NFPA 70 Underground Feeder Sizing 3

[tloussaert]

I have a recent application where I was sizing underground feeders. I sized the feeders according to Table 310.16. My first assessment was that the conductors were either direct burial in encased concrete and they were in metallic conduit in ambient air. About 75% of the run is concrete encased PVC schedule 40 and GRC bends/sweeps, the other 25% is GRC in ambient air. I have used all 4" conduit.

I will be protecting these feeders with a 1600A adjustable trip circuit breaker.

Based on the above facts I selected from Table 310.16 an ampacity rating of 335A for a 400kcmil cable. I chose the 75 Deg C cable rating because the terminals that the cables are being terminated on are rated for 75 Deg C.

If we were to use 6 conductors per phase, we would have a total ampacity of 335Ax6=2010A.

I had figured two conductors per phase in each conduit, this would give us 6 conductors per conduit.

I derated the total ampacity for the number of conductors per conduit 4-6 conductors derate at 80%, from table 310.15(B)(2)(a).
2010Ax0.80=1608A

When I had an independent engineering company review these calculations they informed me that they believe I did not correctly apply the table 310.16 for this application, they feel that I am required to follow the formula listed for 310.15(C), I believe that this formula is an optional calculation to table 310.16. This formula directs the user to Appendix B, I do not believe that this formula is required for underground installations however is unclear. In appendix B it states "This annex is not a part of the requirements of this NFPA document but it is included for informational purposes only."

Could you provide some additional clarification on this subject?

Tom

 

[jolt]

You are correct the calculation is optional and unnecessary... and your calculation method is correct (or nearly correct, see below.)

If this is a 3-phase, 4-wire circuit, the neutral is considered a current-carrying conductor and therefore you should use a 70% derating factor.

 

[tloussaert]

Thank you. Jolt. I have had some long discussions regarding the necessity of this calculation.

 

[dpc]

Table 310.16 is only applicable up to 2000V. If your circuit is less than 2000V, then I agree that the NEC does not **require** additional derating of the ampacity due to duct bank heating. But you might want to consider it anyway.

This derating IS required for circuits over 2000V. There was a Code revision in the 1980's that required similar derating for 2000V and below circuits, but it was quickly rescinded due to outcry from contractors that this was too difficult to figure out, especially if duct bank configuration did not match the few mentioned in the tables.

Of course, the physics are the same regardless of voltage, so heating is a concern. I think the feeling was that Table 310.16 is already conservative in conductor ampacities and that there was no history of cable damage in duct banks due to overheating.

The 3 circuit and 6 circuit ampacities given in Tables 310.77 through 310.86 are based on the Neher-McGrath equations using the thermal resistance stated in the tables.

 

[alehman]

I agree that 310.16 is conservative and probably acceptable for many situations. If the load is a commercial application with typical daily load cycles that never exceeds 70 to 80% of the ampacity, I would go with 310.16. This is the "code requirement". If the load is continuous and approaching the rating of the conductors, then I think a close look at appendix B is appropriate (although not required). There is commercial software available to help with these calculations, but it must have valid data on thermal properties of the surroundings.

 

[splitbolt]

You can derate off of the 90 deg column as long as your final ampacity is not over the 75 deg colomn value per 110.14(c). At 80% 350 kcmil would work.

 

[mc5w]

Appendix B of National Electrical Code is mandantory in Canada and some states in the U.S. because of the popularity of underground services. The laws of thermnodynamics also make appendix B mandantory.

In 1987 National Electrical Code the NFPA tried to make both appendix B and 4-wire electric range and dryer outlets mandantory. Both attempts were repealed in 1990 because of the high cost of copper back then. In order to properly ground an indoor swimming pool in 1988 I had to go to one of my buddies and beg for some copper only split bolts and grouding devices.

Tloussaert, why don't you do the service at primary voltage and use an integrated high voltage switch, dry transformer, metering compartment, 1600 amp circuit breaker, and switchboard lineup from SquareD or General Electric? Dry transformer technology for indoor or outdoor fully dead front industrial substations goes all the way up to 46,000 volts - I think that the company who actually makes the dry transformer is ABB, what is known as brand name labeling.

This was the way that Coca-Cola did their third electrical service. They did the first service as an underground duct bank off of a padmounted transformer. They did their second service using 3 bars in parallel weatherproof busway coming out of the low voltage compartment of a padmounted transformer. They did their third one using a SquareD indoor industrial substation with a 12,470 volt delta primary 277Y480 volt secondary dry transformer with forced air cooling to bump up the rating from 2,500 KVA to 3,330 KVA.
Mike Cole, mc5w@earthlink.net