When wiring a 45KVA transformer, the secondary conductors must terminate in a single disconnect, right? Or to a 100 Amp Main Breaker Panel. There is protection on the 480V primary side. I looked at one job, and they have a three phase 480V to 208V 45 KVA transformer. The secondary conductors go to a Wireway, and they spliced feeders to two- 100 amp panels. This would be a violation, right? Thanks

The conductors on the secondary of a transformer need to follow the 'tap' rules in 240.21(C).

Depending on the distances and cable sizes involved, it is possible that this is a valid installation.

If the protection on the primary is less than 125% of the transformer rating, per NEC Table 450.3(B), no secondary protection is required. If the primary OCP is larger than 125%, the secondary protection cannot be more than 125%, and two 100A breakers is in violation.

So... if that transformer is fed from a 60A breaker, I think you're OK. At least from a transformer protection standpoint; ampacity, etc, is a different issue.

Though the transformer may be protected by the primary side circuit breaker, all the conductors involved still need to be protected. It is not permissible to protect the secondary conductors of this installation with a primary side breaker only, see the last sentence of 240.21(C)(1)

A transformer can be loaded to 100% of its capacity but you;ll have to size the conductors and OCPD at 125% of the secondary full load current in order to take advantage of this. Note 2 to table 450.3(B) says you can have multiple circuit breakers on the secondary side, but they cannot total more than what a single breaker would be.

IMO, the two 100 amp panels described by SJT are not in compliance.

You are mis-reading 450.3.

A transformer is required to have a primary protective device rated at not more than 125% unlessthere is secondary protection. So, if the 45kVA primary side is protected at not more than 67A, all secondary protection may be ignored.

But, the secondary conductors need to be protected under 240.21(C). I can envision this particular installation as being acceptable under 240.21(C)2, 3, or 6.

45kVA 3-phase XFMRs are suitable to feed ONE 3-phase 100A panel...

OP did not mention whether or not the 100A twins were 3-phase or single phase...

It is also not clear whether these panels have internal OCPD -- that is a back-fed 100 dual-pole breaker...

OP does not provide enough information to determine Code compliance.

According to what NEC article?

Tom, I'm with JBD on this one. The main OCPD's in 2-100A panels fed by a single 45KVA XFMR aren't added together that way any more than you would count the 20A CB's in a panel to determine it's load. If the conductors to each are sized properly and the primary OCPD is less than 125% of the XFMR's rating, multiple panels could be fed depending on computed load. And I would also echo the comment to Tesla. It's the most common set-up but that doesn't make it the only code-compliant arrangement.

OK, what I'll check are the size of the Fuses in the Disconnect on the Primary side, and if they are within the 125% of the XFMR's rating, I should accept the 2 - 100AMP Panels then. Sounds like if there was an Overload, the Primary fuses would go, to protect the Transformer. There are other Electric utility rooms in the same Building, with similar situations, but the key would be those fuses in the Primary side. Code wise, but not practical, there could be 4- 100AMP Main Breaker Panels on the secondary, as long as the Primary fuses are correct? Good Weekend

You may NOT use only primary side protection for a 3-phase set-up.

Primary side only is permitted only by the exceptions which focus on buck-boost and single phase set-ups.

Further study will reveal that UNDER CERTAIN CONDITIONS the collapsing field of the secondary can generate staggering voltages which the OCPD is there to drain. Otherwise you get the exact same type of physics that causes your spark-plug to fire off at 35,000 Volts even though the automobile system voltage is only 12 to 14 VDC.

IF THE WIRE SIZE meets the tap rules you might be looking at a transformer set up for isolated power for Point Of Sale or the back-office. In such situations it may occur to the EC to use TWIN single phase panels working off of alternate windings of a WYE secondary to produce plenty of cheap 1-phase slots in an otherwise 3-phase installation.

FYI, when you thumb through Ugly's -- or any manufacturer's XFMR line you will note that a 40kVA or 50kVA is NOT ON OFFER.

The industry standard sizes are listed in Ugly's:
3
6
9 Code provision cut-offs at this size...
15
30
45 sized for 100A/125A situations ... special Code provision permits this popular size to hang above grid ceilings...

75 suitable for 200A/225 situations ... the largest commonly occurring for just that reason -- sometimes feeds mother-daughter panels when the isolated circuits are many but draw low power... this size is also a favorite with Poco's all over for small establishments...

100
150 More Code provision cut-offs inre wire-type/insulation
225
300
500
750 The largest my Poco will permit in a vault -- their vault that is... enough to supply a substantial hotel, etc... any loads bigger than this must use twined XFMRs.

So as to why 45kVA = 100A or 125A set-ups ... talk to the NEMA panel.

Tesla, OCP does not protect against the type of transient spikes you're talking about, because they happen far too quickly for a fuse or breaker to interrupt, and are over. You need a surge suppressor for this. Suffied to say, NEC does not require secondary OCP if primary is fed from an appropriately sized OCP. The 450.3(B) tables and secondary OCP are related more to bus taps and such.

2005 NEC 240(F)...

TRANSFORMER SECONDARY CONDUCTORS:

Single-phase (other than 2-wire) and multi-phase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary over-current device.....

This phrase has been in the Code for years -- for the reasons detailed above.

Tesla, 450.2 talks about any transformer, single or polyphase. 450.3(B) is table for Primary and secondary protection.

You are still stuck with OPCD on the Secondary under 450.3(B)...

Primary only protection is disallowed by 240(F) which means that you must use line 2 from the table: PRIMARY & SECONDARY PROTECTION.

I have in my day installed a ton of dry-type delta-wye transformers. Without exception the AHJ looked for the secondary OCPD --- RIGHT OFF THE BAT EVERY TIME.

It remained on their hot list of common electrician errors.

I can't speak for your AHJ, but out here they are on this like white on rice. You'll get red-tagged every time.

Wrong, it is not about 3-phase transformers, it is about transformers with multiple output voltages.

Yes, 240.4(F) says the secondary conductors need protection except for single voltage secondaries.
But it is 240.21(C)that says where the conductor over current protection must be located on the secondary of a transformer and what size that protection can be.

450.3(B) only says the transformer primary must be protected at not more than 125% except when a properly sized secondary protection is provided.

There are two issues here. The protection of the transformer as found in 450.3 and the protection of the secondary conductors as found in 240.21(C). These are two independent (in most cases) issues. If the primary protection of the transformer is 125% or less of the rated primary current, there is no code requirement to provide protection for the secondary of the transformer. The conductors connected to the secondary of the transformer will require protection based on the rules found in 240.21(C).

I must refer you to the NEC Handbook for elaboration.

2005 NEC Handbook page 162 explaining 240.4(F):

The fundamental requirement of 240.4 specifics that conductors are to be protected against over-current in accordance with their ampacity, and 240.21 requires that the protection be provided at the point the conductors receive their supply. Section 240.4(F) permits the secondary circuit conductors from a transformer to be protected by over-current devices in the primary circuit conductors of the transformer only in the following two special cases:

1. A transformer with a 2-wire primary and 2-wire secondary, provided the transformer primary is protected in accordance with 450.3

2. A 3-phase, delta-delta-connected transformer having a 3-wire, single-voltage secondary, provided its primary is protected in accordance with 450.3

Except for these two special cases, transformer secondary conductors must be protected by the use of over-current devices, because the primary over-current devices do not provide such protection....

-----

The basic math of 4:1 current multiplication of primary-to-primary vs line-to-neutral is a cooker.

Again and again I must proclaim: AHJ -- handbook in hand -- will be shooting down delta-wye transformers without OCP at the secondary.

In the 09 NEC, 240.21(C) talks about a set of conductors feeding a single load. The two 100Amp panels do not have the conductors going directly back to the transformer. If they did, It might be compliant. I'm gonna talk to the AHJ on this one. Thanks for your inputs.

And again I will say that the secondary of the transformer itself does not require protection if the primary protection is sized at 125% or less of the rated primary current. There is no question that protection of the secondary conductors is required, except in the two special cases you cited. My issue is only with the choice of words, as the end result is the same...there will be an OCPD on the secondary side of the transformer, but it is often only for the protection of the secondary conductors and not for the protection of the secondary winding. Article 450 only applies to the transformer itself. Article 240 only applies to the protection of the conductors and not to the protection of the transformer. Even your quote from the handbook states "transformer secondary conductors must be protected by the use of over-current devices".

I've re-read the original post and the OCPD size on the primary was not mentioned and I don't believe that SJT posted it in any of his follow ups.

That being the case, I'll stand by my answer if the primary side OCPD exceeds 60 amps. If the primary OCPD is 60 amps or less, I definitely agree that there can be as many OCPDS of any size on the secondary side as you can afford to buy, keeping in mind that all conductors must be protected.

I believe that Don's answer is correct, there will always be an OCPD on the secondary of a delta-wye connection and that it is there for the protection of the conductors , not the transformer, as long as the primary side OCPD does not exceed 125%.

In a left-handed way (no insult meant to you southpaws), Tesla is correct that an inspector will write a violation if there is no OCPD device on the secondary side of a delta-wye, but it isn't required to be there for the protection of the transformer.

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

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.

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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.

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

Scott

Bump

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

Stay tuned for further developments!

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.

JBD;



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)




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.

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)?

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.



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)




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

Scott

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.

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, Good drawings. Did you create those on Autocadd? Thanks

SJT,

Yes, the drawings posted here at ECN were compiled within an AutoCAD Environment (drawn with AutoCAD).

BTW:
In regards to...


Did you ever find out what size fuses were installed for Primary OCPD?

Let me know if there are any questions or comments.

Thanks for the reply.

Scott

I have not checked the Primary fuse size, but I will. The Bldg. is only 2 years old, and the original design should of had 3 circuits to the cubicle sections. there are 2 sections we will be running an additional circuit to each. Learned a few things about cubicle pre-fab modular wiring on this one. Think I'll open a Sam Adams cold one.