Table 119.26(A)(1) as applied to transformers

[birddogger]

If a 13.8kV-480/277 2500 kVA AA/FA transformer is placed facing the 277/480V, 2500A switchboard it serves, does condition 2 or 3 of Table 110.26(A)(1) apply? Since a transformer is never worked on live, I would think the enclosure could be considered grounded and thus it would fall under condition 2 instead of 3. I’m trying to find out if the working space between the two pieces of equip. could be reduced from 4 ft. to 3 ft. because of this.

If the transformer is indeed considered live, then does it actually qualify as over 600V since the primary is at 13.8kV, and thus I would have to have 9 ft. working space (per Table 110.34(A)) between it and the front of the switchboard? If that’s the case, I’m in trouble.

Also, regarding an overhead feeder busduct betweem the two - is this realistic, to try to land 5 sets of #500 between the busduct flange and the bus terminals on both sides of the duct without extra bending space? I would imagine that the cable bending radii might make this impossible without the use of a top hat or a separate pull section, in which case we might just go ahead and run conduit under the floor between the two, if there has to be a pull section either way.

 

[davidbeach]

"Since a transformer is never worked on live"

It used to be that one could consider that transformers would only be worked on (serviced, inspected, etc.) while deenergized. Present widespread use of thermographic imaging (IR scanning), which is only of value if done while energized, suggests that the transformer should be considered as being serviced while energized. The higher voltage would also apply.

On the other hand, if your 13.8kV system is grounded, then your Table 110.34(A) distance for condition 3 is actually only 6 feet. The Table is based on voltage to ground, not line-to-line voltage.

 

[birddogger]

Isn't that just a matter of doing your IR scan through the vents at the top of the transformer, while the live parts are still officially enclosed? Or can a transformer be specified with an IR window like switchgear, for such purposes?

 

[davidbeach]

I have not seen transformers that would allow a good IR scan without being opened. Windows would be an interesting concept, but again, have not seen anybody offering them.

 

[advidana]

How do you know the transformer will not be worked on live? If you quite and somebody else works on it will they always work on it dead. or is it impossible to access the high voltage section.

 

[birddogger]

I'm the consulting engineer laying out the room, so I'm never going to be working on it myself. It's a college campus, so it will only be their maintenance crew. Any IR testing would likely be done by an outside firm (not that this changes the working space requirements any).

I suppose it's worth a call to the local inspector for the job to see what he allows. Other firms I've worked for, and inspectors signing off on other jobs, seem to allow for the transformer to be considered under condition 2, but I've always had my doubts.

 

[resqcapt19]

In my opinion condition 3 applies. Look at 110.26(A)(1)(c). When this section first appeared in the NEC it was an exception to the main rule that. The addition of this wording made it clear that even when the equipment on the opposite side of the aisle is closed, that the code making panel considered any aisle type installation a condition 3 installation. I guess if you can meet the following from the code section: "where conditions of maintenance and supervision ensure that written procedures have been adopted to prohibit equipment on both sides of the aisle from being open at the same time and qualified persons who are authorized will service the installation", then you could apply condition 2.
Don

 

[birddogger]

To resqcapt19 - I don't think you can use 110.26(A)(c) here since this is not an existing installation. As for your suggestion that the NEC is making it clear that any aisle-type installation is considered Conditon 3, note how that paragraph specifically calles out dead-front switchboards, panelboards, or motor control centers, and not transformers. That doesn't mean you (or the other posters here) are incorrect in stating that Condition 2 doesn't apply here, just that arguing it based on that paragraph is a bit suspect.

I suppose you could specify a transformer with safety interlocks on the doors prohibiting access under load to satisty the AHD, but I'm not sure if this option exists for most manufacturers.

If I am overruled and told that a transformer is considered accessible live parts and therefore I have to comply with 110.34(A), I suppose I could "effectively guard" both pieces of equipment with "suitable wood or other insulating materials", this getting me down to a 5 foot allowance under Condition 1. Does anyone know how exactly you go about guarding parts with insulating materials to satisfy this requirement? Or is this kind of farcical condition that is rarely used in practice?

 

[advidana]

If people can access the live parts they will. Give them the required clearnce.

 

[advidana]

You need to get out in the field a bit.