NEC handbook

[brian john]

Re: NEC handbook

I've owned two handbooks one was a Christams gift 8-10 years ago and I used it once or twice. Just bought a new hanbook last month. It sat on my desk two days when an employee was eying it , so now it is his. I liked the smaller softbound (old style) code books, but alas progress.

 

[hurk27]

Re: NEC handbook

Bennie, Charlie and many other on the forum I have learned so much form you guys and wish to just say thank you. I wish I had the time to go to some of the meetings and be able to talk face to face as my tpyeing sometimes keeps me from getting in on alot of questions that I have. but everyone here have been great. Keep up the great work.

 

[pierre]

Re: NEC handbook

Wayne

I like and totally agree with your two posts, 7/5, 7/6. Typing can be a pain in the.... fingers, shoulders, neck, back.... Semantics, a cruel fate to us who do not have the time.
The ideas are so varied here it really makes one think. Do you ever refer to the ROP, ROC for info?

Pierre

 

[Ed MacLaren]

Re: NEC handbook

The NEC is now referring to multiple separately derived systems, this is impossible, only one per premises

Do we agree that a "wiring system" consists of 1) the voltage source, 2) the associated conductors, and 3) the load devices?

If so, how many "systems" are there in the sketch below?

How many of them are separately derived?

Ed

 

[bennie]

Re: NEC handbook

There is a ground/neutral in the service that will be electrically connected to the secondary of every transformer.

My final answer is...None of them are separately derived, because the transformer is connected to the source by means other than magnetic coupling.

The low side neutral is an extension of the utility MGN system. Therefore it is not separate.

Read the definition of premises wiring system. The power transformers in your single line are part of the premises wiring system.

[ July 06, 2003, 10:02 AM: Message edited by: bennie ]

 

[bennie]

Re: NEC handbook

Can anyone answer this...A generator, to qualify as a separately derived system, must have all electrical connections, including a solidly grounded circuit conductor, to supply conductors originating in another system, disconnected.

How come a transformer is not treated the same way?

 

[iwire]

Re: NEC handbook

Originally posted by bennie:
Can anyone answer this...

To give us something to talk about?

 

[Ed MacLaren]

Re: NEC handbook

Can anyone answer this...A generator, to qualify as a separately derived system, must have all electrical connections, including a solidly grounded circuit conductor, to supply conductors originating in another system, disconnected.

How come a transformer is not treated the same way?

Bennie, I know you don't need this explanation, and I don't expect you to change your mind on this, but just for the sake of any others who may find this confusing, I believe transformers and generators are treated the same. In the sketch below of a typical delta-wye transformer, there is no primary neutral run to the transformer.

All secondary unbalanced load current returns to the secondary winding.

The (green) EGC run with the primary feeder is not a "circuit conductor". It will carry no current, except in the event of a primary feeder ground fault.

Ed

[ July 06, 2003, 12:55 PM: Message edited by: Ed MacLaren ]

 

[bennie]

Re: NEC handbook

Ed: Your description is the popular explanation. This explanation is a fabrication to explain a separately derived system, from someone who does not know the true definition, and history.

The explanation that an equipment ground conductor, is not a circuit conductor, is likely a fabrication by the same person. 200.3 contradicts that explanation. An electrical connection only has to be capable of carrying current to qualify as a circuit conductor. Basic electrical theory. The possibility of the conductor not carrying current under normal conditions is irelevant.

The same individual is probably responsible for the erronous explanation for disconnecting the neutral of a generator. This requirement was a long time before a GFI was introduced.

A properly connected generator, for a separately derived system, will not affect the operation of the GFI's. The statement is ludicrous.

 

[roger]

Re: NEC handbook

Ed, if we removed the egc back to the service source and established the the grounded conductor (grounding and bracing the X-O exclusivly on the secondary side) then we would meet the definition of SDS.

The (green) EGC run with the primary feeder is not a "circuit conductor". It will carry no current, except in the event of a primary feeder ground fault.

This is kind of true, but it is physically connected to the primary winding at some point upstream, so it is a "circuit connected conductor" and can not be ignored as "more than" a magnetically coupled common connection".

Ed, I don't mean to sound as though I'm trying to explain something to you, (lord knows I know better than that ) I'm simply explaining my rational.

Roger

 

[bennie]

Re: NEC handbook

Roger: Let me run this by you...

The primary of a transformer supplying a separately derived system is supplied by an ungrounded secondary.

Paraphrased;

An isolation transformer has a grounded primary, with an ungrounded secondary.

A source for a separately derived system has an ungrounded primary, with a grounded secondary.

A door bell transformer is an isolation transformer, even if the secondary is not electrically connected to the source. The secondary does not supply a premises wiring system, so it is excluded.

 

[roger]

Re: NEC handbook

Bennie, this is my opinion also. To use the word isolated in any instance, one side or both sides of a transformer has to be ungrounded or unbonded.

This could be control transformers, door bell transformers, medical equipment iso systems, encapsulated marine transformers, delta delta, ungrounded wye ungrounded wye, UK shaver receptacles, etc...

I know the systems we use are not going to change but, I do think we could fine tune and better define them.

Roger

[ July 06, 2003, 03:05 PM: Message edited by: roger ]

 

[bennie]

Re: NEC handbook

Roger: I thought you would agree, you must be reading the same engineering publications as I do.

All of my statements are not from my own discovery, or creation. They are taken from credible and acceptable engineering standards and practices.

I have asked the authors of the handbook and other official publications to furnish engineering data to substantiate their position on various subjects. I have yet to receive any, mainly because they don't exist except in someones imagination.

I can furnish documented proof regarding my statements, and can counter any support of the handbook and Soares version of a separately derived system.

I am hoping to get some of the code making officials to respond and prove me wrong.

 

[gwz2]

Re: NEC handbook

What about the equipment grounding of the metal transformer enclosure ?

With ungrounded system circuit conductors, the metal parts of any system is to be grounded ( 250.4(A) or (B) ) and the EGC installed with the circuit conductors, i.e. 250.134)(B).

 

[roger]

Re: NEC handbook

Glenn, This is part of the problem and the reason I always mention, if we are going to use an Iso System for premises wiring we will have to monitor and alarm it at some value deemed safe. This could be the sum of the accumulated capacitive / inductive coupling (overall impedance, end of condutor to end of bus ) at a value of say 30ma. I don't know, maybe somebody else has a better idea.

We could still ground one side of the transformer to guard that side, taking care of that part of the problem.


Roger

 

[bennie]

Re: NEC handbook

The grounding of transformers is why a delta/wye distribution, or power transformer, can not be a separately derived system. Delta/wye distribution systems are also MGN systems, therefore can not be considered a separately derived system.

The secondary or wye winding is electrically connected to the supply conductors originating in another system. The wye or star points are solidly connected and grounded.

[ July 06, 2003, 07:50 PM: Message edited by: bennie ]

 

[bennie]

Re: NEC handbook

Much to the surprise of the grounding gurus, many pad mounted transformer enclosures are not connected to the system ground.

These transformers may have as high as 34.5/19.9 KV connected.

[ July 06, 2003, 08:37 PM: Message edited by: bennie ]

 

[gwz2]

Re: NEC handbook

What is wrong with accepting the NEC definitions?

Article 100 Scope, page 70-33 of the 2002 NEC, covers definitions not covered in the dictionary etc.

Hopefully, the 2005 ROP's will have some better recognitions of this "separately derived system", fiasco , but until then - accept what the NEC has.

With the 250.4(A) or (B) and the 250.134 requirements, there will always be the EGC connection problem when metal enclosures are used.

The NEC definition of Separately Devired System does mention ' transformer ' and this definition is for the NEC and the NEC installation(s), not for the Webster's Dictionary.

The ROP's should be being delivered in about 10 more days. I'll surely peruse the proposals that have been the subject(s) of many of these BB topics.

 

[bennie]

Re: NEC handbook

I have no problem with the NEC, until the multiple systems was added. A premises system is one system according to the NEC.

A transformer and generator are not separately derived systems according to the NEC.

A separately derived system is a premises wiring system according to the NEC.

A separately derived system is a premises wiring system without external connections. According to the NEC.

A transformer with winding to winding fault containment can not be considered as coupled only by the magnetic field. According to the NEC.

A transformer, after the main service, is a transformer, nothing else. According to the NEC.

The code is fine, its the understanding by the expert authors that are screwing things up.

 

[charlie]

Re: NEC handbook

Originally posted by Benny:
Q1 Is it true that article 250 is being configured and rewritten by a committee? This would explain the nonsense in 250.30 (2)(b).

Q2 250.104(4) Makes sense. This appears to be an attempt to clean up the ambiguous text of 250.30(3).

Q3 I eat so much shoe leather that I'm going to change my diet and be a vegetarian. I am joining PETA.