Table for ampacity?

[wonda]

I am a junior electrical engineer. I have to parallel 4 ? core 500ckmil size cables for a 2000A main breaker. The supply voltage is 415Y/230V which consists of 3 phases, a neutral and an earth. Each cable is to be installed a 4? conduit which is buried at 24? below the ground level. I have use table 310.16 to calculate the ampacity of the cables, using the following assumption:
? ambient temperature of30oC
? Termination rated at 75oC
? The cable is THHN/THWN rated @ 75oC.
From this table, 1No. 4 ? core 500ckmil cable has the ampacity of 380 which then gives a total of 6No. 4 ? core 500ckmil cable for the 2000A main.
Is this table the correct table to use for this type of installation or should I be using table 310.6 in Annex B?
Do I have to derate the cable for 4 current carrying conductors give that the system is a 415Y/230V 3 phase with neutral?

 

[jumper]

Look at 310.15(4)(c). Do you have a large nonlinear load?

 

[augie47]

most of us can't add "P.E." after our name and would use 310.16 as you did. From your profile, you may have the credentials and expertise to use Annex B per 310.15(C).

 

[charlie b]

This is not as easy a question as you might hope. For starters, I would ask how you came to select 2000 amps as the main breaker size. I say that, because if you began the process with an article 220 load calculation, and came up with about 1.4 MVA (equivalent to 2000 amps at 415 volts), then the load value will have had some conservatism built into it. The NEC load calculation process has us provide much more power than a facility will generally use, once it is constructed and in operation.

Because of this fact, it is generally considered acceptable to use the ampacity values of table 310.16, even though it speaks of only three conductors, and those being in raceway or in earth. In other words, we ignore the fact that each of your six conduits, each of which has a set of 500 MCM conductors, is not isolated from the rest of the world. Each conduit has 5 neighboring conduits, and they are all adding heat to the same surrounding dirt. We can ignore that fact, because we know the calculated value of 2000 amps is conservative.

So under these circumstances, my answer would be that 6 sets of 500 MCM THHN is acceptable, and you don't have to use the table in Annex B.

Also, you are unlikely to have so much harmonics that it will be a major part of the load at the service level. Therefore, you don't have to count the neutral as a current-carryng conductor, and you don't have to derate.

Welcome to the forum. Or I should say, since this is your first post and you have been registered for quite a while, welcome to active participation in the forum.

 

[charlie b]

From your profile, you may have the credentials and expertise to use Annex B per 310.15(C).

But even if he did have that credential, he might not want to apply it here. The ampacity values that a PE can calculate are never as high as the values shown in table 310.16. So if you can justify using that table, it will be your best bet.

 

[wonda]

The utilization factor of this installation is unknown and 2/3 factor is applied. The harmonic currents on the system is assumed to be less then 40%.
This 2000A breaker is for a 1.4MVA generator which is to be set up to parallel with other generators.
310.15 (4) c say when a major portion of the load consists of nonlinear loads and harmonic currents that the neutral is consider a current carry conductor.
When it say major portion does does mean over 60 or 70% of the total load.