Working Distance, Arc-Flash Boundary, Incident Energy -4160V, 500 KVA XFMR

[tybkeith]

Regarding the installation of a 4160V, 500 KVA transformer in a school electric room, it appears NFPA-70 Table 110.34(A) requires a Minimum Clear Distance of 5 ft. Is this the correct minimum distance required in front of this transformer? Assume a non-cast coil Square-D transformer with cover.

The issue is, it is a small electric room and there is only 3 ft. from of the face of this transformer to equipment mounted on the opposite wall.

 

[jim dungar]

Regarding the installation of a 4160V, 500 KVA transformer in a school electric room, it appears NFPA-70 Table 110.34(A) requires a Minimum Clear Distance of 5 ft. Is this the correct minimum distance required in front of this transformer? Assume a non-cast coil Square-D transformer with cover.

The issue is, it is a small electric room and there is only 3 ft. from of the face of this transformer to equipment mounted on the opposite wall.

The NEC distances are for working clearance not necessarily for Arc Flash Incident Energy distances.

As it stands the transformer would be allowed as long as the cover is never removed while it is energized. This means you would not be able to perform any IR scanning unless you had appropriate viewing windows installed in the transformer cover.

 

[tybkeith]

The NEC distances are for working clearance not necessarily for Arc Flash Incident Energy distances.

As it stands the transformer would be allowed as long as the cover is never removed while it is energized. This means you would not be able to perform any IR scanning unless you had appropriate viewing windows installed in the transformer cover.

Thank you for the quick reply Jim. But I am still unclear about why Table 110.34(A) shows 5 ft. Minimum Clear Distance (depth of clear working space). We will only have 3 ft. between our 5kv transformer cover and the equipment on the opposite wall. Are you sure that is not an problem?

The contractor will not do an Arc-Flash analysis until after the transformer is installed. My concern is, that will be too late if it is determined this transformer can not go where they are purposing.

 

[rambojoe]

Exactly what type of xformer? And who owns it and installs it?
And why is it in a "school room"? And what kind of a school ? If you dont mind... You also say 500 kva then 5va... Please clarify

 

[don_resqcapt19]

Thank you for the quick reply Jim. But I am still unclear about why Table 110.34(A) shows 5 ft. Minimum Clear Distance (depth of clear working space). We will only have 3 ft. between our 5kv transformer cover and the equipment on the opposite wall. Are you sure that is not an problem?

The contractor will not do an Arc-Flash analysis until after the transformer is installed. My concern is, that will be too late if it is determined this transformer can not go where they are purposing.

Only your AHJ can answer that, I know many who will insist of the full 5' for the transformer, and others who will say that the transformer will never be worked on or examined while energized and will not require the 5' working clearance specified in the code.

As far as the arc flash study, part of that is the available fault current study, and that needs to be done before the equipment is purchased to make sure the equipment is suitable for the available fault current.

 

[jim dungar]

Thank you for the quick reply Jim. But I am still unclear about why Table 110.34(A) shows 5 ft. Minimum Clear Distance (depth of clear working space). We will only have 3 ft. between our 5kv transformer cover and the equipment on the opposite wall. Are you sure that is not an problem?

The contractor will not do an Arc-Flash analysis until after the transformer is installed. My concern is, that will be too late if it is determined this transformer can not go where they are purposing.

If you are not installing viewing windows in the transformer cover, I would bet most AHJs will not approve the location based on the NEC, as it is not possible to perform some likely preventative maintenance.

Again arc flash distances are a separate issue, as NFPA 70E would like the transformer to be deenergized.

I agree with Don, in that the arc flash analysis should be done prior to and then modified after installation. I would not be surprised to find 25% of after the fact new project arc flash studies end up requiring changes to the newly installed equipment.

 

[steven765]

The NEC distances are for working clearance not necessarily for Arc Flash Incident Energy distances.

As it stands the transformer would be allowed as long as the cover is never removed while it is energized. This means you would not be able to perform any IR scanning unless you had appropriate viewing windows installed in the transformer cover.

Right the engineer of record should calculate the arc flash boundary. That's not a simple table lookup because it depends on the transformer feeding that one, distance from it, and wire size.