Raceway Fill and Ampacity Calculator
Check mixed-conductor raceway fill and review conductor ampacity after ambient and conductor-count adjustments.
Installation details
Starts with a listed manufacturer dimension. Verify or replace it with the exact product's inside diameter.
The conductor-count exception applies only when this is a nipple between enclosures no longer than 24 in.
Use the temperature around the conductors. Supported range: -40°F to 185°F.
Use equipment markings and applicable terminal rules.
Applies the short-nipple fill and conductor-count exceptions when the length is 24 in. or less.
Conductor group 1
Set to zero for equipment grounding conductors or other conductors that do not count as current carrying.
Prefilled for selected Southwire series and sizes where listed. Verify the exact product and edit when needed.
Optional. Use the base ampacity requirement from the feeder sizing worksheet.
Optional. Use the adjusted-load requirement from the feeder sizing worksheet.
Conductor group 2
Set to zero for equipment grounding conductors or other conductors that do not count as current carrying.
Prefilled for selected Southwire series and sizes where listed. Verify the exact product and edit when needed.
Optional. Use the base ampacity requirement from the feeder sizing worksheet.
Optional. Use the adjusted-load requirement from the feeder sizing worksheet.
Calculation results
Entered conductor area is within the selected fill limit. Raceway area: 0.533 in².
| Group | Conductor | Qty. / CCC | Base ampacity | Ambient factor | CCC factor | Adjusted ampacity | Load checks |
|---|---|---|---|---|---|---|---|
| 1 | copper 12 AWG | 3 / 3 | 25.0 A | × 1.00 | × 1.00 | 25.0 A | Entered checks pass |
| 2 | copper 12 AWG | 1 / 0 | — | — | — | — | Not current carrying |
Fill includes every conductor in the groups. Ampacity adjustment uses the combined current-carrying count and reports available ampacity per loaded conductor.
About this calculator
Check a mixed group of insulated conductors in EMT, IMC, RMC, or Schedule 40/80 PVC. The worksheet calculates total conductor fill, current-carrying conductor count, ambient correction, conductor-count adjustment, and terminal-limited ampacity for each loaded conductor group.
Use manufacturer dimensions for the exact raceway and conductor products. The provided dimensions are editable references, not a guarantee that another manufacturer's product has the same inside or outside diameter.
Method and assumptions
Fill area is the sum of the cross-sectional areas of all entered conductors. The general limits are 53% for one conductor or cable, 31% for two, and 40% for more than two. A marked nipple between enclosures that is no longer than 24 in. can use the 60% fill allowance.
The tool adds the current-carrying quantities entered for all groups and uses that count to adjust ampacity. The conductor-count exception applies only to an identified nipple between enclosures no longer than 24 in.; ambient correction and terminal limits still apply. Each group's ampacity uses its selected size, material, and insulation rating with the shared raceway conductor count.
When entered, the base ampacity requirement is compared with the uncorrected table ampacity, limited by the selected terminal rating. The corrected-load requirement is compared with ampacity after ambient and conductor-count adjustments. Use the corresponding per-conductor outputs from the feeder-sizing calculator; this page does not derive the building load.
Conductor and raceway dimensions are manufacturer-specific. The copper THHN/THWN-2 and aluminum XHHW-2 defaults reference Southwire product data. Raceway IDs reference Wheatland EMT/IMC/RMC and JM Eagle Schedule 40/80 PVC product data. Verify the exact listed product before relying on the fill percentage.
Example
Three 12 AWG copper THHN/THWN-2 conductors and one equipment grounding conductor, each with a 0.132 in. outside diameter, in 3/4 in. EMT with a 0.824 in. inside diameter occupy about 10.3% of the raceway area. Only the three phase conductors count as current carrying in this example. At 86°F, using 90°C insulation and 75°C terminals, the group has a 25 A terminal-limited ampacity before any conductor-count adjustment.
This example is a dimension and ampacity illustration, not a conductor-selection result. Verify the neutral count, grounding conductor, product dimensions, load, and locally adopted code for the actual pull.
Interpret the results
The fill result includes every conductor in the entered groups. The ampacity section shows each loaded group's available ampacity after the selected corrections and terminal limit. When load requirements are entered, both the base and corrected-load comparisons must pass for the group to show a pass.
A passing fill or ampacity comparison does not verify the complete installation. Raceway bending, pull tension, equipment grounding, overcurrent protection, conductor arrangement, neutral harmonics, physical damage, and local amendments require separate checks.
FAQs
What conductor dimensions does the calculator use?
The calculator prefills outside diameters for supported Southwire copper THHN/THWN-2 and aluminum XHHW-2 sizes. Raceway inside diameters are prefilled from selected manufacturer product data. Verify both dimensions for the exact listed products and overwrite the values when they differ.
How does it count current-carrying conductors?
Enter the current-carrying quantity for each conductor group. The calculator sums those counts for the raceway adjustment factor. The NEC includes neutral and other-conductor exceptions; apply the rules for the actual circuit rather than assuming every conductor counts or is excluded.
What happens for a nipple between enclosures that is 24 inches or shorter?
When you identify the segment as a nipple between enclosures, the calculator applies the 60% fill allowance and does not apply the current-carrying-conductor adjustment. Ambient-temperature correction still applies. A short raceway segment does not receive these exceptions unless it meets the nipple conditions.
How do I compare the result with a feeder design?
The ampacity results are available per conductor group. To run an optional load comparison, enter the base ampacity requirement and corrected-load requirement per conductor from the feeder-sizing worksheet. The raceway tool does not calculate demand, continuous-load sizing, or motor allowances.
Does the calculator determine whether the raceway is code compliant?
No. It screens entered conductor area and reports adjusted ampacity. It does not select an overcurrent device, determine which conductors count, verify raceway suitability, or evaluate every code exception and installation condition.
Sources
- NFPA 70, National Electrical Code, 2026 edition — Chapter 9 raceway fill, Article 310 conductor adjustment and ampacity, and applicable exceptions. Confirm the edition adopted by the authority having jurisdiction.
- NFPA 70 Chapter 9 Second Draft Report, preliminary report — public code-development material for raceway fill provisions.
- NFPA 70 Chapter 3 Second Draft Report, preliminary report — public code-development material for conductor ampacity and adjustment rules.
- Wheatland EMT, IMC, and RMC dimensions — manufacturer's listed raceway dimensions.
- JM Eagle PVC electrical conduit product data — Schedule 40 and Schedule 80 dimensions.
- Southwire SIMpull CoilPAK copper data, copper THHN/THWN-2 data, and aluminum XHHW-2 data — conductor outside-diameter defaults.