"Phase or line and system" 210.5

[George Stolz]

Hypothetical: I have a 480/277 service. The service feeds two transformers that output 120/208.

How many systems are present?

 

[jeremysterling]

You could put this in poll format, so I could take a multiple choice guess.

 

[George Stolz]

I can do that.

 

[chris kennedy]

I vote 3, but I'm an idiot according to my wife and employer.

 

[hurk27]

I voted 3 as in my opinion I use current paths as the reason, current from the 480/277 "system" will only flow back to its source, and the same goes for each of the 208/120 "systems" also. but of course this is also my opinion of what should be the definition of a SDS also, not the one we now have.

 

[chris kennedy]

but of course this is also my opinion of what should be the definition of a SDS also, not the one we now have.

No direct electrical connection, whats wrong with that def?

 

[jwjrw]

It sounds like 3 systems to me.

 

[acrwc10]

The question may not be "how many systems" but rather when is the "one two punch" going to give the second punch. It seems too easy of a question.

 

[acrwc10]

George is sneaky that way.

 

[George Stolz]

Let's say there's 3 systems.

What means of identification do you use for the two 120/208v systems?

 

[don_resqcapt19]

There are three systems, but for the purpose of 210.5(C) there are only two systems as there are only two "nominal voltage systems".

(C) Ungrounded Conductors. Where the premises wiring system has branch circuits supplied from more than one nominal voltage system, each ungrounded conductor of a branch circuit shall be identified by phase or line and system at all termination, connection, and splice points. The means of identification shall be permitted to be by separate color coding, marking tape, tagging, or other approved means. The method utilized for conductors originating within each branch-circuit panelboard or similar branch-circuit distribution equipment shall be documented in a manner that is readily available or shall be permanently posted at each branch-circuit panelboard or similar branch-circuit distribution equipment.

 

[acrwc10]

How close are they? Or is this a hypothetical installation?

 

[George Stolz]

How close are they? Or is this a hypothetical installation?

Hypothetical. I think Don's logic is sound - but I think it could be written clearer.

 

[acrwc10]

Hypothetical. I think Don's logic is sound - but I think it could be written clearer.

:grin::grin::grin: LMAO.

Have you noticed just how many "run on sentences" there are in the code. It is not written by English professors.

 

[jusme123]

Let's say there's 3 systems.

What means of identification do you use for the two 120/208v systems?

what if you identified them as sub-systems, since they both are dependent on the original system 277/480

 

[iwire]

Hypothetical: I have a 480/277 service. The service feeds two transformers that output 120/208.

How many systems are present?

3 Systems

But only two voltage systems.

 

[hurk27]

There are three systems, but for the purpose of 210.5(C) there are only two systems as there are only two "nominal voltage systems".

210.5(C) Ungrounded Conductors. Where the premises wiring system has branch circuits supplied from more than one nominal voltage system, each ungrounded conductor of a branch circuit shall be identified by phase or line and system at all termination, connection, and splice points. The means of identification shall be permitted to be by separate color coding, marking tape, tagging, or other approved means. The method utilized for conductors originating within each branch-circuit panelboard or similar branch-circuit distribution equipment shall be documented in a manner that is readily available or shall be permanently posted at each branch-circuit panelboard or similar branch-circuit distribution equipment.

What would be the problem with the above code in the case of having two derived 480/277-208/120 systems in the same building?

I can see one major one, if both gets marked with the same identification.

I believe the two word's I highlighted in red should be removed, any separate system needs separate markings.

But then again 210.5(C) can be read as saying more then one system that has a nominal voltage?

210.5(C) Ungrounded Conductors. Where the premises wiring system has branch circuits supplied from more than one nominal voltage system of a different voltage, each ungrounded conductor of a branch circuit shall be identified by phase or line and system at all termination, connection, and splice points. The means of identification shall be permitted to be by separate color coding, marking tape, tagging, or other approved means. The method utilized for conductors originating within each branch-circuit panelboard or similar branch-circuit distribution equipment shall be documented in a manner that is readily available or shall be permanently posted at each branch-circuit panelboard or similar branch-circuit distribution equipment.

The above rewording is what I think many of us are reading into 210.5(C)

 

[don_resqcapt19]

What would be the problem with the above code in the case of having two derived 480/277-208/120 systems in the same building?

I can see one major one, if both gets marked with the same identification. ...

Yes, mixing the systems of the same nominal voltage would be a problem, but where do you draw the line on this? A large building could easily have 20 or more systems.

 

[hurk27]

No direct electrical connection, whats wrong with that def?

Because of the words used for SDS (Separately Derived System) is to describe any system to which is the source of the current is separate from any other current source or system, current will only will flow back to it source, it is the isolation characteristics of a transformer that makes it a new source/system.

The only system that would not be separately derived is an autotransformer, as it is not isolating, many buck/boost transformers are configured as such.

Here's another point, if we are to say that any system is not a SDS if the secondary is bonded to the building or grounding that is electrically connected to the supplying system's grounded conductor, then much of 250.30 could not even be used, as soon as you bond the secondary, 250.30 would not apply?

Separately Derived System. A premises wiring system whose power is derived from a battery, from a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

If you look at a diagram of a autotransformer, the above in red makes sense, but just the grounding of the secondary of a transformer should not change an SDS to a NON-SDS, and no normal current will flow across this connection, a ground is not a circuit conductor.

 

[hurk27]

Yes, mixing the systems of the same nominal voltage would be a problem, but where do you draw the line on this? A large building could easily have 20 or more systems.

I agree this is a major problem in identifying each systems conductors in a large installation.

But what other methods are available? I think allowing one often used method should also be added to 210.5(C) and that is keeping system conductors in separate raceway systems, and or marking each raceway at each termination to which system it belongs to?

To me this is one of those codes that kind of dumb down the NEC, like the multi-wire requirement of disconnect, if we are professionals, we should be making sure where the wires are being fed from when we work on them.