ECN Forum Forums Code Related Discussion NEC & other Code issues 3 phase vs single

Roger has brought up a great example of yet another possible "Monkey Wrench", for which NFPA may toss into the cogs of the grand machinery.

The Article designates "Grounded Circuit", instead of (IMHO) should have been designated "Grounded System".

To me, the intentions sure point towards the ideal of using dual rated, or slash rated Breakers on Branch Circuitry which would include a Circuit having higher Voltage to Ground than the device is rated (such as a 2 wire single phase 240 volt circuit connected to B & C on a 4 wire delta, or B & C on a 3 wire corner grounded delta).
However, the inclusion of "Grounded Circuits" leaves room for several interpretations!

I see this as a valid argument, and never second-guessed it until now!

Scott35

You can debate about the use on the poor NEC wording. However, the information from most manufacturers says that the UL testing of slash rated breakers deals with the voltage phase to ground.

So using a slash rated breaker on any circuit that exceeds the voltage to ground rating would be a violation of the manufacturer's instructions.

JBD, that's just another part of the problem, if the circuit is not referenced to ground there is no voltage from the given conductor to ground.

And to further the problem, read the definition of "Voltage to Ground" in the NEC.

Roger

Roger,

IMHO, if there is a ground in the system then all circuits have a reference to ground regardless if any particular circuit actually contains a ground.

This is also an issue with the testing and listing of a single pole interruption of the breaker not just with the wording of the NEC. For example, Square D says their 120/240 slash rated breakers can only be used where the "Maximum voltage to ground is 120Vac".

The NEC definition of "voltage to ground" says that if the circuit does not have a ground then you look at the voltage between any two of the ungrounded conductors.

JBD, it actually says So with a two wire ungrounded 240 V circuit what is the greatest voltage between the two conductors? It is the same voltage if you measure A-B, B-C, or C-A.

So, with that being the case we would have to consider how much voltage is dropped at the center of a 240 V circuit from each phase of the circuit, and that would be 120 V

Roger



[This message has been edited by Roger (edited 11-03-2006).]

Roger,

How are you coming up with this?

Are you saying that you have a 120/240 3W system that is not grounded (in violation of 250.20(B) unless allowed in 250.22)? Regardless the greatest voltage between two conductors would be 240V so a 120/240 breaker is not allowed.

JBD, you keep saying system, this is not the problem, the problem is where the NEC and UL specifically use the word "Circuit".

We are not questioning whether the system is grounded, it is, the "circuit" is not.

Roger

I used to have a contractor friend of mine who would not install a single phase breaker in a three phase panel and he would install two panels- One for single phase loads and one for 3 thase loads when he dealt with a Delta 240/120 3 phase service. Not cost effective I agree but this was his standard and I'm not to sure he was wrong. He's gone to his reward now but I always knew why he installed things this way and he never would change. That being the case we can argue all day about system/circuit/ground. He never had any issues doing it his way. My final comment on this issue is a quote from the White Book:

Edited for spelling

[This message has been edited by George Little (edited 11-03-2006).]

Roger,
You keep focusing on the word circuit in 240.85 like it requires a grounded conductor or else it doesn't apply.

Can't you run an ungrounded circuit in a grounded raceway? If so don't you have a voltage to ground? If you say no, then you need to determine which two circuit conductors have the greatest voltage between between them (see the definition of Voltage to Ground).

JBD, you are thinking of the EGC, which is not a part of the circuit in function as a current carrying conductor.

Looking at a 240 volt phase to phase (ungrounded) circuit, do you think the high leg (referenced to ground) actually provides 208 volts and the other phase will see this and shed 88 volts so it will only provide 32 volts to the load?

Both of the phases will drop 120 volts each, so neither is a higher voltage than the other is it?

Now, I'm going to try to follow Georges lead and let this be my last post here.

No promises though.

Roger