Check whether your THHN conductors fit inside EMT — fill percentage vs. the NEC Chapter 9 limit, before you pull a single foot.
NEC Chapter 9, Table 1 sets the maximum fill percentage for conductors in a raceway. The limit depends on how many conductors you're running: one conductor may fill up to 53% of the conduit's internal cross-sectional area, two conductors up to 31%, and three or more conductors up to 40%. These limits exist to prevent overheating and to make future pull-through possible.
Table 4 gives the internal cross-sectional area for each conduit type and trade size. Table 5 gives the cross-sectional area of each conductor by type and size. Multiply the conductor area from Table 5 by your quantity, then divide by the conduit area from Table 4 — that's your fill percentage. Compare it to the Table 1 limit for your quantity.
| Conductors | Max fill % | NEC Reference |
|---|---|---|
| 1 | 53% | Ch.9 Table 1, Note 1 |
| 2 | 31% | Ch.9 Table 1 |
| 3 or more | 40% | Ch.9 Table 1 |
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