Transformer Requirements

[lazlowcv]

Hello All,
First of all this isn't my area of expertise and I am unfortunately stuck with providing a solution. Any thoughts or comments are greatly appreciated.
I have been tasked with providing a standalone transformer with primary breaker and enclosure to provide 400V directly to the equipment breaker . This transformer is intended to be placed next to the equipment but can be up to 25FT away. I was asked to also provide the cable connection between the transformer and the equipment breaker.

My Questions:
1. Since this transformer (Wye-Wye Autotransformer) does not meet NEC 240.4(F) requirements it must have secondary protection. Correct?
2. Is a secondary breaker required in the transformer enclosure? Or can Equipment breaker be considered the secondary? There will be approx. 25Ft of cable between the transformer enclosure and equipment breaker.
3. Any NEC requirements that prevent the use of flexible conduit or Portable cable between the transformer and equipment? I haven't found any yet and I don't have the capability to provide rigid conduit unless the customer does it themselves.

I have attached a google drive link to an image of my planning document for anyone who would like some of the details.
Google drive Image

Thank You for your time!

 

[Strathead]

(Wye-Wye Autotransformer)... I won't swear to it and someone will correct me here if I am wrong, but I don't believe and autotransformer is delta or wye. You may use autotransformers wired together as three phase, but you aren't "generating" a new neutral. It also carries completely different rules in the NEC. Verify you have an autotransformer first. Second, you say you are tacked with providing a solution. Are you, or are you tasked with wiring up a solution partially specified by others? If the first, provide us with more information, primary voltage, amperage, the equipment you are talking about, whether its load is constant or variable. If you have a requirement to provide an isolation vs. an autotransformer. You will get all the guidance you need here, from that information. Refer to 450.4 regarding what and where for overcurrent protection.

 

[jim dungar]

Your transformer does not need secondary protection but the secondary conductors do.
Start with 240.21(C). If your equipment meets these requirements you are good to go for your question #2.

 

[don_resqcapt19]

In the provided drawings, neither cable show in the top option is compliant. Any cable of conductor used for that option must have an ampacity of at least 225 amps as the rule in 240.4(B) does not apply to transformer secondary conductors.

Also it would be my opinion that a supply side bonding jumper, in addition to the circuit conductors is required between the secondary of the transformer and the equipment.

 

[electrofelon]

(Wye-Wye Autotransformer)... I won't swear to it and someone will correct me here if I am wrong, but I don't believe and autotransformer is delta or wye. You may use autotransformers wired together as three phase, but you aren't "generating" a new neutral. It also carries completely different rules in the NEC. Verify you have an autotransformer first. Second, you say you are tacked with providing a solution. Are you, or are you tasked with wiring up a solution partially specified by others? If the first, provide us with more information, primary voltage, amperage, the equipment you are talking about, whether its load is constant or variable. If you have a requirement to provide an isolation vs. an autotransformer. You will get all the guidance you need here, from that information. Refer to 450.4 regarding what and where for overcurrent protection.

Autotransformers can be connected in a delta or wye configuration, but since they only have one winding, it would just be either delta or wye connected not "wye-wye".

 

[Strathead]

Your transformer does not need secondary protection but the secondary conductors do.
Start with 240.21(C). If your equipment meets these requirements you are good to go for your question #2.

In the provided drawings, neither cable show in the top option is compliant. Any cable of conductor used for that option must have an ampacity of at least 225 amps as the rule in 240.4(B) does not apply to transformer secondary conductors.

Also it would be my opinion that a supply side bonding jumper, in addition to the circuit conductors is required between the secondary of the transformer and the equipment.

I didn't see the attachment the first time. My question still stands. Do you have an autotransformer, or an isolation transformer? As drawn it is an isolation transformer, and as such, option 2 is ok but in that situation 25Ft is the maximum length allowed and the conductor size must be 225 amps (no next larger breaker size rule here) as stated by don, and also it is a fact that you need a supply side bonding jumper sized per 250.102 in lieu of a 250.122 equipment grounding conductor.

If you have an autotransformer, none of what we are saying.

 

[don_resqcapt19]

I didn't see the attachment the first time. My question still stands. Do you have an autotransformer, or an isolation transformer? As drawn it is an isolation transformer, and as such, option 2 is ok but in that situation 25Ft is the maximum length allowed and the conductor size must be 225 amps (no next larger breaker size rule here) as stated by don, and also it is a fact that you need a supply side bonding jumper sized per 250.102 in lieu of a 250.122 equipment grounding conductor.

If you have an autotransformer, none of what we are saying.

sure appears to be an isolation transformer in the attachment

 

[Strathead]

sure appears to be an isolation transformer in the attachment

Yeah but that is just a quick sketch and the OP used the word autotransformer. I am just speculating here that it is possible. Constant load equipment and a budget, I would go with an autotransformer all day long.

 

[lazlowcv]

In the provided drawings, neither cable show in the top option is compliant. Any cable of conductor used for that option must have an ampacity of at least 225 amps as the rule in 240.4(B) does not apply to transformer secondary conductors.

Also it would be my opinion that a supply side bonding jumper, in addition to the circuit conductors is required between the secondary of the transformer and the equipment.

Thanks, I recently noticed this as well and up'd the cable to 2/0 and 3/0, corrected my 3C to 4 C typo. I don't understand why the equipment breaker is so large when the equipment consumption has a 30 min peak rating of 120A. Seems a bit excessive to me.

I feel like my second option pictured is a waste of a breaker but does allow a smaller cable to the equipment. Not sure yet what would be more cost effective. A cable rated at more than 225A and no additional breaker or an additional breaker and smaller cable.

Do you know any NEC rules that prevent the use of portable cord or flexible conduit for this?

Thank You

Yeah but that is just a quick sketch and the OP used the word autotransformer. I am just speculating here that it is possible. Constant load equipment and a budget, I would go with an autotransformer all day long.

It is an autotransformer. It will potentially be used as a step up transformer if needed in the future. I don't have the approval drawings yet but general specs are as follows:
HPS
Product Line: Auto Transformer
Duty: General Duty
Phase: 3
Rating: 112 kVA
Primary Voltage: 460V,380V,230V,200V
Primary Connection: Wye
Secondary Voltage: 400V
Secondary Connection: Wye-N
Neutral (Full Rated): Yes

Thank You

 

[wwhitney]

I'm a bit confused by the voltages listed in the previous post. Seems like the primary voltages listed are at the utilization level, while the secondary voltage of 400V is at the nominal level? So I'll assume the autotransformer is 480V nominal to 400V nominal, i.e. a single coil with an intermediate tap at the 5/6 point of its length. If I'm mistaken, just change 5/6 to the appropriate fraction in the following:

For autotransformers, do we apply 240.21(C) the same as for isolation transformers? If so, and if this autotransformer is actually 3 distinct single phase autotransformers in a wye arrangement, then each individual autotransformer would be said to have a 2-wire secondary. That means 240.21(C)(1) applies. So if the secondary conductors are sized to have an ampacity of 6/5 times the primary protection, or 6/5 * 175 = 210A, no secondary conductor protection would be required.

If, however, this autotransformer is one 3 phase wye to wye unit, then 240.21(C)(1) as written does not apply. I'm actually unclear on why wye-wye is excluded from 240.21(C)(1), seems to me the math works out the same as delta-delta or two-wire to two-wire. But seeing as how it is excluded, secondary OCPD would be required to protect the secondary conductors, at a location close enough to the transformer to satisfy one of the other options under 240.21(C).

Cheers, Wayne

 

[lazlowcv]

I'm a bit confused by the voltages listed in the previous post. Seems like the primary voltages listed are at the utilization level, while the secondary voltage of 400V is at the nominal level? So I'll assume the autotransformer is 480V nominal to 400V nominal, i.e. a single coil with an intermediate tap at the 5/6 point of its length. If I'm mistaken, just change 5/6 to the appropriate fraction in the following:

For autotransformers, do we apply 240.21(C) the same as for isolation transformers? If so, and if this autotransformer is actually 3 distinct single phase autotransformers in a wye arrangement, then each individual autotransformer would be said to have a 2-wire secondary. That means 240.21(C)(1) applies. So if the secondary conductors are sized to have an ampacity of 6/5 times the primary protection, or 6/5 * 175 = 210A, no secondary conductor protection would be required.

If, however, this autotransformer is one 3 phase wye to wye unit, then 240.21(C)(1) as written does not apply. I'm actually unclear on why wye-wye is excluded from 240.21(C)(1), seems to me the math works out the same as delta-delta or two-wire to two-wire. But seeing as how it is excluded, secondary OCPD would be required to protect the secondary conductors, at a location close enough to the transformer to satisfy one of the other options under 240.21(C).

Cheers, Wayne

Hi Wayne,
I am not sure why this is listed as a Wye-Wye autotransformer in the basic specs. I am still waiting on the actual approval drawings with connection diagrams but I speculate it will be similar to this below.
Transformer image

I can see both of your arguments above as well. While I am still not 100% confident I am leaning toward providing conductors capable of more than 225A at 25Ft length and allowing the equipment breaker to be the secondary OCPD, so to speak. If rated high enough I cant see any risk to the conductors by doing this.

Thanks

 

[Strathead]

I can see both of your arguments above as well. While I am still not 100% confident I am leaning toward providing conductors capable of more than 225A at 25Ft length and allowing the equipment breaker to be the secondary OCPD, so to speak. If rated high enough I cant see any risk to the conductors by doing this.

Thanks

Again, I first express my lack of direct experience, but reading the code, specifically 450.4 and 450.5, I don't feel 240.21 applies at all. The conductors are not tap conductors for purposes of that section, and 450 specifically prohibits "secondary" overcurrent protection That said, I have never looked for an article that tells you how to size the shunt conductors on autotransformer. I have always just assumed they are sized per the ampacity and never looked.

 

[wwhitney]

Again, I first express my lack of direct experience, but reading the code, specifically 450.4 and 450.5, I don't feel 240.21 applies at all.

Nothing in 450 can impact whether 240.21 applies, as Chapter 4 articles are not allowed to amend Chapter 2 articles.

The conductors are not tap conductors for purposes of that section, and 450 specifically prohibits "secondary" overcurrent protection

450.4 does not prohibit "secondary" overcurrent protection. Look at diagram 450.4(A) and notice the language in 450.4 about "between points A and B."

Cheers, Wayne

 

[jim dungar]

Again, I first express my lack of direct experience, but reading the code, specifically 450.4 and 450.5, I don't feel 240.21 applies at all. The conductors are not tap conductors for purposes of that section, and 450 specifically prohibits "secondary" overcurrent protection That said, I have never looked for an article that tells you how to size the shunt conductors on autotransformer. I have always just assumed they are sized per the ampacity and never looked.

Protection per 450.3 is not the 'shunt' protection you are referring to. If you are only dealing with the input and output terminals of a factory assembled transformer (bank), it makes no difference the actual internal construction of the transformer.

240.21(C) always applies when ever transformers are involved in the circuit.