Transformer secondary conductors

[Grouch1980]

HI all,
Say I have a transformer with a 250% maximum rated primary OCPD, and a 125% rated (or next size up) secondary OCPD based on table 450.3(B); my voltages are below 600 volts. The 125% rated OCPD takes care of protecting the transformer secondary.

Now I have to protect the secondary conductors. I look at 240.21(C)(1)... if my transformer is delta delta, then my secondary conductors are protected by the primary OCPD.

What now dictates the location of my 125% rated OCPD on the transformer secondary? Can it be 10 feet away, or 25 feet away?

 

[jim dungar]

The governing section is 240.21(C) which covers transformer secondary conductors. If your delta secondary is only wire, secondary conductor protection would not be required, therefore there is no distance requirement.

 

[Grouch1980]

The governing section is 240.21(C) which covers transformer secondary conductors. If your delta secondary is only wire, secondary conductor protection would not be required, therefore there is no distance requirement.

Thanks. Did you mean to say "if your delta secondary is only three wire"?

 

[Engser18]

When dealing with Xformer protection, the 450 for protect the Xformer, and the 240 for protect primary and secondary conductors. it is required to meet both section. if you can provide more detail that would help.

if ampacity of primary conductors less than primary OCPD (NA if next size up <800A), it consider tap conductors, then 25' rule apply. 240.21 (B) (3). then secondary OCPD is also required (protection of primary and secondary conductors). next sized up are prohibited 240.21 (C)

 

[don_resqcapt19]

HI all,
Say I have a transformer with a 250% maximum rated primary OCPD, and a 125% rated (or next size up) secondary OCPD based on table 450.3(B); my voltages are below 600 volts. The 125% rated OCPD takes care of protecting the transformer secondary.

Now I have to protect the secondary conductors. I look at 240.21(C)(1)... if my transformer is delta delta, then my secondary conductors are protected by the primary OCPD.

What now dictates the location of my 125% rated OCPD on the transformer secondary? Can it be 10 feet away, or 25 feet away?

That is permitted, but will require that the ampacity of the secondary conductors be equal to the rating of the primary OCPD times the secondary-to-primary transformer voltage ratio.
You would be better off to find a way to use one of the other rules in 240.21(C).

 

[augie47]

You might also keep in mind IF your secondary conductors are terminating in a panelboard, 408.36 requires a OCP device ahead of or in the panel so even on your delta-delta there would need to be a secondary OCP device.

 

[Grouch1980]

Thanks everyone!

 

[Grouch1980]

That is permitted, but will require that the ampacity of the secondary conductors be equal to the rating of the primary OCPD times the secondary-to-primary transformer voltage ratio.

Just for argument sake:
Let's say secondary protection is not required. Using 450.3(B), my primary OCPD is sized at 125%, so no secondary protection is needed for the transformer. And following 240.21(C)(1), I have a delta delta transformer AND the secondary conductor ampacity equals what Don describes above, so no secondary protection is needed for the conductors. If I still decide to use a secondary OCPD, meaning this is now an optional installation, am I required to follow the distance requirements found in 240.21(C)(2) or (C)(6)? Or since it's optional I don't have to follow these requirements?

 

[ggunn]

I have a related question. If there is a 480/277V feeder that is tapped to feed a transformer, and there is a fused disco between the tap and the transformer primary, then I believe that 240.21(B)(3)(5) that says the transformer secondary must be connected to a single OCPD should not apply. It seems to me that the primary side of the transformer is connected to a feeder, not a tap, since there is OCP between it and the tap.

What say you?

 

[jim dungar]

Just for argument sake:
Let's say secondary protection is not required. Using 450.3(B), my primary OCPD is sized at 125%, so no secondary protection is needed for the transformer. And following 240.21(C)(1), I have a delta delta transformer AND the secondary conductor ampacity equals what Don describes above, so no secondary protection is needed for the conductors. If I still decide to use a secondary OCPD, meaning this is now an optional installation, am I required to follow the distance requirements found in 240.21(C)(2) or (C)(6)? Or since it's optional I don't have to follow these requirements?

You need to follow the one subparagraph of 240.21(C) that applies to your situation. In your example you can stop reading after 240.21(C)(1).

 

[jim dungar]

I have a related question. If there is a 480/277V feeder that is tapped to feed a transformer, and there is a fused disco between the tap and the transformer primary, then I believe that 240.21(B)(3)(5) that says the transformer secondary must be connected to a single OCPD should not apply. It seems to me that the primary side of the transformer is connected to a feeder, not a tap, since there is OCP between it and the tap.

What say you?

You must follow the NEC definition of tap. Industry slang for connections or splices is not germane. Your conductors are protected by an appropriate OCPD therefore you have a feeder.

 

[david luchini]

You might also keep in mind IF your secondary conductors are terminating in a panelboard, 408.36 requires a OCP device ahead of or in the panel so even on your delta-delta there would need to be a secondary OCP device.

The exception to 408.36(B) says a secondary OCP device for a panelboard might not be required on a delta-delta transformer

 

[ggunn]

You must follow the NEC definition of tap. Industry slang for connections or splices is not germane. Your conductors are protected by an appropriate OCPD therefore you have a feeder.

Where would I find that definition? It's not in Chapter 1 Article 100.

 

[jim dungar]

Where would I find that definition? It's not in Chapter 1 Article 100.

In 2017, you could find it in 240.2.

 

[ggunn]

Thanks for all your replies. Here is a more complete description of the design:

480/277V conductors from a fused service disconnect enter a gutter where Polaris blocks are used to connect them to a 100A panelboard and a 400A disco fused at 300A. We established in another thread that the conductors to the panelboard and the disco are both load side feeder taps. A neutral is run to the panelboard but not to the disco.

Conductors from the disco feed a 300kVA transformer that is 480V delta primary to 240V delta high leg secondary. Full load current for the transformer is 361A on the primary side and 722A on the secondary side.

The secondary conductors enter another gutter where a set of Polaris blocks feeds three 200A discos with 200A fuses. My initial assumption was that there would have to be a single 600A OCPD on the transformer secondary ahead of the gutter to be compliant with 240.21(B)(3)(5), but since there is OCP between the transformer primary and the 480/277V tap, the primary side conductors are not tap conductors, they are a feeder, so 240.21(B)(3)(5) does not apply.

Have I got that right?

 

[Engser18]

Just for argument sake:
Let's say secondary protection is not required. Using 450.3(B), my primary OCPD is sized at 125%, so no secondary protection is needed for the transformer. And following 240.21(C)(1), I have a delta delta transformer AND the secondary conductor ampacity equals what Don describes above, so no secondary protection is needed for the conductors. If I still decide to use a secondary OCPD, meaning this is now an optional installation, am I required to follow the distance requirements found in 240.21(C)(2) or (C)(6)? Or since it's optional I don't have to follow these requirements?

You got it, 1 primary ocpd will do the job, it is only truly work on delta-delta (single voltage).

 

[don_resqcapt19]

Just for argument sake:
Let's say secondary protection is not required. Using 450.3(B), my primary OCPD is sized at 125%, so no secondary protection is needed for the transformer. And following 240.21(C)(1), I have a delta delta transformer AND the secondary conductor ampacity equals what Don describes above, so no secondary protection is needed for the conductors. If I still decide to use a secondary OCPD, meaning this is now an optional installation, am I required to follow the distance requirements found in 240.21(C)(2) or (C)(6)? Or since it's optional I don't have to follow these requirements?

If is is not required, it is not required and there are no rules that apply to added a non-required OCDP.

 

[wwhitney]

The secondary conductors enter another gutter where a set of Polaris blocks feeds three 200A discos with 200A fuses. My initial assumption was that there would have to be a single 600A OCPD on the transformer secondary ahead of the gutter to be compliant with 240.21(B)(3)(5), but since there is OCP between the transformer primary and the 480/277V tap, the primary side conductors are not tap conductors, they are a feeder, so 240.21(B)(3)(5) does not apply.

Have I got that right?

Basically--my only quibble is that feeder tap conductors are feeders. They are just also feeder tap conductors. In your case, the transformer primary is not being directly supplied by feeder tap conductors, so that's right.

a 400A disco fused at 300A. . . . Conductors from the disco feed a 300kVA transformer that is 480V delta primary to 240V delta high leg secondary. Full load current for the transformer is 361A on the primary side and 722A on the secondary side.

I have read here that you may an issue with the 300A fuses blowing on startup, from the magnetizing current, since you have sized the fuses under the transformer full load current. That's the reason Article 450 allows >100% on the primary side OCPD (although then 240.2 requires the primary side conductors to be accordingly upsized).

In fact since it's a 400A service, maybe there's a chance of occasionally tripping the service OCPD? I.e. a 361A full load current transformer is too large for a 400A service? Not sure.

The secondary conductors enter another gutter where a set of Polaris blocks feeds three 200A discos with 200A fuses.

I don't believe that complies with any of the options in 240.21(C). 240.21(C)(1) is not applicable as your transformer has a 4-wire secondary. I believe it is more typical to run 3 separate sets of secondary conductors, each of with terminates directly on its own disconnect.

Cheers, Wayne

 

[ggunn]

I don't believe that complies with any of the options in 240.21(C). 240.21(C)(1) is not applicable as your transformer has a 4-wire secondary. I believe it is more typical to run 3 separate sets of secondary conductors, each of with terminates directly on its own disconnect.

It seems to me that that would only relocate the tap point from the gutter to the secondary terminals of the transformer; I don't see how that would make any difference. My main concern is that we would like to avoid having to have a 600A OCPD on the secondary conductors ahead of the three 200A discos..

 

[wwhitney]

It seems to me that that would only relocate the tap point from the gutter to the secondary terminals of the transformer; I don't see how that would make any difference.

Because each set of secondary conductors would then individually comply with 240.21(C). [Note: I've done zero installations with transformers. My comments are based on carefully integrating 15+ years of what I've read here with what I read in the NEC directly.]

So the plan with the single set of secondary conductors to a gutter to 3 disconnects would require a single 600A disconnect because of 240.21(C), not because of 240.21(B)(3)(5). That 600A disconnect enclosure would presumably be large enough to eliminate the gutter; you'd be using the feeder tap rules in 240.21(B) starting at its load side terminals.

Cheers, Wayne