Okay, this is a good introduction to medium voltage distribution, and it's smart to ask questions early. Let's break down your points

1. Can 6.9 kV be stepped down to 208V, 3-phase?
Yes, absolutely. Transformers are designed to step voltage up or down. A 6.9kV primary voltage and a 208V secondary voltage is a perfectly standard transformer configuration. You just need to specify the correct primary and secondary voltages, vector group (more on that later), and kVA rating when ordering the transformer.2. If so, can this only be done via pole mounted transformers, or can we provide a pad mounted transformer? You can use either pole-mounted or pad-mounted transformers. The choice depends on several factors: Aesthetics

Pad-mounted transformers are generally considered more aesthetically pleasing.

Space

Pole-mounted transformers require a utility pole structure and associated clearances. Pad-mounted transformers require a concrete pad, which consumes ground space.

Accessibility

Pad-mounted transformers are typically easier to access for maintenance and repairs. Pole-mounted transformers require specialized equipment for access.

Cost

Generally, pole-mounted transformers are less expensive, but this can be offset by installation costs (pole setting, conductor stringing, etc.). Pad-mounted transformers require a pad, enclosure, and typically more sophisticated grounding.

Safety

Both types are safe when properly installed and maintained. Pad-mounted transformers offer more inherent protection against unauthorized access.

Site Conditions

Soil conditions, flood plains, and other site-specific factors can influence the best choice.

Local Regulations

Check your local codes and regulations, as some jurisdictions may have preferences or restrictions on transformer types.

Overhead line configuration

If the overhead line doesn't go where your transformer location is, you can tap it and run a short underground section to the pad mount transformer.

In your situation, given it's an industrial site, a pad-mounted transformer is often the preferred choice. It's more robust, easier to maintain, and offers better protection.3. Any rule of thumb for sizing this medium voltage transformer(s)? This is where things get more complex. The "rule of thumb" is: Don't just use the NEC minimum; load the transformer based on the actual anticipated load profile.Here's a breakdown of the sizing process:1. Load Calculation (The Foundation)



Detailed Load Analysis

This is the most critical step. You need to go beyond the NEC's Article 220 and look at what the actual building loads will be. Consider:

Lighting

Use actual fixture schedules and lighting power densities, not just the NEC allowances.

Mechanical Equipment

HVAC systems (chillers, pumps, air handlers), process equipment, compressors, etc. Get nameplate data and understand the duty cycle (how often it runs and at what load).

Motors

Large motors can have significant inrush current. Consider using soft starters or VFDs if necessary to mitigate this.

Receptacles

Estimate receptacle loads based on the type of business and the equipment expected to be plugged in. Don't just assume 180 VA per outlet.

Future Expansion

Factor in potential future expansion. It's cheaper to slightly oversize the transformer now than to replace it later. A 10-20% margin is a good starting point, but it depends on the likelihood and magnitude of future load growth.2.

Demand Factors

Apply appropriate demand factors based on the types of loads and their expected usage patterns. For example, lighting might have a demand factor close to 100%, while receptacles might have a much lower demand factor. Article 220 does give guidance on this.3.

Diversity Factors

Diversity factors account for the fact that not all loads will be operating at their maximum demand simultaneously. This is most applicable where you have several tenants.4.

Calculate the Estimated Peak kVA Demand

Sum up the calculated demands for each load type, taking into account demand and diversity factors. This gives you the estimated peak kVA demand. This is what you should load the transformer to.5.

Transformer Selection



kVA Rating

Select a transformer with a kVA rating slightly above your estimated peak kVA demand. This provides a safety margin and allows for some future growth.

Voltage Rating

Ensure the transformer has the correct primary (6.9 kV) and secondary (208V) voltage ratings.

Impedance (%Z)

The transformer impedance affects short-circuit current levels. Work with your electrical engineer to coordinate the transformer impedance with the available fault current on the 6.9 kV system and the ratings of your downstream protective devices (breakers, fuses). A typical value might be around 5-7%. Higher impedance limits fault current, but also increases voltage drop.

Vector Group

This is critical for paralleling transformers (if you ever plan to do that) and for ensuring proper phase relationships. The most common vector group for step-down transformers is Dyn11 (Delta-Wye, with a 30-degree phase shift). It is essential to know the vector group of the existing 6.9 kV source to avoid phase mismatches. This is generally information you can only obtain by hiring an electrician to conduct tests.

Temperature Rise

Transformers are rated for a specific temperature rise (e.g., 65°C). Consider the ambient temperature at the site when selecting a transformer. If the ambient temperature is high, you may need to derate the transformer.

Transformer Type

Dry-type or liquid-filled (oil-filled, or less commonly, other types of insulating fluid). Pad-mounted transformers are typically liquid-filled. Dry type transformers are suitable for some padmount applications when installed within a ventilated enclosure. Dry type transformers cannot be used in outdoor environments.6.

Coordination with Existing System



Fault Current Study

Perform a fault current study to determine the available fault current at the transformer location. This is critical for selecting appropriately rated protective devices (fuses, breakers).

Protective Device Coordination

Coordinate the protective devices (fuses, breakers) on the primary and secondary sides of the transformer to ensure proper operation and prevent nuisance tripping.

Grounding

Proper grounding is essential for safety and to minimize voltage transients. Consult with a qualified electrical engineer to design a suitable grounding system.

Important Considerations



Hire a Qualified Electrical Engineer

Medium voltage design requires specialized knowledge and experience. This is not a DIY project. Engage a qualified electrical engineer who has experience with medium voltage systems.

Utility Coordination

Even though the line is privately owned, it's a good idea to inform the utility company of your plans. They may have requirements or recommendations that you need to follow.

Codes and Standards

Comply with all applicable codes and standards, including the NEC (National Electrical Code), IEEE (Institute of Electrical and Electronics Engineers) standards, and local regulations.

Short Circuit Study You'll have to perform a short circuit study with a software like SKM Power Tools to determine the available fault current.

Example

Let's say you calculate the following: Lighting: 50 kVA (after demand factors) Mechanical: 100 kVA (after demand factors) Receptacles: 20 kVA (after demand factors) Motors: 80 kVA (after demand factors)Total calculated demand: 50 + 100 + 20 + 80 = 250 kVAAdd 20% for future growth: 250 0.20 = 50 kVARecommended transformer size: 250 + 50 = 300 kVA. You would likely specify a 300 kVA transformer.

In summary: While stepping down 6.9kV to 208V is straightforward in principle, the design and implementation of a medium voltage distribution system require careful planning, detailed load analysis, and coordination with relevant codes, standards, and qualified professionals. Don't cut corners; it's worth investing in expert advice to ensure a safe, reliable, and efficient electrical system. Flag for review