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

Roger,

I still dont' see how you are coming up with statements like:
. To where are they dropping the voltage? And, why it is important?

If the circuit is ungrounded and there is any combination of any two circuit conductors that exceeds the lower voltage then a slash breaker cannot be used. The reason for this is that one of these conductors may accidentally become grounded.

I would include the failure to follow manufacturers instructions and warnings when noting a violation.

[This message has been edited by JBD (edited 11-04-2006).]

Roger,

I disagree with this point:

If your _circuit_ consists of two ungrounded conductors from a grounded _system_, I contend that this is in fact a _grounded_ circuit. The voltage between these ungrounded conductors and ground will be well defined, and this remains true even if the circuit does not include a grounded conductor.

I agree that the code is not clear in that the term solidly grounded _circuit_ is used, rather than 'circuit derived from a solidly grounded _system_' or some similar verbiage, however what other meaning would you apply to 'soildly grounded circuit'?

-Jon

< memory book = open >

Just a quick response to George's post:


When I was very green in the field, having starry eyed dreams of being an EEE-Lecktrickle Injun-ear (and really not seeing the connected craziness that comes with the job,... but that's another story ~:-O...), every new and existing power system was a beautiful lesson at hand.

In the more designated Industrial areas (as opposed to Commercial/light Industrial) - such as Gardena, Santa Fe Springs, and the portion of East LA known as "Bandini" (remember "Bandini Mountain"?), Delta systems covered the earth.
Many 3 Wire Deltas, and a large 1Ø 3 Wire system used for multiple Customers.

Where convenient or economical, customers were serviced with 4 Wire 240/120VAC 3Ø 4 Wire systems - typically warehouses and low demand machine shops.

The thing I found to be extremely bright, was the use of 1Ø 3 Wire Panelboards where L-N or smaller L-L single phase loads where connected, and 3Ø 3 Wire Panelboards (fed without the Grounded Neutral Conductor) where the heavier L-L loads (and, of course, the 3 phase loads) would be connected.

Made complete sense to me, and thought it was like a "standard" technique done by eee-lecktrikle injunears (sorry, I just can't stop jokingly mis-spelling the word "Electrical Engineer").



Never got any straight answer myself, if this was "Bad Practice" or not - as even though it made 100% sense to me, it still may have been a poor choice somehow.

That was circa 1984 to 1987, and by the beginning of 1988 I was taking on the first (of what turned out to be many) Design/Build Projects - as the King Dingdong in charge of creating the Electrical Power and Lighting systems + generating the Construction Documents (drawing the plans).

When any 4 wire delta systems were encountered, I took the "Old School Approach" so commonly seen, and designed using the "1 Phase / 3 Phase" Panelboards concept.

Never had any issues with Plans Examiners or Inspectors (had many complimenting the idea as "Thinking Outside The Box").

So far, after searching the NEC, querying the ideas to building departments + inspectors, and asking a few "open minded EEs" (explained later), all signs point towards "Good and Ethical Design Practice".

Currently - and maybe as long as 10 years current, a 4 Wire Delta is rarely seen, let alone being involved with one of my designed projects.

What seemed to be so "normal" back then, is more or less nostalgic, and in a way - legendary stuff, when looked at in the field today.

Just felt really compelled to mention this after seeing George's reply, as just recently I was describing these very same things to field personnel - those of which were in Elementary School at the time I was first starting to perform designs involving the 4 wire delta systems.

< memory book = closed >

Scott35

btw:

"open minded EEs"

Referring to those EEE-Lecktrikkle Injun-ears, who refused to place themselves in Ivory Towers, and kept their minds open to remember how things in the field went; - like remembering the simple fact of how rapidly smoke is lost from equipment, intended to be connected to a 120V circuit, when it's connected to a circuit having a potential in the 185 - 215 Volts range!

Most had Trade Knowledge (worked as Electricians before going the EE route), a few didn't - all had excellent skills, nonetheless.

Rant is over

Scott, that is exactly the way the computer installations I saw were wired. They brought the 3p panel into the computer room without the grounded conductor, only grounding for the equipment. The regular office space and the single phase (lights and 5-15s on the wall) in the computer room came from a separate single phase panel. The only thing we were careful of on the 3P panel was that the 2 wire L/L loads were not on the red leg.

The first inspectors I came in contact with required seperate panels because of the idiot factor. When I was an electrical knee high, I found out about the wild leg the painful way, but them lights sure was bright for a while...
I have run into several open delta services and the pocos like AEP/SWEPCO and ENTERGY have it available, but so many installations are specing pad mount xfmrs, which, to my knowledge either aren't available or more probable, the pocos dont keep them in stock and should one fail, they could not get your building up without a special order. I like 240 personally, as opposed to 208, but ya get what ya get. The Master who was my first boss must have had leather for skin because he would walk up to a switchboard and touch the bus with his two fingers, rocking them back and fourth, and say, "boys, this'un is 208, not 240..."

In reply to an early part of this post, according to the 1968 EE manual I own (for a look back, I wasn't even born then) the convention at the time was 277/480 style, not the reverse.

That obsolote terminology may have been in effect back in the late 60's (my 1969 handbook also used it). However, the 1993 edition (the oldest one I have)of the IEEE Red Book discusses ANSI C84.1-1989 as the standard that defines the nominal voltages in the US.

Usage of obsolete voltages (i.e. 110 and 220) is discouraged when refering to supply systems.

Looks like I lost track of this thread for a while......

My take is that a 240V 2-wire circuit (without neutral) does not have a solidly grounded conductor as part of the actual circuit, but the circuit itself is still quite solidly referenced to ground by the grounded neutral at the xfmr.

So I'd say the circuit is solidly grounded, it's just that it doesn't include a conductor which is directly connected to ground.


Thanks for that. That's the similar convention to the way we would normally show 3-phase voltages here, e.g. 240/415V.


Use 'em all the time over here......