Transformers Secondary protection

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Dexie123

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I'm sure this subject has been discussed to death but I still hear a lot of different opinions to this question.

Is there any situation where a transformers secondary conductors can be protected by the primary's ocpd and then terminate in a 42 circuit panel without a main circuit breaker in that panel? I am aware the primary ocpd protects the transformer if it's 125% rated and it can protect the secondary conductors for 10ft or 25feet depending but I believe there are no exceptions where a panel can not do without a main ocpd to protect the panel.

Thanks for your input.
 
The secondary conductors require protection also the panelboard requires protection {408.36}. You can use a panel with a main or an external OCPD.
 
There are two different things to deal with, one is conductor protection the other panelboard protection.

As far as the conductors 240.21(C)(1) tells us when the primary OCPD can protect the secondary conductors


240.21(C)(1) Protection by Primary Overcurrent Device. Conductors
supplied by the secondary side of a single-phase transformer
having a 2-wire (single-voltage) secondary, or a
three-phase, delta-delta connected transformer having a
3-wire (single-voltage) secondary, shall be permitted to be
protected by overcurrent protection provided on the primary
(supply) side of the transformer, provided this protection is in
accordance with 450.3 and does not exceed the value determined
by multiplying the secondary conductor ampacity by
the secondary-to-primary transformer voltage ratio.

Single-phase (other than 2-wire) and multiphase (other
than delta-delta, 3-wire) transformer secondary conductors
are not considered to be protected by the primary overcurrent
protective device.
Click to expand...

Next is the panelboard, I do not know of anyway to avoid protecting the panelboard.

408.36 Overcurrent Protection. In addition to the requirement
of 408.30, a panelboard shall be protected by an
overcurrent protective device having a rating not greater
than that of the panelboard. This overcurrent protective device
shall be located within or at any point on the supply
side of the panelboard.
Click to expand...

I do not believe that the transformers primary side overcurrent protection can be used to protect the panelboard.
 
Smart $ said:
I agree... but I do not see it as literally prohibited.
Click to expand...

Yes, it is interesting that the code specifically spells out when the secondary conductors can be protected by the primary OCPD, but there is not specific allowance to use this fact for panelboard protection. Seems like it should be allowed. One argument would be that they should just delete the rule allowing the secondary conductors to be protected by the primary OCPD and just let the electrical theory govern when it can be applied. Another ambiguous situation is where you have a step up step down transformer. Can the primary OCPD of the first transformer be the protection for the second transformer?
 
infinity said:
The secondary conductors require protection also the panelboard requires protection {408.36}. You can use a panel with a main or an external OCPD.
Click to expand...

Consider a situation where the primary OCPD can protect the secondary conductors by proxy, such as a 3-wire delta system on both sides.

If you populate an MLO panelboard on this transformer's secondary, such that the branch circuits do not exceed the busbar ampacity, would the panelboard be considered protected?
 
Carultch said:
Consider a situation where the primary OCPD can protect the secondary conductors by proxy, such as a 3-wire delta system on both sides.

If you populate an MLO panelboard on this transformer's secondary, such that the branch circuits do not exceed the busbar ampacity, would the panelboard be considered protected?
Click to expand...

In reality yes, to the NEC I say no.
 
408.36

(B) Supplied Through a Transformer. Where a panelboard
is supplied through a transformer, the overcurrent protection
required by 408.36 shall be located on the secondary side of
the transformer.
Exception: A panelboard supplied by the secondary side
of a transformer shall be considered as protected by the
overcurrent protection provided on the primary side of the
transformer where that protection is in accordance with
240.21(C)(1).
 
dkidd said:
408.36

(B) Supplied Through a Transformer. Where a panelboard
is supplied through a transformer, the overcurrent protection
required by 408.36 shall be located on the secondary side of
the transformer.
Exception: A panelboard supplied by the secondary side
of a transformer shall be considered as protected by the
overcurrent protection provided on the primary side of the
transformer where that protection is in accordance with
240.21(C)(1).
Click to expand...
:thumbsup:
 
dkidd said:
408.36

(B) Supplied Through a Transformer. Where a panelboard
is supplied through a transformer, the overcurrent protection
required by 408.36 shall be located on the secondary side of
the transformer.
Exception: A panelboard supplied by the secondary side
of a transformer shall be considered as protected by the
overcurrent protection provided on the primary side of the
transformer where that protection is in accordance with
240.21(C)(1).
Click to expand...

Thanks
 
OK, time for a bit of "role playing." I am going to play the role of the electrical inspector that everyone hates, the one who will stretch the words of the code as far as he needs to, in order to promote his own viewpoint. I'll start here:
408.36 Overcurrent Protection. In addition to the requirement of 408.30, a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supply side of the panelboard.
Click to expand...
I will use the words in large, red, bold text as my basis for refusing to allow a panelboard to be protected by a breaker on the primary side of a transformer. And exactly how far will I need to stretch the words? Not that far, actually. I would submit for your consideration that the "supply side of the panelboard" does not extend beyond (i.e., further upstream) than the secondary terminals of the transformer. Put another way, the breaker that protects the primary is not supplying the panelboard, it is supplying the transformer. It is the transformer that supplies the panelboard, and the supply side of the panelboard is the conductors from the transformer's secondary terminals to the panelboard's main lugs. Therefore, an overcurrent device must be installed somewhere between those two points.

Now, how do you like them apples? :)
 
charlie b said:
OK, time for a bit of "role playing." I am going to play the role of the electrical inspector that everyone hates, the one who will stretch the words of the code as far as he needs to, in order to promote his own viewpoint. I'll start here: I will use the words in large, red, bold text as my basis for refusing to allow a panelboard to be protected by a breaker on the primary side of a transformer. And exactly how far will I need to stretch the words? Not that far, actually. I would submit for your consideration that the "supply side of the panelboard" does not extend beyond (i.e., further upstream) than the secondary terminals of the transformer. Put another way, the breaker that protects the primary is not supplying the panelboard, it is supplying the transformer. It is the transformer that supplies the panelboard, and the supply side of the panelboard is the conductors from the transformer's secondary terminals to the panelboard's main lugs. Therefore, an overcurrent device must be installed somewhere between those two points.

Now, how do you like them apples? :)
Click to expand...

I may be a little confused on the OPs situation but I assumed that he was referring to a 120/240, 3 wire or a 3 phase Y secondary. In that case there is no question that a OCPD would be required on the secondary. But in the odd event that he had a 2 wire single phase or 3 wire, 3 phase delta transformer 408.36(B) would clearly allow the omission of secondary protection.
 
texie said:
I may be a little confused on the OPs situation but I assumed that he was referring to a 120/240, 3 wire or a 3 phase Y secondary. In that case there is no question that a OCPD would be required on the secondary. But in the odd event that he had a 2 wire single phase or 3 wire, 3 phase delta transformer 408.36(B) would clearly allow the omission of secondary protection.
Click to expand...

OP asked
Dexie123 said:
Is there any situation where a transformers secondary conductors can be protected by the primary's ocpd and then terminate in a 42 circuit panel without a main circuit breaker in that panel?
Click to expand...

The situation has been described. Nothing he said leads to any assumptions.
 
dkidd said:
408.36

(B) Supplied Through a Transformer. Where a panelboard
is supplied through a transformer, the overcurrent protection
required by 408.36 shall be located on the secondary side of
the transformer.
Exception: A panelboard supplied by the secondary side
of a transformer shall be considered as protected by the
overcurrent protection provided on the primary side of the
transformer where that protection is in accordance with
240.21(C)(1).
Click to expand...

Oh....surprised we missed that. I mean I could see iwire or smart$ missing it but me? ;)
 
Smart $ said:
Not quite sweet enough for me. :D See 408.36(B) Exception quoted and requoted twice over immediately preceding your post.
Click to expand...
This ornery old (pretending to be) inspector ain’t giving up that easily. Let’s design a system.

  • A distribution system can provide a single phase, primary voltage of 480 volts.
  • My calculated load is 100 amps, non-continuous, single phase, 120 volts.
  • I will select a panel rated at 100 amps, single phase, 120 volts.
  • The load equates to 12 KVA.
  • I select the next standard size transformer of 15 KVA.
  • The primary rated current is 31.25 amps.
  • Table 450.3(B) allows me to protect it at 125%, or the next higher standard size. So I select a 40 amp breaker.
  • The exception to 408.36 requires the protection to comply with 240.21(C)(1).
  • That article requires the primary protection to comply with 450.3. It does. So far, so good.
  • That article also requires limits primary protection to a value equal to the ampacity of the secondary conductors times the secondary to primary voltage ratio. Turning that formula around and solving for the minimum ampacity, I get 160 amps by taking 40 times (480/120).
  • So I select a 2/0 conductor for the transformer secondary. It has an ampacity of 175 amps.

You will note that I followed all the requirements of the articles under discussion. You will note that I have properly protected the secondary conductors, as is required by 240.21(C)(1). You may also notice that I have not adequately protected the panel itself. That is, I could run the load up to 150 amps and more on my 100 amp panel, and the primary breaker would not trip.

Q.E.D. ;)
 
charlie b said:

This ornery old (pretending to be) inspector ain’t giving up that easily. Let’s design a system.

  • A distribution system can provide a single phase, primary voltage of 480 volts.
  • My calculated load is 100 amps, non-continuous, single phase, 120 volts.
  • I will select a panel rated at 100 amps, single phase, 120 volts.
  • The load equates to 12 KVA.
  • I select the next standard size transformer of 15 KVA.
  • The primary rated current is 31.25 amps.
  • Table 450.3(B) allows me to protect it at 125%, or the next higher standard size. So I select a 40 amp breaker.
  • The exception to 408.36 requires the protection to comply with 240.21(C)(1).
  • That article requires the primary protection to comply with 450.3. It does. So far, so good.
  • That article also requires limits primary protection to a value equal to the ampacity of the secondary conductors times the secondary to primary voltage ratio. Turning that formula around and solving for the minimum ampacity, I get 160 amps by taking 40 times (480/120).
  • So I select a 2/0 conductor for the transformer secondary. It has an ampacity of 175 amps.

You will note that I followed all the requirements of the articles under discussion. You will note that I have properly protected the secondary conductors, as is required by 240.21(C)(1). You may also notice that I have not adequately protected the panel itself. That is, I could run the load up to 150 amps and more on my 100 amp panel, and the primary breaker would not trip.

Q.E.D. ;)
Click to expand...

But,but, wait. I'm going to use a 25 amp breaker on the primary by choice and now I am compliant with your interpretation. Of course we will both fired for designing such a non traditional system.:lol:
Q.E.D.
 
iwire said:
Thanks
Click to expand...
I will show my ignorance here,:? but so be it. What does protecting the panel board really mean???? Does it mean if the bus bars are shorted together they will be protected?? Is not that all it means??? And lets face it that should be a rare occurrence......someone drops a crescent wrench into a live panel.....
 
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