Three Related Questions: #1

[charlie b]

Common information for all three: Transformer within outdoor substation. Cables connect secondary to busbars. Breakers serve loads both inside and outside the adjacent building. Primary is over 600 volts, secondary is 120/208V.

Question:
Since each breaker protects its downstream conductors, the tap rules do not come into play for them. But what about the conductors from the secondary to the busbars, and what about the busbars themselves? Do they fall within the tap rules? What protects them?

 

[marti smith]

Fuse cutout in the primary circuit?

 

[david luchini]

I don't think tap rules come into effect. My original thought was "outdoor transformer secondary conductors," but it seems that the secondary conductors are located within a structure, the outdoor substation.

I think you'd be looking at 240.21(C)(3), and the secondary conductors (cables and busbars) must have an ampacity not less than the secondary rated current and not less than the sum of the OCPDs.

 

[Mike01]

Question..

Question..

Common information for all three: Transformer within outdoor substation. Cables connect secondary to busbars.

Are the cables factory or field installed??

 

[charlie b]

I believe the substation is all one entity, supplied as such by the manufacturer. There is a possibility that it might be owned by the utility, and that may alter the situation entirely.

 

[mull982]

I've always thought that a transformers secondary conductors or bus were considered a tap conductor and thus that is why you cannot tap them.

 

[augie47]

Boy, the old "structure" definition confuses things again (to me anyway)...
My first thoughts are:
(a) Not taps, but transformer secondary conductors, thus 240.21(C)
(b) Buss and secondary conductors are protected from short circuit by the
transformer primary OCP
(c) conductors are protected from overload by the secondary protection under the 240.21(C) rules....

I'm inclined to use 240.21(C)(4) (if other than at termination all these conductors are outside the "building")
..but,,,,
To do so we have to accept that the outdoor substation is not a structure.

 

[david luchini]

I've always thought that a transformers secondary conductors or bus were considered a tap conductor and thus that is why you cannot tap them.

Deja Vu (I think we had this discussion in a post recently.)

The code doesn't say "you can't tap a tap." It says "Conductors supplied under the provisions of 240.21(A) thru (H) shall not supply another conductor except through an overcurrent device meeting the requirements of 240.4"

(A) thru (H) includes, Branch-circuit conductor (taps,) Feeder Taps, Transformer Secondary Conductors, Service Conductors, Busway Taps, Motor Circuit Taps, Conductors from Generator Terminals, and Battery Conductors.

Some of these groups are taps, some are not. Transformer Secondary Conductors are not.

 

[charlie b]

..but,,,, To do so we have to accept that the outdoor substation is not a structure.

Help me understand this one. How does it possibly being a structure alter the situation?

 

[dana1028]

Deja Vu (I think we had this discussion in a post recently.)

The code doesn't say "you can't tap a tap." It says "Conductors supplied under the provisions of 240.21(A) thru (H) shall not supply another conductor except through an overcurrent device meeting the requirements of 240.4"

(A) thru (H) includes, Branch-circuit conductor (taps,) Feeder Taps, Transformer Secondary Conductors, Service Conductors, Busway Taps, Motor Circuit Taps, Conductors from Generator Terminals, and Battery Conductors.

Some of these groups are taps, some are not. Transformer Secondary Conductors are not.

IN your opinion. As you said, we've had this discussion and many of us feel they are indeed taps and one of our code sections to support that position is 240.4(E).

Go ahead and argue - I accept this code section and the many industry magazine code articles that consider secondaries as taps...and the bottom line...my boss [AHJ] also considers them taps.

 

[david luchini]

Help me understand this one. How does it possibly being a structure alter the situation?

The rules for "outside" transformer secondary conductors say the conductors must be outdoors of a building or structure, except at the point of load terminations.

Your secondary conductors are located within the outdoor substation. IMO, the substation is a "structure" per the NEC definition, therefore, 240.21(C)(4) will not apply.

I think 240.21(C)(3) is better suited to your application.

 

[david luchini]

IN your opinion. As you said, we've had this discussion and many of us feel they are indeed taps and one of our code sections to support that position is 240.4(E).

Go ahead and argue - I accept this code section and the many industry magazine code articles that consider secondaries as taps...and the bottom line...my boss [AHJ] also considers them taps.

Yes, in my opinion, and in the text of the code. A tap conductor must have overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors protected as described elsewhere in 240.4. Transformer secondary conductors do not have any protection ahead of their point of supply.

In addition, 240.21(A),(B),(E) and (F) all relate to tap conductors. 240.21(C),(D),(G) and (H) do not relate to tap conductors. If they were tap conductors, why wouldn't 240.21(B) apply to them?

You can consider them what ever you want. Its what the code considers them that matters.

 

[dana1028]

You can consider them what ever you want. Its what the code considers them that matters.

That's your opinion of what the code says...others have a different opinion. When you become an AHJ you can have an opinion that carries weight on your turf.

 

[david luchini]

That's your opinion of what the code says...others have a different opinion. When you become an AHJ you can have an opinion that carries weight on your turf.

Only an AHJ can have an opinion on code? I've known several that have had the wrong opinion.

Where does the code text support transformer secondary conductors are taps?

 

[augie47]

The rules for "outside" transformer secondary conductors say the conductors must be outdoors of a building or structure, except at the point of load terminations.

Your secondary conductors are located within the outdoor substation. IMO, the substation is a "structure" per the NEC definition, therefore, 240.21(C)(4) will not apply.

I think 240.21(C)(3) is better suited to your application.

I don't necessarily disagree.. like other posts in this thread, the AHJ will be the final authority,,,
I would say you are correct by "letter of the law", but I feel some inspectors might view it as not being a structure.

 

[augie47]

That's your opinion of what the code says...others have a different opinion. When you become an AHJ you can have an opinion that carries weight on your turf.

That is true, but there are opinions and then there are opinions based on some factual information. If you look at the definition of a "tap conductor" in 240.2:
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.
I find it difficult to consider a transformer secondary as a tap.

But, as David wisely points out, the opinion of an AHJ or Inspector is still and opinion and subject to be wrong... just look at some of my recent posts

 

[RUWired]

Charlie, Is this an industrial site and do you meet all the conditions in 240.21(C)3?

Rick

 

[charlie b]

Charlie, Is this an industrial site. . . ?

It is not. The building under consideration is for housing of enlisted personnel at a US military base overseas.

 

[RUWired]

It is not. The building under consideration is for housing of enlisted personnel at a US military base overseas.

If that is the case then a main OCPD is required on the secondary side.

 

[charlie b]

If that is the case then a main OCPD is required on the secondary side.

Would it, perhaps, be more accurate to say that overcurrent protection is required, and that it can consist of up to six breakers (i.e., you don't need a single main breaker)? The design I am reviewing has four breakers connected to the bus bars, and all of it (including the transformer) is enclosed within this outdoor substation.