Medium voltage tap rule

[pistolpigg]

I have 25KV that feeds our faclity to a service disconnect switch from the load side of switch goes to multiple substations and they are all over 25 feet away. Our electrical engineer says that they can only be 25 feet away according to the tap rule. Is this a true statement? If so how this did this pass elctrical inspection that was done in 1988? or can you please tell me where I can find this informatuon in the code book.

 

[masterinbama]

Are the taps all outside?

 

[charlie b]

The tap rules are all within 240.21. There are no separate tap rules for medium voltage. But you have not given us enough information to confirm that these really are taps. What is the rating of the overcurrent device at the service disconnect switch (or other point upstream of the feeders to the various substations), and what is the ampacity of each of these feeders?

 

[pistolpigg]

All the switches are inside and are fused with 40 e fuses and the wire is all compact 4/O

 

[charlie b]

You are still not giving enough information. Let's start with the 25kV feed to the service disconnect switch. Is there an overcurrent device that is part of the service disconnect switch? If so, what is its current rating?

Next, do all of the feeders to the substations connect to the service disconnect switch directly, or is there an overcurrent device between the service disconnect and each feeder? If there is an OCPD, what is its current rating?

Next, I do not know the notation, "40 e fuse." Does the "40 e" represent a type of fuse, or are you saying each fuse is rated at 40 amps?

Finally, do you know the ampacity of each feeder? You say it is 4/0 compact, but you do not give us the insulation type, nor whether it is copper or aluminum.

 

[pistolpigg]

The 25 kv switch is fed directly from the power company recloser to the building through a cable tray and another conduit to the building as soon as each come through the building exterior wall they're connected to square hvl 25 kv fused switches. They were put in at two different times the switches are fused with Ge40 e fuses large glass have to assume that amps and class? All the wire is 4/0 compact copper 35 kv wire and from The bottom side of these switches they fed other switches exactly like the main switch fuses and all. They are coupled to square d transformers on these second switches load side. On the top of each transformer disconnect switch they bolted the same wire feed side to feed the top of the next transformer and then the next top to the next switch then the same 2 more times

 

[charlie b]

All the wire is 4/0 compact copper 35 kv wire

I don't see that type in any ampacity table, so I don't know what number to assign as its ampacity. But I will bet it is way, way higher than 40 amps.

. . . the switches are fused with Ge40 e fuses large glass. . . .

I cannot find the meaning of "40 e" in this context. I did see tables and charts from several manufacturers, listing the current rating of fuses with the annotation of "40 e." So unless anyone can correct me, I will infer that this means the fuse is rated for 40 amps.

The bottom side of these switches they fed other switches exactly like the main switch fuses and all.

What you are telling me is that each and every cable within this installation (not counting the utility feed, which is outside the scope of the NEC) has an upstream fuse rated at 40 amps, and each cable has an ampacity that is well above 40 amps. If that is the case, then this is not a "tap" situation at all. That is what I have been trying to discover. If you protect a conductor from overcurrent at the point at which it gets its power source (i.e., at the beginning of the run), and if the overcurrent rating is lower than the ampacity, then you can run that conductor for miles and miles, and never run into a tap rule violation. Certainly, the 25 foot limit that appears in the tap rules is not applicable to such an installation.

An example of a tap rule situation would be where you start with a 400 amp overcurrent device, and run a conductor rated for at least 400 amps, and somewhere in the middle of that run you want to connect a conductor rated for only 200 amps, and you don't want to put a 200 amp overcurrent device at the point of that connection, but wish instead to run the 200 amp conductor some distance and land it on a panel that has a 200 amp main breaker. The overcurrent protection upstream of the 200 amp conductor is rated at 400 amps, and would not, by itself, be able to protect the 200 amp conductor from overcurrent. We are allowed, under certain constraints, to use the breaker at the end of the run, indeed at the downstream panel, to protect the upstream conductor. The constraints are provided in the tap rules of 240.21.

 

[pistolpigg]

Ok thanks so where in the code book can I tell him to go to to show him that this is true? We are getting ready to put in two more medium voltage transformers and he keeps saying 25 foot. I showed him the existing setup and he said that it was wrong and had kept him up at night trying to come up with the best solution to the existing situation.

 

[charlie b]

Ok thanks so where in the code book can I tell him to go to to show him that this is true?

The 25 foot rule appears in 240.21(B)(2). You don't get to read what is in that section, until you first read the beginning of the article in 240.21. It tells us to put overcurrent protection at the point where the conductor gets its supply, EXCEPT as provided in the sub-paragraphs that follow. The 25 foot rule is one of the exceptions. But from your description, each conductor has overcurrent protection, in the form of a 40 amp fuse, and that protection is at the beginning of the run. Therefore, you have complied with the opening statement of 240.21, and don't need to get into its exceptions.

Perhaps it is time for your engineer to become familiar with "Charlie's Rule." :happyyes:

 

[david luchini]

The tap rules are all within 240.21. There are no separate tap rules for medium voltage.

I'm not sure I agree with Charlie here. The tap rules in 240.21 (in fact all of Parts I thru VIII in article 240) apply to circuits of not more than 600V nominal. Part IX in Article 240 applies to circuits of 600V, nominal. 240.101(B) is specific to Feeder Taps over 600V.

That being said...

What you are telling me is that each and every cable within this installation (not counting the utility feed, which is outside the scope of the NEC) has an upstream fuse rated at 40 amps, and each cable has an ampacity that is well above 40 amps. If that is the case, then this is not a "tap" situation at all.

I agree that that doesn't appear to be a "tap" situation here at all.

 

[charlie b]

240.101(B) is specific to Feeder Taps over 600V.

Except that that article is not a "tap rule." It does not allow a tapped conductor to be protected at the end of the run, in the manner that is allowed under 240.21. Actually, that article is nothing but double-speak. All it says is that a conductor is allowed to be protected by an OCPD, as long as the OCPD protects the conductor. That does not meet the definition of "tap conductors," as provided in 240.2.

 

[david luchini]

Except that that article is not a "tap rule." It does not allow a tapped conductor to be protected at the end of the run, in the manner that is allowed under 240.21. Actually, that article is nothing but double-speak. All it says is that a conductor is allowed to be protected by an OCPD, as long as the OCPD protects the conductor. That does not meet the definition of "tap conductors," as provided in 240.2.

I agree that 240.101(B) isn't an actual "tap rule" per the manner allowed under 240.21. Then again, the definition of "tap conductors" in 240.2 do not apply to circuits over 600V, only to those not more than 600V.

But Part IX allows overcurrent protection that is 3 times or 6 times the ampacity of the conductor to start with. So I understand the point of 240.101(B) as allowing, say a #8Awg cu, 5kV MV-90 feeder to be "tapped" from a #4Awg cu, 5kV MV-90 (T310.73) feeder protected by a 150A fuse.

 

[charlie b]

Then again, the definition of "tap conductors" in 240.2 do not apply to circuits over 600V, only to those not more than 600V.

Yes it does. I concede that the defintion of "tap conductors" appears within Part I of article 240, specifically in 240.2. However, that definition begins with the phrase, "as used in this article." It does not say, "as used in parts I through VIII of this article." The "article" is the entirety of 240. So the definition applies to circuits over 600 volts.

 

[david luchini]

Yes it does. I concede that the defintion of "tap conductors" appears within Part I of article 240, specifically in 240.2. However, that definition begins with the phrase, "as used in this article." It does not say, "as used in parts I through VIII of this article." The "article" is the entirety of 240. So the definition applies to circuits over 600 volts.

I'd have to disagree. Section 240.1 makes it clear that section 240.2 only applies to installations of not more than 600V. They could have said "As used in Parts I thru VIII of this article, a tap conductor is..." but it would have been redundant as they already made clear that section 240.2 only applies to installations of not more than 600V.

In addition, as you have pointed out, in Part IX the code uses the term "tap conductor" in a manner contrary to the definition in 240.2. It seems clear from this that the definition of "tap conductor" in 240.2 doesn't apply to Part IX.

 

[pistolpigg]

So what section would this type of wiring be confirmed to be legal in?

 

[david luchini]

So what section would this type of wiring be confirmed to be legal in?

Look at 240.100 and 240.101 to see if your installation is legal.

 

[pistolpigg]

so could this be considered a medium voltage circuit since its not truly tapped

 

[david luchini]

so could this be considered a medium voltage circuit since its not truly tapped

I would think it would be considered a medium voltage circuit since is operating at 25kV.