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#206368 06/20/12 05:28 PM
Joined: May 2007
Posts: 55
Up2code Offline OP
Both section 240.4(F) & 240.21(C)(1) clearly state "multiphase (other than delta-delta 3W) transformer secondary conductors are NOT considered protected by the primary OCPD." I have been looking over some past approved electrical plans and none of the 480V, 3 phase, 3 wire, delta primary to 120/208V, 3 phase, 4 wire, Y secondary's have a seperate OCPD for the secondary conductors. Instead, the secondary conductors are approved & installed as tap conductors, protected only by the primary OCPD. Am I missing something or reading this wrong?

Last edited by Up2code; 06/20/12 05:30 PM.
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Joined: Jul 2004
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Likes: 17
I am curious why those installations were approved myself. Are the "taps" protected at their ampacity on the load end?

Greg Fretwell
Joined: May 2007
Posts: 55
Up2code Offline OP

So far what I've looked over, Yes, the secondary conductors' ampacity does match up with the OCPD they terminate into. So that should not be an issue at this point.

In NEC 2008, where does it actually state WHERE the secondary conductors have to recieve their overcurrent protection once they leave the transformer, since they are NOT considered protected by the primary OCPD?

Joined: Jul 2004
Posts: 9,801
Likes: 17
240.14 says the OCPD "shall be located at the point where the conductors receive their supply except as specified in 240.21(A) through (H)."

That makes these a tap. This might be "hold your nose legal" but it is not the intent of taps.

I will defer to the guys here who do the plan reviews to see if they think this is OK or normal. A lot would depend on how long the unprotected "tap" was and how it was protected. If this is a fairly short run, in pipe that terminates in a OCPD sized to the conductor I would probably be OK with it. You are really only worried about short circuit protection there, not overloads since the load side OCPD provides that. In a bolted fault situation I imagine the primary OCPD would operate before the transformer melted.

Greg Fretwell
Joined: Apr 2002
Posts: 7,336
Likes: 7
"Normal" for a 75Kva, 480 delta primary, 120/208 Y secondary is 100 amp, 3 pole, primary OCP, and secondary conductors treated as a tap; termination at a 200 amp, 3 pole OCP.

3/0 Cu minimum conductors on secondary w/200 amp OCP.

BTW,some plans come in with 4/0 Cu THHN/THWN-2 spec for secondary, 200 amp OCP in 42 circuit panel. Primary w/#1 Cu, 100 amp, fused, 600 volt primary disco adjacent to xfr.

EDit.....ALL of the above are conduit/raceways.

Last edited by HotLine1; 06/20/12 10:31 PM.

Joined: Jun 2004
Posts: 1,273
Plowing through the NEC...

Typical 75kVA secondary systems primary OCPD is a 90 Amp 480Y277 VAC circuit breaker feeding the dry-type transformer in DELTA.

Do spend some time going over Secondary Derived Systems....

The tap rules permit secondary taps of such size to extend up to 25-feet before hitting a Secondary OCPD.

I've never seen that set-up. Instead, the OCPD is commonly mounted directly to the side of the dry-type transformer -- as a NEMA1 General Duty Safety Switch with time-delay 200A fuses.

When that was not done, the Secondaries hit a Circuit Breaker -- 200A or even 225A VAC mounted on a 3-phase 250A Bus.


It is possible to cheese it past the sole-C/B approach by using the SIX OCPD rule. In these cases, the Secondaries land on MLO bussing -- which, then is restricted to just SIX THROWS. This latter requirement requires limitations on the bus length so that the rule can't be worked around -- except crudely.

I've never seen even one real world installation that cheesy.


The reason for the OCPD on the Wye Secondaries: fault currents in the event of critical failures in the windings can reach astounding levels... way beyond the imaginations of the ordinary electrician.

However, experience has shown that the most improbable accidents happen -- like forklifts tipping over onto pad-mounted transformers.

Failures on the secondary side of the transformer DO NOT trip the primary circuit breaker. Normally, such arcs are fed as if everything is okay -- just a motor load going on here... move along... move along.

In the panic, no one ever seems to know which primary circuit breaker needs to be thrown.


Look to see if the six throw rule is being used - -or if you need to buck-up on SDS rules.

Joined: Oct 2000
Posts: 2,722
Broom Pusher and
Adding to Tesla's Posted information above;

One of the most important reasons behind 240.4, and Table 450.3(B) restriction of the Primary OCPD's ability to properly protect the Secondary Conductors from Overcurrent, where the Transformer's OUTPUT will be a "Dual Voltage" ( Slash-Rated Voltage, or simply L-N + L-L Circuitry), under Normal Designed Operating conditions, comes from the higher Load Amperes -vs- KVA per Phase on L-N output Circuitry.

75 KVA Transformer; 480V 3 Phase Delta Connected Primary; 208Y/120V 3 Phase 4 Wire Secondary:
Primary OCPD only (Primary Protection for Secondary Conductors)

Primary (input) FLA: 90.252 Amps (75,000VA / 831 = 90.252)

Primary Feeder Conductors Ampacity figured *1.25 FLA, which equates to 112.8 Amps, so we will use 3 #1 THHN Cu's, with a 110/3 MCCB.
(to keep things simple, I will keep the Primary maximum OCPD rating at 125%, as would be done with a single Voltage output Transformer, along with protecting the #1s at 110 Amps instead of 125 Amps).

Secondary (output) FLA: 208.34 Amps (75,000VA / 360 = 208.3333...)

Secondary Feeder Conductors Ampacity figured *1.25 FLA, which equates to 260 Amps, so we will Terminate at 250 Amps, using 4 #250 MCM THHN Cu's

Primary L-L Voltamps:

a: Nominal: 43,296VA (equals rated Amperes of 90.2A at 480V),

b: At 100% OCPD Trip rating: 52,800VA (at 110 Amperes)

*** Note: Primary Feeder could draw 100% of the OCPD's Trip rating (110 Amps), and continue drawing 100% until the Sun Supernovas, without Tripping from an Overload.***

Secondary L-L Voltamps:

a: Nominal: 43,296VA (equals 208.15 Amperes at 208V - which is the rated FLA),

b: At 100% Primary OCPD Trip rating: 52,800VA (110 Amperes L-L at Primary side).. this equals 253.8 Amps at 208V.


Secondary L-N Voltamps:

a: Nominal Primary L-L Volt Amps: 43,296 VA (equates to 90.2 Amps at 480V 1 Phase 2 Wire),

b: Amperes for the same 43,296 VA per an L-N 120V Circuit: 360.8 Amps.
Conductor = 250 MCM THHN CU.
Can 'ya smell the smoke yet? cry

Even worse would be the Available KVA (52.8 KVA), since the L-L Primary Amperes may rise to 110 Amps for eternity!

Apply 52.8 KVA to a 120V L-N Circuit:
52,800 VA / 120V = 440 Amps!!!

Now the smoke can be smelled, seen, and is accompanied by Large Flames!!! shocked

The above example is applicable to any SDS with a Dual Voltage Output.
This includes:
208Y/120V 3 Phase 4 Wire Wye,
240/120V 3 Phase 4 Wire Delta,
480Y/277V 3 Phase 4 Wire Wye,
120/240V 1 Phase 3 Wire.

*** Note: These are Systems which are _DESIGNED_ for Loads to be connected either L-N, L-L, or L-L-L during normal operation.
These Systems have (2) different Voltages; One equals the L-L Voltage, One equals the L-N Voltage.

Systems which are _DESIGNED_ for Loads to be connected L-L, or L-L-L during normal operation have only (1) System Voltage.
Grounding of the System does not automatically make the System a Dual Voltage System.

So, to keep the smoke from leaking out, Secondary Conductor OCPD's are used to protect the Secondary Conductors of Dual Voltage rated Systems.
Smoke may leak out of Secondary Conductors, Secondary Windings, or both.

-- Scott (EE)

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: May 2007
Posts: 55
Up2code Offline OP
Thanks for all the input.

Question #1) If the transformer secondary's are installed as tap conductors, then section 215.2 (125% rule) does NOT apply right? Since they are tap conductors and not feeder conductors.

Question #2) I still am not clear on my original question. If 240.4(F) states that my secondary's are NOT considered protected by the primary OCPD, then why are engineers' submitting plans with the primary OCPD sized per Table 450.3(B) at 250%, for primary AND secondary protection? Even though the secondary's do terminate into an OCPD rated at their ampacity.

To be clear, yes, these transformers in question are SDS'.

Joined: Apr 2002
Posts: 7,336
Likes: 7
I have not seen any plan submitted as you describe within question #2.

If such submission was made, a letter of rejection would go to said engineer for clarification & calcs.

There are two (2) items that require protection related to transformers, the pri & sec conductors, and the transformer itself. That may be where the confusion starts.

Scotts methodology uses the transformer to the max allowed, which is compliant based on his conductors and calcs.

As I posted above, that is how I see plans come in, and how I did transformer installs over the years. For 3 phase 480 delta, to 3 phase 'Y' 120/208 SDS installs.


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