Transformer Secondary Protection

[mull982]

I came across a 150kVA 480V Delta primary / 208V wye seconary 3-phase transformer the other day. The primary was proteced by a 150A fused switch which is not adequate for the primary full load capacity of the transformer however my focus here is more on the secondary of the transformer.

The secondary cables from the transformer are 600MCM and feed a 208V switchboard with no main protective device. There are six fused switches in this switchboard whose vaules add up to more than 600A. Note 2 of table 450.3(B) allows for the transformer to not have a secondary protective device if it terminates somewhere where there are not more than 6 protective devices in one location that do not add up to more than 125% of transformer secondary full load. With the secondary full load of this transformer being 416A, 125% or this value would be 521A and therefore the fact that the six protective devices in this switchboard add up to more than 600A is a violation. Would you agree? Would you also agree that a main breaker needs to be added into this switchboard?

In additional to this tapped off the main bus in the switchboard with no protective device is a set of 400MCM cables which is tapped to feed two panelboards about 5ft away. The tap off of this 400mcm cable is made in a trough and the tap conductors to the panels are 4/0 each. Each panel has a 225A main breaker. To me this just further violates the secondary protection exception for 3 reasons being that it now adds more than six breakers that the transformer terminates into, these breakers are all not in one loction (5ft away in different equipment) and the additional breaker size only increases the combined value of protective devices.

So in summary I see three problems here why the transformer secondary is not adequately protected:

1) Protective devices are all not in one location
2) There are more than six protective devices that the secondary of the transformer terminates into
3) Combined value of protective devices is over 125% of transformer secondary current.

Would you guys agree with all of this?

Also how would the tapped conductor off of the switchboard to the panelboards be viewed. Wouldn't his be a tap violation as well?

 

[mayanees]

...hmmm

...hmmm

Mull,
A quick look at this has me thinking that the transformer protection is okay since the note says where protection is required, and the xfmr could get by with primary-only-protection being less than 125%, so sec'y protection is not required. But the 420-amp 600 kCM cable is not protected by the summation of the ocpds. My quick thought is to upgrade the cable to the sum of the ocpds and it's safe.
John M

 

[david luchini]

I think the 600MCM secondary conductors to the switchboard would be OK if the switchboard is rated 400A or less and the conductors are not over 10ft long, per 240.21C)(2)(1)(b). However, the 400MCM tap would be a violation of 240.21(C)(2)(2).

 

[mull982]

Wait a second.

Isn't secondary protection of the conductors required per 240.4(F) for a delta/wye transformer? According to this you cannot simply use primary protection only and therefore secondary protection is required?

David

The switchboard is s 400A switchboard. Would the tap rule that you cited overide 240.4(F) and thus not require secondary protection?

 

[charlie b]

In additional to this tapped off the main bus in the switchboard with no protective device is a set of 400MCM cables which is tapped to feed two panelboards about 5ft away. . . .To me this just further violates the secondary protection exception for 3 reasons being that it now adds more than six breakers that the transformer terminates into, these breakers are all not in one loction (5ft away in different equipment) and the additional breaker size only increases the combined value of protective devices.

I agree. However, if the taps for these two extra loads had been made at the transformer secondary, instead of in the switchboard, then that part of the violation would have gone away. That is because of the phrase, ". . . or each set of conductors feeding separate loads. . . ," at the beginning of 240.21(C).

 

[charlie b]

According to this you cannot simply use primary protection only and therefore secondary protection is required?

I agree.

The switchboard is s 400A switchboard. Would the tap rule that you cited overide 240.4(F) and thus not require secondary protection?

It would not. You do need secondary protection. To be clear, we are talking about protecting the conductors that attach to the transformer secondary, and go from there to the switchboard. Protecting the transformer's secondary windings is a completely different matter. As for the secondary conductors, it is OK to use 600 MCM cables, and to connect to a 400 amp switchboard. But the total of the six fused switches cannot exceed 400 amps, and you can't tap the tap to go to two additional panels.

 

[mull982]

I agree.
It would not. You do need secondary protection. To be clear, we are talking about protecting the conductors that attach to the transformer secondary, and go from there to the switchboard. Protecting the transformer's secondary windings is a completely different matter. As for the secondary conductors, it is OK to use 600 MCM cables, and to connect to a 400 amp switchboard. But the total of the six fused switches cannot exceed 400 amps, and you can't tap the tap to go to two additional panels.

A few additional questions/clarifications:

1) We have said that the secondary conducor ampacity must be equal to or greater than the secondary OCPD per 24.21(C) and that we cannot use the provisions of 240.4(B). Can the rated secondary protective device however be rounded up based on calcuated 125% of transformer secondary current. For instance lets say 150kVA unit with 208V secondary has secondary rated current of 416A . 125% of this value would be 521A. Can you select a breaker that is rated at the next size up which I believe in this case is 600A (maybe 550A) or must you choose a breaker rated at 500A? I understand it that you select your breaker size first then select matching cable size so if you are allowed to choose a 600A breaker then you must size cables to have an ampacity equal to or greater than 600A. Is this correct? Do you choose breaker first then cables to match?

2) Is there anything preventing the use of 240.4(B) for the breakers and cables on the primary of the transformer? Similar to my first question can you use the next size up philosophy for the primary breaker if 250% of the transformers rated current does not come out to a standard breaker size? I assume same order here applies with sizing breaker first then sizing cable to breaker?

3) Do the rules or 240.4(F) only apply to protection of the transformer secondary "conductors" and not necessarily protection of the transformer itself? If so then is there anything that requires secondary protection of the "transformer" Are the rules in 450.3 referring to protecting the transformer or are just another place that shows required protection for the secondary conductors? If nothing dictates secondary protection of the atcual transformer itself is the secondary protection in this section just an option rather than a requirement?

4) Following up on my previous question if the exception in 450.3 allows for up to 6 devices in one location to be equal to 125% of transformer secondary current rating must the secondary cable feeding these devices be rated equal to or greater then the combined ampacity of these devices or can the provision of 240.4(B) "next size up" be used here?

5) Are transformer secondary conductors always considered a tap? So tapping a transformer secondary conductor as I mentioned in my OP is always a violation of 240.21?

 

[david luchini]

A few additional questions/clarifications:

1) We have said that the secondary conducor ampacity must be equal to or greater than the secondary OCPD per 24.21(C) and that we cannot use the provisions of 240.4(B). Can the rated secondary protective device however be rounded up based on calcuated 125% of transformer secondary current. For instance lets say 150kVA unit with 208V secondary has secondary rated current of 416A . 125% of this value would be 521A. Can you select a breaker that is rated at the next size up which I believe in this case is 600A (maybe 550A) or must you choose a breaker rated at 500A? I understand it that you select your breaker size first then select matching cable size so if you are allowed to choose a 600A breaker then you must size cables to have an ampacity equal to or greater than 600A. Is this correct? Do you choose breaker first then cables to match?

If you are using "primary & secondary" transformer protection, you are allowed to size the secondary at 125% of the rated current, using the next standard size up, 600A as you mention. The secondary conductors must have an ampacity of at least 600A.

2) Is there anything preventing the use of 240.4(B) for the breakers and cables on the primary of the transformer? Similar to my first question can you use the next size up philosophy for the primary breaker if 250% of the transformers rated current does not come out to a standard breaker size? I assume same order here applies with sizing breaker first then sizing cable to breaker?

No, nothing prevents using 240.4(B) for the primary. But using "primary & secondary" protection for the transformer, you are not allowed to go up to the next standard size. Figure 250% of the primary rated current and select the next OCPD size down.

3) Do the rules or 240.4(F) only apply to protection of the transformer secondary "conductors" and not necessarily protection of the transformer itself? If so then is there anything that requires secondary protection of the "transformer" Are the rules in 450.3 referring to protecting the transformer or are just another place that shows required protection for the secondary conductors? If nothing dictates secondary protection of the atcual transformer itself is the secondary protection in this section just an option rather than a requirement?

240.4(F) applies to secondary conductors only, not the transformer protection. The rules in 450.3 are for transformer protection, not conductor protection. Transformer secondary protection is only required if the primary protection exceeds 125% of the primary rated current (or the next standard size up.) This would then require the "primary & secondary" protection in 450.3

4) Following up on my previous question if the exception in 450.3 allows for up to 6 devices in one location to be equal to 125% of transformer secondary current rating must the secondary cable feeding these devices be rated equal to or greater then the combined ampacity of these devices or can the provision of 240.4(B) "next size up" be used here?

If you had 6 individual devices grouped in one location, then each device would require its own set of secondary conductors each with an ampacity of at least the rating of the OCPD.

5) Are transformer secondary conductors always considered a tap? So tapping a transformer secondary conductor as I mentioned in my OP is always a violation of 240.21?

There is some disagreement, but IMO, transformer secondary conductors are never considered a tap. They do not meet the definition of tap conductors in Sect. 240.2. In addition, you will see that "tap" is mentioned in Section 240.21, parts (A), (B), (E) and (F), but "tap" is not mentioned in 240.21, parts (C), (D) and (G). But tapping a transformer secondary conductor is not allowed per 240.21.

 

[mull982]

No, nothing prevents using 240.4(B) for the primary. But using "primary & secondary" protection for the transformer, you are not allowed to go up to the next standard size. Figure 250% of the primary rated current and select the next OCPD size down.

So why can you use 240.4(B) when you only have primary protection but cant use it for the primary breaker when you have primary and secondary protection?

If you had 6 individual devices grouped in one location, then each device would require its own set of secondary conductors each with an ampacity of at least the rating of the OCPD.

Would a secondary breaker terminiating in a MLO panel or switchboard with 6 breakers who's ratings did not exceed the ampacity of the secondary conductors adequately protect the secondary conductors? Or for the secondary conductors must you always follow the guidelines of 240.21 and have the cables terminate in a single OCPD only?



So esentially you are saying that for a transformer if the primary does not exceed 225% then no secondary protection is needed?

Am I right in saying that with transformers you size the breaker sizes first and then size the cables to match the breakers? This is kind of backwards from other feeder sizing methods where you usually size the cable first based on load and then size breaker to protect.

 

[david luchini]

So why can you use 240.4(B) when you only have primary protection but cant use it for the primary breaker when you have primary and secondary protection?

No, you are misunderstanding. There were separate questions being responded to.
1) You can use 240.4(B) for primary conductor protection. It doesn't matter if the transformer is "primary only" or "primary & secondary" protected. 240.4(B) applies to primary conductors for both cases.

2) The "next up" philosophy cannot be used for a primary circuit breaker size if 250% does not correspond to a standard circuit breaker size.

For example a 112.5kVA, 480-208/120V transformer has a primary rated current of 135.3A. Using primary and secondary protection, 250% of that is 338.3A. The primary OCPD cannot be the next size up (or 350A), it has to be the next size down, 300A. The conductors from the primary OCPD to the xfmr can be 300mcm (285A) per 240.4(B).

Would a secondary breaker terminiating in a MLO panel or switchboard with 6 breakers who's ratings did not exceed the ampacity of the secondary conductors adequately protect the secondary conductors? Or for the secondary conductors must you always follow the guidelines of 240.21 and have the cables terminate in a single OCPD only?

240.21(C)(2), for instance, doesn't require that the secondary conductors terminate in a single OCPD only. They are allowed to terminate at an OCPD or a device. (240.21(C)(2)(1)b.) The switchboard would be a device, so the secondary conductors would have to have an ampacity of not less than the switchboard rating.

So esentially you are saying that for a transformer if the primary does not exceed 225% then no secondary protection is needed?

Am I right in saying that with transformers you size the breaker sizes first and then size the cables to match the breakers? This is kind of backwards from other feeder sizing methods where you usually size the cable first based on load and then size breaker to protect.

I think you mean 125%, not 225%. But yes, per 450.3 if the transformer primary OCPD doesn't exceed 125% (or the next higher standard rated OCPD) of the primary rated current, then secondary transformer protection is not required. Secondary conductor protection would still be required.

I would say its somewhat accurate to say with transformers you size the breakers first, then the conductors to match. But don't forget, you are sizing the transformer to handle the load, so the conductors should be adequately sized, but you still must verify they are acceptable for the load.

For example, if you had a 208V load that was 205A (73.85kVA) and I used a 75kVA transformer with a 250A secondary c/b and 250mcm secondary conductors. It would seem to be OK, but if the 205A load was 200A continuous and 5A non-continuous, I would need the OCPD and conductors to have an ampacity of 200*1.25+5=255. The 250A c/b is too small. I would have to use a 300A secondary c/b with 350mcm conductors for that load.

 

[mull982]

240.21(C)(2), for instance, doesn't require that the secondary conductors terminate in a single OCPD only. They are allowed to terminate at an OCPD or a device. (240.21(C)(2)(1)b.) The switchboard would be a device, so the secondary conductors would have to have an ampacity of not less than the switchboard rating.

But what if the switchboard they are terminating in is a MLO switchboard. Would the ampacity of the secondary conductors have to be equal to or greater than the combined rating of all the feeder breakers located in the switchboard? Is there a maximum number of how many feeder breakers can be in the "device" into which it terminates?

For instance if a transformer secondary conductors terminate into a 100A MLO panel the combined ratings of all the branch breakers in a 100a panel (assuming upwares of 30 breakers) will surely have a combined rating of more than 100A. So in this case sizing the conducors for the "device" rating would not be protected by the combined breaker ratings.

 

[david luchini]

For instance if a transformer secondary conductors terminate into a 100A MLO panel the combined ratings of all the branch breakers in a 100a panel (assuming upwares of 30 breakers) will surely have a combined rating of more than 100A. So in this case sizing the conducors for the "device" rating would not be protected by the combined breaker ratings.

This wouldn't be allowed because of 408.36, but I understand your point.

But what if the switchboard they are terminating in is a MLO switchboard. Would the ampacity of the secondary conductors have to be equal to or greater than the combined rating of all the feeder breakers located in the switchboard? Is there a maximum number of how many feeder breakers can be in the "device" into which it terminates?

From what I read, if there is primary only protection of the transformer (so that secondary protection is not required) there is no limit to the number of OCPDs in the switchboard or no limit to the sum of the ratings of the switchboard. In my mind, I would assume the limit is 6 OCPDs, but I don't know what section of code says that. Maybe someone can point out which section I'm missing.

240.21(C)(2) says the ampacity of the secondary conductors must not be less than the calculated load, and not less than the rating of the device (switchboard in this case) that they supply. So if the load on the secondary is 502A, and the secondary conductors were two sets of 350mcm supplying an mlo switchboard that had 6 125A c/b's, that would seem to be OK under 240.21(C)(2).

Edit: Maybe its the last sentence of 240.41(C) that requires the breakers in the MLO switchboard to not exceed the secondary conductor rating?

 

[mull982]

In 240.21(C)(2)(4) it states that the secondary conductors must be at least 1/10 the primary breaker rating multiplied by the transformer ratio. If this condition is met does this only allow you to locate the secondary breaker somewhere other than the trasformer secondary terminals (within 10ft) This 1/10 requirement is not intented to take the place of secondary protection for the conductor correct?

 

[david luchini]

In 240.21(C)(2)(4) it states that the secondary conductors must be at least 1/10 the primary breaker rating multiplied by the transformer ratio. If this condition is met does this only allow you to locate the secondary breaker somewhere other than the trasformer secondary terminals (within 10ft) This 1/10 requirement is not intented to take the place of secondary protection for the conductor correct?

No, this section does NOT allow you to locate the secondary breaker somewhere other than within 10ft of the transformer secondary terminals. Not that 240.21(C)(2) says "and complies with ALL of the following."

That section just sets a minimum limit on the secondary conductors. For instance, if you had a 112.5kVA 480-208/120V transformer with a 175A primary only protection, and you wanted to run 10 secondary sets of #10Awg to a 30A c/b for each of 10 separate loads, 240.21(C)(2)(4) would prevent that because the primary OCPD x prim/sec voltage ratio exceeds 10 times the secondary conductor ampacity.