Transformer to panelboard & transformer secondary tap rules

[Jpflex]

There’s a few things I have questions on regarding such a set up using a 30 kVA single phase 480 primary / 120/240 volt secondary step down transformer to feed a breaker sub pannel

First, on the floor mounted transformers such as in the picture shown, I have not encountered a set up where the secondary windings x1 and x2 terminals would terminate directly a 2 pole breaker before feeding the secondary wires.

Without an upstream LINE-SIDE OCPD as explained above, often smaller phase wires terminating directly to transformer secondary terminals x1 and x2 and now become TAPS. These taps are then subject to transformer secondary tap rules 240.21 such as limited to 10’ spans.

One of my thoughts is that transformers such as these DO NOT provide such a way to connect a LINE SIDE OCPD before transformer secondary wires. If you attempt to do so you will just be creating another tap secondary situation with no line side OCPD. For this reason OCPD often terminated at the secondary wire LOAD END under tap rules

This fact limits secondary windings to 10’ but NEC WORK SPACE / DEBTH DISTANCE live parts to live parts requirements between the transformer and breaker panel may require more distance between panel and transformer (entire secondary length including sections inside equipment but not just outside).

Therefore, these two pieces of equipment may not be code compliant if located so close together to not exceed a total of 10’ feet secondary conductor span (NOT JUST EQUIPMENT SPAN) between the two. It seems one code can interfere with the second code. Thaughts?

 

[augie47]

Are you referencing once of the newer Eaton Breaker-Intergrated-Transformers where the secondary breaker is mount in the transformer ??

 

[Jpflex]

Are you referencing once of the newer Eaton Breaker-Intergrated-Transformers where the secondary breaker is mount in the transformer ??
View attachment 2570671

No but this is cool. The ones I did were most like the one in the picture I uploaded. If you open the cover you only have H and X terminals but no OCPD

 

[ptonsparky]

Use of smaller primary OC protection can extend the secondary protection. Max of 125%.

 

[ptonsparky]

A max of 80 amp on primary would allow for no OC on the secondary.
The secondary conductors would need to be rated @ min of 144 amp and could extend to the secondary OC protection.

IMO, the secondary conductors would not be considered taps if protected at their ampacity. The panel main breaker.

 

[infinity]

Therefore, these two pieces of equipment may not be code compliant if located so close together to not exceed a total of 10’ feet secondary conductor span (NOT JUST EQUIPMENT SPAN) between the two. It seems one code can interfere with the second code. Thaughts?

Is it possible that they are using the 25' rule for the secondary conductors? In that case working space for the panel shouldn't be an issue.

 

[david luchini]

I can't figure out why working space requirements would require more space between the transformer and panelboard???? We see panelboards next to transformers all the time.

 

[infinity]

I can't figure out why working space requirements would require more space between the transformer and panelboard???? We see panelboards next to transformers all the time.

Even with the 10' you have quite a bit of room with 10' of secondary conductors.

 

[augie47]

Agree. I've wired a bunch of transformers and, other than a few that met the "outside rule" or were under Part VIII (supervised industrial), almost all met the 10ft or 25ft rule with no problem.

 

[don_resqcapt19]

There’s a few things I have questions on regarding such a set up using a 30 kVA single phase 480 primary / 120/240 volt secondary step down transformer to feed a breaker sub pannel

First, on the floor mounted transformers such as in the picture shown, I have not encountered a set up where the secondary windings x1 and x2 terminals would terminate directly a 2 pole breaker before feeding the secondary wires.

Without an upstream LINE-SIDE OCPD as explained above, often smaller phase wires terminating directly to transformer secondary terminals x1 and x2 and now become TAPS. These taps are then subject to transformer secondary tap rules 240.21 such as limited to 10’ spans.

One of my thoughts is that transformers such as these DO NOT provide such a way to connect a LINE SIDE OCPD before transformer secondary wires. If you attempt to do so you will just be creating another tap secondary situation with no line side OCPD. For this reason OCPD often terminated at the secondary wire LOAD END under tap rules

This fact limits secondary windings to 10’ but NEC WORK SPACE / DEBTH DISTANCE live parts to live parts requirements between the transformer and breaker panel may require more distance between panel and transformer (entire secondary length including sections inside equipment but not just outside).

Therefore, these two pieces of equipment may not be code compliant if located so close together to not exceed a total of 10’ feet secondary conductor span (NOT JUST EQUIPMENT SPAN) between the two. It seems one code can interfere with the second code. Thaughts?

There are multiple transformer secondary conductor rules in 240.21(C). The most commonly used one is 240.21(C)(6) which permits 25' of conductor between the transformer secondary terminals and the line side of the first overcurrent protective device.

I can't think of a time where I have ever used the 10' transformer secondary conductor rule in 240.21(C)(2), but have used the 10' feeder tap rule in 240.21(B)(1) many times.

Note that while transformer secondary conductors are often called taps, there are two sets of rules in 240.21. 240.21(B) for feeder taps, and 240.21(C) for transformer secondary conductors.

 

[Jpflex]

There are multiple transformer secondary conductor rules in 240.21(C). The most commonly used one is 240.21(C)(6) which permits 25' of conductor between the transformer secondary terminals and the line side of the first overcurrent protective device.

I can't think of a time where I have ever used the 10' transformer secondary conductor rule in 240.21(C)(2), but have used the 10' feeder tap rule in 240.21(B)(1) many times.

Note that while transformer secondary conductors are often called taps, there are two sets of rules in 240.21. 240.21(B) for feeder taps, and 240.21(C) for transformer secondary conductors.

All I have is an electrical calculations book and it only mentioned a 10’ rule for transformer secondary taps. Based on only this rule I’m saying that the distance requirements for working space and 10’ foot secondary conductors can conflict

 

[augie47]

All I have is an electrical calculations book and it only mentioned a 10’ rule for transformer secondary taps. Based on only this rule I’m saying that the distance requirements for working space and 10’ foot secondary conductors can conflict

In a normal installation how would this occur ? Looking at the illustration in your original post the panel would appear to have plenty of working room.

 

[Jpflex]

In a normal installation how would this occur ? Looking at the illustration in your original post the panel would appear to have plenty of working room.

The 10 foot rule is total conductor length including as much as inside the transformer cabinet and as much as is inside the breaker panel. Additionally the bends of the small run for wall corners etc also results in length shrink. Subtracting this can give you much less than a 10’ space between equipment

 

[jim dungar]

The 10 foot rule is total conductor length including as much as inside the transformer cabinet and as much as is inside the breaker panel. Additionally the bends of the small run for wall corners etc also results in length shrink. Subtracting this can give you much less than a 10’ space between equipment

But the transformer can be mounted directly adjacent to the panel, as long as there is enough room on the other side of the panel to accommodate the working space.
If not, there is always the more commonly used 25' rule for transformer secondaries.

 

[curt swartz]

The 10 foot rule is total conductor length including as much as inside the transformer cabinet and as much as is inside the breaker panel. Additionally the bends of the small run for wall corners etc also results in length shrink. Subtracting this can give you much less than a 10’ space between equipment

Where are you finding a requirement that there be 10' between equipment?

Ignore the book you are reading an follow the NEC. As has been stated in most cases the conductor length limit is 25'. Even if you are limited to 10' its easy to do when the transformer and secondary over current device are adjacent to each other.

 

[don_resqcapt19]

All I have is an electrical calculations book and it only mentioned a 10’ rule for transformer secondary taps. Based on only this rule I’m saying that the distance requirements for working space and 10’ foot secondary conductors can conflict

This is a code forum...we only work with what the NEC says, not what some other book seems to think that the NEC says.

However even the picture you posted likely meets the 10' transformer secondary conductor rule.

About the only use of the 10' rule is the case where you have multiple small sets of transformer secondary conductors each running to their own OCPD, or where the capacity of the transformer far exceeds the load that will be supplied by the transformer.

I don't think I have ever seen the 10' secondary conductor rule used in my 40+ years in the trade. I have seen the 10' feeder tap rule used many times in industrial control panels, but not the 10' secondary conductor rule.

 

[infinity]

Like Don stated the 10' rule is very rarely applicable. The 25' rule makes the distance length between the panel and the transformer pretty much moot.

 

[Jpflex]

This is a code forum...we only work with what the NEC says, not what some other book seems to think that the NEC says.

However even the picture you posted likely meets the 10' transformer secondary conductor rule.

About the only use of the 10' rule is the case where you have multiple small sets of transformer secondary conductors each running to their own OCPD, or where the capacity of the transformer far exceeds the load that will be supplied by the transformer.

I don't think I have ever seen the 10' secondary conductor rule used in my 40+ years in the trade. I have seen the 10' feeder tap rule used many times in industrial control panels, but not the 10' secondary conductor rule.

I would have to say your first comment is not correct because the NEC code book is NOT an instructional book nor a guide on how to apply or interpret code. This is why there are additional books to expand along side the NEC code book.

Vocational schools do not mandate attendance with only the NEC code book, rather they often require students to purchase additional material. I do also own a copy of the NEC code book but it is 4 hours away at work site.

On your post mentioning that the only case you ever saw the 10 foot rule was for multiple transformer secondary terminating into their own OCPD. How then can you not have a tap if a transformer only provides terminals at the secondary x side of the transformer without provisions of an OCPD (such as transformer secondary windings terminating directly to an OCPD and no reduction or splice of smaller conductor between windings and OCPD)?

What I mean is that not every transformer has an OCPD where the secondary conductors originate for each ungrounded conductor. If you terminate the smaller secondary conductors at its load end only, then this resembles more of a tap connection subject to specific rules

 

[don_resqcapt19]

I would have to say your first comment is not correct because the NEC code book is NOT an instructional book nor a guide on how to apply or interpret code. This is why there are additional books to expand along side the NEC code book.

Vocational schools do not mandate attendance with only the NEC code book, rather they often require students to purchase additional material. I do also own a copy of the NEC code book but it is 4 hours away at work site.

On your post mentioning that the only case you ever saw the 10 foot rule was for multiple transformer secondary terminating into their own OCPD. How then can you not have a tap if a transformer only provides terminals at the secondary x side of the transformer without provisions of an OCPD (such as transformer secondary windings terminating directly to an OCPD and no reduction or splice of smaller conductor between windings and OCPD)?

What I mean is that not every transformer has an OCPD where the secondary conductors originate for each ungrounded conductor. If you terminate the smaller secondary conductors at its load end only, then this resembles more of a tap connection subject to specific rules

I am sorry but the rules are only found in the NEC and it does not matter what some other publication thinks those rules may be. Sure there are instructional books that help to explain the rules, but they are not the rules. You have suggested that your book only provides for a 10' secondary conductor and that is 100% WRONG.

If you can post here, you can read the free version of the NEC on the NFPA site...granted, not a very user friendly version, but all of the code is there to view for free.

First, you can never have a tap on the secondary of a transformer...those are not tap conductors, but transformer secondary conductors.

No matter where the OCPD is, there are always transformer secondary conductors, even where the OCPD is in the same enclosure as the transformer. There is nothing in the transformer secondary protection rules about "no reduction or splice of a smaller conductor". There are simply rules that require protection at the load end of the transformer secondary conductors. The 10' rule is rarely used because if you are using it, you are not using very much of the capacity of the transformer, or you are using multiple sets of transformer secondary conductors.

Have you ever made an installation where the 10' secondary conductor rule applied? If so please post the details so we can understand what you are talking about.

Not sure what you mean by "if you terminate a smaller conductor" as, except in the rare cases of a two wire to two wire transformer or a 3 wire delta to 3 wire delta transformer, the secondary conductors always require overcurrent protection. In those two cases, the transformer primary OCPD is permitted to provide the required protection for the transformer primary conductors, the transformer primary winding, the transformer secondary winding, and the transformer secondary conductors. In general, the transformer secondary conductors always require overcurrent protection on their load end, and by far the most common rule that applies is the 25' transformer secondary conductor rule.

 

[Jpflex]

In general, the transformer secondary conductors always require overcurrent protection on their load end, and by far the most common rule that applies is the 25' transformer secondary conductor rule.

I get what you’re saying for the most part and I didn’t intend to imply that other educational books were to be followed as code but only a source for further detail, theory, instructions and explanations.

The protection of the secondary conductor only at its load end but not its line side would not protect the conductor to its ampacity if a short were upstream of the load side terminating breaker, and if it were not for the primary side breaker / counter electromagnetic force resistance between primary and secondary.