Okay, let's break down the transformer sizing and breaker selection for your dehumidifier project.1. Calculate Total Load



Single Dehumidifier Load

277V 9A = 2493 VA (Volt-Amps)

Total Load (All 15)

15 2493 VA = 37,395 VA (37.4 kVA)

2. Transformer kVA Sizing



Worst-Case Scenario (All 15 Running)

37.4 kVA

Consider Start-Up

Dehumidifiers, like many motors, can have a higher inrush current during startup (typically 3-5 times the running current, but can be higher depending on the model). Since you expect up to 3 to start simultaneously, we need to account for that. A common practice is to derate for startup, but let's focus on the worst case scenario.

Transformer Selection



I recommend a 45 kVA transformer. This provides a safety factor for voltage drop under load, future expansion (even though you say you won't expand, it's a good practice), and any unexpected surges. It also falls into a common standard transformer size. It's better to be slightly oversized than undersized.3. Transformer Specifications



Type

Three-Phase, Dry-Type Transformer (Generally the most suitable for indoor applications)

Primary Voltage

208V Delta

Secondary Voltage

480V Wye (277V phase to neutral)

kVA Rating

45 kVA

Frequency

60 Hz

Enclosure

NEMA 2 (Dripproof) or NEMA 3R (Outdoor) depending on the installation location. Typically these will be installed indoors so NEMA 2 is fine.

4. Primary Breaker Sizing (208V Panel)



Transformer Primary Current

(kVA 1000) / (√3 Primary Voltage) = (45 1000) / (1.732 208) = 125 amps

Breaker Sizing

You need a breaker that is sized to handle at least 125 amps continuously.

Select a 150 Ampere, 3-Pole circuit breaker. This is a standard size above the calculated current and provides a safety margin.5. Secondary Breaker Sizing (480/277V Panel)



Branch Circuit Breakers

Each dehumidifier circuit will require a dedicated circuit breaker. Dehumidifier Current: 9 Amps

Recommended Breaker Size

Use a

15 Ampere, 1-Pole circuit breaker for each dehumidifier circuit. This is a standard size and is suitable for the 9A load.6. Availability and Manufacturers



Common Manufacturers

Eaton Schneider Electric/Square D ABB Siemens Hammond Power Solutions

Availability

These manufacturers typically have a good distribution network. Check with your local electrical supply houses (e.g., Graybar, Rexel, Platt Electric) for pricing and availability. Lead times can vary, especially with current supply chain issues. Be sure to check on lead times and availability when you request quotes.

Online Retailers

Websites like Grainger, Zoro, and Amazon Business can sometimes be options, but for a transformer of this size, your local electrical supply house is generally the best bet.

Important Considerations and Cautions



NEC Compliance

All wiring, breaker sizing, and transformer installation must comply with the National Electrical Code (NEC) and any local codes.

Consult with a licensed electrician to ensure proper installation and compliance. Overcurrent Protection

Ensure both the primary and secondary sides of the transformer have appropriate overcurrent protection (circuit breakers).

Grounding

Proper grounding of the transformer and the 480/277V panel is critical for safety. Follow NEC requirements for grounding.

Ventilation

Transformers generate heat. Ensure the transformer is installed in a well-ventilated area, free from obstructions, to prevent overheating. Consult the manufacturer's installation instructions for specific ventilation requirements.

Voltage Drop

Calculate the voltage drop from the transformer to the furthest dehumidifier. While 45 kVA should help, long wire runs can still cause voltage drop. Size the conductors appropriately to minimize voltage drop.

Transformer Taps

Some transformers have taps that allow you to adjust the output voltage slightly. This can be helpful if you experience voltage drop issues.

Summary of Recommendations



Transformer

45 kVA, Three-Phase, 208V Delta Primary, 480V Wye Secondary, Dry-Type

208V Primary Breaker

150 Ampere, 3-Pole*

480/277V Dehumidifier Branch Circuit Breakers

15 Ampere, 1-Pole (one for each dehumidifier)

Next Steps


Contact Your Electrician

Review these recommendations with your licensed electrician. They can verify the calculations and ensure proper installation.2.

Obtain Quotes

Get quotes from multiple electrical supply houses for the transformer, breakers, and other necessary components.3.

Check Lead Times

Inquire about lead times on the transformer, as they can vary significantly.4.

Review Installation Instructions

Carefully review the manufacturer's installation instructions for the transformer before installation.5.

Ensure Proper Ventilation: Make sure the transformer will be installed in a well-ventilated area.This should provide you with a good starting point. Good luck with your project! Flag for review