I have seen some circuit breakers marked "Replacement Use Only - Not for CTL assemblies" What is a CTL system? What do the letters "CTL" represent? What is the danger of using "Replacement Use Only - Not for CTL assemblies" in a CTL assembly?
The history of CTL is somewhat clouded. You will hear many comments about what CTL means (but most are wrong). If memory serves, CTL means "Circuit Total Limiting". "Circuit Total Limiting" (CTL) was introduced when the words "A lighting and appliance branch-circuit panelboard shall be provided with physical means to prevent the installation of more overcurrent devices than that number for which the panelboard was designed, rated, and approved" was added to Section 384-15 in the 1965 NEC.
Here is a specific example:
Lighting and Appliance Panelboards (load centers) made since 1967 have been marked "Class CTL Panelboard". Some of these CTL panelboards (load centers) will accept a breaker that is other than full size (called tandems, half-size, wafers, etc.). These breakers have varying means to limit the number of them that can be installed in the load center. One manufacturer has a hooked foot on the tail end of the breaker that must mate up with a slot in the load center-mounting rail. Others use a rejection feature in the breaker that must mount up with a "fork" in the busbar to be installed. There are other arrangements as well.
When you hear somebody talk about a 30-40 load center (panelboard). It has 30 spaces for full size breakers and 10 of those spaces have slots to that allow the installation of the "other than full size" breakers. This 30-40 panelboard can contain 30 full size breakers and no tandem breakers, or it can contain 20 full size breakers with 10 "other than full size" breakers for a total of 40 circuits.
What are the dangers on improperly using "Replacement Use Only - Not for CTL assemblies" in a CTL assembly?
The bottom line is... use the breakers that are intended to be installed in the panelboard (load center). Your job will go much smoother.
Thanks to Jim Pauley of Square D for the short history of CTL circuit breakers.
From Mike Holtís Free Electrical Newsletter
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