Raceway
Conduit Fill Calculator
Pick the conduit and the conductors. This looks up both areas and applies the right fill limit for your conductor count.
Reference data and planning arithmetic. Does not apply ambient or bundling derating, motor rules, or local amendments. Not a substitute for the NEC edition your jurisdiction has adopted, an inspection, or a licensed electrician.
How many wires fit in 1/2 inch conduit?
Nine 12 AWG THHN conductors fit in 1/2 inch EMT. Half-inch EMT has 0.304 in² of internal area, 40% of which is 0.1216 in², and 12 AWG THHN is 0.0133 in² each — so nine fit at 0.1197 in² and ten do not.
For 14 AWG THHN it is twelve conductors; for 10 AWG, five.
The 40% limit only applies to three or more conductors. NEC Chapter 9 Table 1 gives 53% for a single conductor and 31% for exactly two — the two-conductor case is the tightest of the three and the one most often got wrong.
Conduit type matters at the same nominal size: 1/2 inch Schedule 40 PVC is 0.285 in² and Schedule 80 PVC only 0.217 in², 29% less than EMT.
The 40% limit is not always 40%
Everyone quotes 40% conduit fill. That figure is correct for three or more conductors, and it is wrong for one or two. NEC Chapter 9 Table 1 gives three limits:
| Conductors in the raceway | Fill limit | Usable area in 1/2 in EMT | 12 AWG THHN that fit |
|---|---|---|---|
| 1 conductor | 53% | 0.1611 in² | 1 |
| 2 conductors | 31% | 0.0942 in² | 2 |
| 3 or more conductors | 40% | 0.1216 in² | 9 |
Why the limits go that way
It looks backwards — fewer conductors, tighter limit — until you think about packing. The percentages come from the geometry of how round objects nest inside a round tube, and from leaving room to pull.
A single conductor can use a larger share because it sits in the middle with clearance all round. Two round conductors in a round conduit pack very inefficiently and jam against each other, so the allowance drops sharply. Three or more nest into the space better, and 40% is the long-standing compromise between pulling tension and heat dissipation.
The limits exist for two reasons: you have to be able to pull the conductors without damaging the insulation, and the bundle has to shed heat. Overfilled conduit fails on both counts, and a nicked conductor inside a wall is a fault waiting to happen.
Where the numbers come from
Three tables, all in NEC Chapter 9, and this calculator holds all three:
- Table 1 — the fill percentage, by conductor count.
- Table 4 — total internal area of each conduit type and size. Note that EMT, rigid, PVC 40 and PVC 80 all differ at the same nominal size: half-inch EMT is 0.304 in² while half-inch PVC 80 is only 0.217 in², a 29% difference.
- Table 5 — area of each conductor including insulation. THHN is notably slimmer than XHHW or THW at the same gauge, which is why THHN is so often specified in tight conduit.
| Size | THHN/THWN-2 | XHHW-2 | THW/THHW | Fit in 1/2 in EMT |
|---|---|---|---|---|
| 14 AWG | 0.0097 | 0.0139 | 0.0139 | 12 |
| 12 AWG | 0.0133 | 0.0181 | 0.0181 | 9 |
| 10 AWG | 0.0211 | 0.0243 | 0.0243 | 5 |
| 8 AWG | 0.0366 | 0.0437 | 0.0437 | 3 |
| 6 AWG | 0.0507 | 0.059 | 0.0726 | 2 |
| 4 AWG | 0.0824 | 0.0814 | 0.0973 | 1 |
| 3 AWG | 0.0973 | 0.0962 | 0.1134 | 1 |
| 2 AWG | 0.1158 | 0.1146 | 0.1333 | 1 |
| 1 AWG | 0.1562 | 0.1534 | 0.1901 | none |
| 1/0 AWG | 0.1855 | 0.1825 | 0.2223 | none |
| 2/0 AWG | 0.2223 | 0.219 | 0.2624 | none |
| 3/0 AWG | 0.2679 | 0.2642 | 0.3117 | none |
| 4/0 AWG | 0.3237 | 0.3197 | 0.3718 | none |
| 250 kcmil | 0.397 | 0.3904 | 0.4596 | none |
| 300 kcmil | 0.4608 | 0.4536 | 0.5281 | none |
| 350 kcmil | 0.5242 | 0.5153 | 0.5958 | none |
| 500 kcmil | 0.7073 | 0.6984 | 0.7901 | none |
What this does not cover
Conductor bundling derates ampacity. This is the thing most likely to catch you: more than three current-carrying conductors in a raceway reduces how much current each can carry — 80% for four to six, 70% for seven to nine, 50% for twenty to thirty. A conduit can pass the fill calculation and still be unusable because the conductors in it no longer have the ampacity you need. Neutrals and equipment grounds usually do not count as current-carrying.
Also outside this calculation: nipples shorter than 24 inches are allowed 60% fill; conduit bodies and boxes have their own rules; and every bend adds pulling tension, with 360° the maximum between pull points.
Common questions
How many 12 AWG wires fit in 1/2 inch EMT?
Nine THHN conductors. Half-inch EMT has 0.304 in² total area, 40% of which is 0.1216 in², and 12 AWG THHN is 0.0133 in² each — so nine fit at 0.1197 in² and ten do not.
Is conduit fill always 40%?
No. NEC Chapter 9 Table 1 gives 53% for one conductor, 31% for two, and 40% for three or more. The two-conductor case is the one people get wrong, and it is the tightest limit of the three.
Do equipment grounding conductors count toward conduit fill?
Yes, for fill they count like any other conductor — they take up space. They usually do not count as current-carrying for ampacity derating, which is a separate calculation.
Why does the same size PVC hold less than EMT?
Thicker walls. Half-inch EMT has 0.304 in² of internal area, Schedule 40 PVC has 0.285, and Schedule 80 PVC only 0.217 — 29% less than EMT at the same nominal size.