3-phase 3-wire vs 4-wire


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Old 03-21-17, 09:53 PM
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3-phase 3-wire vs 4-wire

It's always been my understanding that saying "3-phase 3-wire" means you have 3 phases and no neutral. And "3-phase 4-wire" meant that there was in fact a neutral. I've always thought that the ground conductor does not count in the number of "wires".

Is this right?

Thanks.
 
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Old 03-21-17, 10:20 PM
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That is correct. The ground is never counted in the wire count.

3 phase 3 wire would be for the most part a delta system.
3 phase 4 wire would be for the most part a wye system.
 
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Old 03-22-17, 04:32 PM
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4 wire could also be delta with a stinger (high) leg. You most commonly find this in old smaller commercial and some older residential (I have it for example) where older/previously installed A/C units and well pumps are 3 phase. A 240V system will be 240V phase to phase, 120V between A-N and C-N, and 208V between B-N. These are love/hate, because you end up with higher phase voltage therefore lower current draw, but you can't connect any 120V loads to the B phase so you end up with wasted spaces in the panel.. So you either have to get a larger panel than you'd otherwise need or you put in a separate split phase panel for that..

A small wye system would be 208V phase to phase and 120V from any phase to N.

(and yes I did see "for the most part", just going farther into it.. )
 
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Old 03-23-17, 11:02 AM
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I've always thought that the ground conductor does not count in the number of "wires".
This isn't always true. In 240 volt and 480 volt grounded delta systems you have 3-wires and no neutral conductor, but one phase, usually the "B" phase, is grounded. You have a grounded phase conductor, but not a ground conductor. Regardless, it's still a 3-phase 3-wire system.
 
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Old 03-24-17, 06:11 AM
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A 240 volt 3 phase system with one phase grounded (a 240 volt corner grounded system) cannot be used readily for 120/240 volt power. A transformer is needed to provided 120/240 volt power. Specifically the transformer primary would be connected to any two conductors of the delta 3 phase source and a center tapped transformer secondary would provide the 120/240 volt power with the center tap grounded and connected to all of the "neutral" conductors.

Any 240 volt only loads should be fed directly by two legs of the corner grounded system to allow the use of a not so large transformer. A separate panel is needed since the 120/240 volt transformer secondary conductors must be kept separate (isolated).
 
 

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