Derated Ampacity due to Rho Protected by OCPD

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Andrew445

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I have a solar question as it relates to the NEC. We typically run ETAP with specified loading to determine the steady-state temperature of the DC conductors in order ensure it does not exceed insulation temperature limits (90°C). There is also an option in the program that determines the maximum ampacity of the DC cables before exceeding temperature limits. My question is: does this simulated max. ampacity value need be protected by the next higher standard OCPD rating?

For example, circuits are sized to be 400 kcmil Aluminum, protected with a 300A fuse. There are a bunch of direct-buried circuits in the same trench with poor soil conditions. The program results in an ampacity of 230A. This is technically not protected by the 300A fuse. Do I need to mess with the sizes/arrangement until the ampacity is greater than 250A? Meanwhile, the steady-state temp. at specified normal-loading values hit a maximum of 80°C.

I think it is a little different with PV DC circuits as well because the OCPD is mainly protecting the cable from backfeed parallel contribution in the event of a short-circuit. It is highly unlikely a short circuit will result in a value less than 250A. I guess my initial thought is NO, this ampacity value need not be protected by the fuse. Parallel contributions are always at least double the normal loading on that circuit. A short-circuit would be a quick event, and the transient temperature response will be very slow in comparison to the fuse tripping time.

Open to all thoughts, thanks!
 
While 310.15(C) permits ampacity determination under engineering supervision, I believe the determined ampacity must still adhere to other associated requirements, i.e. considered protected by the OCPD. That said, in some cases, such an engineering determination yields a lower ampacity than determined by the tables per 310.15(A)(1). For these cases, the engineer of record is burdened with making the decision whether to use the tables or change installation parameters...
 
Smart $ said:
While 310.15(C) permits ampacity determination under engineering supervision, I believe the determined ampacity must still adhere to other associated requirements, i.e. considered protected by the OCPD. That said, in some cases, such an engineering determination yields a lower ampacity than determined by the tables per 310.15(A)(1). For these cases, the engineer of record is burdened with making the decision whether to use the tables or change installation parameters...
Click to expand...

What is a practical reason for even using the more complicated formula, if it is known to yield stricter results?
 
Appreciate the thoughts. Normally I would agree. The reason I am second-guessing myself is because of the Exceptions in NEC 690.9(A). You are not even required to protect the conductor at all if you have no parallel contribution, because the cable is already required to be sized to carry the maximum possible short-circuit current produced from its source. You can consider the rho and soil temp. to be a "Condition of Use" and adjust the ampacity tables accordingly via thermal modelling, but this derated ampacity need not be compared to the OCPD rating, because there is no OCPD to compare to.

The only protection needed is at the end of the cable, and only if the parallel contribution is greater than the source contribution of the given circuit. And the only way that the cable sees this contribution is in the event of a short-circuit—it does not occur under normal operation. In this short circuit, the fuse will instantly see at least double the current for which it was sized and blow very quickly.

The other wrinkle is that per code, you are not required to derate for thermal resistivity for <2000V system. Many thoughts exist on the applicability of this, and I don't know myself. I guess I think it should be done because I have never seen a geotech report come back to me with a rho 90 or less.
 
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