Adding to Tesla's
Posted information above;
One of the most important reasons behind 240.4, and Table 450.3(B) restriction of the Primary OCPD's ability to properly protect the Secondary Conductors from Overcurrent, where the Transformer's OUTPUT
will be a "Dual Voltage" ( Slash-Rated Voltage, or simply L-N + L-L Circuitry), under Normal Designed Operating conditions, comes from the higher Load Amperes -vs- KVA per Phase on L-N output Circuitry.
75 KVA Transformer; 480V 3 Phase Delta Connected Primary; 208Y/120V 3 Phase 4 Wire Secondary:
Primary OCPD only (Primary Protection for Secondary Conductors)
Primary (input) FLA: 90.252 Amps (75,000VA / 831 = 90.252)
Primary Feeder Conductors Ampacity figured *1.25 FLA, which equates to 112.8 Amps, so we will use 3 #1 THHN Cu's, with a 110/3 MCCB.
(to keep things simple, I will keep the Primary maximum OCPD rating at 125%, as would be done with a single Voltage output Transformer, along with protecting the #1s at 110 Amps instead of 125 Amps).
Secondary (output) FLA: 208.34 Amps (75,000VA / 360 = 208.3333...)
Secondary Feeder Conductors Ampacity figured *1.25 FLA, which equates to 260 Amps, so we will Terminate at 250 Amps, using 4 #250 MCM THHN Cu's
Primary L-L Voltamps:
a: Nominal: 43,296VA (equals rated Amperes of 90.2A at 480V),
b: At 100% OCPD Trip rating: 52,800VA (at 110 Amperes)
*** Note: Primary Feeder could draw 100% of the OCPD's Trip rating (110 Amps), and continue drawing 100% until the Sun Supernovas, without Tripping from an Overload.***
Secondary L-L Voltamps:
a: Nominal: 43,296VA (equals 208.15 Amperes at 208V - which is the rated FLA),
b: At 100% Primary OCPD Trip rating: 52,800VA (110 Amperes L-L at Primary side).. this equals 253.8 Amps at 208V.HERE IS THE ISSUE!!!
Secondary L-N Voltamps:
a: Nominal Primary L-L Volt Amps: 43,296 VA (equates to 90.2 Amps at 480V 1 Phase 2 Wire),
b: Amperes for the same 43,296 VA per an L-N 120V Circuit: 360.8 Amps.
Conductor = 250 MCM THHN CU.
Can 'ya smell the smoke yet?
Even worse would be the Available KVA (52.8 KVA), since the L-L Primary Amperes may rise to 110 Amps for eternity!
Apply 52.8 KVA to a 120V L-N Circuit:
52,800 VA / 120V = 440 Amps!!!
Now the smoke can be smelled, seen, and is accompanied by Large Flames!!!
The above example is applicable to any SDS with a Dual Voltage Output.
208Y/120V 3 Phase 4 Wire Wye,
240/120V 3 Phase 4 Wire Delta,
480Y/277V 3 Phase 4 Wire Wye,
120/240V 1 Phase 3 Wire.
*** Note: These are Systems which are _DESIGNED_ for Loads to be connected either L-N, L-L, or L-L-L during normal operation.
These Systems have (2) different Voltages; One equals the L-L Voltage, One equals the L-N Voltage.
Systems which are _DESIGNED_ for Loads to be connected L-L, or L-L-L during normal operation have only (1) System Voltage.
Grounding of the System does not automatically make the System a Dual Voltage System.
So, to keep the smoke from leaking out, Secondary Conductor OCPD's are used to protect the Secondary Conductors of Dual Voltage rated Systems.
Smoke may leak out of Secondary Conductors, Secondary Windings, or both.
-- Scott (EE)