Somewhere on the NACHI site (can’t remember where right now) I came across the following comment or statement: “Some AFCI breakers have been known to overheat when stacked, causing premature tripping when outlet(s) are used.” Does anyone know any brand names or specifics?
BTW, I am starting to see some AFCI’s being separated in the panels.
Thanks.
If you ask any manufacturer, they say it’s not a problem. Some electricians have experienced this to be a problem (not life-safety, but a nuisance). I have not personally had this problem nor have I observed it to be a problem. That said, it might be an issue more in panels that are mounted in higher ambient operation environments, such as outdoor panels in the southwest or FL.
I have Sqaure-D Homelines, stacked(soon to be more if Marc can get me CAFCI’s cheap 
), and no problems since installed.
tom
I’m not familiar with NEC regs on AFCI stacking in a SE panel, but I do know that AFCIs (due to their intricate circuitry), generate more heat than a standard breaker. When these are placed together, I’ve seen them register 130 degrees on my laser thermometer. (Granted this was a hot day, but the center breakers were 30 degrees hotter than the regular breakers on either side)
There is a very simple but necessary fix. The AFCIs are interspersed with standard breakers in between. This keeps the temperature down to a more reasonable 10 or so degrees warmer than the standard breakers.
Since breakers are calibrated at 40c (104f) I thought I might get some traction with this in the comment phase of the 2008 Ref 210.12 but NFPA basically said any concern about this was unfounded.
Not sure what you mean by “calibrated” at 40° C
They are tested at that ambient temp for insulation resistance to moisture and maximum operating temp of parts.
That is the ambient temperature that they use on the trip curve charts. Presumably the thermal element will operate differently if it is hotter. I was really hoping NFPA would expand on that a bit in the NOC but they basically just blew off the question.
Gotcha, thanks
So… How hot, is too hot? 150, 175, 200, ? Where should we draw the line, and call these (or any other breaker for that matter) out?
You might be crossing some sort of line that ought not be crossed if you’re asking that question. Your answer, however, is in the NETA testing specs.
I just looked in the U/L marking guide to see what they say the 40c means.
“A circuit breaker that is acceptable for use in ambients up to 40c is marked 40c”.
Stacking is not a problem ( That I am aware of )…have had stacked AFCI’s and GFCI’s together for nearly 3 years now ( in excess of 10 stacked ) and no problems.