Question

Question: I have inherited someone's headache. The set up so far is 37 separate 12-2 mc home runs. I have to land them in a trough then bring them down thru emt to the panel.I understand that I am limited to not using more than 20 percent of the raceway cross section. Furthermore I am over the maximum of 30 current carrying conductors so I will have to derate. What is confusing me is the conductor fill. If the wires are spliced, does that mean my count now goes up to 74 current carrying conductors? The trough they purchased is 6x6 by 24 inches. Who am kidding, im confused as to all of it. I wonder now how my derating is going to affect my ampacity. If someone can give me hand here I would appreciate it.Thanks

Answer 1

Conduit Fill With splices

Each conductor counts as one current-carrying conductor for conduit fill calculation. So, with 37 home runs and splices at each end, you have 74 current-carrying conductors.

Without splices

If the home runs are not spliced and are terminated directly, each home run would count as one current-carrying conductor, resulting in 37 current-carrying conductors.

Derating Derating is necessary when the number of current-carrying conductors exceeds 30. The allowable ampacity must be derated by the following factors:

Number of Current-Carrying Conductors, n:31-40: 80%41-50: 70%51-60: 60%61-70: 50%71-80: 40%Over 80: 30%

Trough Cross Section The maximum fill allowed in a raceway (e.g., trough) is 20% of the cross-sectional area. For a 6x6x24 inch trough, the cross-sectional area is 36 square inches. Therefore, the maximum number of current-carrying conductors that can be installed in the trough is:
36 square inches x 0.20 = 7.2 square inches
Considerations If the raceway is filled to 20% and you have over 30 current-carrying conductors, you will need to derate the ampacity of each conductor. To determine the ampacity of each conductor after derating, multiply the ampacity given in the NEC by the appropriate derating factor. It's important to note that the derating factors are based on the number of current-carrying conductors in the raceway, not the number of home runs.

Recommendation Calculate the actual number of current-carrying conductors in the trough, taking into account splices or terminations. Determine the percentage fill of the raceway based on the number of conductors. If the percentage fill exceeds 20%, derate the ampacity of each conductor according to the NEC guidelines.

Additional Resources [NEC Article 310.16 - Raceway Fill](https://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards/detail?code=70)* [NEC Table 310.16 - Allowable Current-Carrying Capacities for Three Single Insulated Conductors Rated 0-2000 Volts, Installed in Raceway](https://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards/detail?code=70) Flag for review

Answer 2

Conduit Fill With splices

Each conductor counts as one current-carrying conductor for conduit fill calculation. So, with 37 home runs and splices at each end, you have 74 current-carrying conductors.

Without splices

If the home runs are not spliced and are terminated directly, each home run would count as one current-carrying conductor, resulting in 37 current-carrying conductors.

Derating Derating is necessary when the number of current-carrying conductors exceeds 30. The allowable ampacity must be derated by the following factors:

Topic: Trough fill calculations