid conduit — choosing the raceway type before you size it.
- Conduit, raceway and cable support — the product category.
- Wire, cords and cables — conductors and insulation types.
Sources and edition note
Figures on this page follow NEC Chapter 9, Tables 1, 4, 5, 5A and 8, and Annex C, as reported by electrical code reference publishers writing against the 2023 edition. NEC editions are adopted state by state and amended locally. Confirm every value against the edition adopted in your jurisdiction before relying on it for an installation. This page is general guidance and does not replace the National Electrical Code or the judgement of the authority having jurisdiction.
Send a conduit schedule, a drawing set or a bill of materials. ATEK Distribution supplies conduit, fittings and wire to contractors, industrial facilities and government buyers nationwide from Minneapolis, Minnesota. Quote responses go out within one business day, often the same day.