Ask Shorty

Conduit fill calculator

Choose the raceway, the insulation type, the wire size, and how many you're pulling. You get the smallest trade size that fits and the maximum count for each size.

Built on the 2026 NEC · Free, no sign-up · Updated October 2026

AWG (for example 14, 12, 10) or kcmil (for example 250, 350).

All conductors must be the same type and size.

The result updates as you type.

Result

Smallest fit: 3/4 (21)

Maximum conductors per trade size (Annex C)

1/2 (16) trade size
9
3/4 (21) trade size
16
1 (27) trade size
26
1-1/4 (35) trade size
45
1-1/2 (41) trade size
61
2 (53) trade size
101
2-1/2 (63) trade size
176
3 (78) trade size
266
3-1/2 (91) trade size
347
4 (103) trade size
443
5 (129) trade size
666
6 (155) trade size
941

How it was calculated

  1. Locate the THHN 12 row in table-C.1.
  2. Smallest trade size whose max-count ≥ 16 → 3/4 (21).

Assumptions

  • EMT conduit per Annex C table-C.1.
  • All conductors are THHN 12 AWG/kcmil (mixed sizes aren't supported by Annex C lookup — that case requires Chapter 9 Table 4/5 area math).
  • Counts are for concentric-stranded conductors. For compact-stranded conductors, use Table C.1(A).
  • Annex C is informational; the controlling rule is the percent-fill limit in Chapter 9 Table 1.

Sections used

Chapter 9 Note 1Annex C, Table C.1

These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.

How conduit fill works

Chapter 9, Table 1 limits how much of a raceway's inside area the conductors can take up: 53% for one conductor, 31% for two, and 40% for three or more. Annex C applies that 40% limit for you. It lists how many conductors of one type and size fit in each trade size of each raceway.

This calculator looks up your conductor in the Annex C table for the raceway you pick and finds the smallest trade size where your count fits.

When you need the long method

Annex C only works when every conductor is the same type and size. If you're pulling mixed sizes, such as three 10 AWG and a 12 AWG ground, add up each conductor's area from Chapter 9, Table 5 (or Table 8 for bare conductors). Then pick the trade size whose 40% column in Table 4 is at least that total.

A nipple 24 in. or shorter between enclosures can be filled to 60%, and the more-than-three-conductor derating doesn't apply to it.

Fill is not the whole story

  • A full raceway is hard to pull. Long runs and lots of bends call for a bigger size than the minimum.
  • More than three current-carrying conductors in one raceway means you have to derate their ampacity. Run the numbers with the ampacity derating calculator.
  • Equipment grounding conductors count toward fill, even though they don't count for derating.

Worked example

Sixteen 12 AWG THHN conductors in EMT:

  • Conduit or tubing type: EMT
  • Conductor insulation type: THHN
  • Conductor size: 12
  • Number of conductors: 16

Result

Smallest fit: 3/4 (21)

Maximum conductors per trade size (Annex C)

1/2 (16) trade size
9
3/4 (21) trade size
16
1 (27) trade size
26
1-1/4 (35) trade size
45
1-1/2 (41) trade size
61
2 (53) trade size
101
2-1/2 (63) trade size
176
3 (78) trade size
266
3-1/2 (91) trade size
347
4 (103) trade size
443
5 (129) trade size
666
6 (155) trade size
941

How it was calculated

  1. Locate the THHN 12 row in table-C.1.
  2. Smallest trade size whose max-count ≥ 16 → 3/4 (21).

Assumptions

  • EMT conduit per Annex C table-C.1.
  • All conductors are THHN 12 AWG/kcmil (mixed sizes aren't supported by Annex C lookup — that case requires Chapter 9 Table 4/5 area math).
  • Counts are for concentric-stranded conductors. For compact-stranded conductors, use Table C.1(A).
  • Annex C is informational; the controlling rule is the percent-fill limit in Chapter 9 Table 1.

Sections used

Chapter 9 Note 1Annex C, Table C.1

These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.

Questions electricians ask

What does a conduit fill calculator do, and does it size conduit under the NEC?

It finds the smallest conduit size that holds your conductors. You pick the raceway (EMT, IMC, RMC, or PVC Schedule 40), the insulation type, the wire size, and the count. This calculator follows the NEC 2026 method: the 40% limit in Chapter 9, Table 1, applied through the Annex C tables.

How many 12 AWG THHN fit in 3/4 in. EMT?

16. Annex C, Table C.1 allows sixteen 12 AWG THHN conductors in 3/4 in. EMT. The calculator lists the maximum count for every trade size, so you can read off any size you're considering.

Does the ground wire count toward conduit fill?

Yes. Every conductor in the raceway counts toward the fill percentage, including equipment grounding conductors.

Can I mix wire sizes?

Yes, but not with Annex C. Add each conductor's area from Chapter 9, Table 5 and compare the total to the 40% column of Table 4 for your raceway.

Why is the conduit fill limit 40%?

Table 1 in Chapter 9 sets 40% for three or more conductors so the wires can be pulled without damaging the insulation and so heat can dissipate.

What is the fill percentage for one or two conductors?

Chapter 9, Table 1 sets 53% for one conductor and 31% for two. Three or more conductors get 40%. Annex C is built on the 40% limit, so for exactly one or two conductors, add up the conductor area from Table 5 and compare it to the 53% or 31% column of Table 4.

Does it work for data cable, low-voltage cable, or the Canadian code?

No. It covers the building-wire types in Annex C, such as THHN, THWN, and XHHW, in the four raceways listed above. It does not size conduit for communications or low-voltage cable, and it is built on the NEC, not the Canadian code.

Practice it hands-on

Drop wires into a pipe drawn to scale, and pick the smallest pipe that stays under the fill limit.

Try Conduit Fill

Teach it in one class

Free slides, quiz, and instructor note: Conduit fill (NEC 2026 Chapter 9, Tables 1, 4, and 5).

Get the lesson

How this calculator was built

The calculator runs the same code as the Ask Shorty iPhone app. Automated tests check it against hand-worked answers, and every section it names is checked against the 2026 NEC text. It shows section numbers, not code text. Read the sections in your own code book and confirm which edition your AHJ enforces.

How we test the calculators. See something wrong? Tell us and we will check it against the code.

Related calculators

Have a code question the calculator doesn’t cover?

Ask Shorty in plain English. You get a straight answer with the NEC 2026 section it came from.

Ask a question