timm333
Senior Member
- Location
- Minneapolis, MN
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- Electrical Design Engineer
Please see attached. Is it ok in NEC to protect three 15kV transformer cables by a single upstream utility fuse (F1)?
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Cable Protection
- Thread starter timm333
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Please see attached. Is it ok in NEC to protect three 15kV transformer cables by a single upstream utility fuse (F1)?
If this is your cable, wouldn't this be your fuse? Who is responsible for it? Where is the point of delivery?
In 240.100 (A)(2) a fuse shall be connected in series with each conductor, but read a little farther. Utilities fuse differently than NEC wants. You really need to look at available fault currents and fuse TCC's. Get the right size fuse for the application. I'm not talking about just the amp ratings. there must be over a dozen types of 6 amp fuses and all will have different clearing times.
timm333
Senior Member
- Location
- Minneapolis, MN
- Occupation
- Electrical Design Engineer
The fuse and all MV cables are the responsibility of utility.The single incoming feeder cable on the primary side of primary-enclosure is # 1/0 AWG, and the three cables on secondary of the primary-enclosure are also each # 1/0 AWG. The transformers have internal primary fuses. The points of delivery are three MCC's that are located on the secondary side of the three transformers.
Section 240.101(B) states that conductors tapped to a feeder don't need additional protection. Does it mean that the three cables on the secondary side of primary-enclosure don't need individual fuse (because they are protected by upstream fuse F1)?
Also 240.101(A) says that the continuous rating of the fuse shall not exceed three times the ampacity of cable. Why do they allow fuse rating to be more than the ampacity of the cable here?
Section 240.101(B) states that conductors tapped to a feeder don't need additional protection. Does it mean that the three cables on the secondary side of primary-enclosure don't need individual fuse (because they are protected by upstream fuse F1)?
Also 240.101(A) says that the continuous rating of the fuse shall not exceed three times the ampacity of cable. Why do they allow fuse rating to be more than the ampacity of the cable here?
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The fuse and all MV cables are the responsibility of utility.The single incoming feeder cable on the primary side of primary-enclosure is # 1/0 AWG, and the three cables on secondary of the primary-enclosure are also each # 1/0 AWG. The transformers have internal primary fuses. The points of delivery are three MCC's that are located on the secondary side of the three transformers.
Section 240.101(B) states that conductors tapped to a feeder don't need additional protection. Does it mean that the three cables on the secondary side of primary-enclosure don't need individual fuse (because they are protected by upstream fuse F1)?
Also 240.101(A) says that the continuous rating of the fuse shall not exceed three times the ampacity of cable. Why do they allow fuse rating to be more than the ampacity of the cable here?
Under no circumstances should you depend on the utility to provide your OCP. I am still not clear as to the setup. The utility comes into a transformer and then goes to a MCC? What is the secondary voltage? I would think your protection should start at the point of delivery, as though there were no protection anywhere else upstream. A utility will normally size a fuse for the transformer, not the conductor. Some curves are higher than others with regards to fuses, so you really don't know what protection is being offered, unless you get the fuse type, along with the amps, and look at some TCC's.
As far as the 300% for the fuse, you need to look at 240.100 (C) first. Take the OCPD TCC's, along with the cable damage charachteristics, and the cables normal permitted ampacity. These should be coordinated to prevent damage (per 240.100 (C))
240.101 (A) just tells you that once you have done your calcs, no matter what you have come up with, that you cannot exceed the ampacity of the cable by no more than 300%. You don't have to go that high, it just says you are allowed to.
Hey, use a breaker and you can exceed the ampacity by 600%...
Do you consider the secondary side of the transformer as a tap? You need to have some protection on a cable. after your OCPD, a tap can be installed wthout additional protection if you wish.
timm333
Senior Member
- Location
- Minneapolis, MN
- Occupation
- Electrical Design Engineer
Yes the utility comes into the transformer and then goes to MCC. They have provided primary fuses inside the transformer. The transformer secondary is 480V. Our responsibility starts at the MCC.
The secondary side of the transformer is not a problem. But they are not putting three individual fuses at the starting point of three cables within the secondary of primary enclosure. So I will put a note for the utility that they need to perform coordination study of fuse and conductors as per NEC 240.100 (the conductor damage curve must fall to the right of fuse TCC curve.)
The secondary side of the transformer is not a problem. But they are not putting three individual fuses at the starting point of three cables within the secondary of primary enclosure. So I will put a note for the utility that they need to perform coordination study of fuse and conductors as per NEC 240.100 (the conductor damage curve must fall to the right of fuse TCC curve.)
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Yes the utility comes into the transformer and then goes to MCC. They have provided primary fuses inside the transformer. The transformer secondary is 480V. Our responsibility starts at the MCC.
The secondary side of the transformer is not a problem. But they are not putting three individual fuses at the starting point of three cables within the secondary of primary enclosure. So I will put a note for the utility that they need to perform coordination study of fuse and conductors as per NEC 240.100 (the conductor damage curve must fall to the right of fuse TCC curve.)
I don't think you understand the relationship between the utility and the NEC. The utility does not have to go by NEC regulations. Check out NEC90.2(B). If they want to, they can go by NESC. The transformers have fuses in them, internal or Bay-O-Net depending on age. You don't need to worry about the fusing until it gets to you.
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