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Monday, February 6, 2017
Question:
Re: CQD answer published Thursday, January 19, 2017 -Phase
We received several comments about the phase discussion so here are some of them.
(1) Your answer is a nonresponsive answer at best. Single phase consists of 1 phase conductors with 1 neutral or grounded conductor. Single phase may also consist of 2 phase conductors with or without an neutral conductor, i.e. 240 volt single phase or 120/240 volt single phase. Two phase power is typically limited to the system as designed by Nikolai Tesla where the output of 2 single phase generators had the neutral or grounded conductors tied together yielding 2 phase 5 wire which has 4 phase conductors and a single neutral or grounded conductor. The generators at Niagara Falls are two phase 5 wire, each phase is 90 degrees out of sync with the other phases not 120 degrees. You can still purchase two phase 4 wire motors that will operate on this system or you may inefficiently operate a three phase motor on 3 of the phase conductors. Three phase power has 3 separate phase conductors operating 120 degrees out of sync that may or may not utilize a neutral aka grounded conductor. The neutral is used to connect single phase loads to the three phase system. Cordially, Bill Nolte, C.S.H.O.
(2) Hi Guys, Being from Philadelphia I just wanted to mention that there is indeed 2-phase electric and it is in fact still in use today as some buildings in the ‘Olde’ section of Philadelphia were and are permanently wired for it. It has two hots and two neutrals and is 90° out of phase. Thanks for carrying on the tradition! Dan Haruch
(3) from an old electrician who worked on 5 wire two phase services in Hartford Connecticut. you would have 240 between phases and the neutral was shared for 120 voltages. Some of those services are still around. The code still has two phases. Neutrals had to be 140% of the ungrounded conductors. Fred Valente
A
Answer:
Thanks for all the additional information and comments. The 140 % is stated in 220.61(A) Exception for 2 phase feeder and service neutral loads. Single phase AC motor currents are Table 430.248, two phase AC motor currents are Table 430.249, and three phase AC motor currents are Table 430.250.