Conduit Fill Calculator (NEC): THHN in EMT, PVC and RMC
Pick the conduit type and trade size, enter how many THHN or THWN-2 conductors of each size you are pulling, and the calculator shows the fill percentage against the NEC limit, plus the smallest trade size that would take the same conductors. Up to four groups of different sizes can be combined, so a typical circuit with line, neutral and equipment grounding conductor goes in as two rows.
How conduit fill is calculated
Conduit fill is the cross-sectional area of all the conductors divided by the internal area of the conduit. The NEC caps it in Chapter 9, Table 1:
| Number of conductors | Maximum fill |
|---|---|
| 1 | 53 % |
| 2 | 31 % |
| 3 or more | 40 % |
The odd-looking 31 % for two conductors is deliberate. Two round conductors side by side in a round pipe jam more easily than one or three, so the limit is tighter.
The internal areas come from Chapter 9, Table 4, which lists every conduit type separately. EMT, PVC Schedule 40, PVC Schedule 80 and rigid metal conduit of the same trade size all have different internal diameters, which is why changing the conduit type changes the answer. The conductor areas come from Table 5, which gives the total area of each conductor including its insulation. The calculator uses the THHN/THWN-2 column, the conductor found in most commercial pulls.
Worked example
A 20 A circuit in 3/4″ EMT: two 12 AWG THHN for line and neutral and one 12 AWG THHN for the equipment grounding conductor, plus a second circuit sharing the run, another two 12 AWG.
- Conductors: 5 × 12 AWG = 5 × 0.0133 in² = 0.0665 in².
- 3/4″ EMT internal area: 0.533 in².
- Fill: 0.0665 ÷ 0.533 = 12.5 %, well inside the 40 % limit.
Conduit fill is rarely the limit on a run like this. The thing that catches you is derating: with four current-carrying conductors in the same raceway, NEC 310.15(C)(1) reduces their ampacity to 80 %. Fill tells you whether the wires will physically go in; it says nothing about whether they will run cool once they are there.
Maximum number of THHN conductors in EMT
When all the conductors are the same size, Chapter 9 Note 7 lets you round up to the next whole conductor if the calculation ends in 0.8 or more. The table includes that rounding, which is why it matches the figures in NEC Annex C.
| EMT size | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG | 4 AWG |
|---|---|---|---|---|---|---|
| 1/2″ | 12 | 9 | 5 | 3 | 1 | 1 |
| 3/4″ | 22 | 16 | 10 | 6 | 4 | 2 |
| 1″ | 35 | 26 | 16 | 9 | 7 | 4 |
| 1-1/4″ | 61 | 45 | 28 | 16 | 12 | 7 |
| 1-1/2″ | 84 | 61 | 38 | 22 | 16 | 10 |
| 2″ | 138 | 101 | 63 | 36 | 26 | 16 |
Notes from the field
- Nipples are an exception. A conduit section no longer than 24 inches between enclosures may be filled to 60 %, and derating does not apply to it (Chapter 9, Note 4).
- A legal fill is not always an easy pull. Long runs with several bends can be at 30 % and still need a lot of lubricant and patience. Three conductors whose combined diameter is close to the conduit’s internal diameter can jam at a bend; the jam ratio of conduit ID to conductor OD is worst between about 2.8 and 3.2.
- Bare and compact conductors use different areas. A bare copper grounding conductor is smaller than an insulated one; its area is in Chapter 9, Table 8. Compact stranding has its own column in Table 5A. The calculator treats every conductor as standard THHN, which errs on the safe side for a bare ground.
- Check your code edition. The figures here follow the current NEC tables. Local amendments occasionally differ, and outside North America conduit fill is handled by the cable manufacturer’s data and national rules rather than these tables.
Related tools: voltage drop calculator, AWG calculator, wire gauge chart.