DC Circuit Sizing Question

[Andrew445]

I am sizing the circuit from the combiner box to the inverter. The Isc is 9.24A, and there are 18 strings in parallel. Aluminum conductors are being used. The applicable temp. derate is 0.82.

The SMA inverter I am using is DC 1000V and has the following fuse sizes available:
200A
250A
315A
350A
400A
450A

Typically, I simply take 9.24*1.56*18 = 259A --> 300A fuse. In this case I am left with using a 315A fuse.

My question is: does the derated ampacity of the conductor need to be greater than the standard fuse rating of 300A, or can it simply be greater than 250A? The former pushes the size to 600 kmil, while the latter leaves it at 400 kcmil.

On a side note: Using the ASHRAE 20-yr max. dry-bulb temps leaves the derate at 0.82. Dropping to the 10-yr max. leaves a 0.87 derate, so some engineering judgment needs to happen here as well.

 

[jaggedben]

My question is: does the derated ampacity of the conductor need to be greater than the standard fuse rating of 300A, or can it simply be greater than 250A?

Great than your calculated 259A. (I'll assume 250A was a typo). The next-size-up fuse rating is irrelevant.

Note that in the 2011 code you use either 156% of Isc without further derating, or 125% of Isc with derating, whichever is greater. Or to put it another way, simply use 259A if your conditions of use derating would be not less than .80, which you've said it isn't.

So your 400 kcmil is okay under the 2011 code.

The 2008 code was a bit more ambiguous as to conditions of use derating. In my experience an AHJ usually just looks for 156%, but I suppose an AHJ might ask for 156% plus derating, which would push you to 500 or 600 kcmil depending on your derate factor. I haven't done permitting for a system this big, maybe others will chime in.

 

[Andrew445]

I like the way you look at the two currents. I can see how if the derate is greater than 0.80, then the 1.56 will always be higher.

After this is all done, I still think it is important to ensure that the conductor is protected by the fuse.

Yes, the Maximum circuit current was calculated to be 259A. When normal standard fuse sizes are available, I would use a 300A fuse, and then check to ensure the derated ampacity of the conductor is still protected by that 300A fuse (greater than 250A). So I look at the 90? column 305 * 0.82 = 250.1A. The next higher standard size is 300A so I'm good. The problem is that I need to use 315A. 300A is not an option. So in this case, technically the next higher standardsize is 300A, not 315A. If the current falls in the 300-315 range, the conductor could be damaged. So would I be forced to ensure that the derated ampacity is at least 300A? (for 600 kcmil, 385*0.82 = 315A).