About Cable Tray & Conduit Sizing Calculator
The cable tray and conduit sizing calculator checks the fill ratio of a raceway and the derated ampacity of the cables it carries per NEC Article 392, NEC Chapter 9, IEC 61537, and AS/NZS 3008. It is used by electrical engineers to confirm that a conduit or tray is not overfilled and that cables are not overloaded after grouping and temperature derating.
Add each cable size and quantity, choose the installation type, and set the ambient temperature. The tool returns the total cable area, the fill ratio against the code limit, the combined derating factor, the derated current of each cable, and a recommended conduit or tray size, all updating in real time.
How It Works
- Sum the cross-sectional area of every cable as pi * (OD/2)^2 * quantity.
- Divide by the raceway area to get the fill ratio and compare it with the code limit (40% for more than two conductors in conduit, 50% for cables in tray).
- Look up the grouping factor from NEC Table 310.15(C)(1) and the temperature factor from NEC Table 310.15(B)(1), interpolating between ambient temperatures.
- Multiply the two factors for the combined derating, apply it to each base ampacity, and recommend the smallest conduit or tray size that satisfies the fill limit.
Worked Example
Six current-carrying conductors in a raceway at 40 deg C ambient: NEC Table 310.15(C)(1) gives a grouping factor of 0.80 for 4 to 6 conductors, and NEC Table 310.15(B)(1) gives a temperature factor of 0.88 at 40 deg C for 75 deg C conductors. The combined derating is 0.80 * 0.88 = 0.704, so a 100 A cable carries 100 * 0.704 = 70.4 A.
Formulas
- Cable area
A_cable = pi * (OD/2)^2 * quantity- Fill ratio
fill% = (sum(A_cable) / A_raceway) * 100- Combined derating
derating = grouping * temperature- Derated ampacity
I_derated = I_base * derating
Standards & References
- NEC (NFPA 70) Article 392
- NEC Chapter 9
- IEC 61537
- AS/NZS 3008
Frequently Asked Questions
What is the maximum allowable fill?
For conduit with more than two conductors the NEC Chapter 9 limit is 40% fill, and for multi-conductor cables in a cable tray the NEC 392 limit is 50%. The tool flags a pass or fail against the applicable limit.
How does derating work?
Two factors are multiplied: a grouping factor from NEC Table 310.15(C)(1) based on the number of current-carrying conductors, and a temperature factor from NEC Table 310.15(B)(1) based on the ambient temperature. For example six conductors at 40 deg C give 0.80 * 0.88 = 0.704.
Why does adding more cables reduce ampacity?
Bundled conductors cannot dissipate heat as easily, so the NEC grouping factor steps down from 1.0 for one to three conductors to 0.80 for four to six and lower for larger groups, reducing the allowable current per cable.
Does it recommend a size?
Yes. The tool suggests the smallest standard conduit diameter or tray width and depth whose internal area keeps the total cable area within the fill limit.