high leg *stinger* to ground. Usable? 1

[gmelectric]

I am trying to install a piece of kitchen equipment in a restaurant. There is 3 phase power with a stinger leg (red and black to ground is 120, purple to ground is 208, any hot leg to any other hot leg is 240). The red and black legs are pretty much maxed out in terms of amperage, but the stinger leg is barely used. If I have a straight 208 single phase requirement for a piece of equipment, can I use to stinger leg and a neutral to power it? This is defienitely not a 208/120 piece of equipment.

 

[davidbeach]

Draw a diagram of the transformer, notice the winding and a half between the high leg and the neutral. If that half winding is already maxed out with its share of the 240V load. how is it going to carry the 208V load?

 

[EEJaime]

This is an old type of system. No equipment used in the US is designed to run on 208V. phase to ground. it is a "bastard" voltage that results from the center tapped transformer. Any piece of equipment that is "200", "208","220" or "240V" rated, is rated for this phase to phase. This might be different in Europe, but I am not familiar with those systems.

The three transformers are connected in a delta configuration with one of them, usually the larger sized transformer center tapped to provide 120 Volt power. All three phase loads and single 240V loads are fed by connecting between A-B, B-C or C-A nodes. All 120 volt loads are fed from the B to "high leg" or center tapped neutral or the neutral to C connection. It was common practice in the past to install these systems where both 120 and 240V loads existed. Utility companies started phasing these out as wye connected systems became more popular and available. They have the disadvantage, (as do 120/240 single phase services), of being difficult to balance on the distribution system.

 

[rbulsara]

To answer the basic question, I do not see why it wont work, not do i know of it being prohibited in NEC.

davidbeach: If transformer capacity is used up, it wont take any kind of load, whether be it 208V or 240V, without being overloaded.


eejamie: It may be old, but still a valid congfiguration and meets Code.

 

[davidbeach]

rbulsara: Agreed. My point was that it's not just one winding in use. You could get one of these where there is enough 120V single phase and 240V three phase to completely load up the center tapped winding without loading up the other one or two windings (depending on number of transformers in use) and it looks like there ought to be capacity on the high leg. In that case you could add 240V single phase, only on one winding, but not 208V which uses two windings.

EEJamie: Any load that can run on 208V single phase would not see any difference between being connected phase-to-neutral on a 240V delta and being connected phase-to-phase on a 208Y system.