Cable entry options for switchgear & transformers

[IanOwens]

I am trying to figure out what options are available for cable entries into switchgear and transformers on an NEC project.
We will be using both non metalclad 'Tray Cable' and non metalclad MV-90 cable. On a typical IEC project for this application I would use a brass industrial cable gland with metric parallel threads. The switchgear and transformers would have gland plates with suitably sized clearance holes, and the gland would be secured with a locknut.

The project is to be in line with NEC/NEMA standards, and I am having difficulty finding suitable cable glands (sometimes referred to as cable connectors / cord grips ?) or other suitable means of cable entry.

It seems that options include:

(1) 'Cord and cable fittings' type products such as Cooper Crouse Hinds CGB product, although this does not appear to be available in aluminium (for use with single core power cables), neither does it appear to be rated as NEMA 4X for use outdoors in transformer bay location. Appleton manufacture a similar product in Aluminium or steel material but again, it does not appear to meet NEMA 4X, and neither of these manufacturer's show the product with a locknut for cable entry into enclosures with clearance holes.
(2) Killark manufacture a cord connector (model ZS108) which is shown in the catalogue with a locknut. I have concerns about the ability of this arrangement in terms of maintaining integrity of enclosure when using tapered NPT threads. I have been told that with a tapered thread the locknut can become tight on the thread before it has formed a tight seal between the connector and the enclosure.
(3) The final option appears to be to pass the non-armoured cable through an 'insulating bushing' such as o-z gedney type ABB. These do not appear to provide any sort of seal between the cable and the enclosure, so would be unsuitable for cable entry to transformers, and since our switchgear is to be top entry they do not appear to be suitable in terms of ingress protection of the switchgear enclosure.

There seems to be limitations and restrictions with all of the above methods none appears to fully meet our requirements.

I would be grateful for advice as to how cable entry to switchgear and transformers is typically carried out with NEMA equipment and in accordance with NEC standards.

 

[dpc]

Any non-armored medium voltage cable will have to be in conduit. So that part is easy - you'll be using conduit fittings, not cable fittings. I do not think NEC allows you to run medium-voltage cable out of conduit unless it is armored or metal-clad cable.

Normally, all non-armored cable entering metal-clad switchgear are in conduit. Standard cord fittings can be used for TC cable penetrations, if this wiring method is acceptable to the local authority having jurisdiction.

When applying the NEC in the US, keep in mind that the NEC does not have force of law until adopted by the applicable state and/or local government. Most adopt it, but many have variations and amendments that are tacked on. And the NEC always gives the final say to the local "authority having jurisdiction" (AHJ), normally your friendly local electrical inspector, who may, or may not, be able to find his backside with both hands.

 

[IanOwens]

I cannot find anywhere in the NEC where it requires non metal-clad MV (or TC for that matter) to be run in conduit. Article 328 states that MV cable is permitted in cable tray and even direct buried (which we also propose to do). Article 328 does not make any reference to what cable construction should be.

I accept that in practice metal-clad cable is used for direct buried and non-conduit installations, but is this a situation of following historical practice rather than meeting a particular requirement of the code ? - I stand to be corrected if anyone can tell me the section in NEC where it states metal-clad MV cable is required if not in conduit.

The project in question is located in Peru.

Thanks

 

[dpc]

It's in Article 300. See 300.37 for the medium-voltage cable. There is actually an exception that would it allow it for MV cable in "locations accessible to qualified persons only", which would be a locked electrical equipment room in NEC-speak. So it appears I overstated things a bit, although I honestly can't recall seeing it done on an NEC installation over the past 35 years. In utility power plants and substations, it is sometimes done. I have seen it done in IEC land, of course.

But see also 300.3 (C)(2). This says basically, that MV cable cannot occupy the same raceway or enclosure as low voltage cables. Inside the switchgear, MV conductors must be separated from LV conductors by a grounded metal barrier.

See also 300.4.

Peru I know nothing about.

 

[rcw retired EE]

One problem with cable tray in top entry gear is how to seal the cable entry from water. Using a short vertical conduit with a 90 degree bend to feed the tray is one way to doit. The conduit fittigns provide a semi-watertight seal.

It is very difficult to find a single conductor cable gland except for the ones you already mentioned. You could switch to armored cable and use armored cable fittings in tray.

Another method frequently used is to build a metal top hat or junction box on top of the switchgear taht allows the cabel tray and cabels to enter the top hat horizontally. It is easier to flash and seal a horizontal penetration.