A commercial office space has a 120/208V, 3-phase, 4-wire panelboard. The panelboard has a main circuit breaker rated for 225A. The calculated demand load for the panelboard is 180A. What is the minimum required rating for the panelboard busbars?
- A175A
- B225A
- C150A
- D200A
Show answer & explanationAnswer & explanation
Correct answer: B. 225A
According to NEC 408.30, the ampere rating of a panelboard must not be less than the calculated load. In addition, the rating of a panelboard (which refers to its busbar rating) generally matches or exceeds the rating of the overcurrent device protecting it. Since the main circuit breaker is rated for 225A, the panelboard busbars must also be rated for a minimum of 225A to safely carry the current that the breaker allows to pass. While the calculated demand load is 180A, the panelboard's rating is dictated by the upstream protection.
Why the other options are wrong
- A. 175A is too low; it would be overloaded by a 225A main breaker.
- C. 150A is too low; it would be overloaded by a 225A main breaker.
- D. 200A is too low; it would be overloaded by a 225A main breaker.
Panelboard Rating
The ampere rating of a panelboard, referring to its busbars, must be not less than the calculated load and must be protected by an overcurrent device with a rating no greater than the panelboard's rating, as specified in NEC 408.30.
- Panelboard rating must be ≥ calculated load.
- Panelboard rating must be ≥ rating of its main OCPD.
- Ensures safe operation and prevents overload of busbars.
- Standard ratings align with standard OCPD ratings.
Memory trick: Panel's Power: Match the breaker, exceed the load, keep it safe and sound.