Mounting a Dry-Type Transformer

Did you hang your dry-type transformer in a location and in a position that will reduce its performance and shorten its life?

Dry-type transformers account for a large range of capacities and applications. It's true they don't have the heat capacity of liquid-filled transformers, and so can't withstand momentary overloads nearly as well. However, they're small, relatively light, cheap, durable, and applicable to thousands of applications.

Codes and Standards

The National Electrical Code (NEC), UL, and the IEEE all address installing dry-type transformers, and some of the requirements seem counter-intuitive. For example: per the NEC, dry types under 50kVA don't need to be readily accessible; you can install them in hollow building spaces. Every junction box in the building must be readily accessible, but a transformer can hang in a closed chase as long as it gets ventilated air. The UL1591 wants 6 in. of clearance all around the outside of the enclosure.

Several ANSI/IEEE standards apply to installing dry-type transformers. Standard C57.94 is the general standard, Standard 344-I 987 deals with seismic issues, and C37.81 covers seismic capabilities of equipment in nuclear plants. UL has standard 1591 for the installation of dry-type transformers.


Do you know which earthquake zone you are in? Refer to the Figure, in original article, to find out. If you are designing transformer mounts for zones other than 0, consult seismic standards. Severe earthquakes happen in the heartland as well as both seaboards (See sidebar). It's the transformer manufacturer's job to build a unit that will survive an earthquake. It's the building designer's job to provide strength enough so the building survives. It's the transformer installer's job to connect the two so the transformer works after an earthquake.

The Uniform Building Code, The California Building Code, and IEEE C37.81 have estimates of the earthquake motions expressed as an acceleration versus frequency graph. The capabilities of the equipment must exceed the curves on this graph. At least one manufacturer tests their equipment on a shake table using accelerations larger than those shown in the graph. When you buy a transformer, you often have the option of buying a manufacturer-designed mounting kit for it. This option will probably save you time and money, and is usually the best way to go.

Tips for trouble-free transformers

Wherever you locate the transformer, mount it as though it will always be there. If it goes on the floor, place it on a solid foundation clear of incidental water and other corrosive materials. It's good to install floor-mounted transformers on low concrete pads (or housekeeping pads) added on top of the floor.

Check your transformer's mounting holes. If they're for wall mounting, mount the unit on a wall. If they're for floor mounting, mount the unit on the floor. Don't build adapting brackets unless you're willing to accept liability for them. Most manufacturers will work with you to provide a suitable bracket. Plan on using all the mounting holes, not just some of them. Use anchors appropriate for the substrate material and follow the manufacturer's installation recommendations. Most wall constructions suitable for use with expanding anchors are also suitable for through-bolts and large washers. Use grade 5 bolts or stronger. See sidebar above. Contact the structural designer before attaching anything significant to beams or walls.

Connect the transformer to a building member strong enough to absorb the earthquake loads predicted for your zone rating. What about constructing the mounting frame? A 45kVA transformer will weigh about 350 lb, and a 75 kVA will weigh about 500 lb; plan accordingly. Manufacturers of strut beams publish load data charts that tell you (by channel type or model number) how long a given beam can be to hold a given weight. They also publish charts to tell you the design loads of the fasteners that go with the beams.

If you hang the transformer, the mounting system must have "excess strength" in the structural members. What is "excess strength?" All transformers vibrate, causing possible fatigue in the support material. This increases the material's brittleness and eventually can cause it to crack and break. You must assume that transformer is going to hang there for 20 or 30 years. The mounting must be beefy enough to resist fatigue for at least that long. A manufacturer's mounting kit, if available, resolves this concern for you. The first two photos (in original article) show examples of extremely solid structural mounting. But location is also important, as photo 3 (in original article) shows.

The same vibration that causes metal fatigue can transfer to the air as noise. The noise can come directly from the unit, or the building structure can carry it over considerable distances before it's audible. Most transformers have built-in vibration isolators, but additional isolation (e.g., flexible conduit to the unit) will reduce noise.

Good workmanship and attention to detail can make or break an installation. Plan for maintenance accessibility. The transformer should be level, plumb and square, of course. UL1591 says it must have 6 in. clearance from the nearest surface; even if the enclosure is non-ventilated. If you follow the standards listed here and use the strut manufacturers' application tables, you'll have a cool-running transformer that stays where you put it.

It's dangerous to think of earthquakes as a "West Coast thing"; severe quakes hit other places, too. Though a 9 on the Richter scale means the ground actually liquefies, lesser quakes cause considerable damage. In 1812 , an earthquake in Missouri hit 8.7 on the Richter scale; and changed the course of the Mississippi River. In 1897, Virginia got a 6.3. Southern Illinois had a 5.7 in 1968. In 1985, Painesville, Ohio, was almost the epicenter of a 5.0 that radiated for hundreds of miles. The Perry Nuclear Power Plant, only 18 miles away, experienced no damage; thanks to extensive seismic design. Some facilities, not so well prepared, fared poorly.


SIDEBAR: Seismic Requirements

Most transformer manufacturers have seismic information. Don't assume the torque ratings out of a reference book apply to the bolts you are using. The correct torque depends on the type of bolt, size of bolt, hardness of bolt, and the type of lubricant either embedded in or applied to the threads. For seismic applications, do not re-use fasteners (it's okay to re-use flat washers).

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