What defines a sub-panel?

Question

Options

• AAn enclosure consisting of buss-bars and circuit breakers located at the service entrance to a building
• BAn enclosure consisting of buss-bars and circuit breakers with the fundamental purpose of distributing power to receptacles and the load
• CAn enclosure consisting of buss-bars and circuit breakers designed to accept two sources of power and operate as a best source selector
• DAn enclosure consisting of buss-bars and draw-out circuit breakers with the fundamental purpose of distributing power to branch distribution panels

• A
  4% (1)
• B
  81% (22)
• C
  11% (3)
• D
  4% (1)

Explanation

Option B is correct because a sub-panel is, by definition, a downstream distribution point that takes power from the main panel and delivers it to branch circuits serving receptacles and other loads - it is the last panel before power reaches end-use devices. Option A describes the main service panel, which is located at the service entrance where utility power first enters the building, not a sub-panel. Option C describes an automatic transfer switch or source-selector panel, which is a specialized device for choosing between two power inputs (such as utility and generator), a function unrelated to sub-panels. Option D describes a main distribution panel or switchboard - a higher-tier device that feeds other panels rather than directly serving loads, and the mention of draw-out breakers points to large commercial switchgear, not a sub-panel.

Memory tip: The prefix "sub" means beneath or subordinate. A sub-panel is subordinate to the main panel, sitting one level below it in the distribution hierarchy and delivering power directly to the final loads like receptacles - making "distributing power to receptacles and the load" the defining phrase.

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