Transformer secondary - sizing OCPD incorrectly in panel

[wwhitney]

Which I already alluded to in the same post. That either could be argued. It would be on the person enforcing it to read it to mean whether the secondary protection is required or whether the secondary protection exists.

But you make the idea that it is still required seem like a plausible reading. It is not.

(2020 NEC) 450.3(B) says "Overcurrent protection shall be provided in accordance with Table 450.3(B)." The title of Table 450.3(B) is "Maximum Rating or Setting of Overcurrent Protection for Transformers . . .". When the table entry says "Not required" that means there is no maximum. Any other reading is implausible.

Cheers, Wayne

 

[jim dungar]

Let's say instead of a 150 amp MCB, there's a 500 amp MCB in the panel connected to the transformer secondary with 600 kcmils and no code issues - right? How could this be allowed on a 30 kVA transformer secondary??

How about during a period of remodeling, when a temporary transformer is being used to feed an existing panelboard.

I have had to do this a few times in my career.

 

[ptonsparky]

Before I went with no OCPD on the secondary side, I'd make sure that transformer powers up with only a 125% on the primary side.

(FWIW, I'd most likely have a single secondary protection anyway.)

 

[Elect117]

But you make the idea that it is still required seem like a plausible reading. It is not.

(2020 NEC) 450.3(B) says "Overcurrent protection shall be provided in accordance with Table 450.3(B)." The title of Table 450.3(B) is "Maximum Rating or Setting of Overcurrent Protection for Transformers . . .". When the table entry says "Not required" that means there is no maximum. Any other reading is implausible.

Cheers, Wayne

I didn't say transformer secondary protection was required.

I said where secondary protection is installed, it is required to meet the table's second row. That is my interpretation of it.

Where it states the type of protection installed, it specifies "Only Primary" and "Primary and Secondary". So, I read that as, where secondary protection is installed, it is both "Primary and Secondary" and needs to meet that requirement. There are only a few applications where secondary protection is not installed. So in the case of a panelboard, I would argue that the protection for the panelboard would need to suffice in meeting 408.36, 450.3's "Primary and Secondary" protection, along with 240.21(C), because that OCPD now has an overlapping job. It is common for one OCPD to be for more than one reason and need to met requirements of more than one section.

No different to when a wire-type EGC is installed. You then have to meet the rules for 250.122. You wouldn't ignore them just because a wire type isn't needed since you already have EMT or other means. If it is installed, you have to meet the requirements for a wire type EGC.

I see it as the same for table 450.3.

Where Primary and Secondary protection exist, you have to meet the second row. Where there is an application that requires only primary protection, you wouldn't have any OCPD on the secondary side of the transformer.

And, in temporary applications, I believe you have some allowances for temporary oversizing. Especially if all the load isn't installed yet. I would hope the AHJ would make an exception. But I don't think anyone is arguing that a 30kVA transformer's secondary windings, bushing, terminations, would hold up being protected by a 400A MCB and 400A panelboard. That seems frivolous to me. The transformer will obviously fail provided a sustained load of 300A. I would need to see the argument that the primary side is capable of protecting it. From my understanding, the heat in the secondary is not always translated to the primary since they are magnetically coupled. I have also had a few calculations where a large unbalance single phase load could overload the wye side's LV terminal of a phase before the resultant line current in the delta high side would trip.

So to argue that, where secondary side protection is required (delta - wye), but the primary side is sized to "Primary only" of 125%, means you could then size the secondary side to anything is just bad engineering practice and, in my opinion, a misinterpretation of 450.3. The primary side is not going to suitably protect against overload on the low voltage side wiring or terminations. That is my understanding for the existence of 240.21(C).

 

[wwhitney]

I said where secondary protection is installed, it is required to meet the table's second row. That is my interpretation of it.

I would agree if the column heading of the first column on Table 450.3(B) were "Protection Installed". But it's not, it's "Protection Method".

So you choose a method to protect the transformer. You have a choice of complying with the first row or the second row; just because secondary protection is installed does not require you to choose the second row. "Not required" does not mean "not installed". Your interpretation is stricter than what the wording of Table 450.3(B) says.

If there are transformer topologies that are not suitable for primary only protection (I need to think more about that part of your post), then Article 450 needs some additional requirements restricting the protection method choice in the table. [If those topologies are a subset of the topologies for which separate secondary conductor protection is required by article 240, the heading title of Table 450.3(B) could be changed to "Protection Installed" in line with what you propose. But it would be cleaner for the requirement to be more explicit in Article 450, rather than indirectly relying on another article.]

Cheers, Wayne

 

[infinity]

Just to clarify from the OP this 30 kva transformer doesn't require both primary and secondary protection?

For a 30 kVA transformer, 480V primary 120/208V secondary, is having a 150A main breaker on the transformer secondary 120/208V panel a code violation or just bad engineering practice??

30,000 VA / 360 = 83.3 amps.

 

[wwhitney]

No different to when a wire-type EGC is installed. You then have to meet the rules for 250.122. You wouldn't ignore them just because a wire type isn't needed since you already have EMT or other means. If it is installed, you have to meet the requirements for a wire type EGC.

I think I disagree. If you have an EMT EGC, I see no prohibition on installing another wire and bonding it to the EGC at both ends. That wire would not be a wire-type EGC if it is too small or the wrong color, it's just a bonus wire.

E.g. I install (4) black #10 Cu in an EMT installed as required to be an EGC. Two are for a 30A 2-wire circuit, and 2 are spare. Are you saying it's prohibited to bond the spares to metallic boxes at each end rather than leaving them capped and floating, because they aren't green?

Cheers, Wayne

 

[wwhitney]

I have also had a few calculations where a large unbalance single phase load could overload the wye side's LV terminal of a phase before the resultant line current in the delta high side would trip.

Seems like that can happen with a single phase 120V/240V secondary and primary only transformer protection, as the secondary could be loaded just L1-N (either intentionally if stupidly, or via a not so low impedance fault that happens to draw the right current), and you won't reach rated primary current until that half of the secondary reaches a 2x overcurrent.

So the question is (a) is that an acceptable problem or an unacceptable problem; and (b) if unacceptable, is it prohibited by anything in the NEC other than your interpretation of 450.3(B)?

I'm leaning towards "acceptable problem", since for example the feeder tap rules would permit similar situations with a 3x or 10x sustained overcurrent.

Cheers, Wayne

 

[jim dungar]

I read that as, where secondary protection is installed, it is both "Primary and Secondary" and needs to meet that requirement. There are only a few applications where secondary protection is not installed. So in the case of a panelboard, I would argue that the protection for the panelboard would need to suffice in meeting 408.36, 450.3's "Primary and Secondary" protection, along with 240.21(C), because that OCPD now has an overlapping job. It is common for one OCPD to be for more than one reason and need to met requirements of more than one section.

Not according the the rules of grammar taught to me by the sisters at St Mary's.

Simply being present does not mean an OCPD must meet the requirements of 450.3.
Just like an OCPD does not need to meet 240.21 simply because it exists.

 

[Elect117]

I would agree if the column heading of the first column on Table 450.3(B) were "Protection Installed". But it's not, it's "Protection Method".

So you choose a method to protect the transformer. You have a choice of complying with the first row or the second row; just because secondary protection is installed does not require you to choose the second row. "Not required" does not mean "not installed". Your interpretation is stricter than what the wording of Table 450.3(B) says.

If there are transformer topologies that are not suitable for primary only protection (I need to think more about that part of your post), then Article 450 needs some additional requirements restricting the protection method choice in the table. [If those topologies are a subset of the topologies for which separate secondary conductor protection is required by article 240, the heading title of Table 450.3(B) could be changed to "Protection Installed" in line with what you propose. But it would be cleaner for the requirement to be more explicit in Article 450, rather than indirectly relying on another article.]

Cheers, Wayne

I agree 450.3 needs some love. I think it is better suited to be written like 408 where it pushes you to other relevant sections or has informational notes. I think too many people get hung up on art. 450 and forget about art. 240 and art. 408.

I think I disagree. If you have an EMT EGC, I see no prohibition on installing another wire and bonding it to the EGC at both ends. That wire would not be a wire-type EGC if it is too small or the wrong color, it's just a bonus wire.

E.g. I install (4) black #10 Cu in an EMT installed as required to be an EGC. Two are for a 30A 2-wire circuit, and 2 are spare. Are you saying it's prohibited to bond the spares to metallic boxes at each end rather than leaving them capped and floating, because they aren't green?

Cheers, Wayne

I think this will veer a little off course, but yes. I do have an issue with using spare or previously abandoned cables as wire-type EGCs. The wire type equipment grounding conductor, even if used on top of another established method, such as EMT or RMC, if installed, should be properly identified and installed to meet the relevant code sections.

I think this would fall under a 110.7 violation where you are intentionally bonding unused conductors, initially intended as phase conductors, to the equipment. Though, this could probably be argued to great lengths lol.

Not according the the rules of grammar taught to me by the sisters at St Mary's.

Simply being present does not mean an OCPD must meet the requirements of 450.3.
Just like an OCPD does not need to meet 240.21 simply because it exists.

Is that an eye be four e joke? lol

That is how I read it.

Under the column that reads, protection method there are two options. Primary protection only or primary and secondary protection.

The way I read it, the presence of secondary protection requires you to use the second row. Since the first options says, "primary protection only". Other sections or engineering might require that secondary protection be installed.

And correct me if I am wrong here, but the way you and Wayne are reading it is, If you install primary protection to 125% then secondary protection is not required.

Or perhaps you are saying that the presence of an OCPD on the secondary side does not mean it is intended to protect the transformer?

Or maybe it is a combination of the two? The presence of an OCPD does not mean it protects the transformer and if you size the primary to 125% then there is no requirement for secondary protection?

 

[wwhitney]

I think this will veer a little off course, but yes. I do have an issue with using spare or previously abandoned cables as wire-type EGCs.

I don't think bonding the spare wire means you are using it as a wire-type EGC. It's just a spare wire that is bonded.

Or maybe it is a combination of the two? The presence of an OCPD does not mean it protects the transformer and if you size the primary to 125% then there is no requirement for secondary protection?

Yes, precisely, just like the bonded spare wire.

Cheers, Wayne

 

[jim dungar]

And correct me if I am wrong here, but the way you and Wayne are reading it is, If you install primary protection to 125% then secondary protection is not required.

Absolutely.
This is expressly stated in the text where it uses the words "Not Required".

Or perhaps you are saying that the presence of an OCPD on the secondary side does not mean it is intended to protect the transformer?

The presence of an OCPD is not the determining factor. That device must meet the threshold, of 125%, in order for it to qualify as valid secondary protection.

When I started in this industry it was not uncommon for transformer to be installed without any secondary protection at all. These practices led to the changes and the text we have now for 240.21(C) and 408.36, but there were no changes to 450.3.

 

[Elect117]

Absolutely.
This is expressly stated in the text where it uses the words "Not Required".


The presence of an OCPD is not the determining factor. That device must meet the threshold, of 125%, in order for it to qualify as valid secondary protection.

When I started in this industry it was not uncommon for transformer to be installed without any secondary protection at all. These practices led to the changes and the text we have now for 240.21(C) and 408.36, but there were no changes to 450.3.

I don't think bonding the spare wire means you are using it as a wire-type EGC. It's just a spare wire that is bonded.


Yes, precisely, just like the bonded spare wire.

Cheers, Wayne

I think I see where you both are coming from. I won't disagree that your interpretation makes sense. I don't know if I agree with the engineering side of it but I would have to sit down and work through some calculations. I have always been under the impression that the high side protection is not a reliable way of protecting against overloading, even if sized to 125% (+ round up). I guess that would also need to include the fact that transformers can handle being overloaded. So any calculation would need to consider that as well.

I appreciate your time in explaining your positions. It has helped me gain a better understanding.

 

[jim dungar]

I have always been under the impression that the high side protection is not a reliable way of protecting against overloading

You are correct.
Primary side only protection is often more about removing a failed transformer from the grid/system than it is about preventing the failure in the first place.
But just because something is a poor design does not mean it is against code.

Many decades ago the DOE did a survey of transformers and found most of them had only an average loading of roughly 35%.

 

[kwired]

Just to clarify from the OP this 30 kva transformer doesn't require both primary and secondary protection?

I think it does in most cases. Strictly from recollection seems about the only situations where primary protection can also be the secondary protection is with two wire to two wire single phase or 3 wire delta/delta setups. Multiwire with neutral for a secondary can overload a portion of the windings and not trip the primary device but will trip a properly selected secondary device.

 

[wwhitney]

I think it does in most cases. Strictly from recollection seems about the only situations where primary protection can also be the secondary protection is with two wire to two wire single phase or 3 wire delta/delta setups.

That requirement is in Article 240, not in Article 450. So it is secondary conductor protection, not transformer protection. Article 450 does not require both primary and secondary protection for transformers of 1000V or less, per Table 450.3(B) top row.

Cheers, Wayne

 

[kwired]

That requirement is in Article 240, not in Article 450. So it is secondary conductor protection, not transformer protection. Article 450 does not require both primary and secondary protection for transformers of 1000V or less, per Table 450.3(B) top row.

Cheers, Wayne

So OP will need 150 amp secondary conductors if he has 150 amp overcurrent protection unless maybe is supplying motor(s) or other loads that may have other allowances outside of the general rules.