Medium Voltage transformer secondary protection

[philly]

I'm looking at an application trying to determine if the secondary of a medium voltage transformer is adequately protected per the NEC. From what I can tell all of the same rules with MV transformers apply as do with LV transformer the only difference being that the allowed values of protective devices are higher.

The situation I have is a 2500kVa transformer with a 12.47kV delta primary and a 4.16kV wye secondary at 5.46%Z. The primary of the transformer is protected by a 200A fused switch. The secondary of the transformer has (4) different feeder circuit tapped off of the secondary terminals of the transformers landing on (4) individual pieces of equipment. One feeder terminates on a fused switch which feeds another transformer and (3) other feeders go on to feed (3) individual MV motor starters. I'm looking at this application to see if it meets the requuired code and here is what I see:

The primary is adequately protected since the 200A fuse falls within the allowable 300% fuse rating of the 112A full load on the primary of the transformer.

The first thing that I question on the secondary is the fact that the secondary conductors all terminate in diferent locatins. FPN 2 of table 450.3(A) states that if there is no secondary protection on the transformer then secondary protection can consist of six sets of breakers or fuses grouped in one location. Although at each of the 4 location the transformer secondary terminates there are fuses these fuses are not all "in the same location" Since these do not all terminate in one location would this be considered a violation of code?

Secondly the same FPN requires that when protected by multiple fuses on the secondary the total of these fuse ratings shall not exceed the allowed value of a single overcurrent device which for this transformer would be 225% of transformer secondry FLA of 347A or 867A. In this cases the 3 MV motor fuses are 100A, 100A and 170A while the fuesed switch feeding a transformer has a 200A fuse. This gives a combined seconday fuse total of 570A which falls below the allowable total of 867A thus providing the adquate protection. Do you agree?

Lastly I am verifying that the secondary conductors are protected per 240.21(C). With LV conductors I understand that secondary conductors terminating into an OCPD on the secondary must have an ampacity equal to or greater than the OCPD and the "next size up" rule cannot be used. With MV conductors however are the secondary conducotrs allowed to be protected by 240.101(A) and thus be protected by an fuse that has a rating that is 300% of the cable ampacity. For example on of the conducotrs that feeds on of the motors starters is a #6 MV cable in conduit thus having an ampacity of 58A. This cable terminates in a 100A fuse however this 100A fuse falls within the 300% rating of the cables ampacity. Would this conductor be adquately protected. The other cables are #6, #2, and #1. Does the combined ampacity of these cables come into play in regards to the secondary current rating of the transformer?

Thanks for the help!

 

[jim dungar]

Be careful to not take sections of the Code out of context.

450.3 is about protecting the transformer not the secondary conductors.
450.3(A) Note 2 uses the phrase "where required'. As you pointed the transformer has appropriate primary side protection, therefore Note 2 is not applicable, in regards to protecting the transformer and it should be ignored entirely.

240.21(C) is the location for transformer secondary conductor protection (600V and less).
Each set of secondary conductors is considered individually, therefore they are allowed to terminate in different locations. However, each location must comply with the provisions of 240.21(C).

240.92 is for overcurrent protection in "supervised industrial" locations.
240.92(C) allows transformer secondary conductor to be much longer than the limits of 240.21(C).

240.100 is the actually the correct area for protection of conductors >600.
240.100(A) allows the location of the conductor protection to be determined under "engineering supervision" that considers the conductor damage curves.

It seems most engineers use the limits of 240.21 & 240.92 even when dealing with >600V installations.

 

[philly]

450.3 is about protecting the transformer not the secondary conductors.
450.3(A) Note 2 uses the phrase "where required'. As you pointed the transformer has appropriate primary side protection, therefore Note 2 is not applicable, in regards to protecting the transformer and it should be ignored entirely..

Yes but I thought that all transformers except for Delta-Delta transformers require secondary protection per 240.4(F). So wouldn't this case require secondary protection and thus the guidelines for secondary protection in 450.3 apply?

240.21(C) is the location for transformer secondary conductor protection (600V and less).
Each set of secondary conductors is considered individually, therefore they are allowed to terminate in different locations. However, each location must comply with the provisions of 240.21(C)...

O.k. so are you saying that 240.21 doesn't apply to MV secondaries? And it sounds like you are saying that even with LV secondaries each set of secondary conductors can terminate in a seperate location as long as each set of feeders complies with 240.21? Is that correct?

240.92 is for overcurrent protection in "supervised industrial" locations.
240.92(C) allows transformer secondary conductor to be much longer than the limits of 240.21(C).)

This is interesting. I have never really realized this section was applicable in some situations. So in situations such as an industrial facility these rules would apply and trumph those of 240.21 even for LV installations? So for instance with both LV and MV the requirements of 240.92(C)(2) take presitance over those of 240.21?


240.100 is the actually the correct area for protection of conductors >600.
240.100(A) allows the location of the conductor protection to be determined under "engineering supervision" that considers the conductor damage curves.

It seems most engineers use the limits of 240.21 & 240.92 even when dealing with >600V installations.

This may answer my previous question. So with dealing with MV installations typically the greater value between 240.21 & 240.92 is chosen? Does this apply for LV systems too when dealing with supervised locations?

 

[jim dungar]

Yes but I thought that all transformers except for Delta-Delta transformers require secondary protection per 240.4(F). So wouldn't this case require secondary protection and thus the guidelines for secondary protection in 450.3 apply?

Section 450 is about protecting the transformer windings, it has absolutely nothing to do with 240.4. Likewise, 240.4 and 240.21 have nothing to do with the transformer.
Confusion usually comes from using a single secondary protective device to meet two different code sections.

O.k. so are you saying that 240.21 doesn't apply to MV secondaries? And it sounds like you are saying that even with LV secondaries each set of secondary conductors can terminate in a seperate location as long as each set of feeders complies with 240.21? Is that correct?

Yes.

This is interesting. I have never really realized this section was applicable in some situations. So in situations such as an industrial facility these rules would apply and trumph those of 240.21 even for LV installations? So for instance with both LV and MV the requirements of 240.92(C)(2) take presitance over those of 240.21?

I would not say 240.92 trumps a previous section, I am saying that either section, but not both, could be applicable.

This may answer my previous question. So with dealing with MV installations typically the greater value between 240.21 & 240.92 is chosen? Does this apply for LV systems too when dealing with supervised locations?

I don't think there is a single 'always' correct method. For example, many installations that were adequately supervised, 30 years ago, would no longer qualify.

 

[philly]

Section 450 is about protecting the transformer windings, it has absolutely nothing to do with 240.4. Likewise, 240.4 and 240.21 have nothing to do with the transformer.
Confusion usually comes from using a single secondary protective device to meet two different code sections.

So for both LV and MV transformers if the transformer has primary protection that falls within the "primary protection only" category then secondary protection is not required per the 450.3 tables? So then the 450.3 tables are only for protection of transformer windings? So then even if secondary protection is not required for protection of transofmrer windings, then we still need to look at 240.21 to see if conductors are adequately protected?

So how do we determine if an particular location falls under the category of a "supervise location" in order to use 240.92? Would an industrial facility with a full time electrical staff be considered a "supervised location"? If so then 240.92 could apply to both LV and MV systems in this facility?

 

[jim dungar]

So for both LV and MV transformers if the transformer has primary protection that falls within the "primary protection only" category then secondary protection is not required per the 450.3 tables? So then the 450.3 tables are only for protection of transformer windings? So then even if secondary protection is not required for protection of transofmrer windings, then we still need to look at 240.21 to see if conductors are adequately protected?

Correct.

So how do we determine if an particular location falls under the category of a "supervise location" in order to use 240.92? Would an industrial facility with a full time electrical staff be considered a "supervised location"? If so then 240.92 could apply to both LV and MV systems in this facility?

It is detailed in 240.2.
But, this may be why this section is not widely used.

 

[philly]

It is detailed in 240.2.
But, this may be why this section is not widely used.

Wow I thought I had a firm understanding of 240.21 until you pointed out this 240.92 section (very interesting)

So in an industrial system which meets the "supervised location" requirements of 240.2 I have an 480V 800A breaker with a 4/0AWG feeder (150ft long) which then terminates on a 225A breaker. Under the provisions of 240.21 this 4/0AWG feeder with an ampacity of 235A would not be allowed to be adequately protected by the 800A breaker. Now however since we are introducting 240.92 I'm a bit confused how to treat a situation like this. For feeders 240.92 seems to reference the equations in table 240.92(B) which appear to be equations for cable damage curves. If I'm using software though that shows the cable damage curves can these cables be considered protected as long as the cable damage curev is above the breaker?

Is using 240.92 a good practice over using 240.21?

For this particualr case even with 240.21 and the 150ft 4/0AWG feeder is this a violation since the cable is over 25ft long? I see in 240.21(B)(5) there is a provision for taps of unlimited length but this seems to apply to feeders that are outdoors. I'm not sure if the inside of an industrial facility would be considered outdoors? The application described above would seem to be a violation to me.

Thanks for the help in understanding this.

 

[jim dungar]

Is using 240.92 a good practice over using 240.21?

In my opinion - generally no.

 

[philly]

In my opinion - generally no.

O.k. with that said how would you approach the following situation:

4.16kV transformer secondary conductors indoors 50ft long are #6 and terminate into an 100A fuse.

Would you:

1) Cite 240.21 and say that 50ft is longer than permitted in 240.21C and cannot be adequately protected by 100A fuse.

or

2) Cite 240.92 and look at at a TCC curve for the cable and fuse to see if cable damage curve is above fuse curve. If it is then you would consider the cable protected by the fuse.

or

3) Cite 240.101 and say that a MV feeder can be protected by a fuse tha has a rating of 3x the ampacity of the cable. Since #6 has ampacity of 58A in conduit then the 100A fuse is within the allowed 3x rating and therfore adequately proteccts the cable.

I'm just trying to get an idea of the thought process for such MV applications in regards to conductor protection.

 

[jim dungar]

Typically I choose my MV protection for the load, and then choose a conductor.

 

[philly]

Typically I choose my MV protection for the load, and then choose a conductor.

Here is another situation that I keep coming across that I am going back and forth with and has me confused :?

For LV applications where there is a feeder tap or a secondary transformer secondary conductor. Both 240.21(B)(3) and 240.21(C)(3) give provisions for conductors whose lenghts dont exceed 25 ft. For anything over 25ft located indoors there is no provision for providing protection for these conductors so I interpret this as any tapped conductor or transformer secondary conductor having a distance of over 25 ft is a violation.

However as you stated earlier 240.92 does not give any specific length for tapped condcutors or transformer secondary conductors. So then if I want to use an unlimited length for tapped conductors are there any guidelines I must follow for the ampacity of the cables and their protection by an OCPD? It appears this section says to use table 240.92(B) for tapped feeder conductors, but what is typically done here? Even with a tapped feeder conductors of unlimited length do you still follow the provisions of 240.21(B) dictating the requiremenets for the size/ampacity of the conductors? Same with following the provisions of 240.21(C) dictating the requirments for the size/ampacity of the secondary transformer conductors?

Thanks for helping clear up my confusion!

 

[philly]

Let me give (2) examples related to my previous post to see how others would address them.

1) A 480V 800A breaker with a 4/0 cable connected to the breaker terminating into a 225A breaker 150ft away. The 150ft is longer than any of the distances in provisions 240.21(B)(1-4) and the conducors are located indoors so 240.21(B)(5) does not apply. Although these conductors do terminate into a single OCPD which limits the current to 225A which is below the conductor ampacity how do we determine if this is legal based upon the distance?

2) A second situation is the same 800A breaker and 4/0 conductor fed from it which terminates into a panel with no main breaker. This is a violation of all provisions of 240.21(B) since the conductors dont terminate into a single OCPD. So if we install a 225A main breaker in the panel so that the feeder terminates into a single OCPD limiting the current to below the conductor ampacity how do we justfify this (what code section) based on the fact that the cable length is 150ft and greater than any distance provision in 240.21(B)?

 

[philly]

Can anyone offer any input or share their design experiences in regards to this cable protection issue?

 

[david luchini]

Let me give (2) examples related to my previous post to see how others would address them.

1) A 480V 800A breaker with a 4/0 cable connected to the breaker terminating into a 225A breaker 150ft away. The 150ft is longer than any of the distances in provisions 240.21(B)(1-4) and the conducors are located indoors so 240.21(B)(5) does not apply. Although these conductors do terminate into a single OCPD which limits the current to 225A which is below the conductor ampacity how do we determine if this is legal based upon the distance?

240.4 says that conductors shall be protected against overcurrent in accordance with their ampacity. 240.21 says that the overcurrent protection shall be located "at the point the conductors receive their supply" except as specified in 240.21(A) thru (H). 240.21(B) relates to the "tap conductors" as you have described. 240.21(B) says conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) thru (B)(4) (ignoring (B)(5) as you have.)

240.21(B)(1) specifies tap conductors not longer than 10'.
240.21(B)(2) specifies tap conductors not longer than 25'
240.21(B)(3) specifies tap conductors supplying a transformer with primary and secondary conductors not longer than 25'
240.21(B)(4) specifies tap conductors not longer than 25'.

Your example shows tap conductors with a length of 150'. These conductors are clearly not installed per the provisions of 240.21(B)(1), (2), (3) or (4), so 240.4 & 240.21 require these conductors to be protected in accordance with their ampacity at the point the conductors receive their supply. Your 4/0 conductors are protected at the point they receive their supply by an 800A OCPD which is clearly a violation of 240.4 and 240.21.

2) A second situation is the same 800A breaker and 4/0 conductor fed from it which terminates into a panel with no main breaker. This is a violation of all provisions of 240.21(B) since the conductors dont terminate into a single OCPD. So if we install a 225A main breaker in the panel so that the feeder terminates into a single OCPD limiting the current to below the conductor ampacity how do we justfify this (what code section) based on the fact that the cable length is 150ft and greater than any distance provision in 240.21(B)?

Your second example has the same violation of 240.4 and 240.21. It cannot be justified.

If you had a "Supervised Industrial Installation" per Section VIII of Art. 240, you would have the same violation. 240.92(A) says feeder conductors shall be protected at the point the conductors receive their supply, except as permitted in 240.92(B) for feeder taps. 240.92(B) says that for feeder taps specified in 240.21(B)(2), (B)(3) and (B)(4)...that is to say feeder taps conductors not longer than 25', the taps conductors shall be permitted to be sized in accordance with T240.92(B).

 

[philly]

240.4 says that conductors shall be protected against overcurrent in accordance with their ampacity. 240.21 says that the overcurrent protection shall be located "at the point the conductors receive their supply" except as specified in 240.21(A) thru (H). 240.21(B) relates to the "tap conductors" as you have described. 240.21(B) says conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) thru (B)(4) (ignoring (B)(5) as you have.)

240.21(B)(1) specifies tap conductors not longer than 10'.
240.21(B)(2) specifies tap conductors not longer than 25'
240.21(B)(3) specifies tap conductors supplying a transformer with primary and secondary conductors not longer than 25'
240.21(B)(4) specifies tap conductors not longer than 25'.

Your example shows tap conductors with a length of 150'. These conductors are clearly not installed per the provisions of 240.21(B)(1), (2), (3) or (4), so 240.4 & 240.21 require these conductors to be protected in accordance with their ampacity at the point the conductors receive their supply. Your 4/0 conductors are protected at the point they receive their supply by an 800A OCPD which is clearly a violation of 240.4 and 240.21.

Yes but doesn't 240.92 allow for other provisions above and beyone 240.21(B) 1-4? For feeder taps in 240.92(B) which is what this example would fall under it says to use Table 240.92(B)? Is it accaptable to use this table in a supervised enviornment to protect the above conductors?

Your second example has the same violation of 240.4 and 240.21. It cannot be justified.

If you had a "Supervised Industrial Installation" per Section VIII of Art. 240, you would have the same violation. 240.92(A) says feeder conductors shall be protected at the point the conductors receive their supply, except as permitted in 240.92(B) for feeder taps. 240.92(B) says that for feeder taps specified in 240.21(B)(2), (B)(3) and (B)(4)...that is to say feeder taps conductors not longer than 25', the taps conductors shall be permitted to be sized in accordance with T240.92(B).

Where do you see that table 240.92(B) is restricted to feeder taps less than 25ft as you mentioned?

 

[david luchini]

Yes but doesn't 240.92 allow for other provisions above and beyone 240.21(B) 1-4? For feeder taps in 240.92(B) which is what this example would fall under it says to use Table 240.92(B)? Is it accaptable to use this table in a supervised enviornment to protect the above conductors?

240.92(B) allows the tap conductors to be size in accordance with T240.92(B). To me, this suggests that in a supervised industrial installation (Part VIII of Art. 240) you could create a 100A tap from an 800A feeder as long as the tap conductors are protected per T240.92(B).

Where do you see that table 240.92(B) is restricted to feeder taps less than 25ft as you mentioned?

240.92(B) says "for feeder taps as specified in 240.21(B)(2), (B)(3) and (B)(4)..."

The taps specified in each of those 3 sections are as follows:

240.21(B)(2): Taps not over 25ft long.
240.21(B)(3): Taps supplying a Transformer [Primary plus Secondary not over 25ft long]
240.21(B)(4): Taps over 25ft long (High bay manufacturing) - Taps can be not over 25ft horizontally and not over 100ft total length.

The tap lengths specified in 240.21(B)(2), (B)(3) and (B)(4) apply to the taps in a Supervised Industrial Location, per 240.92(B). Nothing in 240.92(B), or Part VIII, allows taps to be 150' or 200' or any particular length you want.

 

[philly]

240.92(B) allows the tap conductors to be size in accordance with T240.92(B). To me, this suggests that in a supervised industrial installation (Part VIII of Art. 240) you could create a 100A tap from an 800A feeder as long as the tap conductors are protected per T240.92(B).
.

Does this still require the tapped feeders to be under a certain length (25ft?) as you stated earlier?

240.92(B) says "for feeder taps as specified in 240.21(B)(2), (B)(3) and (B)(4)..."

The taps specified in each of those 3 sections are as follows:

240.21(B)(2): Taps not over 25ft long.
240.21(B)(3): Taps supplying a Transformer [Primary plus Secondary not over 25ft long]
240.21(B)(4): Taps over 25ft long (High bay manufacturing) - Taps can be not over 25ft horizontally and not over 100ft total length.

The tap lengths specified in 240.21(B)(2), (B)(3) and (B)(4) apply to the taps in a Supervised Industrial Location, per 240.92(B). Nothing in 240.92(B), or Part VIII, allows taps to be 150' or 200' or any particular length you want.

O.k. it sounds like since you are saying that since 240.92(B) references 240.21(B)(2), (3), and (4) that the tap conductors here still have to fall within the maxium allowable distance dictated by these sections which is 25ft?

Does this same 25ft distance apply to transformer secondary conductors as stated in 240.92(C). This section talks about a max distance of 100ft for short circuit but doesn't really talk about a lenght related to overcurrent protection in 240.92(C)(2). This section also talks about limiting the load to the conductor ampacity. Does this allow the use of the "next size up" provision of 240.4(B)

Thanks!

 

[david luchini]

O.k. it sounds like since you are saying that since 240.92(B) references 240.21(B)(2), (3), and (4) that the tap conductors here still have to fall within the maxium allowable distance dictated by these sections which is 25ft?

Does this same 25ft distance apply to transformer secondary conductors as stated in 240.92(C). This section talks about a max distance of 100ft for short circuit but doesn't really talk about a lenght related to overcurrent protection in 240.92(C)(2). This section also talks about limiting the load to the conductor ampacity. Does this allow the use of the "next size up" provision of 240.4(B)

No, the length requirements in 240.21(C) would not apply to the transformer secondary conductors in a Supervised Industrial Installation. 240.90(C) says overcurrent protection is not required at the secondary connection where the conditions of 240.92(C)(1), (C)(2), and (C)(3) are met. (There is no reference to 240.21(C)).

240.92(C)(1)(1) has a 100' secondary conductor limit, but (C)(1)(2) and (C)(1)(3) do not have a specific length limit.

I don't believe that 240.92(C)(2) allows the "next size up" provision.

 

[jim dungar]

I don't believe that 240.92(C)(2) allows the "next size up" provision.

I agree.

 

[mull982]

So to sumarize here.

In supervised locations:

Feeder taps can be protected according to table 240.92 however the lenghts for these feeder taps must adhere to the rules of 240.21(B) and therefore no feeder tap can be longer than 25ft.

For transformer secondary conductors there is no limit on the distance these secondary conductors can be without protection at the transformer secondary. For the OCPD into which the transformer secondary conductors terminate into (or group of devices) the device must not have an rating higher than the ampacity of the secondary conductors, and the "Next size up" provision cannot be used here.

Does this sound correct?