Rating Plug
A rating plug is a removable, calibrated device installed in certain molded-case or insulated-case circuit breakers to establish or limit the breaker’s ampere rating and long-time trip setting. It allows the breaker’s practical current rating to be adjusted without changing the breaker frame, ensuring the effective rating matches the protected load and conductor ampacity.
On a manufacturing shop floor, rating plugs are used in main distribution switchboards and feeder breakers to match breaker ratings to conductor ampacity and load requirements. For example, a 400 A frame may use a 250 A rating plug to protect a feeder cable sized for 250 A. They enable selective coordination by tuning long-time trip settings, reducing nuisance trips. When facility loads change, qualified electricians can replace the rating plug within the frame’s range, provided feeder conductors and bus are verified. This practice supports NEC/CEC compliance and WorkSafeBC requirements for equipment use within rated capacity, ensuring worker safety and regulatory adherence.
Does the breaker’s ampere rating come from the frame nameplate or the rating plug value?
For rating-plug-equipped breakers, the effective ampere rating is the rating plug value, not the frame maximum. The frame defines the maximum available rating range; the rating plug defines the actual installed rating.
How does the rating plug interact with NEC/CEC and WorkSafeBC requirements?
NEC/CEC requires conductors to be sized at or above the overcurrent device rating, which is defined by the rating plug. WorkSafeBC requires equipment not exceed marked rated capacity; the breaker’s effective rating set by the plug is the limiting capacity for that circuit.
How should rating plugs be handled in SafeDesk electrical compliance audits?
Audits should verify the installed rating plug value, confirm conductor ampacity and bus ratings are ≥ rating-plug rating, check that panel schedules and labels reflect the current setting, and ensure any prior changes have documentation and engineering approval.