208V Y to 400V Y Transformer Questions

[CCWest1]

I would greatly appreciate you all checking me on this:

My client has an infrared paint cure system for his spray booth. The system is 400V 3 phase 60Hz with FLA at 43.3. The machine manufacture says a 30KVA transformer will power their system. The voltage coming into the building is 208V Y 3 phase. Marcus transformer can build me a transformer 208V Y to 400V Y rated 83.3 primary fusing with secondary output at 43.4A. I realize the breaker or fusing on the primary side should be at least 125%, which would mean the breaker should be at least 104A. I am more inclined to go with a 125A fused disconnect. I can achieve this with a 125A rated sub feed lug coming from the 200A panel that will be less than 25ft from the disconnect and transformer location. (the 200A panel uses push in breakers, the sub feed lug will push in)

My biggest concern and question I have for you, is that the machine's load is essentially the same as the secondary output of the transformer. Should I be concerned about that if using a 30KVA transformer or should I step up to a 45KVA transformer. Again, the machine manufacturer says the 30KVA will do. If I go with the 45KVA transformer, then I will most likely have to change out the panel to a feed through panel so I can connect to a 150A primary side disconnect, which makes it a much bigger install.

Your thoughts are greatly appreciated!

 

[Smart $]

Welcome. :thumbsup:

System uses infrared heating elements, right? They're resistive, so only a little increase over FLA at startup. A 30kVA transformer with adequate ventilation should be able to power this system 24/7/365 for a long time.

If you are getting the transformer custom wound, go with 208V delta PRI, 400V wye SEC.

Fuse secondary at 60A, protect primary with any rating up to 225A, preferably at least 110A (this rating determines wire size). See Primary and Secondary Protection, Table 450.3(B).

You do not need a disconnect between source panel and the transformer, with the primary ocpd in the panel.

 

[charlie b]

400 volts times 43.3 amps times the square root of 3 comes out almost exactly at 30 KVA. I know of no rule that requires us to include allowances for future load growth or that would prohibit us from operating a component at 100% of its design rating.

 

[CCWest1]

Reply to own post

Reply to own post

I would greatly appreciate you all checking me on this:

My client has an infrared paint cure system for his spray booth. The system is 400V 3 phase 60Hz with FLA at 43.3. The machine manufacture says a 30KVA transformer will power their system. The voltage coming into the building is 208V Y 3 phase. Marcus transformer can build me a transformer 208V Y to 400V Y rated 83.3 primary fusing with secondary output at 43.4A. I realize the breaker or fusing on the primary side should be at least 125%, which would mean the breaker should be at least 104A. I am more inclined to go with a 125A fused disconnect. I can achieve this with a 125A rated sub feed lug coming from the 200A panel that will be less than 25ft from the disconnect and transformer location. (the 200A panel uses push in breakers, the sub feed lug will push in)

My biggest concern and question I have for you, is that the machine's load is essentially the same as the secondary output of the transformer. Should I be concerned about that if using a 30KVA transformer or should I step up to a 45KVA transformer. Again, the machine manufacturer says the 30KVA will do. If I go with the 45KVA transformer, then I will most likely have to change out the panel to a feed through panel so I can connect to a 150A primary side disconnect, which makes it a much bigger install.

Your thoughts are greatly appreciated!

Replying to my own post here. I neglected to look at the secondary fusing which also needs to be 125% of the load, correct? So that would mean I need at least a 60A disconnect to cover the 43.3 FLA. Which means I would have to go to the next size transformer. Am I right about that?

 

[CCWest1]

Reply to Smart$

Reply to Smart$

Welcome. :thumbsup:

System uses infrared heating elements, right? They're resistive, so only a little increase over FLA at startup. A 30kVA transformer with adequate ventilation should be able to power this system 24/7/365 for a long time.

If you are getting the transformer custom wound, go with 208V delta PRI, 400V wye SEC.

Fuse secondary at 60A, protect primary with any rating up to 225A, preferably at least 110A (this rating determines wire size). See Primary and Secondary Protection, Table 450.3(B).

You do not need a disconnect between source panel and the transformer, with the primary ocpd in the panel.

Thanks!

 

[CCWest1]

Reply

Reply

400 volts times 43.3 amps times the square root of 3 comes out almost exactly at 30 KVA. I know of no rule that requires us to include allowances for future load growth or that would prohibit us from operating a component at 100% of its design rating.

Thank you Charlie!

 

[charlie b]

I neglected to look at the secondary fusing which also needs to be 125% of the load, correct?

Needs to be? No. Allowed to be? Yes.

 

[texie]

Replying to my own post here. I neglected to look at the secondary fusing which also needs to be 125% of the load, correct? So that would mean I need at least a 60A disconnect to cover the 43.3 FLA. Which means I would have to go to the next size transformer. Am I right about that?

As others have mentioned, the 30 KVA should be fine as this is a resistive load and a transformer does not need to be rated for 125%.
Your secondary OCPD would be a 60 (43.3 x 1.25 = 54-next size up = 60) You would need conductor ampacity of 60 @ 75 degree as the next size up rule does not apply to transformer secondary conductors.
You will also need to ground the X0 per Art. 250 and have a grounding electrode system. Running it ungrounded along with the NEC requirements of same I would not recommended.
And as was said, you want that 208 primary winding to be a delta.

 

[david luchini]

Replying to my own post here. I neglected to look at the secondary fusing which also needs to be 125% of the load, correct? So that would mean I need at least a 60A disconnect to cover the 43.3 FLA. Which means I would have to go to the next size transformer. Am I right about that?

No, having a 60A disconnect on the secondary does not require you to go to the next size transformer.

 

[CCWest1]

Please review my calcs for conductor & bonding sizing.

Please review my calcs for conductor & bonding sizing.

Welcome. :thumbsup:

System uses infrared heating elements, right? They're resistive, so only a little increase over FLA at startup. A 30kVA transformer with adequate ventilation should be able to power this system 24/7/365 for a long time.

If you are getting the transformer custom wound, go with 208V delta PRI, 400V wye SEC.

Fuse secondary at 60A, protect primary with any rating up to 225A, preferably at least 110A (this rating determines wire size). See Primary and Secondary Protection, Table 450.3(B).

You do not need a disconnect between source panel and the transformer, with the primary ocpd in the panel.[/

Based on your recommendations for transformer type (delta to wye) Here are my conductor calcs. At the outset, there will be no neutral conductors either on the primary or secondary side. Sizing of feeder conductors at 125% listed in table 310.15(B)(16) based on 75 degree C. Manufacturer's listed supply side amps = 85A. 85A x 1.25 = 106A. Use next size up fusing = 110A. Use conductors next size up from 110A = 115A. Use #2 AWG for supply side conductors.

Size of equipment grounding (bonding) per table 250.122(A)= 110A primary protection requires a #6 AWG copper conductor for bonding.

Size of secondary conductors. Machine load is 43.3A. 43.3A x 125% = 54A. Use a 60A fused disconnect. Size of conductors at 75 degree C. Use next size up from 60A = 65A. Use #6 AWG for secondary conductors.

Size of equipment grounding, EGC, per table 250.122(A). 65A secondary protection requires a #8 AWG copper conductor for bonding
Bond all metal parts, EGC, and connect to the xfmr X0 terminal. Bonding jumper sized from table 250.66 and based on largest size of hot conductor which is #2 AWG, so bonding jumper is #8 AWG.

Grounding electrode conductor, GEC, to be based on total area of the largest secondary hot conductor and terminate at the same point where the neutral to case bonding jumper is installed, X0 terminal.

For GEC, use #8 AWG based on table 250.66 and connect to either cold water pipe, structural steel member or electrode.

My Questions: There is structural steel nearby however I cannot verify whether the structural steel meets 250.52(2) 1 & 2. I am more inclined to connect to the building's service electrode, which is actually a galv. pipe, located within' 20 feet of the xfmr location. Does this sound right to you?

Next question: I came across an article Mike Holt wrote, where he shows an illustration showing a supply side disconnect, xfmr, and secondary panel. He is showing an EGC from the primary disconnect to the xfmr, and no EGC from the main panel to the primary disconnect. Is that because installing an EGC with the conductors from the main panel would produce a parallel path to ground? If so, should I also NOT install an EGC from the main panel to the primary disconnect.? Last question, which may be dumb one, is it okay to use THHN 90 degree c for all my conductors?

 

[Smart $]

...

My Questions: There is structural steel nearby however I cannot verify whether the structural steel meets 250.52(2) 1 & 2. I am more inclined to connect to the building's service electrode, which is actually a galv. pipe, located within' 20 feet of the xfmr location. Does this sound right to you?

Not if there is a verifiable water pipe electrode [250.30(A)(4)].

Otherwise, yes. But I caution you if there is only #6 from service to the pipe, where if not for the pipe electrode a larger GEC would be required per Table 250.66. Check your service to see if there is a "full size" GEC. If so, I recommend running a GEC and bonding to the service GEC.

Next question: I came across an article Mike Holt wrote, where he shows an illustration showing a supply side disconnect, xfmr, and secondary panel. He is showing an EGC from the primary disconnect to the xfmr, and no EGC from the main panel to the primary disconnect. Is that because installing an EGC with the conductors from the main panel would produce a parallel path to ground? If so, should I also NOT install an EGC from the main panel to the primary disconnect.?

Got a link to that Mike Holt article? That doesn't sound right. You are required to run an EGC with the primary conductors.

Last question, which may be dumb one, is it okay to use THHN 90 degree c for all my conductors?

Yes... as long as none of it will be in a wet location.... but most THHN nowadays is dual rated either THWN or THWN-2, which would be okay in a wet location.

 

[CCWest1]

Link Mike Holt Illustration/Article

Link Mike Holt Illustration/Article

Not if there is a verifiable water pipe electrode [250.30(A)(4)].

Otherwise, yes. But I caution you if there is only #6 from service to the pipe, where if not for the pipe electrode a larger GEC would be required per Table 250.66. Check your service to see if there is a "full size" GEC. If so, I recommend running a GEC and bonding to the service GEC.


Got a link to that Mike Holt article? That doesn't sound right. You are required to run an EGC with the primary conductors.


Yes... as long as none of it will be in a wet location.... but most THHN nowadays is dual rated either THWN or THWN-2, which would be okay in a wet location.

http://ecmweb.com/contractor/transformer-installation-made-easysort

 

[Smart $]

http://ecmweb.com/contractor/transformer-installation-made-easysort

I believe the conduit coming into the top of the primary disconnect serves as the EGC. Assume the connector uses a bonding locknut. Otherwise, it would seem strange for the graphic to show a load-side EGC and an EBJ from EGC bus terminal to the enclosure.

Note there is also a section in Code (can't recall exact location at this time) which allows some equipment to be considered grounded by being mechanically fastened to [adequately] grounded metal. In this case, though the image is overcropped, the column to the left appears to be a structural steel grounding electrode.