sundance-communications.com/forum Forums General General Telephone & Systems Information A question for any prior sparkies out there

Hal, just so you know and to clarify my response, here in Quebec, it has to be on a 2-pole breaker or 2 singles can be used, but MUST be tied together; and yes on different phases.

I figured as much. I guess Canada has been ahead of us on that. Only the latest '08 NEC (which few states have adopted yet) requires that, unless both phases feed a device on the same yolk. Then it's always been required.

Another change in the '08 is that if one of those three wire cables enters the panel through a duplex connector in a knockout with another cable, the three conductors from the three wire cables must be ty-rapped together so that someone can tell what hots go with what neutral.

Both changes are very good ideas.

-Hal

Hey, I like the ty-rap idea as well and makes good sense.

I'd actually like to take it one step further and do it anytime that two cables are in the same connector.

Question. Are they REALLY phases in the same context as 3-phase, or really just 'legs'? After all, aren't they really just opposite ends of a grounded-centertap transformer winding? And the other winding on the transformer is, at most, 1 phase of a 3-phase line, and more probably, a 2800V or whatever feed to the pole. My understanding of 3-phase is quite different than that! John C.

Well my understanding is that it is split phase. It's just two legs of power and a neutral.

On commercial jobs we run 3 phase, but residential - at least here in NYC is what they call "single phase 208" (No 110/220 here, it's 120/208).

As I recall, Long Island was 220V and Westchester was 240V but residentially they were all the same -two opposing phases - opposite sides of the sine wave.

Sam

Sam, how would it be single phase? The 208 comes from the three 120V phases being 180 degrees out of phase from each other...
Here it's all 120/240 single phase residential, and 120/208Y and 277/480Y for commercial...

Sam as I recall "single phase" is deriveed from a 3 pahse circuit feeding a transformer wired either Delta or Y which provides 1 leg "hot and neutral" as for 220 feeds to homes or such it is 2 legs "hot" with a common neutral.they are not the "opposite sides of a sine wave".

Jeff -

It's the same here 120/208 residential, 277/480 3 phase for commercial. I've never done residential feeder wiring personally. All I'm saying is that they call it "single phase 208". Put a meter across the two phases and you read 208 Volts. From either phase to the Neutral it's 120 Volts.

Jim -

When I said opposite sides of the sine wave I meant 180 degrees out of phase. On an O-Scope I believe when you would see one phase at the top peak, the other phase would be at the bottom peak. Sorry if I'm not explaining it well. I'm a telephone man who occasionally works as an electrician.

Sam

Jeff, since a single-phase service would take the two hots from A-B, B-C or C-A with a center-tapped neutral, it is still only single phase. Unless all three hot legs are included, it is still considered a single-phase service. A or B to neutral is still 120 volts. Thank Goodness that I paid attention in geometry class in high school.

Delta services are very uncommon these days since the "wild" leg resulting in 240 volts to neutral leaves way too much room for error. There are still plenty of them out there, but the trend is to phase them out for obvious reasons.

Most of today's services are "Y", where the neutral is established at the mid-point of the "Y". Any end of the "Y", as in A, B or C will provide 120 volts. Due to triangulation, 120 volts is multiplied by 1.732 (the square root of 3) to result in the 208 volts between phase letters.