Electrician: Conduit Fill Percent

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The NEC caps conduit fill at 40% for three or more conductors, and going over means a hard pull and derated wire. Total the conductor area, divide by the conduit's inside area, and let the sheet flag the overfills before you pull.

Quick formula: Conductor area over conduit area, rounded:

=ROUND(B2*C2/D2,4)

Three #12 THHN at 0.0133 in² each in 1/2-inch EMT (0.304 in²) fills 13.1% — well under the 40% limit.

Functions used (tap for the full reference guide):

ROUND IF

The example

Three runs with their conductor count, per-conductor area, and conduit inside area. The status flags anything over 40%.

	A	B	C	D	E	F
1	Run	Wires	Wire in²	Conduit in²	Fill	Status
2	Lighting	3	0.0133	0.304	13.13%	OK
3	Receptacles	8	0.0133	0.304	35.00%	OK
4	Subpanel feed	4	0.0507	0.304	66.71%	Over

The formula

The total conductor area divided by the conduit area is the fill; IF compares it to the 40% limit:

=ROUND(B2*C2/D2,4) // conductors x wire area, over conduit inside area

How it works

Area over area, then a code check:

B2*C2 is the total copper area — the conductor count times each conductor's cross-section.
/D2 divides by the conduit's inside area to get the fill as a fraction.
ROUND(...,4) keeps four decimals so the percent format shows two.
=IF(E2>0.4,"Over","OK") flags any run past the 40% ceiling for three or more conductors.

The limit changes with conductor count: 53% for one conductor, 31% for two, and 40% for three or more. Swap the 0.4 in the IF to match the run.

Try it: interactive demo

Interactive

Enter the conductor count, per-conductor area, and conduit inside area.

Conductors

Wire area (in²)

Conduit area (in²)

Variations

Adjust the limit by count

One conductor allows 53%, two allow 31%; pick the ceiling from the count.

=IF(B2=1,0.53,IF(B2=2,0.31,0.4))

Spare capacity left

How much conduit area remains under the 40% cap.

=D2*0.4-B2*C2

Pitfalls & errors

Use the conductor's total area including insulation, not the bare copper. NEC Chapter 9, Table 5 lists the approximate area for each wire type — THHN and XHHW of the same gauge differ.

Mixed wire sizes in one conduit? Sum each size's area separately instead of multiplying one area by the count. The single-area shortcut only works when every conductor matches.

Practice workbook

📊

Download the free Electrician: Conduit Fill Percent practice workbook

Edit the yellow wire-count, wire-area, and conduit-area cells; fill and the code check recalculate.

Frequently asked questions

Where do the conduit and wire areas come from?

NEC Chapter 9: Table 4 lists the inside area of each conduit type and size, and Table 5 lists the area of each conductor. Copy the sizes you actually stock into a small lookup and point the formula at it.

Is 40% a hard limit?

For three or more conductors, yes — it is a code maximum, not a guideline. Go over and the wire may need derating or simply will not pull. Stay under it and leave room for future adds.

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Function references: ROUND IF