transformer taps

[ODonnell]

Looking for advice on how-to for a motor breaker panel (VFDs).
I want to give a 100-amp fused disconnect 480V3-ph, feeding a small 6-circuit panel for use with VFDs, 20A 3-ph individual VFDs, but much smaller loads.
I have a 480V 3-ph Delta to 208Y/120v 4 wire transformer that is supplying well under the overload protection.
I want to come off the primary (480V-Y) taps of the transformer to feed the disconnect maybe 50 feet away and up one level.
1. Can i do this?
2. Can the transformer lugs accept more than one wire?
3. would it be ok to use stacked taps on the transformer supply side lugs to keep a decent space between connections?

help appreciated.

 

[templdl]

Looking for advice on how-to for a motor breaker panel (VFDs).
I want to give a 100-amp fused disconnect 480V3-ph, feeding a small 6-circuit panel for use with VFDs, 20A 3-ph individual VFDs, but much smaller loads.
I have a 480V 3-ph Delta to 208Y/120v 4 wire transformer that is supplying well under the overload protection.
I want to come off the primary (480V-Y) taps of the transformer to feed the disconnect maybe 50 feet away and up one level.
1. Can i do this?
2. Can the transformer lugs accept more than one wire?
3. would it be ok to use stacked taps on the transformer supply side lugs to keep a decent space between connections?

help appreciated.

No.
I don't have a clue as to what the lugs are. Did you consult the transformer manufacture??
Did you consult with the transformer manufacture?

Often times asking the manufacturer of the transformer is avoided because they may you an answer that you don't want to hear noting that neither the manufacturer or the KVA of the transformer has been provided.

 

[augie47]

I would agree that in all likelihood you would encounter Code problems.
The rating of the lugs would be one primary concern. The distance to your disconnect is another.
Can you provide more detail and perhaps someone will have a suggestion.
How big is the transformer ? What Size over-current protection and wire are used for the primary ?

 

[ODonnell]

more info

more info

the lugs will be replaced with two-slot lugs for the second set of wire. that should solve that.
the wire feeding the primary is unknown and i cannot find out until tuedsay when it can be shut down again.
the transformer is 480 delta primary-208y/120 secondary, 150kVA rating, 250A breaker primary circuit protection. the primary supply is 480Y with ground, no neutral, but it is not needed for the transformer, or for our new panel. the current load on the transformer is small, it was sized to provide for future expansion, provding for relatively few operational circuits at this time, never going to get much more use than that in reality, which is why we believe there is plenty of availability for providing for the new panel.
the distance to the disconnect is probably about 40 feet if that.

thanks for continued help!

 

[augie47]

I may be overlooking some important Code section, but if you have the proper lugs and you install your feeder to your 100 amp disconnect per the applicable 240.21 rule (I can't see you going 40 ft if its a "tap".) I find no problem. (templdl may teach me something here)
Keep in mind, even with a minor load, your transformer will still have an inrush current and your added load may cause a nuisance trip.

 

[templdl]

I may be overlooking some important Code section, but if you have the proper lugs and you install your feeder to your 100 amp disconnect per the applicable 240.21 rule (I can't see you going 40 ft if its a "tap".) I find no problem. (templdl may teach me something here)
Keep in mind, even with a minor load, your transformer will still have an inrush current and your added load may cause a nuisance trip.

I go back to my question to the xfmr manufacturer, is the transformer UL listed and if so would any modifications to the lugs violate their UL listing? Often time there is an effort made to avoid consulting with the manufacturer for fear that they will give an answer that is not favorable to the user. Also, if the user does proceed whether or no the manufacturer is consulted if the AHJ has the responsibility to approve a modification as such. Also, would this be turning a transformer enclosure into a terminal compartment? Since this was not considered in the transformer design before one proceeds to step 2 this should be verified first. Work with the xfmr mfg first to assure that they would allow you to change the lugs and to assure that you can enter and exit the enclosure that are permitted by the transformer design. This may not be an issue but I would certainly discuss this with the mfg first.
If you can do that then consider the location of the OCPD, tap rule if applicable, xfmr inrush consideration, etc.

 

[hurk27]

What size is the circuit feeding the transformer, what is the KVA of the transformer, Does the transformer have a local disconnect ahead of it?

240.21(B) will limit taps to 25' for most installations and limit the ratio of 1/3rd the rating to the circuit feeding the transformer, 240.21(B)(4) can allow a tap up to 100' if you meet the requirements of that code but it has some strict requirements that in most cases most you will not meet.

If there is a disconnect then why not set another disconnect next to the transformer disconnect and tap it from the disconnect?

If not, as was said asking the manufacture if double lugs can be installed on the terminal connection bar on the primary, if the bar has dividers that limit the width of the lug, then stacked double lugs are available, do not just put two single lugs on the same bolt in the terminal bar.

if the transformer feeder circuit is 300 amps or above then you can not tap the feeder anyways with 100 amp conductors unless you install fused disconnect within 10' of the tap which will allow up to 1/10 the rating of the feeding circuit which would allow up to a 1k amp circuit tap for 100 amps.

Another option is to use Polaris insulated taps sized for the conductors to tap ahead of the terminal strip in the transformer, again most likely you wont be able to run 40 feet with out hitting a fused disconnect so a disconnect at the transformer for the new feeder will be a must, but will allow you to forgo the disconnect or main breaker at the sub panel which will need to be a 3-phase no neutral panel.

Do not use the tap terminals on the transformer that are design to change the ratio of the transformer as they are not design to supply a load as they will had heat in the core of the transformer and its windings.

Also taps are required to be enclosed in a raceway, so you can not run cable between the tap and the disconnect.

 

[kwired]

Looking for advice on how-to for a motor breaker panel (VFDs).
I want to give a 100-amp fused disconnect 480V3-ph, feeding a small 6-circuit panel for use with VFDs, 20A 3-ph individual VFDs, but much smaller loads.
I have a 480V 3-ph Delta to 208Y/120v 4 wire transformer that is supplying well under the overload protection.
I want to come off the primary (480V-Y) taps of the transformer to feed the disconnect maybe 50 feet away and up one level.
1. Can i do this?
2. Can the transformer lugs accept more than one wire?
3. would it be ok to use stacked taps on the transformer supply side lugs to keep a decent space between connections?

help appreciated.

answers to questions:

1. maybe

2. aren't any lugs usually field installed, so if otherwise allowed to do what you propose, just install appropriate lugs.

3. I don't understand the question.

If the supply conductors are protected at or above their ampacity you do not have tap conductors, you simply have feeder conductors or branch circuit conductors and there is no length limitation. How high the overcurrent protection is in relation to the rated primary current of the transformer could be an issue, but you can have 250% in all cases where there is also secondary protection.

 

[ODonnell]

Square D consulted

Square D consulted

I spoke with a square D rep (after waiting on hold for 25 minutes, i think they need more reps). He informed me they typically ship without lugs and the installer provides what is needed. Any lug that will fit this is OK (within required specs, of course). In fact, he said what most people do not know is they do allow for putting lugs on the back side of the hole as well, allowing you to utilize that for the extra place for wire if needed, as long as local codes are observed, and there is room for everything.
My plan then is to place the fused disconnect within 25 feet (of wire length), either replace the lugs or use additional lugs on the back side of the terminal since the manufacturer allows this, and follow all the tap rules. the primary wire is 300Kcmils aluminum, the transofrmer is 180A on the primary, which i had overlooked before, and the breaker is 250A. The fuses will be 100A.
overall, since this is a live theatrical production, we have a lot of diversification of load, as of the completion and implimentation of the show utilizing the new panel, our MAX load during a show that would be possible, but not ever likely to happen, would be 10 amps @480 on the new panel, and 100amps @208 on the transformer. Overall, it just gives us the ability to loacte scenery that may have a 480V 3-ph motor on it in more loactions around the stage allowing the designers to put anything wherever they need to!

thanks for all the help. it was a learning experience!

 

[petersonra]

It is not clear to me that replacing a lug with a different UL listed connector is much of an issue given how common it is.

 

[templdl]

Fantastic ODonnell, and straight from the horses mouth. Now you can with all confidence provide the lugs that you need to accomplish what you want to do. It gets back to the manufacturer providing lug provisions with NEMA 2, 4 hole etc. And the customer provides the lugs based upon the cable that they will be using.