If you are under NEC jurisdiction, taps must be fused unless adequately protected by the feeder overcurrent device. Fuses may be sized up to 3 times the rated ampacity of the cable.
That is really not the point in this case as the "tree" of shielded #2, protected as a whole by an 80e or 100E dip will never theortically exceed the insulation thermal. It is for an industrial yard, and the trade of outages. Sure to be cheap in the upgrade. The outfit can go down anytime a "tree" branch fails. This is not really about the NEC, but rather the common practice or strategy of employing fuses for underground, with above ground load-break-elbow based green boxes. I do not want to fill all the parking space with green, dead front load-break fused elbow boxes. Plus cost, the almighty, is always an issue.
MV fuses are there only for short circuit protection not oveload. Therefore only check necessary is that the smallest cable section on that fused circuit is capable of withstanding the available fault current. This is done by checking the cable damage curver vs. the fuse.
This is very common in MV distribution systems, specially underground feeders. I belive IEEE buff book also discusses this..I don't have one in front of me.
E rated fuses are desinged to open within 5 minutes at 200% of the E rated current. That is a 100E rated fuse will not not open at 100A or little more, but if it draws 200A it will open in 5 mins. Anything under that it takes forever to open , if at all. Refer to some MV fuse manufacutuer's application guides.
Thanks Tinfoil,I will look into those using handholes. I wanted to avoid putting them in manholes, as it requires quite an operation nowdays, sniffing, setting up A-frames, harnesses etc. to do work in them. And I was worried someone would catch the 100E fu-pa, Rublska gets the prize. Actually the in new design I was going to use 50E fuses. Currently they are using 100E in order to pick up the load of ~2MVA of connected transformers. The 100E currently feeds a lead-sheathed paper insulated #6-3. They have had some blow outs in the lead cable in the past, but it really has held up all-in-all well for being installed in 1944, inside "orangeburg", -asphalt paper duct.
I think my cart got ahead of my horse. I will definitely need to do a full coordination study to complete the job.
Mr. alehman, I am sorry that I did not at first fully comprehend you statement about fusing and cable size. I have noticed that you are never wrong. What am I missing? are we talking about the instantaneous cable withstand rating, allowable OCP for transformers, or load diversity?
I would never fuse a feeder with 3X-cable capacity normally, unless it fell under a tap rule, or diversity factor. Thank You.
