Transformer SBJ + GEC Size

[Elect117]

I got sent prints to review and the designer is planning on feeding two panelboards off of one dry type.

150kVA (480-208/120 3PH)

250kcmil to 250 MCB

600kcmil to 400A MCB

I already noted that the secondary protection didn't meet Table 450.3(B) Note 2.

But I didn't know if I needed to also mark the SBJ or GEC size for correction either. They have the SBJ and GEC sized at 1/0.

My questions is, should they have sized the SBJ and GEC as 600kcmil + 250kcmil = 850kcmil => 2/0 or is it based on the largest single conductor from the transformer (600kcmil => 1/0)?

If they needed 2/0, could you help point me to the section? Is it the "parallel" language that I am not interpretating right? I always thought parallel meant they started and ended at the same point like from chapter 310. The max current would be based on the largest conductor but do you need to consider them both faulting at the same time?

 

[infinity]

My questions is, should they have sized the SBJ and GEC as 600kcmil + 250kcmil = 850kcmil => 2/0 or is it based on the largest single conductor from the transformer (600kcmil => 1/0)?

The SSBJ is sized according to the conductors in each raceway based on T250.102(C)(1). What you have are not parallel secondary conductors but even if they were you still would size the SSBJ based on what is in each raceway.

 

[don_resqcapt19]

I got sent prints to review and the designer is planning on feeding two panelboards off of one dry type.

150kVA (480-208/120 3PH)

250kcmil to 250 MCB

600kcmil to 400A MCB

I already noted that the secondary protection didn't meet Table 450.3(B) Note 2.

But I didn't know if I needed to also mark the SBJ or GEC size for correction either. They have the SBJ and GEC sized at 1/0.

My questions is, should they have sized the SBJ and GEC as 600kcmil + 250kcmil = 850kcmil => 2/0 or is it based on the largest single conductor from the transformer (600kcmil => 1/0)?

If they needed 2/0, could you help point me to the section? Is it the "parallel" language that I am not interpretating right? I always thought parallel meant they started and ended at the same point like from chapter 310. The max current would be based on the largest conductor but do you need to consider them both faulting at the same time?

What is the rating of the transformer feeder OCPD? Secondary protection of the transformer may not be required.

The GEC will be based on the total area of the secondary conductors per Table 250.66, and will require 2/0 copper.

As Rob said, the SSBJs will be sized based on the conductors in each raceway, so a 2 AWG for the 250s, and a 1/0 for the 600s.

 

[augie47]

For the SBJ I would say you need to follow 250.28 and, if the SBJ is for both supplies size it based on the sum of both conductors.

 

[don_resqcapt19]

For the SBJ I would say you need to follow 250.28 and, if the SBJ is for both supplies size it based on the sum of both conductors.

There are two panels on the secondary so 250.28(D)(3) applies. (assuming you are installing system bonding jumpers in each panel) If there is a single system bonding jumper in the transformer is would be sized on the sum of the conductors.

 

[Elect117]

The SSBJ is sized according to the conductors in each raceway based on T250.102(C)(1). What you have are not parallel secondary conductors but even if they were you still would size the SSBJ based on what is in each raceway.

They had sized it based on 250.122 of the load side OCPD and I already marked it for correction.

What is the rating of the transformer feeder OCPD? Secondary protection of the transformer may not be required.

The GEC will be based on the total area of the secondary conductors per Table 250.66, and will require 2/0 copper.

As Rob said, the SSBJs will be sized based on the conductors in each raceway, so a 2 AWG for the 250s, and a 1/0 for the 600s.

Primary OCPD was 225A w/ 4/0. But the idea of it being required to protect the transformer and it being required for other sections like 240.21(C) or 408.36 just leaves it to interpretations. If art. 450 defined when secondary protection was non required or was required then I would be more inclined to ignore the oversizing. It only specifies where it is required, then blah blah blah.

I wish Table 250.66 didn't specify "service entrance" in the notes. I feel like it just muddied it up for me. I wish it just said it how you put it, total area of conductors. The way that table is worded just leaves it open. Maybe it is the first sentence of 250.30(A)(5), "A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived ungrounded conductors." Since they made it plural, "derived ungrounded conductors" leads you to combining a phase from each set for use of table 250.66.

For the SBJ I would say you need to follow 250.28 and, if the SBJ is for both supplies size it based on the sum of both conductors.

You found it for me! thanks. 250.28(D)(3).

There are two panels on the secondary so 250.28(D)(3) applies. (assuming you are installing system bonding jumpers in each panel) If there is a single system bonding jumper in the transformer is would be sized on the sum of the conductors.

250.28(D)(3) covers both. They are going to have the SBJ in the dry type so 2/0 would be it. If they were bonding at the panels then SBJs would be sized independently. Like the SSBJ to each panel that hey are going to install.

 

[don_resqcapt19]

...
Primary OCPD was 225A w/ 4/0. But the idea of it being required to protect the transformer and it being required for other sections like 240.21(C) or 408.36 just leaves it to interpretations. If art. 450 defined when secondary protection was non required or was required then I would be more inclined to ignore the oversizing. It only specifies where it is required, then blah blah blah.

...

First you have to understand that the rules in Article 450 apply ONLY to the protection of the transformer windings. The top line of Table 450.3(B) makes it very clear that where the transformer primary protection does not exceed 125% of the primary current the transformer does not require any secondary overcurrent protection.

Then you have the issue of protecting the secondary CONDUCTORS in accordance with one of the rules found in 240.21(C).
As you mentioned there is also the rule in 408.36 requiring protection for the panelboard.

In most case the OCPD that protects the secondary conductors also protects the panelboard.

 

[wwhitney]

Primary OCPD was 225A w/ 4/0. But the idea of it being required to protect the transformer and it being required for other sections like 240.21(C) or 408.36 just leaves it to interpretations.

To reiterate Don's comments, if the primary OCPD is 125% or less, the secondary OCPD is unlimited by Article 450. You use larger secondary OCPD, you just need to use larger secondary conductors to comply with 240.21(C).

Cheers, Wayne

 

[Elect117]

First you have to understand that the rules in Article 450 apply ONLY to the protection of the transformer windings. The top line of Table 450.3(B) makes it very clear that where the transformer primary protection does not exceed 125% of the primary current the transformer does not require any secondary overcurrent protection.

Then you have the issue of protecting the secondary CONDUCTORS in accordance with one of the rules found in 240.21(C).
As you mentioned there is also the rule in 408.36 requiring protection for the panelboard.

In most case the OCPD that protects the secondary conductors also protects the panelboard.

To reiterate Don's comments, if the primary OCPD is 125% or less, the secondary OCPD is unlimited by Article 450. You use larger secondary OCPD, you just need to use larger secondary conductors to comply with 240.21(C).

Cheers, Wayne

The phrase of every other section, mainly 240.21(C) and 408.36 specifically say secondary protection is required. I understand that those sections do not cover the transformer itself but it makes it hard to separate them.

I have always enforced needing to meet primary and secondary protection rule since secondary protection is required elsewhere in the code. When it is not required, like in the delta-delta or single phase applications, I have allowed primary only protection. It is the difference between where installed and where required is what I have a problem with in the table.

I guess I should stop enforcing it like that then lol.

 

[wwhitney]

The phrase of every other section, mainly 240.21(C) and 408.36 specifically say secondary protection is required. I understand that those sections do not cover the transformer itself but it makes it hard to separate them.

240.21(C) covers protection of secondary conductors. It always applies, regardless of what Article 450 says. It also imposes no relationship on secondary conductors and their OCPD with respect to the transformer rating.

Likewise, 408.36 covers protection of a panelboard. You may not have a panelboard on the secondary of a transformer. If you do, it applies per panelboard, and again there is no relationship between the panelboard and OCPD rating and the transformer rating.

450.3 is the only thing that could impose a limit on the secondary OCPD with respect to the transformer rating. But when 450.3(B) is applicable, and you comply with the first row of the table, it does not impose any limit on the secondary OCPD. In this scenario, there is no limit on the secondary OCPD size compared to the transformer rating.

[Note that 450.3 and 215.3 jointly impose a minimum on the rating of the transformer based on the calculated load connected to it.]

Cheers, Wayne

 

[don_resqcapt19]

The phrase of every other section, mainly 240.21(C) and 408.36 specifically say secondary protection is required. I understand that those sections do not cover the transformer itself but it makes it hard to separate them.

I have always enforced needing to meet primary and secondary protection rule since secondary protection is required elsewhere in the code. When it is not required, like in the delta-delta or single phase applications, I have allowed primary only protection. It is the difference between where installed and where required is what I have a problem with in the table.

I guess I should stop enforcing it like that then lol.

Please read the language closer.

The parent text in 240.21(C) very clearly tells you that the section applies to the protection of the secondary conductors and has nothing to do with the protection of the secondary winding of the transformer.

The language in 408.36 only talks about the protection of the panelboard itself. That language does not even have the word transformer in it.

 

[Elect117]

The language in 408.36 only talks about the protection of the panelboard itself. That language does not even have the word transformer in it.

It does but that is neither here nor there.

"(B) Supplied Through a Transformer.
Where a panelboard is supplied through a transformer, the overcurrent protection required by 408.36 shall be located on the secondary side of the transformer."

The parent text in 240.21(C) very clearly tells you that the section applies to the protection of the secondary conductors and has nothing to do with the protection of the secondary winding of the transformer.

That was a miscommunication. I meant OCPD is required on the secondary side of the transformer in both cases. Not implying those sections superseded something like 450.3 for transformer protection. Just the implication that they required secondary protection be a part of the "protection method". Just not necessarily the only method.

 

[don_resqcapt19]

It does but that is neither here nor there.

"(B) Supplied Through a Transformer.
Where a panelboard is supplied through a transformer, the overcurrent protection required by 408.36 shall be located on the secondary side of the transformer."





That was a miscommunication. I meant OCPD is required on the secondary side of the transformer in both cases. Not implying those sections superseded something like 450.3 for transformer protection. Just the implication that they required secondary protection be a part of the "protection method". Just not necessarily the only method.

Sorry...I did not look at (B).

My point is that it does not require secondary protection...it requires protection of the secondary conductors. Those are totally independent concepts, even though, when transformer secondary protection is required, a single device can supply the secondary protection and the secondary conductor protection.