Enter conduit internal area and the sum of conductor cross-sectional areas, including insulation. The conductor count used for fill includes conductors such as an equipment grounding conductor even when they do not count as current-carrying for an ampacity adjustment. The standard example limits are 53% for one, 31% for two and 40% for more than two, subject to the applicable rule and exceptions.
The usual conductor-count fill limits have exceptions, including particular nipples and cable configurations. Fill and ampacity adjustment are separate calculations using potentially different counts. The result does not check pulling force, bend limits, conductor protection, temperature or local adoption.
Do not infer electrical approval from a spare-area number.
Conduit fill from conduit internal area
| Conduit internal area | 0.533 |
|---|---|
| Total conductor area | 0.04 |
| Number of conductors | 3 |
| Conduit fill | 7.5% |
| Spare conductor area at the allowed fill (sq in) | 0.173 |
Common error: Using the copper area instead of the full area with insulation. The insulation takes up room, and the fill limit counts the whole conductor.
Conduit Fill: formula
Conduit Fill: changing Conduit internal area
| Conduit internal area | Conduit fill | Spare conductor area at the allowed fill (sq in) |
|---|---|---|
| 0.43 | 9.3% | 0.132 |
| 0.48 | 8.3% | 0.152 |
| 0.53 | 7.5% | 0.172 |
| 0.59 | 6.8% | 0.196 |
| 0.64 | 6.2% | 0.216 |
Conduit Fill: compare input ranges
| Input | Tested from | To | Conduit fill: low input | Conduit fill: high input | Swing |
|---|---|---|---|---|---|
| Conduit internal area | 0.48 | 0.59 | 8.3% | 6.8% | 1.5% (21%) |
| Total conductor area | 0.036 | 0.044 | 6.8% | 8.3% | 1.5% (20%) |
| Number of conductors | 2 | 4 | 7.5% | 7.5% | 0.000% |
Conduit Fill: individual input comparisons
| Input | Value | Conduit fill | Spare conductor area at the allowed fill (sq in) |
|---|---|---|---|
| Conduit internal area | 0.43 | 9.3% | 0.132 |
| 0.48 | 8.3% | 0.152 | |
| 0.53 | 7.5% | 0.172 | |
| 0.59 | 6.8% | 0.196 | |
| 0.64 | 6.2% | 0.216 | |
| Total conductor area | 0.032 | 6.0% | 0.181 |
| 0.036 | 6.8% | 0.177 | |
| 0.04 | 7.5% | 0.173 | |
| 0.044 | 8.3% | 0.169 | |
| 0.048 | 9.0% | 0.165 | |
| Number of conductors | 1 | 7.5% | 0.243 |
| 2 | 7.5% | 0.125 | |
| 3 | 7.5% | 0.173 | |
| 4 | 7.5% | 0.173 | |
| 5 | 7.5% | 0.173 |
Conduit fill: Conduit internal area against Total conductor area
| Conduit internal area / Total conductor area | 0.032 | 0.04 | 0.048 |
|---|---|---|---|
| 0.43 | 7.4% | 9.3% | 11.2% |
| 0.48 | 6.7% | 8.3% | 10.0% |
| 0.53 | 6.0% | 7.5% | 9.1% |
| 0.59 | 5.4% | 6.8% | 8.1% |
| 0.64 | 5.0% | 6.3% | 7.5% |
Conduit Fill: values and definitions
| Quantity and definition | Value used |
|---|---|
| Conduit internal area — Internal cross-sectional area of the conduit, in square inches (from NEC Chapter 9 Table 4). | 0.533 |
| Total conductor area — Combined cross-sectional area of all conductors, in square inches (from NEC Chapter 9 Table 5). | 0.04 |
| Number of conductors — Total insulated/covered conductor count for the applicable fill rule, including grounding conductors; this is different from the current-carrying count used for ampacity adjustment. | 3 |
| Conduit fill | 7.5% |
| Spare conductor area at the allowed fill (sq in) | 0.173 |
Conduit internal area: half and double
| Conduit internal area | Conduit fill |
|---|---|
| 0.267 | 15.0% |
| 0.533 | 7.5% |
| 1.1 | 3.7% |
Conductors in a conduit need room for heat to escape and for pulling, so the code limits how much of the conduit's cross-section they may fill. For more than two conductors the limit is commonly 40 percent, for two it is 31 percent and for one it is 53 percent. Three conductors with a combined area of 0.0399 square inches in a conduit with 0.533 square inches inside fill about 7.5 percent, leaving about 0.173 square inches of spare room at the limit.
Use the conductor areas from the code table for the insulation type, and confirm the limits in the edition your area has adopted. Bends and length affect how hard the pull will be.
See wire ratings on the wire ampacity calculator, box capacity on the electrical box fill page and voltage loss on the voltage drop calculator. The watts to amps and three-phase power calculators size circuits, and the circle area page relates to conduit cross-sections.