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Joined: Jul 2002
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SJT Offline OP
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Tom, I am a lefty, but I throw with my Right. No wonder I'm all mixed up. I have to check the size of those Primary fuses, to make sure it's under that 125%. Am I correct in saying, there could be up to 6 disconnects on the secondary of a transformer, could be more than six? Thanks

Joined: Feb 2003
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Originally Posted by SJT
Tom, I am a lefty, but I throw with my Right. No wonder I'm all mixed up. I have to check the size of those Primary fuses, to make sure it's under that 125%. Am I correct in saying, there could be up to 6 disconnects on the secondary of a transformer, could be more than six? Thanks


Well ., I am both lefty and righty anyway.,

If you are right if under 6 disconnects you do not need a master OCPD at all but once you get over that number then oui you will need it anyway.

Merci,Marc


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

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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:

  1. 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.
  2. Each 100 Amp Panelboard is fed via Conductors tapped off the Secondary "Main" Feeder mentioned in #1 above.
  3. 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.

-------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------

SIDE NOTE

Keep 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.

-------------------------------------------------------------------------------------------------

*** Forum Members ***
Please review this post, and comment where applicable.

Scott


Scott " 35 " Thompson
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Bump

I have been compiling drawings, and should have them posted soon!

Stay tuned for further developments! grin

Scott


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
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JBD Offline
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Originally Posted by Scott35
SJT;
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").
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".
Scott


Scott,

I think your above statements might be misleading.

The connection at the transformer defines the system voltages. The system voltages define whether the system is 'single voltage' or 'multi-voltage.

If the neutral point (i.e. X0 terminal) is solidly grounded it is multi-voltage, regardless if the panelboard does or does not contain the grounded conductor.

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JBD;

Quote


The connection at the transformer defines the system voltages. The system voltages define whether the system is 'single voltage' or 'multi-voltage.



While this is true for measured L-L and L-N / L-G Voltages, in the case where only the Line "A", "B", and "C" Conductors are brought from the Transformer to the first Panelboard (not including the System Conductor typically referred to as the "Neutral"), the SDS would be a Single Voltage System.

Reason: Only one Voltage is being Utilized for Normal Circuitry and operation of Loads.
This defines a System as "Utilizing" a Single Voltage.

In the case of a Transformer with Wye connected Secondary Coils, the Star Point would be Grounded, so as to offer the lowest possible Voltage-To-Ground on that System; however the only Conductor which would extend beyond the Star Point (X0) would be an Equipment Grounding Conductor (EGC).

As an EGC is not an Active Circuit Conductor, it would not be considered a System defining Conductor (i.e.: 3 Phase 4 Wire would have 3 Ungrounded Conductors + 1 Grounded Conductor as "Active" Circuit Conductors)


Quote


If the neutral point (i.e. X0 terminal) is solidly grounded it is multi-voltage, regardless if the panelboard does or does not contain the grounded conductor.



Not necessarily... this just means the System is Solidly Grounded, as opposed to an Ungrounded System.
The reference to whether or not a System has one or two Voltages comes from the Active Circuit Conductors that Loads may be connected across.

Let's use Three separate examples:

Example #1: 1 Phase Isolation Transformer; Split-Coil Windings on Secondary, Single Coil Primary...

120V Secondary Windings setup in Series, with Jumper across X2 & X3, so as to create 240V between X1 and X4.

If we tap that jumper between X2 & X3, bond it to a Grounding Electrode System (GES), plus utilize the tapped Conductor for Active Circuitry, we have created a Grounded Neutral Circuit Conductor, and the System is 120/240V 1PH 3 Wire.

If we tap the jumper, but _ONLY_ bond it to a GES, then we have a 240V 1 PH 2 Wire Grounded System, with maximum of 120V to Ground.

Even though the L-G Potential is 120V from either of the two Ungrounded Conductors, the System is still a Single Voltage System - as the only loads connected to it will be for 240V, not 120V &/or 240V.

----

Example #2: Center Tap Grounded 3 Wire Delta...

Similar to the example above, to achieve a Grounded Delta System without having to "Corner Ground" it, the "X4 Center Tap" of the winding between Phase "A" and Phase "C" would be bonded to the GES _ONLY_, without a System Grounded Conductor brought to the Panelboard from X4.
Only an EGC would be brought to the Panelboard.

In this case, the max Voltage to Ground would be 200V (+/-), and the nominal Voltage to Ground would be 120V.

Lines "A", "B" and "C" would be the Active System Conductors, and the System would be designated 240V 3 Phase 3 Wire.


Example #3: Corner Grounded Delta...

Line "B" on a # Phase 3 Wire Corner Grounded Delta is an Active Circuit Conductor, which is bonded to the GES, identified by White tape, and is that System's Grounded Circuit Conductor.

Voltage between "B" and Ground (EGC) is close to zero, and raises as the distance from transformer increases.
Voltage between Lines A-B, &/or Lines B-C equal the same as measured between Lines A-C.
Also, Voltage to Ground from Line A or Line C will equal the same voltage as measured between A-B, B-C or A-C.

This is, of course, a Single Voltage System.

What defines it the most is the _Single Voltage_ Measured Between Lines A-B, B-C and A-C, and that there is no other Active Circuit Conductor being utilized which a different voltage could be measured (either higher or lower than the L-L Voltage), and which a Load would be connected across "in a correct or proper fashion".
(meaning the Load is not connected to one Line Conductor, and directly to an EGC).

-------------------------------------

To sum things up;
In the case of defining a System as Single Voltage -vs- Two Voltages, the Conductors utilized for Normal Operations Of Designed or Connected Loads determines the scope.

Hope I did not come off sounding arrogant, I just wanted to try to clearly explain the logic behind it, and where Articles 240 + 450 would designate the SDS of having either a Single or Dual Voltage.

All comments are welcome.

Please critique wherever necessary.

Scott.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
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JBD Offline
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A 208Y/120 secondary is multi-voltage regardless if the load is all three wire.

The tap rules for multi-voltage vs single-voltage transformers are about the transformer connections not the load. The transformer connections dictate how fault current on the secondary will be reflected back into the primary.

A solidly grounded wye secondary can have a line to ground fault that is not reflected back to the delta primary simply by the turns ratio of transformer.

Do the math. Given a single L-G fault of 2.0PU on a wye secondary, what is the corresponding Line current on the delta primary. Will a primary side protective device clear this L-G fault in the required time frame (assume a NEMA AB-1 characteristic)?

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JBD,

Please review NEC Articles 240.4(F) and Section 450.
This will explain the reason to define a System's Load characteristics per "Single" or "Dual" Voltage SDS.

It has to do with Overloading the Secondary side Feeders, when only Primary side Over Current Protection is provided.
It has nothing to do with Ground Faults on the Secondary side.

If an SDS with Dual Voltages has only Primary side OCPD, there is a high possibility to Overload the Secondary Feeders with L-N Load Current, without the Primary Feeders' OCPD tripping.

Example:

45 KVA 480V x 208Y/120V 3P 4W Wye Transformer.

Primary Full-Load Amperes (FLA) = 54.1 Amps
Secondary FLA = 125.0 Amps
Primary OCPD only - max. rating = 125% Primary FLA.
(54.1 A * 1.25 = 67.6 A)
Primary Feeder OCPD = 60/3
Primary Feeders: 2#6 THHN cu.
Secondary Feeders: 4#1/0 THHN cu.

Load on Secondary Feeders between Line A and Grounded Neutral Conductor exceeds 125 Amps (15 KVA), goes up to 200 Amps (24 KVA)

Primary Feeder OCPD sees only 50 Amps, while 200 Amps flows on the Secondary side's Feeder.
Saturation will not affect output voltage at this point.

Even with Primary OCPD at 100% (54 Amps), there will only be 50 Amps flowing.

As you can see, there is a possibility to overload Secondary Conductors, on a Dual Voltage SDS having Primary Over Current Protection only.

If the SDS had only One Voltage output, a 208V L-L Secondary Load of 26 KVA (125 Amps L-L) would be reflected back to the Primary, and an L-L Primary Current of 54.2 Amps will be flowing through the Primary side OCPD.

If the Secondary L-L Load increases to 31.2 KVA (150 Amps), the Primary L-L Load will increase to 65 Amps, which will trip the OCPD - likely within 30 Minutes.

Quote

A solidly grounded wye secondary can have a line to ground fault that is not reflected back to the delta primary simply by the turns ratio of transformer.


480x208Y/120V Transformer.

Winding ratio = 4:1

L-G Short Circuit Amperes (SCA) on Secondary = 100% value of the Secondary L-L-L Bolted Fault Value.

L-G Secondary Fault SCA reflected to Primary Windings = 0.25 of the L-L-L Secondary SCA.

L-L-L SCA = 12000 Amps
L-G Fault = 12000 Amps x 1.0 = 12000 Amps
Fault Current reflected back to Primary = 3000 Amps (12000 * 0.25 = 3000)


Quote

Do the math. Given a single L-G fault of 2.0PU on a wye secondary, what is the corresponding Line current on the delta primary. Will a primary side protective device clear this L-G fault in the required time frame (assume a NEMA AB-1 characteristic)?


Using the example I provided (3000 Amps on Primary side), the OCPD should trip within 0.2 Seconds.

Scott


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
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JBD Offline
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240.4(F) clearly says it applies to a delta-delta transformer. It does not allow the primary device to protect the secondary conductors of a delta-wye transformer, regardless if the load is 3-wire only.

As you agree, a L-G fault of only 2PU (using your numbers 2*125A = 250A) on secondary yields only 1.15PU (62.5A) on the primary is not enough to cause the primary device to operate in a timely enough fashion to protect the secondary.

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SJT ;

I have uploaded some drawings to the Technical Reference Section, which cover Transformers and Feeders + OCPDs.

Please refer to the following linked page, for a few examples per your scenario:

Transformers & Feeders: 3 Phase 4 Wire Wye

*** NOTE ***

Click on the underlined text above, to open that linked page.

BTW, Additional setups - such as Deltas, 3 Phase 3 Wire Single Voltage Systems, and 1 Phase Systems, are available for viewing at the Technical Reference Section.

Refer to the Technical Reference Section Main Page
for additional linked pages.

Scott


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
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