The suitable wattage for a secondary distribution box depends on the total connected load, typically calculated from the sum of all branch circuits with a safety margin, often ranging from a few kilowatts for small residential setups to tens of kilowatts for commercial applications.

Determining the Load

A secondary distribution box, or subpanel, distributes power from the main service panel to a specific area, such as a garage, workshop, or office space. To determine the suitable wattage:

• List all circuits: Include lighting, outlets, and heavy equipment. Each circuit's power is calculated as Watts = Volts × Amps for single-phase systems, or Watts = √3 × Volts × Amps for three-phase systems .
• Classify loads: Continuous loads (operating ≥3 hours) require a 125% safety factor on the breaker rating, while non-continuous loads do not .
• Sum the total load: Add all circuit wattages to get the total expected load.

Typical Ratings

• Residential subpanels: Often rated 100–200A, which translates to roughly 12 kW–48 kW at 120/240V single-phase systems .
• Small commercial or workshop subpanels: May require 200–400A, corresponding to 48 kW–96 kW at 240V single-phase or higher for three-phase systems .
• Future expansion: Include 20–25% extra capacity to accommodate additional circuits or equipment .

Safety and Compliance

• Ensure the subpanel's ampacity matches or exceeds the calculated load.
• Use appropriately sized wires and breakers to prevent overheating and fire hazards.
• Follow local electrical codes (NEC in the US, IEC in Europe) for continuous load calculations and breaker sizing .

Practical Example

For a workshop with ten 120V circuits (lighting and tools) and three 240V circuits (heavy machinery):

1. Calculate each circuit's wattage.
2. Sum all wattages.
3. Add 25% for future expansion.
4. Select a subpanel rated slightly above this total, e.g., a 200A panel for a total load of ~40 kW. By following this method, the secondary distribution box will safely handle the load while allowing room for growth and minimizing the risk of overloads .