SJT;If an Over Current Protection Device (OCPD) exists on the Secondary side (used for protecting the Secondary Conductors), then the Primary Fuses may be sized up to 250% of the rated Primary Current.
With a 45 KVA 480V x 208Y/120V 3 Phase 4 Wire Transformer, the rated Primary Amperage is 54.1..., making the maximum OCPD on the Primary side 125 Amps
(54.1 Amperes x 2.5 = 135.25 Amperes)
You may find 125 Amp Fuses in that Switch for the Primary Feeders. This would be compliant provided the Secondary Conductors have Over Current Protection.
Panelboard mounted main Circuit Breakers - one at each Panelboard, would be compliant per the use of a Primary Side OCPD rated 250% of the Transformer's rated Primary Current; and will also be compliant per a Secondary with either a "Single Voltage" (i.e.: 240V, 208V, etc.), or a Secondary with "Two Voltages" (i.e.: 208Y/120V).
If a Wye connected Secondary is used without bringing a System Grounded Conductor (from "X0" Terminal) to the Panelboards, this will be considered a Single Voltage Separately Derived System ("SDS").
Some details per your scenario:- It appears that the Two Panelboards fed from the Secondary of this 45 KVA Transformer are fed via a tapped "Main" Feeder, and this Feeder is derived from the X0, X1, X2 & X3 Transformer Terminals.
- Each 100 Amp Panelboard is fed via Conductors tapped off the Secondary "Main" Feeder mentioned in #1 above.
- Each Panelboard contains a 100/3 Main breaker
If this is correct, then the following would be compliant:
- With Primary Protection rated higher than 125%, but not more than 250% of the rated Primary Full Load Amperes (FLA), the Secondary "Main" Feeder size would be 3/0 THHN CU, with #2 THHN CU Conductors tapped from the "Main" Feeder, Terminating to the Line side of each Panelboard's Main Breaker.
- With Primary Protection rated higher than 125%, but not more than 250% of the rated Primary FLA, each Panelboard's Feeders - being sized #1 THHN CU, could Terminate to the Secondary Lugs of the Transformer.
- With Primary Protection rated no higher than 125% of the rated Primary FLA, the Secondary "Main" Feeder may be size 1/0 THHN CU, with #2 THHN Cu Conductors tapped from the "Main" Feeder, terminating to each Panelboard's Main Breaker.
- With Primary Protection rated no higher than 125% of the rated Primary FLA, each Panelboard's Feeders - being sized #1 THHN CU, could Terminate to the Secondary Lugs of the Transformer.
If the SDS (Panels fed from the Secondary side of the 45 KVA Transformer)
DO NOT utilize a Grounded Conductor derived from Terminal "X0", this will be a "Single Voltage System".
Primary side Protection not exceeding 125% rated Primary FLA would allow each Panelboard to use #1/0 Conductors without the need of Secondary Feeder (Conductor) Protection.
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SIDE NOTEKeep in mind that the OCPDs are used to protect the CONDUCTORS, Terminated to the Primary and Secondary sides of the Transformer; not to protect the Transformer Windings themselves!
The Transformer Windings may withstand Current Levels up to 150% the rated FLA, prior to a Winding failure.
Even at 1.5 x FLA, the core is saturated, and output Voltage will suffer; with a corresponding reduction in output Current.
The overall KVA will be reduced, and the drawn KW within the input KVA "Package" will also be reduced as result of the lower Voltage on the Secondary side.
Bolted Faults on the Secondary side are a different matter, as opposed to a correctly operating intentional Load.
I will post some Drawings regarding this Thread ASAP.
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Scott
