Matching Transformer Primary Conductors to Primary OCPD

[Jpflex]

This isn't always true though. 240.4(B) often allows the OCPD to exceed the conductor ratings in table 350.16 (2020 NEC), though not by a lot.

Motor circuits allow for inrush without requiring the conductors to be sized for it.


I understand, I am asking why the conductors need to match the OCPD (sized for inrush) instead of the FLA, when, in the example I provided, there is primary and secondary protection to ensure that an overload condition won't jeopardize the conductors. An overload condition can only occur based on current flow in the secondary circuit. As long as that is limited by an OCPD on the secondary, the transformer and all conductors should be adequately protected.

An extreme event like a short circuit or ground fault should be arrested by one of the circuit protective devices depending on the location of the fault.


I think I posted that I had decided to use a primary OCPD with adjustable settings so I could keep the breaker closer to the FLA of the transformer.

It is wasteful to size conductors for something that happens rarely and lasts for .1 seconds.

My opinion is that the OCPD contemplated in article 450 are to protect the transformer, not the conductors, and that it is the responsibility of the circuit designer to make sure the conductors have adequate protection from an overload, with the direction given in article 240.21(B) and (C).

In the 2017 Handbook, exhibit 240.9 shows a 110A OCPD, feeding a set of conductors rated at 50A on the primary side. The secondary side of this transformer has protection not exceeding 125% of the transformer current rating, so the primary transformer protection is permitted to be no more than 250%. The 110A fuse shown is 244% of the FLA of 45A.

This would seem to indicate that the primary conductors can have an ampacity lower than the primary protection.

As your probably aware using an OCPD rated higher than the ampacity of the primary conductors ampacity which feed the transformer may be allowed when sizing to the next standard size breaker when the conductors ampacity involves fractions.

Motors allow smaller conductors with oversized ampere breakers for the conductors rated ampacity to deal with inrush current and since separate overload protection is required to protect the wire to its ampacity under an extended overload heat condition.

However, I am not aware that transformers have separate overload protection so I believe the code forces larger wire to be used on the primary for just the small percent of short circuit possibility as you mentioned?

 

[Jim_SWFL]

As your probably aware using an OCPD rated higher than the ampacity of the primary conductors ampacity which feed the transformer may be allowed when sizing to the next standard size breaker when the conductors ampacity involves fractions.

Motors allow smaller conductors with oversized ampere breakers for the conductors rated ampacity to deal with inrush current and since separate overload protection is required to protect the wire to its ampacity under an extended overload heat condition.

However, I am not aware that transformers have separate overload protection so I believe the code forces larger wire to be used on the primary for just the small percent of short circuit possibility as you mentioned?

You're correct about the thermal overload and motors, but wouldn't the secondary OCPD serve a similar purpose? Current flow on the secondary would be limited.

If you have a few minutes, please have a look at exhibit 240.9. Unless I am looking at it incorrectly, there is a 100A fuse ahead of 50A wire on the primary side of that transformer.

 

[wwhitney]

If you have a few minutes, please have a look at exhibit 240.9. Unless I am looking at it incorrectly, there is a 100A fuse ahead of 50A wire on the primary side of that transformer.

David reproduced this exhibit in post #93. This is an example of applying 240.21(B)(3), which I mentioned in post #4. There's no question that that tap rule allows 50A primary conductors to the transformer if you comply with the limits within the rule (primarily a 25' maximum length for primary and secondary conductors combined).

Whether the secondary conductors in exhibit 240.9 are properly sized was the subject of most of the debate in this thread. Their sizing is not contrary to the most obvious reading of 240.21(B)(3), and hence are arguably compliant. Yet their sizing makes them less protected than comparable situations sized to 240.21(B)(2), so I have argued that a more appropriate way to apply 240.21(B)(3) in this situation is to use a 4.0:1 voltage ratio for consistency with the other 25' tap rules.

Cheers, Wayne

 

[Jim_SWFL]

David reproduced this exhibit in post #93. This is an example of applying 240.21(B)(3), which I mentioned in post #4. There's no question that that tap rule allows 50A primary conductors to the transformer if you comply with the limits within the rule (primarily a 25' maximum length for primary and secondary conductors combined).

Whether the secondary conductors in exhibit 240.9 are properly sized was the subject of most of the debate in this thread. Their sizing is not contrary to the most obvious reading of 240.21(B)(3), and hence are arguably compliant. Yet their sizing makes them less protected than comparable situations sized to 240.21(B)(2), so I have argued that a more appropriate way to apply 240.21(B)(3) in this situation is to use a 4.0:1 voltage ratio for consistency with the other 25' tap rules.

Cheers, Wayne

I don't really see a difference whether they are tap feeders or not, there is a 110A fuse ahead of 50A wire.

For a 250KVA single phase 480:240 xfrmr, I might have used an 800A breaker. The FLA of this transformer is 520A, the calculated demand on the secondary is 835A (240) and 417A (480). The secondary conductors terminate on a 1,000A breaker, which is slightly less than 100% of the FLA of 1,041A.

Why would I need 800A of wire on the primary when under conditions of use these conductors would never see more than 500A.

Surely, the justification can't be the inrush or a fault. The wire (in this case a parallel set of #300MCM copper - 285A) should have a withstand rating in the neighborhood of 14,200A for 5 seconds. Check my math, but I just don't see the logic.

 

[wwhitney]

I don't really see a difference whether they are tap feeders or not, there is a 110A fuse ahead of 50A wire.

That's more or less the definition of a feeder tap, and it is allowed under 240.21(B)(3) for transformers.

Why would I need 800A of wire on the primary when under conditions of use these conductors would never see more than 500A.

You don't need 800A of primary conductor, you use could use 240.21(B)(3) with 500A of primary conductor as long as you comply with the limits.

Or is your question "why are transformers subject to this 25' limit, when motor circuits have no length limit?" That's a good question to which I don't have a good answer.

Cheers, Wayne

 

[Jim_SWFL]

That's more or less the definition of a feeder tap, and it is allowed under 240.21(B)(3) for transformers.

Wayne, I am trying to resolve a problem in how I interpret the code surrounding transformers and associated conductors.

My original question doesn't involve a feeder tap, and a tap is just a method to supply power.

What I started out trying to find is a rule-based justification for using 570A of wire on a 1200A breaker feeding a 250KVA 480:240V transformer.

The rules for motors (similar inrush problem) allow for OCPD's that exceed the wire ampacity.

This arrangement relies on motor overload device to protect the circuit from a condition that would damage the wire insulation but still be under the current required to open the OCPD.

It is my opinion that a transformer with a secondary OCPD meeting the requirements in 450.3(B) are protected from overload in a manner similar to a motor. Perhaps I am wrong on that point?

Assuming that I not wrong, I could safely protect the transformer with a OCPD 250% or less of the rated FLA and feed that transformer with wire having an ampacity that covers the FLA of the transformer.

In this case a 1200A breaker on the primary (230% of primary FLA), and a 1,000A breaker on the secondary (96% of secondary FLA). The primary side would be fed with two parallel sets of #300 (570A).

Even though the question I am trying to answer doesn't involve a tap, I feel exhibit 240.9 lends some credence to my position.

In that exhibit there is a 110A fuse on #8 (50A) wire feeding a 37.5KVA transformer primary. How does a tap make this "ok", but feeding the same transformer from a panelboard with a 110A breaker would require #3 wire. Isn't a panelboard a bunch of taps?

 

[wwhitney]

My original question doesn't involve a feeder tap, and a tap is just a method to supply power.

A 1200A breaker directly supplying 570A of wire is a feeder tap (well, unless it's a branch circuit for an enormous motor). You don't need to have a segment of 1200A wire between the 1200A breaker and the 570A wire to be a feeder tap.

With that clarified, please see post #103 again.

Cheers, Wayne

 

[jim dungar]

The NEC says what it says.
Most of the NEC rules do not have deep detailed engineering behind them rather, most of the requirements are based on almost a century of practical applications and safe installations as deemed by the various CMPs.
Do not take the logic from one NEC location and try to use it to rationalize a different situation. Well maybe you can if the same CMP is responsible for those sections.

The OP said there was a transformer with primary protection >125% FLA and secondary protection <=125%. This makes the application conform to 450.3(B).

Note that 450.3(B) does not allow the primary protective device to ever exceed 250%, therefore rounding up per 240.4(B) may not be applicable.

The conductors between the primary protection and the transformer must be protected per 240.4 which talks about protective device settings not loading.

240.21(B)(3) is not applicable unless the combined primary and secondary conductor length is <=25'. The OP does not provide the conductor lengths, so any other discussion would be theoretical at best.

In my experience most installations, over 200A, have a hard enough time meeting the maximum secondary conductor length of 25' for 240.21(C) much less the combined primary and secondary distance of 25'

 

[JoeStillman]

There is an important difference between the 250% max CB you see in 430 and the one in 450. In article 430, the CB referred to is for Short Circuit and Ground Fault protection. Overload protection is specified elsewhere, and it's almost always provided by the starter or drive. The overload protection will always be set below the wire ampacity. In article 450, the primary breaker is providing OL protection as well as SC and GF. You have no other overload protection for the primary conductors and so that breaker has to match your ampacity (tap rules excepted.)

 

[wwhitney]

There is an important difference between the 250% max CB you see in 430 and the one in 450. In article 430, the CB referred to is for Short Circuit and Ground Fault protection. Overload protection is specified elsewhere, and it's almost always provided by the starter or drive. The overload protection will always be set below the wire ampacity. In article 450, the primary breaker is providing OL protection as well as SC and GF. You have no other overload protection for the primary conductors and so that breaker has to match your ampacity (tap rules excepted.)

Not following your comparison. If using 450.4(B) and 250% primary protection, then 125% secondary protection is required. So the secondary OCPD would also be providing OL protection for the primary conductors.

But seeing as how 450 doesn't address conductor protection at all, the more apt comparison is between 430 and 240.21.

Cheers, Wayne

 

[JoeStillman]

Not following your comparison. If using 450.4(B) and 250% primary protection, then 125% secondary protection is required. So the secondary OCPD would also be providing OL protection for the primary conductors.

But seeing as how 450 doesn't address conductor protection at all, the more apt comparison is between 430 and 240.21.

Cheers, Wayne

I was referring to the OP in post #3, where he brought up the reference to Article 430. But you later clarified how the secondary OCP can be considered the Overload Protection for the purposes of 240.21(B)(3), which is why I said "tap rules excepted".

Paragraph 240.21(B)(3) seemed counter-intuitive at first, in light of 240.4(F), but that refers to protecting secondary conductors with the primary breaker, not primary conductors with the secondary breaker.

I confess, I've never taken advantage of 240.21(B)(3). You would have to have the 480V panel and 208V panel or MCB in the same room to be able to use it.