Slash Rated Breakers

Originally Posted by Jerry Peck

while we are on breakers and their ratings, I doubt many electricians or inspectors look for this, and it is beyond the scope for home inspectors (and would typically only be found in the condo building I described which is supplied by a 3-phase service yet each stack of condo units is only supplied with 120 volt/208 volt.

From the UL breaker marking guide.
"Based on the preceding paragraphs, “slant-rated” breakers (120/240, 208Y/120 V, etc., as opposed to 240, 480 V, etc.) are not intended for use on “slant-rated” delta systems. For example, a 3-pole, 120/240 V breaker is not intended for use on a 240/120 V, 3-phase, 4-wire, delta system, because on the high leg, the voltage to neutral is 208 V. In this instance, a 3-pole, 240 V breaker should be used."

I wonder how many 120 volt/240 volt rated breakers are installed on 120 volt/208 volt systems? Should not be any.
To start let's call them slash rated since that is more common and what the NEC uses.

The common 2P breaker used in residential is 120/240V. The lower rating, 120V, is the max voltage L-N or L-G. The higher rating, 240V, is max voltage L-L.

So these 2P 120/240V slash rated breakers are fine for 120/240, 120/208, and 208Y120 systems.

A 3P 120/240V breaker can be used on 208Y/120 also. For both 120/208V and 208Y/120V systems the L-L voltage, 208V, is less the the max rating, 240V, of the slash rated breakers.

The problem arises when installing 120/208V slash rated breakers in hi leg deltas and corner grounded deltas.

I will start with a high leg delta, commonly written as 240/120 or 240D/120.

First a pic:
While you get 240V L-L between any two phases and 120V L-N(G) using A or C phase as L, you get 208V L-N(G) using B phase as L.

208V L-N exceeds the lower value of a 120/240 slash rated breaker.

You use 240V 2P and 3P straight rated breakers for this system. Technically you could use a 2P slash rated breaker if it was on A and C only.

No SP breakers are installed on B phase, only A and C.

Corner grounded deltas are all 240V loads using 2P and 3P breakers. There is no 120V in this system, so all breakers are 240V straight rated. As shown below the 2Ps are special for three apps for this system.

The NEC covers this in 240.85.

240.85 Applications. A circuit breaker with a straight voltage
rating, such as 240V or 480V, shall be permitted to be
applied in a circuit in which the nominal voltage between
any two conductors does not exceed the circuit breaker’s
voltage rating. A two-pole circuit breaker shall not be used for
protecting a 3-phase, corner-grounded delta circuit unless the
circuit breaker is marked 1φ–3φ to indicate such suitability.

A circuit breaker with a slash rating, such as 120/240V
or 480Y/277V, shall be permitted to be applied in a solidly
grounded circuit where the nominal voltage of any conductor
to ground does not exceed the lower of the two values
of the circuit breaker’s voltage rating and the nominal voltage
between any two conductors does not exceed the higher
value of the circuit breaker’s voltage rating.