Re: 3 phase wiring

Posted by John T on October 23, 2008 at 07:29:27 from (66.244.97.31):

In Reply to: 3 phase wiring posted by skip33652 on October 22, 2008 at 16:51:16:

Skip,

QUESTION: what is the difference in 208 volt vs 240 volt 3 phase?

ANSWER A: What you call 208 3 phase is the result of three secondary transformer windings connected in a Y configuration with one common end where the three individual secondary windings are all bonded serving as the NEUTRAL. Then the voltage from the other end of any of the three windings with respect to NEUTRAL is 120 volts (3 legs of 120 volt single phase) while the voltage from either end to any other end is 208 volts. In laymans terms what you end up with is three individual legs of 120 volt single phase, L1 to N,,,,,L2 to N,,,L3 to N PLUS you have 208 volts 3 phase available on the three ends to power 208 volt 3 phase equipment. The technical terms for that configuration is 208 Y 120 Volt 3 Phase 4 Wire...............

ANSWER B: What you call 240 volt 3 phase is one common wiring configuration where the three transformer secondary windings are connected together IN A DELTA (NOT a Y) CONFIGURATION. In such theres 240 volts across each of the three transformer windings (L1 L2,,,,,L2 L3,,,,,,L3 L1) and those three feeds can power 240 volt 3 phase loads. This is a 3 wire NOT a 4 wire system (theres no common Neutral). The technical term would be 240 volt 3 phase 3 wire DELTA. You can ONLY get 240 volts, be it three legs of 240 volt single phase (across any one transformer) or 240 volt 3 phase if you use all three.

NOTE, in the above configuration, its possible to have one of the three transformers center tapped and use that center tap point as a Neutral
such that the one transformer can supply two legs of 120 volt single phase (L1 to N or L2 to N) PLUS still supply 240 volts single phase across its ends PLUS that transformer can still serve as one of the three necesary legs of 240 volt 3 phase. The technical term for this is 120/240 volt 3 phase 4 wire CENTER TAPPED DELTA or High or Red Leg System because while the voltage from either end of the center tapped xformer to Neutral is 120 volts, there would be 208 from the other end to Neutral i.e its the HIGH OR RED LEG

QUESTION: What are the differences and advantages or disadvantages of each?

ANSWER: the difference is the 208 Y 120 volt 3 phase 4 wire system can supply three legs of 120 volt single phase plus 208 volts 3 phase for 208 3 phase loads.......The straight 240 volt 3 phase 3 wire Delta can supply three legs of 240 volt 3 phase plus 240 3 phase.......The 120/240 volt 3 phase 4 wire center tapped Delta (red or high leg system) can supply 2 legs of 120 volt single phase plus 3 phase 240.

QUESTION: also can 208 be used for 240 and 240 for 208?

ANSWER: Some devices can be operated at either while other can NOT.

PRACTICAL ANSWER: I used to design secondary power distribution systems for a living as an electrical engineer but Im long retired n rust now. HOWEVER here is how I chose which system to use.

If I was designing say a primary office use complex that had mostly 120 volt single phase loads Id use the 208 Y 120 volt 3 phase 4 wire system and specify 208 volt 3 phase HVAC equipment. HOWEVER if it was more of an industrial building with lots of motors and construction equipment etc and only minimal 120 volt convenience receptacles, Id use the 120/240 volt 3 phase 4 wire delta and feed my small 120 volt loads off that one center tapped xformer.......or maybe if there was a lot of 240 single phase plus some 240 volt 3 phase I might use a dry xfmr to get my 120 volt.

Hope this helps you understand the different systems

John T Nordhoff BSEE Longggggggg retired

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