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#130624 - 08/29/06 07:23 AM feeder protection  
carranoelectric  Offline
Junior Member
Joined: Aug 2006
Posts: 4
stratford ct
Is the secondary feeder from a 75 kva xfrm protected from the primary breaker? I have a 100 amp 480 v breaker on the primary with a 200 amp 208 v secodary to a panel. Do I need a 200 amp main or can I use main lug and have the 100 amp breaker protect the 200 amp feeder?

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#130625 - 08/30/06 04:27 PM Re: feeder protection  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA

Welcome to ECN!!!

1st off, does the Secondary of the Transformer have "Dual Voltages"?
i.e. is it a 208Y/120V 3Ø 4 Wire Transformer, or is it simply a 208V 3Ø 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 3Ø 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 3Ø 3 WIRE,
SEC = 208Y/120 VOLTS 3Ø 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%, 150°C Rise),
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!

Good luck

edited to correct typo' on a voltage listed! would have really made some heads spin! [Linked Image]

[This message has been edited by Scott35 (edited 08-30-2006).]

Scott " 35 " Thompson
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