3 phase wiring

what is the difference in 208 volt vs 240 volt 3 phase? what are the differences and advantages or disadvantages of each?also can 208 be used for 240 and 240 for 208? thanks Paul

I have seen a lot of 208v being 5 wire (a neutral, and a ground to the shell of component)
220-240v can be just 4 wire (3 hots and a neutral) I have used a 220v supply on 208v.
One better than the other?

We do it all the time at work and it depends on what you're running most new generation 1.15sf energy saver motors can handle the heat because as you know voltage down amps are up just make sure your starters, heaters and relay contacts and wire size is good for the amps and you should be good to go. Trying to get everything for 208volts three phase is hard to come by and expensive to boot. Most new generation VFD's have a broader voltage range also with better cooling.

If you are using three phase 240V. It"s going to apply 139V to any single phase loaded wired from the A,B or C phase to neutral.
228V is the highest three phase voltage that keeps 120V equipment within the 10% under/over voltage range.
While some folks don"t believe it. That bare ground and insulated current carrying white neutral are not the same thing.

I was just answering the question can you use 240volt 3 phase rated equipment on 208volts? Never had the other condition.

208 volts is 120 volts times the square root of three. A 208 wye three phase circuit will give you 120 volts on each leg to neutral. Whether or not you can substitute one for the other depends on what the equipment is. A lot of three phase equipment (such as welders) can be set up to use either one.

208/120 is WYE configuration with the neutral, 4 wires plus the ground. As stated already, connecting to any one of the phase conductors plus the neutral gives 120 volts. 208 is good where many of the loads are 120 volts. 240 volts is Delta configuration. Delta may or may not have 120 volts available. If one (of the three) transformers has a center tap, that tap plus a connection to either end of that transformer secondary winding will give 120 volts. That places all the 120 volt loads on just one phase and thus the 120 volt loads should be limited to avoid excessive phase imbalance. Connecting to the other 3-phase plus the neutral of such a Delta system gives 208 volts Single-phase and should never be done. That is called the "high leg" and is marked with orange tape. Often larger Delta services will have smaller WYE connected transformers inside the premises, owned and installed by the customer, to supply their 120 volt loads. Not cheap, but a really good way to do it.

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

If they are not the same then why are they hooked up together at the main box. The neutral wire is to get 110 from a 220 system it goes to ground rather back to the power source.

The government in its infinite wisdom uses 208 on all Military bases. They have to get special equipment to run on it.
Walt

Jeez guys I'm impressed kinda like asking someone what time it is and they tell you how a watch works. Again the question was will 208volts 3ph work on 240volt 3ph. equipment and the answer is yes. He didn't ask you how to wire it. Personally I haven't tried the other direction. You EE's need to deal with the NEC not me those of us in the field have to do what we gotta do to make things run escpecially when your equipment is down and China made parts are weeks away just get out the calculator do Ohm's law and the conductor chart and make it happen.

The ground wire is not to carry running current, That is what the insulated white is for.
The bare ground wire is used to hold the chassis, not the current path down to ground potential.
The ground is also supposed to keep the neutral from floating above earth potential.
If ground and neutal was the same. Why does the electrical code call for three live lines, an insulated neutral and a bare ground, five wires?
A 120/208 grounded Y system is very flexable and economical system to install and operate.
120V single phase loads, "240V" single phase and three phase motors. Even 347/600 three phase with with a delta/Y transformer.
No wild legs and odd taps as some of those grounded deltas have.