I have noticed that arc-fault circuit-interrupter (AFCI) circuit breakers tend to be warmer to the touch than regular circuit breakers. Now that the 2008 NEC requires most circuits in a dwelling to be protected by a combination-type AFCI circuit breaker, and there will be more AFCIs stacked into a panel, will the added heat affect the operation of the circuit breakers in the panelboard or create a hazard?
The short answer is no; the additional stacked AFCI circuit breakers do not produce enough heat to adversely affect the operation of the circuit breakers in the panelboard or to introduce a hazard.
In order to address possible heating concerns with the expanded use of AFCIs in dwelling unit panelboards as required by the 2008 NEC, UL conducted a research project. This project confirmed that the expanded use of AFCIs in panelboards does not result in temperatures in excess of those permitted in the Standard for Safety for Panelboards, UL 67, and the Standard for Safety for Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures, UL 489.
UL worked with AFCI manufacturers to conduct this testing by evaluating several different circuit breaker configurations as specified in UL 67, as well as a “typical residential” loading configuration.
In order to set up a more severe scenario, the temperature tests out of UL 67 included populating a 200-A, 42-circuit main lug only panelboard, fully loaded to 200 A per phase, with all AFCI circuit breakers (with the exception of the larger ampere breakers, which are not available with AFCI protection). Normally, AFCI breakers would represent half or fewer of the branch-circuit breakers.
The “typical residential” configuration consisted of a 200-A main breaker panelboard populated with circuit breakers to represent a configuration likely to be found in a typical residential panelboard and loaded to 80 percent of the current rating (corresponding to the maximum continuous load permitted by the NEC ). During each of these tests, the UL 67 and UL 489 temperature limits were not exceeded.
Since the “typical” loading resulted in lower temperatures than the UL 67 testing, these results suggest that code-compliant installations of additional AFCI circuit breakers in panelboards will not cause a safety concern due to their thermal characteristics.