ampacity calculation for feeder repair

[bob]

I don't think 310.15(A)(2) applies

Quote:
310.15(A)(2) Selection of Ampacity. Where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest value shall be used.
Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.


To me this states 10% of the 600 feet in the cable tray or 10' whichever is less. 10% of 600 is 60 feet. Thus 10 feet is the max. allowed of the conduit. In the OP's example there is 25 feet of conduit therefore it does not apply.

Dennis
You are correct. I was thinking you could go as far as 10% of the circuit length or in this case 60 ft. Thanks for catching the error.

 

[Dennis Alwon]

Dennis, can you please give me the table reference you used to determine 1300 amps?

So after reading this thread you now probably realized I used T. 310.16 at 75C. I have never used a cable tray but my take as explained early is that the conduit is the limiting factor. If the conduit were 10 feet or less then you could use the cable tray table but since there is 25 feet you must use the weakest link in the ampacity rating and that is the conduit. At least that is my take.

 

[Tfret]

392.11 deals with voltages below 2000. This is a 2.3kv system so that doesn't apply. Table 310.69 is the closest thing I see to our condition. From that, using MV-90, I see 1495 amps for a single run of 1750mcm. If I double this, which is what I have installed right now, that gives me 3500mcm of copper. Using the formula in table B.310.11 for parallel runs of 1750mcm I see a multiplier of 80%. 0.8 x 1495 = 1196 amps. I have two runs per phase so that means 2392 amps per phase total. The system has a 2000 amp breaker. So this seems reasonable doesn't it? So how do I get the same ampacity using a commonly stocked mcm cable, such as 1000mcm?

 

[don_resqcapt19]

392.11 deals with voltages below 2000. This is a 2.3kv system so that doesn't apply. Table 310.69 is the closest thing I see to our condition. From that, using MV-90, I see 1495 amps for a single run of 1750mcm. If I double this, which is what I have installed right now, that gives me 3500mcm of copper. Using the formula in table B.310.11 for parallel runs of 1750mcm I see a multiplier of 80%. 0.8 x 1495 = 1196 amps. I have two runs per phase so that means 2392 amps per phase total. The system has a 2000 amp breaker. So this seems reasonable doesn't it? So how do I get the same ampacity using a commonly stocked mcm cable, such as 1000mcm?

I just don't see how you can use the free air ampacity tables for an installation that includes underground ducts. 310.60(B)(1) says you have to use the lowest ampacity for the run, when the conditions of installation result in multiple ampacities. The only issue is the fact that the underground duct bank ampacity table only goes up to 1000 kcmil. If we extrapolate, we can assume that the 1750 kcmil in the duct has an ampacity of around 1000 amps, but that assumes a single isolated duct with not thermal interaction from other ducts.

 

[Cold Fusion]

--- I am considering using 4 runs of 1000mcm per phase to replace the original circuit. 1000mcm is common and can be delivered right away at the lengths we need. My question is, how do I properly calculate the ampacity of this proposed circuit? It runs in cable tray for 600', then goes under ground in conduit for 25' where it enters the main sub station. ---

---
I should have mentioned this is a 2.3kv system. 392.12 deals with the capacity of the tray. It doesn't look that is a problem. 4 conductors in a bundle per phase all in a single layer seems to fit this code. The max continous load is never over 1000 amps. The system has a 2000 amp breaker. ---

Everyone has had a lot of fun looking at the original 1750kcmil install - and applying <2000V regulations to it. But that wasn't what you asked.

4 - 1000kcmil looks good for 1640A (T310.77, detail 3, MV-90, CU, shielded - terminated on one end). So, you will have to set the CB down to under 1640A. Or look at 310.60.D

As you noted the cable tray section ampacity exceeds the conduit/UG section.

Other sections you may wish to look at are:
300, Part II
310.6 (and ex 1) You may be able to use nonshielded
490.

cf

 

[bob]

I have attached a table from the Okonite Catalog. The maximum size 5 kv cable is 1000 kcm with an ampacity of 720 amps in conduit. That seems like the maxium you are going to get for this size cable. Since you said you load is about 1000 amps, this should do the job.

 

[Cold Fusion]

I have attached a table from the Okonite Catalog. The maximum size 5 kv cable is 1000 kcm with an ampacity of 720 amps in conduit. That seems like the maxium you are going to get for this size cable. Since you said you load is about 1000 amps, this should do the job.

I don't think you get 720A with 90C gear and multiple duct banks.

cf

 

[bob]

Why? I did not read in the post that there were duct banks. I still wonder about the 2.3 kv system. I would like Tfret to describe this in more detail.
Tfret, how about doing this, describe the 2.3 kv system.