transformer secondary tap conductors 2006 nec

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grumpy11

Senior Member
installing one 75 kva transformer in an office buliding
480volt 3 phase step down to 120/208v
primary ocpd is 110amp
secondary is 4 250mcm going well over 100ft to a 250amp main ocpd in a panel.
the engineer is saying that no ocpd is required off the secondary for protection of conductors.

i say he is going well beyond the 25ft tap rule for transformers.
 

augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
The engineer is incorrect.
The length allowed will vary by conditions as shown in 240.21(C), but, other than outside conductors, 25 ft would be the max
 

raider1

Senior Member
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Location
Logan, Utah
The engineer is incorrect. You need to install an OCPD to protect the secondary conductor in accordance with the rules in 240.21(C).

Chris
 

grumpy11

Senior Member
transformer

transformer

thank and sorry for type o its 2005 nec. you think he would understand 240.21 c.
some people dont like to tell eng that they are wrong
but i dont like putting in a violation when i know it is
thank you
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
installing one 75 kva transformer in an office buliding
480volt 3 phase step down to 120/208v
primary ocpd is 110amp
secondary is 4 250mcm going well over 100ft to a 250amp main ocpd in a panel.
the engineer is saying that no ocpd is required off the secondary for protection of conductors.

i say he is going well beyond the 25ft tap rule for transformers.

I don't think any tap rule applies at all as there is no tap involved.

But you can't use the primary OCPD to protect the secondary conductors except for some very narrow circumstances.

This is not a delta to 2W single phase setup so he cannot use that exception.

Having said that I have seen a lot of installs that violate this rule by having a 3W single phase secondary.
 
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augie47

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State Electrical Inspector (Retired)
I find many of us mistakenly refer to the 240.21(C) rules as "tap" rules.
The proximity and similarity to the 240.21(B) tap rules make this a common mistake.
 

iwire

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Location
Massachusetts
I don't think any tap rule applies at all as there is no tap involved.

I find many of us mistakenly refer to the 240.21(C) rules as "tap" rules.
The proximity and similarity to the 240.21(B) tap rules make this a common mistake.

I find many of us mistakenly refer to the 240.21(C) rules as "tap" rules.

But transformer secondary conductors are taps.

That is why we can't tap them a second time. :)
 

jim dungar

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Location
Wisconsin
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Retired Electrical Engineer - Power Systems
But transformer secondary conductors are taps.

That is why we can't tap them a second time.
Maybe not based on 240.4(E) and 240.4(F).:blink:

The 240.2 definition of 'tap conductor' makes reference to 240.4.
 

iwire

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Location
Massachusetts
Maybe not based on 240.4(E) and 240.4(F).:blink:

The 240.2 definition of 'tap conductor' makes reference to 240.4.

Yeah, I had looked at those and the definition of tap conductor

So maybe I am wrong and they are not really tap conductors even though we treat them as such.
 

jim dungar

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Wisconsin
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So maybe I am wrong and they are not really tap conductors even though we treat them as such.
I'm good with this.:thumbsup:
Kind of how we treat the output of a SDS like it was a service entrance.
 

RUWired

Senior Member
Location
Pa.
If transformer secondary conductors are not concidered to be protected by the primary OCPD on multiwire transformers, then there is no protection ahead of the connection at the secondary. This would make the secondary conductors tap conductors.

Tap Conductors. As used in this article, a tap conductor is defined as a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4.

Rick
 

jim dungar

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Location
Wisconsin
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Retired Electrical Engineer - Power Systems
Tap Conductors. As used in this article, ... exceeds the value permitted .... as described elsewhere in 240.4.
So are not transformer secondary conductor installed per 240.4(F) excluded from this definition?
 

grumpy11

Senior Member
transformers

transformers

i alway considered connection to the secondary terminal of a transformer is a tap
as per 240.21 c and 10/25 tap rules and 240.21 c 6
 

david luchini

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Connecticut
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If transformer secondary conductors are not concidered to be protected by the primary OCPD on multiwire transformers, then there is no protection ahead of the connection at the secondary. This would make the secondary conductors tap conductors.

Tap Conductors. As used in this article, a tap conductor is defined as a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4.

Rick

This seems contradictory. You say transformer secondary conductors are "taps" because they have NO protection at the connection at the secondary.

Yet you quote the definition that says "taps" HAVE overcurrent protection ahead of its point of connection that is higher than the value permitted by for similar conductors...

By the definition in 240.2, transformer secondary conductors are not taps.
 

RUWired

Senior Member
Location
Pa.
Dave, If there is no OCPD ahead of the secondary conductors, then the conductors are not protected at its rated current.

Rick
 

david luchini

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Location
Connecticut
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Dave, If there is no OCPD ahead of the secondary conductors, then the conductors are not protected at its rated current.

Rick
That is not quite correct. 240.21(C) requires the conductors to be protected at their rated current at the OCPD at which they terminate (except for the case where the secondaries are considered protected by the primary OCPD.) It is more accurate to say that the conductors are not protected at their rated current "at the point they receive their supply."

Tap conductors have an OCPD ahead of their point of supply which exceeds the value permitted for similar conductors...

Transformer secondary conductors have NO OCPD ahead of their point of supply. Per the definition of "tap conductor" that you have quoted, transformer secondary conductors are not tap conductors because they do NOT have an OCPD ahead of their point of supply.
 

RUWired

Senior Member
Location
Pa.
Dave the way i see it, on 4-wire secondary transformers, the primary OCPD doesn't protect the secondary conductors "at their ampacity" because of the secondary to primary ratio. The primary OCP will be to high for the secondary conductors via this ratio. This would now fit the definition of tap conductors.

Rick
 

david luchini

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Connecticut
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Engineer
Dave the way i see it, on 4-wire secondary transformers, the primary OCPD doesn't protect the secondary conductors "at their ampacity" because of the secondary to primary ratio. The primary OCP will be to high for the secondary conductors via this ratio. This would now fit the definition of tap conductors.

Rick

I think this is where you are going wrong. 240.4(F) say that transformer secondary conductors (other than 3 wire delta) shall NOT be considered to be protected by the primary OCPD. If you had a 45kVA, 480-240V delta (3-wire) transformer with a 60A primary OCPD and #1awg secondary conductors, the secondaries can be protected by the primary OCPD because of the ratio you mention. However, if you had a 45kVA, 480-240/120V delta (4-wire w/a center tap) the secondary to primary ratio is still the same, but the secondaries cannot be protected by the primary OCPD. Or if you had a 45kVA, 480-208/120V (wye) transformer with a 60A primary OCPD and #1/0awg secondary conductors, the primary OCPD again does NOT exceed the value of the secondary conductor ampacity times the secondary to primary ratio. But again, the primary OCPD cannot be used to protect the secondary conductors even though the primary OCPD is not to high. The secondary to primary ratio has nothing to do with whether the secondary conductors are taps.

240.21 says that overcurrent protection shall be provided for each (ungrounded) conductor at the point they receive their supply, except as permitted in 240.21(A) thru (H). 240.21(A) tell us how to deal with branch circuit TAPS, 240.21(B) tell us how to deal with feeder TAPS, and 240.21(C) tells us how to deal with transformer secondary conductors. Transformer secondary conductors are neither branch circuit taps nor feeder taps.

Going back to the definition of Tap Conductors in 240.2, there are two conditions that must be met for conductors to be tap conductors: they must have an OCPD ahead of its point of supply, and the OCPD must exceed the value permitted for similar conductors protected as described elsewhere in 240.4. The point of supply for the transformer secondary conductors is the transformer secondary. There is no OCPD ahead of their point of supply, so by definition, the transformer secondary conductors are not tap conductors.
 

RUWired

Senior Member
Location
Pa.
However, if you had a 45kVA, 480-240/120V delta (4-wire w/a center tap) the secondary to primary ratio is still the same,
Not so, with a 120 secondary voltage, the ratio would be .25.

david said:
Or if you had a 45kVA, 480-208/120V (wye) transformer with a 60A primary OCPD and #1/0awg secondary conductors, the primary OCPD again does NOT exceed the value of the secondary conductor ampacity times the secondary to primary ratio.
Sure it does. The secondary conductor amps(150) x secondary to primary ratio (.25) =37.5. A 60 amp primary OCPD is more than 37.5
david said:
But again, the primary OCPD cannot be used to protect the secondary conductors even though the primary OCPD is not to high.
The primary Ocpd will always be higher than the secondary conductor amps at the ratio quoted.


The way i understand it, even with 2-wire -2-wire transformers, the secondary conductor amps times the ratio number is what the maximum OCPD is allowed to be set at and still be within the limits of article 450.3.

With a 120/208 secondary, the 120 volts ratio is .25 with a 480 volt primary voltage. The .25 ratio x secondary amps would be if allowed,what the maximum primary OCPD is allowed to be set at, and if it was set at the typical 125/250% per 450.3, it would be higher than the secondary conductor amperage and now be a tap conductor.

To be quite honest, to me it doesn't really matter what the conductors are called. They can be secondary conductors or they can be tap conductors. It is a good debate.

Rick
 
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