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1st off, does the Secondary of the Transformer have "Dual Voltages"?
i.e. is it a 208Y/120V 3Ã˜ 4 Wire Transformer, or is it simply a 208V 3Ã˜ 3 Wire Transformer.
If the Transformer's Secondary is only one _USABLE_ Output Voltage (only a single Voltage is usable for driving loads on the Secondary side), then the OCPD (Over Current Protection Device) on the Primary side - for the Primary Feeders, may be used to protect the Secondary Feeders.
This would mean the Transformer is rated as:PRI = 480 VOLTS 3Ã˜ 3 WIRE,
SEC = 208 VOLTS 3Ã˜ 3 WIRE
If the Transformer's Secondary has _TWO USABLE_ Output Voltages for driving loads, then the Secondary Feeders _MAY NOT_ be protected by the Primary Feeder OCPD, so the Secondary Feeders will require their own OCPD - which may be at the first Panelboard (within certain limits of length).
An example of a Transformer with _TWO USABLE_ Secondary/Output Voltages is:PRI = 480 VOLTS 3Ã˜ 3 WIRE,
SEC = 208Y/120 VOLTS 3Ã˜ 4 WIRE
Other examples are:
PRI = 480 VOLTS 3PH. 3 WIRE,
SEC = 240/120 VOLTS, 3PH. 4 WIRE DELTA
PRI = 480 VOLTS 1PH. 2 WIRE,
SEC = 120/240 VOLTS, 1PH. 3 WIRE
If your Transformer has a "Dual Voltage" Secondary - typical of a 208Y/120V 3PH 4 WIRE Wye Secondary, you need to include a Secondary Feeder OCPD somewhere. It may be a Fusible Disconnect (like an EXO / Safety Switch) placed at the Transformer, or it may be in the form of a Main Breaker in the first Panelboard fed from the Transformer.
A typical installation example - where the Transformer is within 25 feet of the Panelboard it feeds:
75 KVA Dry-Type Transformer (Z = 4.25%, 150Â°C Rise),
PRI = 480V 3 PHASE 3 WIRE (DELTA)
SEC = 208Y/120V 3 PHASE 4 WIRE (Wye)
Primary fed from 480V Gear, located either at the Transformer, or across the Building from the Transformer (in this case, the 480V Panelboard will be 10 feet away from the Transformer).
Primary Feeder = 3 # 2 THHN cu., + 1 # 6 cu. EGC (Equipment Grounding Conductor) - 1-1/4" EMT.
Primary Feeder OCPD = 100 Amp, 3 Pole Common Trip Circuit Breaker.
Transformer fed with Primary Feeders, Terminated to lugs at positions "H1", "H2" and "H3".
EGC with Primary Feeder bonded to Transformer's Enclosure via suitable lug.
Secondary Feeders' UNGROUNDED CONDUCTORS derived from Termiantions to lugs at positions "X1", "X2" and "X3".
Secondary Feeder's GROUNDED CONDUCTOR derived from Termination to lug at position "X0".
(Secondary Feeders = 4 # 3/0 THHN cu.)
Grounding Electrode Conductor (1 # 4 cu.), Terminated to lug at position "X0".
Secondary side's MAIN BONDING JUMPER (1 # 4 cu.) derived from Termination to lug at position "X0", and landed to lug bonded to Transformer's Enclosure.
Secondary Feeder's EGC = 1 # 6 cu., terminated to Transformer's Enclosure on same lug strip as the Maim Bonding Jumper's attachment to the Transformer's Enclosure.
Secondary Feeders (4 # 3/0 THHN cu. + 1 # 6 cu. - 2" EMT), run apx. 15 feet to Panelboard "LP1"
Panelboard "LP1" = 208Y/120V 3 PH. 4 WIRE; 42 Space; BUS = 225 AMP MAX. - 240V MAX., NEUTRAL VOLTS = 120V, SCA = 22KAIC MAX.; MAIN = 200/3.
Secondary Feeders for _UNGROUNDED CONDUCTORS_ terminate into Line side lugs of the 200/3 Main Breaker - thus providing Secondary Feeder Over current protection.
The Secondary Feeder's _GROUNDED CONDUCTOR_ terminates to an _ISOLATED_ Neutral Bus kit (Isolated from the enclosure).
The Secondary Feeder's _EGC_ terminates to either:
* A Lug, which is bonded to the Panelboard's metallic enclosure (the "Can"),
* A Ground Bus Bar, which is bonded to the Panelboard's enclosure.
I think this will be helpful for your scenario. If it does not cover what you need to know, feel free to reply.
If this does help, let us know too!
edited to correct typo' on a voltage listed! would have really made some heads spin!
[This message has been edited by Scott35 (edited 08-30-2006).]