Circuit total limitation
present-day standards for electrical panels
Circuit total limitation (CTL) is a legacy standard for electrical panels sold in the United States according to the National Electrical Code. This standard requires an electrical panel to provide a physical mechanism to prevent installing more circuit breakers than it was designed for. This has generally been implemented by restricting the use of tandem (duplex) breakers to replace standard single pole breakers. This was seen as preventing wire overload in smaller panels, but is not a method to prevent electrical overload (any size breaker can still go in a CTL limited panel).
Code Requirement
The 1965 edition of the NEC, article 384-15 was the first reference to the circuit total limitation of panelboards. As of 2008, the location of this language is at Article 408.54 now titled "Maximum Number of Overcurrent Devices.". However, this rule never prevented panel overloads, as any ampacity breaker can be used in any slot. The rule was repealed in 2008.
Non-CTL for replacement only
Circuitboards and panelboards built prior to 1965 did not have circuit total limiting devices or features built-in. To support these old panels, non-CTL circuit breakers that bypass the rejection feature are still sold "for replacement use only." As a result, numerous unsafe situations have resulted where panels were dangerously overloaded because these non-CTL breakers continue to be used.
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