What is the principle behind "circuit total limitation" (CTL) for electrical panels?

Question

Contemporary electrical panels/loadcenters limit the number of tandem circuit breakers that can be used. Is this because areas of the underlying panel bus bars can overheat despite all of the individual breakers (including the main) operating below their rated trip current? Or is it because of the risk of a neutral slot being used by more than one circuit? Both? Something else?

Answer 1

CTL was repealed in 2008

It was originally added in 1965, but was repealed 43 years later in 2008. It required that any single panelboard could not have more than 42 "poles/phases" worth of circuits (counting 120/240V breakers as 2 poles and 3-phase breakers as 3 poles).

I believe the original concern was panel amp loading, however, that is addressed by panel load calculations.

Other than that, the only benefit I can think of to CTL was available neutral bar spots. Code requires only 1 neutral wire per spot on the neutral bar, so for instance a 42 space panel might only have 56 neutral spots (assuming UL approves that panel for 3 ground wires per spot). Populating that to 84 circuits, possible with tandems, would create a problem landing all those neutrals. However, in practice, those grounds can be moved to accessory ground bars... and a good number of circuits are 240V (no neutral at all), 120/240V or MWBC (2 poles 1 neutral). So it tends to work out.

CTL was implemented as notches on the bus stab (usually)

This drawing is typical of all manufacturers except Square-D and GE. The notch in the bus stab matched up to a blocking bar on the breaker, allowing CTL tandems to fit only in spaces with notched stabs. For instance Eaton made the BRD2020 as a "pre-CTL" 20/20 tandem, and a BD2020 as a "CTL" 20/20 tandem with the blocking feature.

Nothing really prevented you from putting a pre-CTL BRD2020 into a forbidden breaker space, except that Eaton charged more for non-CTL breakers. This made the whole exercise rather silly, since anyone could defeat CTL limits at will. Problem: Nothing bad ever happened.

When accident data proves accidents don't happen because of X, NFPA will repeal a rule forbidding X. This also happened with blown insulation around knob-and-tube wiring.

Manufacturers have reacted to CTL's repeal

Especially since the repeal is now old enough to have a driver's license LOL. During CTL, 3 manufacturers still had to support their "prior to 1965" pre-CTL panels, so they made "pre-CTL" tandem and quadplex breakers.

For instance Eaton, no great fan of spurious SKUs (part numbers), wanted very much to stop making a BRD2020 and (now irrelevant) BD2020. So they had UL re-list the BRD2020 as the BR2020, allowing it into all panels.

GE used a completely different method with THQP half-width "thin" breakers rather than tandems. These engaged to vertical "side stabs" which were only installed where CTL permitted them. Hence it was impossible to defeat CTL limits in GE - it was airtight. Interestingly, though, GE only now makes true "Tandem" and "Quadplex" breakers, and are the same breaker type as normal breakers - e.g. THQL2020. So these products exist specifically to defeat CTL limits.

Square D has done nothing on the HomeLine family. CTL limits still function there and there is no tandem option in a CTL-forbidden breaker space.

Answer 2

Circuit Total Limitations comes from a requirement way back in the 1960's where the NEC required panel board manufactures to limit the total number of circuit breakers a panel could contain.

The principle was probably safety and prevention of fire due to the advent of tandem breakers possibility of being overused beyond the panel boards listed capacity.

Answer 3

It is to limit the total number of circuits, and so the total load on the bus etc. It doesn't have anything to do with the neutrals.