Outside Medium Voltage Taps

[vw55]

Feeder originates in a medium voltage switch with 150E fuses. Feeder is 5kv, 3-#1/0 cu conductors that terminate in a 150E fused switch located at a substation approximately 300 feet away. Contractor wants to replace the existing the 150E fuses at the source switch with 250E fuses, replace the conductors with #3/0 only for a length of 200 feet at which point he wants to splice into the existing #1/0 conductors (which are still terminated at the remote substation) and tap at that same point with #4 cu conductors running to a second substation where these conductors will terminate in a 100E fuse set.

Does this installation comply with 240.21?

 

[templdl]

Man, don't mess with them fuses unless a "qualified person" can justify replacing them as has been suggested. Medium voltage fusing is a whole different ballgame than with lowvoltage. The fuse curves must be verified for coordination.

 

[bob]

I would want to know 1. What is the existing load on the cable. I assume that it feeds a transformer. 2. What will be the new load. 3. How old is this cable. Reason is Okonite lists it 5/8 kv 1/0 cable at 195 amps in undergroung duct and 3/0 at 250 amps. This ampacity is listed in the NEC table 310-73 conductor temp 90C and 100% load factor. Just because you are adding load may not mean you need to change the cable.
Is your 4 kv primary from a utility or do you have your own substarion?

Okonite Site http://www.okonite.com/engineering/3-conductor-underground-ducts.html

 

[vw55]

The feeder is approximately 3 months old. The load on the remote switch (feeding a unit substation transformer) is about 120 amps. The added load will be about 90 amps.

This is not a utility company feed.

My question is really dealing with whether or not the tap rule can be applied to a feeder that actually terminates in two taps, not at an overcurrent device.

 

[bob]

My question is really dealing with whether or not the tap rule can be applied to a feeder that actually terminates in two taps, not at an overcurrent device.

In my opinion the NEC does not help much with system over 600 volts.
An example of taps for a system 600 volt or less may be an apartment comlpex with a trough containing a sercice lateral and 6 taps to an apatrment fused disconnect.
You say the load may be 120 + 90 = 210 amps. I would question the wisdom removing the 1/0 195 amps feeder and replacing it with 3/0 rated at 250 amps with a load of 210 amps. It does not give much room for future additional loads. However you know the system better than I do.

 

[masterinbama]

as far as I can tell this is a common situation on underground medium voltage installations.I did a sewer treatment plant that used a simular setup for the aerator motors.We installed two 300 amp feeders from main plant out to the ponds and then T-tapped each one at the manholes with the taps terminating at an outdoor MV starter with a manual transfer switch so each feeder was independent of each other and could be deenergized for maintenance

 

[steve66]

This might be OK per 240.21(B)(5). You need to check through the 4 items listed and make sure your installation meets them all. Also, all your wires have to be rated for the full value of the fuse it terminates in. You can't round up to the next size fuse. For the 250E fuse, the 3/0 has to be rated for the full 250A.

You mentioned:

My question is really dealing with whether or not the tap rule can be applied to a feeder that actually terminates in two taps, not at an overcurrent device.

Although you can't tap a tap, what you are doing sounds OK. The 3/0 is not a tap since its good for a full 250A. So you have 2 separate taps, and each terminates in its own OCP.

Steve