Load requirements for secondary distribution boxes
Secondary distribution boxes should be sized and rated according to the service amperage, voltage, and installation configuration, with typical ratings up to 1600 A for three-phase services.
Electrical Ratings and Load Considerations
Secondary distribution boxes are designed to handle three-phase services at voltages such as 120/208 V or 347/600 V, with maximum service ratings up to 1600 A on private property . The load on each box should be distributed to prevent overloading, and configurations may include simple radial, primary selective radial, secondary selective, or loop selective systems to ensure reliability and minimize outage duration . In secondary selective systems, approximately half of the total load is placed on each bus, allowing service continuity if one primary circuit fails .
Installation and Sizing Requirements
- Box Dimensions: For wall-mounted mid-run pull boxes inside buildings, the minimum size is 900 mm wide × 1800 mm high × 800 mm deep. Floor-mounted or outdoor mid-run pull boxes require a minimum of 632 mm, subject to utility approval .
- Wireway Requirements: Vertically staggered bus extensions require a minimum wireway width of 300 mm, while horizontally staggered bus extensions require 450 mm width and 450 mm depth for 1200 A and 1600 A services .
- Access and Clearance: Building parkade vehicle access should be at least 2100 mm to ensure safe installation and maintenance .
Material and Certification Standards
All materials from the stub-off on private property, including meter sockets, service panels, and switchgear, must be CSA certified. Equipment installed on private property is generally exempt from electrical inspection permits but must comply with utility standards .
Design Considerations
- Proximity to Load: Locating transformers and secondary switchboards close to load centers minimizes cable lengths, reducing voltage drop and improving efficiency .
- Fault Protection: Secondary fault currents and breaker applications are similar to radial unit substations, and proper breaker sizing is essential to protect the system .
- Service Continuity: Using bus tie breakers and selective configurations allows rapid restoration of service in case of primary circuit failure .
References to Standards
- BC Hydro ES54 S2-01: Provides detailed requirements for secondary three-phase services up to 1600 A, including box sizing, wireway dimensions, and installation practices .
- Eaton Secondary Unit Substation Design Guide: Offers guidance on substation configurations, load distribution, and fault management .
- IEEE 525-2007: Covers design, installation, and protection of wire and cable systems in substations, emphasizing reliability and minimizing cable failures . By following these load standards and installation guidelines, secondary distribution boxes can safely handle the intended electrical load while ensuring operational reliability and compliance with regulatory requirements.
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