Underground Generator Feed

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dy11

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Plant Electrical Engineer
We are designing a 3000A underground 480V cable run that will bring standby generator power into a building via an UG ductbank. The Neher-Mcgrath/Engineering Supervision method of sizing the cables was used. The cables are being connected to terminations that are rated for 75C. The cable itself has a 90C rating.

One group is insistent on installing a cable vault under the generator so the final 8 ft of the run can transition from the ductbank to the generator terminations in free air due to this area exceeding 75C in the calculations for the duct bank sweeping up into the generator slab. This group is firm that 75C cannot be exceeded anywhere in the run due to the 75C rating of the terminations (110.14 C)

Another opinion is the 310.14 A (2) Exception applies where the requirement for the higher ampacity can be ignored due to the length in question being under 10'.

I feel like the calculation method is being mixed with the table methods of determining ampacity but I'm having trouble determining the separation of the two. The 2017 NEC applies here.
 
If the Neher/Mcgrath ampacity exceeds the 75°C-in-raceway ampacity [table 310.15(B)(16)], the 75°C-in-raceway ampacity governs the whole circuit, not the N/M. If the 75°C-in-free-air ampacity [table 310.15(B)((17)], is higher, then N/M will apply. Sounds like you need the vault to get the free-air ampacity.
 
IMO, you can use 90 degree C for underground section for calculation or derate purpose, when emerging, you have to use 75 degree C. When using Neher/Mcgrath method, the results is mostly lower 310.15(B)(16) at 75 degree column. If it higher the table, I would use table instead of cal value.
If you use table then 75 degree column apply entire the run.
 
Higher temp can be used for adjustment factor or derate purpose as soon as it not exceed the table see NEC 310.15(B).
 
When we do the Neher McGrath calc, we stop the calc at 75degC anywhere along the feeder, as I have no idea how the temp will propagate out of the duct to the termination, which is rated at 75degC in its assembly (even though the termination may have 90deg stamped on it)
 
ron said:
When we do the Neher McGrath calc, we stop the calc at 75degC anywhere along the feeder, as I have no idea how the temp will propagate out of the duct to the termination, which is rated at 75degC in its assembly (even though the termination may have 90deg stamped on it)
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He did said the terminations are rated at 75C and conductors are rated at 90C. There isn’t wrong by using 75C for entirely circuit just more conservative. If Neher McGrath cal exceed applicable tables, I would use table instead of cal value.
 
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