ART 450 Secondary Protection confusion

[DW98]

Hopefully someone can clear up what's confused me for a long time. I apologize in advance if the following is unclear or rambling.

What is secondary protection for a transformer? what does it physically look like? For instance, I'll use a 125% sized primary breaker, and then secondary protection isn't required. But, for example, I'll put a 200-225A main circuit breaker in a panelboard off a 75kva xfmr, but the circuit breaker is to protect the panelboard as I understand it not the xfmr. The feeders would be indirectly protected against overcurrent by the breaker but not protected from ground faults. Also, sometimes, I'll have a fused disconnect on a secondary side of a xfmr, but again this is for the feeders and the downstream panel. How is secondary xfmr protection different, or is it? Is the fused disconnect or main circuit breaker mentioned above the secondary protection, or is it something different?

 

[pete m.]

Article 450 deals with Transformer protection ONLY. Using table 450.3(B) you can have primary protection only. Now the transformer is protected.

Article 450 has nothing to do with protection of the secondary conductors. 240.21(C) is where you go for secondary conductor protection.

Article 450 has nothing to do with protection of the panelboard that the transformer serves. 408.36 is where you go for panelboard protection.

Three separate articles three separate purposes. You will often find yourself referencing the three to come up with the correct protection scheme.

Pete

 

[iwire]

Nicely put Pete.

 

[charlie b]

Let?s start with the requirement that every chunk of metal that carries current must be protected against overcurrent. That includes the primary feeder serving the transformer, the transformer?s primary windings, the transformer?s secondary windings, the feeder from the transformer to the panelboard, and the panelboard itself. When the book speaks of ?secondary protection,? it is specifically speaking of the transformer?s secondary windings. However, in most of the designs I have done, the main breaker within the panelboard serves to provide overcurrent protection for the transformer?s secondary windings, the feeder from the transformer to the panelboard, and the panelboard itself. A fused disconnect immediately downstream of the transformer can perform those same three functions. Under certain limited circumstances, a single overcurrent device is allowed to protect the primary feeder, the primary windings, and the secondary windings. However, that item is not allowed to also protect the secondary feeder, nor the panelboard.

 

[DW98]

xfmr secondary response to response

xfmr secondary response to response

Thanks for the response. I understand your response and am familiar with the other articles. I just don't understand what xfmr secondary protection is or looks like. primary overcurrent protection protects the primary windings which are coupled to the secondary. If you need secondary protection, where is it located? Is it a circuit breaker or fused switch immediately next to the xfmr?

 

[david luchini]

The transformer secondary protection could be a panel main circuit breaker, it could be a fused switch, it could be on individual enclosed circuit breaker, etc.

The confusing part is that, as you mention, transformer secondary protection isn't always required per 450.3. But secondary conductor protection is required in most cases, except those mentioned 240.21(C)(1). See you will likely see an OCPD on the secondary even when secondary protection isn't required.

In your 75kVA (480-120/208) xfmr example, you could provide a 200A primary OCPD (more than 125%), then the secondary must be protected at not more than 125% (next size up) which could be up to 300A. If you used a 200A secondary c/b, the conductors must have an ampacity of at least 200. If you used a 250A secondary c/b, the conductors must have an ampacity of at least 250, etc.

If you provided a primary OCPD at not more than 125% (next size up) then the max. c/b could be 125A. In this case you don't need to protect the secondary, so you put any size secondary OCPD that you wanted. You could put a 400A MCB panelboard, for instance. The secondary conductors must have an ampacity of at least 400. You could put a 600A secondary OCPD if you wanted.

 

[dkidd]

However, that item is not allowed to also protect the secondary feeder.

It can for a single phase 2-wire transformer.

 

[pete m.]

Under certain limited circumstances, a single overcurrent device is allowed to protect the primary feeder, the primary windings, and the secondary windings. However, that item is not allowed to also protect the secondary feeder, nor the panelboard.

Charlie,

I am with you 100% until the statement highlighted in blue.

240.21(C)(1) does allow the primary OCPD to protect the secondary conductors and 408.36 states, in part, that "This overcurrent protective device shall be located within or at any point on the supply side of the panelboard.

Like you stated there are certain circumstances that must be met but I believe it is possible.

Pete

 

[pete m.]

Nicely put Pete.

It's rare but thanks! (I also see now that I missed the OP's actual question...)

Pete

 

[DW98]

second response

second response

Thanks to both responses. My first response was to Pete's. Charlie's response also sums up and clarifies what I was thinking. The panelboard breaker or fused disconnect are required for panelboard and/or feeder protection, but by defaut protect the secondary windings although not required where primary is 125% sized. So in an instance where you have secondary protection, it can serve by default as the required feeder or panel protection.

Thanks again

 

[dkidd]

The 10 and 25 foot tap rules come into play on how far away the panelboard could be.

 

[dkidd]

Article 450 deals with Transformer protection ONLY. Using table 450.3(B) you can have primary protection only. Now the transformer is protected.

The secondary windings of the transformer must be protected from overload. A device on the primary cannot protect the individual secondary windings from overload, except for single phase 2-winding transformers, and delta-delta connected transformers with no taps.

 

[Machiavelli999]

I thought that if the secondary feed to the panelboard is less than 10 feet than an OCP was not necessary on the secondary due to the tap rule.

Is that wrong?

 

[david luchini]

The secondary windings of the transformer must be protected from overload. A device on the primary cannot protect the individual secondary windings from overload, except for single phase 2-winding transformers, and delta-delta connected transformers with no taps.

Based on what? If the primary of a 75kVA, 480-120/208V transformer is protected with a 125A ocpd (primary only protection per T450.3(B)), then you could have a 600A ocpd on the secondary if you wanted. I don't think a 600A ocpd is going to provide overload protection for the 208A rated secondary current.

 

[david luchini]

I thought that if the secondary feed to the panelboard is less than 10 feet than an OCP was not necessary on the secondary due to the tap rule.

Is that wrong?

Yes, that is wrong. See 240.21(C)(2).

 

[don_resqcapt19]

Yes, that is wrong. See 240.21(C)(2).

David,
That section does not require that the secondary conductors terminate on an OCPD. It only requires that they terminate on a device and that the conductors have an ampacity equal to or greater than the rating of that device.
That being said, the rule in 408.36 would require protection for the panelboard itself. It appears that if you are using a switchboard, you could land the secondary conductors on a MLO switchboard.

 

[david luchini]

David,
That section does not require that the secondary conductors terminate on an OCPD. It only requires that they terminate on a device and that the conductors have an ampacity equal to or greater than the rating of that device.

Maybe I misunderstood the question, but I didn't read it as asking if the secondary conductors needed to "terminate" on an OCPD. I read the question as whether or not overcurrent protection needed to be provided for the secondary conductors where the length is 10ft or less. 210.21(C)(2) doesn't remove the requirement to provide overcurrent protection for the secondary conductors.

That being said, the rule in 408.36 would require protection for the panelboard itself. It appears that if you are using a switchboard, you could land the secondary conductors on a MLO switchboard.

Yes, I agree that 210.21(C)(2)(1)b would allow this. But if the switchboard had two 200A feeder breakers, or four 100A feeder breakers, etc, then the secondary conductors must have an ampacity of at least 400 such that the secondary conductors are protected in "accordance with their ampacity" per 240.4, by the feeder breakers in the switchboard.

I don't believe you could connect 600mcm secondary conductors to a 400A rated switchboard that had three 200A feeder c/bs. The secondary conductors would not be protected in "accordance with their ampacity" in that case.

 

[don_resqcapt19]

It is my opinion that the 10' secondary conductor rule does not require that the conductors have overcurrent protection. It only requires that the conductors have an ampacity equal to or greater than the calculated load and equal to or greater than the rating of the device that they are landed on. I don't see a requirement that they have overcurrent protection. Even the informational note seems to suggest that this section does not require overcurrent protection for the secondary conductors.

 

[david luchini]

It is my opinion that the 10' secondary conductor rule does not require that the conductors have overcurrent protection. It only requires that the conductors have an ampacity equal to or greater than the calculated load and equal to or greater than the rating of the device that they are landed on. I don't see a requirement that they have overcurrent protection. Even the informational note seems to suggest that this section does not require overcurrent protection for the secondary conductors.

Thanks. I remember I discussion a year or two ago that related to the "device" listed in 240.21(C)(2)(1)b. Originally, my reading of 240.21(C)(2)(1)b was that ocp wouldn't be required when connecting to a "device", but that discussion lead me to change my opinion.

The discussion was as follows:

240.4 requires conductors to be protected against overcurrent in accordance with their ampacity unless otherwise permitted or required in 240.4(A) thru (G).

240.4(F) permits protection of the secondary conductors by the primary OCPD for 2-wire single phase or 3-phase, 3-wire delta transformers. It does not permit the primary OCPD to protect the secondary conductors of

240.21 says that the LOCATION of the OCPD shall be the point where the conductors receive their supply, except as specified in 240.21(A) thru (H).

240.21(C) says that the LOCATION of the OCPD for transformer secondary conductors need not be at the point the conductors receive their supply (the transformer secondary) as specified in 240.21(C)(1) thru 240.21(C)(6). Nothing in 240.21(C) says OCP protection isn't required for the secondary conductors, it just tells you where the location of the OCPD can be.

The fine print note which says "For overcurrent protection requirements for panelboards, see 408.36" doesn't suggest to me that overcurrent protection isn't required, it suggests to me that an MLO panelboard cannot be the "device" which the secondary conductors supply.

 

[don_resqcapt19]

David,
You make some good points, but I still don't agree. All of the other sections specifically require an OCPD. This one does not.
It appears to me that the rules in 240.21 cover both the requirement and the location of the OCPD.

240.21 Location in Circuit.
Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in 240.21(A) through (H). Conductors supplied under the provisions of 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4.

It remains my opinion that the "except as specified" clause applies to both the requirement for and the location of the OCPD.

The over riding point may be the general rule in 240.4 that you cited, but it is my opinion that a specific rule as found in 240.21 always over rule a general rule.
I thought there was a proposal and panel comment on this issue, but I have been able to find it.