Follow these basic steps to select the best transformer for each phase of your next project.

Suppose you've got the job of designing the power distribution for an entire building. At some point, you'll have to select the service transformer and the distribution transformers. How do you select the right transformer? Let's answer that question, beginning with the service entrance.

First you must determine your maximum coincident load, which is the largest load you can expect at any given time. For example, you don't normally run electric heat and air conditioning at the same time, so your calculations would include only the larger of the two loads. You might not operate all welding receptacles simultaneously or run some pieces of production equipment at the same time, either. However, you would run certain loads at the same time. Determine what those loads are — based on operational requirements — then determine the kVA each will draw. Be sure to calculate your motor loads per NEC Art. 430. Depending on the application, you'll also need to refer to NEC Articles 440 (HVAC units), 610 (cranes and hoists), 620 (elevators and other “people movers”), 670 (industrial machinery), and 695 (fire pump motors).

Once you've done the individual calculations and added them up to arrive at the maximum coincident load, you should add in contingency factors for reasonable unplanned load growth (20% is common) and for planned additional load.

But don't stop there. The following list includes some additional factors you'll have to consider when choosing a transformer.

  • Pole-mounted, pad-mounted, indoor, outdoor, in a vault or cage

  • Primary voltage: delta or wye (probably dictated by the utility)

  • Secondary voltage (480V for small facilities) or delta-wye (depends on load type)

  • 3-wire, 4-wire, or 5-wire configuration (4-wire is most common)

  • Power factor and efficiency

  • Impedance

  • Temperature rise

  • Case style (consider footprint, height, access, cooling)

  • Location, space requirements

  • Seismic requirements

  • Accessories (including taps) and instrumentation

  • Dry-type or liquid-filled

  • Basic impulse level (BIL), which is the withstand rating in kV

Based on these criteria, you can work with a vendor or the electric utility to select the correct transformer for your service entrance.

You should also consider expansion — locate the service transformer in a way that allows for the addition of another service. For example, if the building has room for an addition on the east side, don't place your transformer there because you'll have to move it to allow for expansion. You must balance these selection factors against price, delivery, and performance requirements of the load to which you are supplying power.

Once you've selected the service entrance transformer, you can supply the feeder circuits. You want to distribute at the highest voltage possible to minimize losses and power quality problems. In a large facility, you'll probably distribute 14.4kV onsite, though many facilities distribute at 25kV. To select the transformers for these feeders, follow the same steps we just used for service entrance transformer selection.

Load transformers

The next layer of distribution is usually 4160V, then 480V — you normally begin to power equipment at these levels. Don't assume loads start at 480V. Some large production equipment requires a 4160V supply. Fire pumps, reactor coolant pumps, chiller pumps, and other high-inertia loads also often run on 4160V.

A transformer's function for distribution is straightforward: to step down the voltage and provide isolation between primary and secondary. When supplying a particular piece of equipment or equipment grouping, however, transformer selection gets more complicated. Motors that run at 4160V, for example, tend to be large, expensive, and mission-critical. Usually, you'll put such a motor on its own transformer and panel with few, if any, other loads. Use the earlier load calculations to size these individual load transformers, and carefully note the load characteristics — this is crucial information to supply to the vendor.

Low-voltage distribution

Before you size your low-voltage distribution transformers, decide how you're going to lay out your branch circuits. For example, you might use one transformer to supply a motor control center (MCC), another for lighting loads, another for convenience receptacles, and so on. You may use one transformer per plant area for nonmotor process loads, or nonmotor buckets from the MCC. You could also provide an individual transformer for infrastructure equipment (plant air, elevators, electric doors), or individual transformers for individual equipment. A good planner will also provide a transformer for emergency equipment (fire control, security, emergency egress lighting) that resides on a UPS and may even have an alternate transformer supplying the switchgear or panels through a transfer switch.

In each case, size your distribution transformer based on the maximum load you can expect — just as you did for the service entrance. Also, a piece of large equipment — such as a robotic machine that automates sequential stamping, cutting, and welding operations — may require its own 4160V supply (integral OEM transformers step down to other voltages) while also requiring a 480V supply to corollary equipment. That corollary equipment should not run off the same 4160V supply as the main machine, so you may have two 4160V transformers with very different operating requirements.

What about distributed vs. centralized and size vs. quantity? By using several small transformers, you can reduce loss of function in the event of a transformer failure. Match transformers to load type and use as few transformers as possible (which means maximizing the load per transformer) to effectively reduce voltage drop. However, blindly following either of these approaches ignores the real issues: The small-transformer approach is expensive and inefficient — the money spent on transformers would go much further for maintenance.

Now let's look at three ways transformers can provide some design flexibility in specialized ways.

Autotransformers

Primary and secondary windings are magnetically isolated from each other, but electrically connected. Advantages include lower cost, smaller size, less weight, higher efficiency, and better voltage regulation.

Buck and boost

These allow you to raise or lower voltage to particular equipment. For example, if you have 230V equipment but 208V/120V distribution, a buck-and-boost transformer can raise the voltage to that equipment from 208V to 228.8V, which is close enough for proper operation.

Zig-zag

Suppose you have 120V lighting fixtures and you want to upgrade to 277V for increased efficiency, but your facility has a 480V/240V 3-phase, 4-wire delta system. How do you get 277V out of this system? You can connect three transformers in a “zig-zag” configuration (Figure on page 28). Or, you can buy a zig-zag transformer, which is usually the preferred approach. However, such a transformer has unusual characteristics that affect, among other things, your selection of OCPD. So don't use a zig-zag unless you take the time to understand all the design ramifications.

When choosing a transformer, your core job is defining the load size and the nature of the load. Often, industry experience makes transformer selection easy once you've done this. But sometimes an application does not lend itself to straightforward decisions. In such cases, you'll need to consult the appropriate standards and references, and work with your vendor. The factors outlined here will help you make that a successful collaboration.

Recommended Resources

IEEE Std 141: Recommended Practice for Power Distribution for Industrial Plants

ANSI C57.92: Guide for Loading Oil-Immersed Distribution and Power Transformers

ANSI C57.96: Guide for Loading Dry-Type Distribution and Power Transformers

NEC Article 450 covers installation criteria for transformers of all types