3-phase

Can someone tell me what I would see with a voltmeter accross the various wires and to ground?
I think I'd see 120, 120, and 240 to ground on each of the wires respectively.

I'm only trying to understand a little better to understand future options. I'm not rewiring anything.

I've pretty thouroughly scanned Comed's service document. I see the choices but not a good explanation.

Thanks - Dave

The post of Dave - ORd is right on except for the last statement about the wild leg possibly going down. That"s why they call it a wild leg, because it doesn"t stay right at 208. When the single phase loading gets unbalanced, the wild leg swings up and down with the unbalance. It drives engineers crazy.

Gary

We have 240 Volt three phase open delta.

Phase voltages read 125, 208, 125 Volts to ground.

With the motor load of the grain dryer setup, and inherent phase imbalance of the open center system, the voltages would read 125 180 125 to ground at full load.

I've heard about the wild phase for years, but about 20 years ago was involved in an office building with 3 phase that didn't have a wild phase - just had 3 120 volt lines and the space planners / consulting engineers had totally screwed that up. Cost my company about 50K to fix their totally eff'd up original installation.

I found exactly what I was looking for at the linked location.

Thanks to all.

• Electrical Supply Configurations

In downtown Toronto during the 1960's there was a push for a simpler cheaper 240/416 service. All lights, office equipment etc was to be 240.Everything in Europe was 240 so they thought no problem here. And motor loads 416V. The very few 120V only loads were fed from a small bank of stepdown transformers. Today the serices have had a whack of extra transformers added for the 120 loads. 416 motors are hard to come by so an oversized US 440V often gets used as an replacement.To cope with the low voltage.

Typically, the center-tapped transformer supplying legs A and C is much beefier than the smaller transformer supplying leg B (the wild leg). Something like a 50KVA main transformer and 5 or 10 KVA phase B transormer is typical.

The electric company sizes the transformers assuming that the bulk of the loads will be single phase loads, with a relatively modest amount of three phase loading. The big transformer drives all of your single phase loads -- the "wild leg" is generally only brought in for three phase loads. The advantage to the power company is that they only need to bring in two of the three high voltage phases, three, and they only need two transformers vs. three. The disadvantage is that the arrangement is less efficient at supplying three phase power for a given wire size, and the phase balance and power quality of what the end user gets is less than with a true 3 phase wye or 3 phase delta arrangement. If you drive down the road past your house and see 4 high voltage wires, then there are already all three phases available at the road. Look at the power poles serving your neighbors. If you see three equally sized transformers, then true three phase is availalbe. If you see a big transformer and a small transformer, you may be looking at an open delta configuration.

The three phases delivered to you should read 120 volts phase to neutral for phase A and phase C, with 208 volts phase to neutral for phase B. I disagree with the posters that say this will go down to 180 volts with loading.

Example:
Underground Service:
The following underground service may be provided:
� Single-phase, 120/240-volt, three-wire, grounded.
� Three-phase, 208Y/120-volt, four-wire, grounded, wye.
� Three-phase, 480Y/277-volt, four-wire, grounded, wye.
� Three-phase, 240/120-volt, four-wire, grounded, open-delta (motor loads limited
to 40HP or less)
� Three-phase, 480/240-volt, four-wire, grounded, open-delta (motor loads limited
to 40HP or less)

Overhead Service:
The following overhead service may be provided:
� Single-phase, 120/240-volt, three-wire, grounded.
� Three-phase, 208Y/120-volt, four-wire, grounded, wye.
� Three-phase, 240/120-volt, four-wire, grounded, delta.
� Three-phase, 480Y/277-volt, four-wire, grounded, wye.
� Three-phase, 480/240-volt, four-wire, grounded, open-delta (motor loads limited
to 40HP or less)
� Three-phase, 480/240-volt, four-wire, grounded, delta ( for motor loads of 40HP
or greater, served from overhead primary lines only)

Be aware of the dangers of the grounded phase in a 3 phase system. I never could get an explanation as to why they used that system.

steveormary

Read thru the above posts again. Some of this stuff is comming back. Alot of 3ph motors will run just fine on 208. I would think the best system to go with would be a 208/120 3ph 4 wire installation. This would give you the 120 for lights and recepts and the single phase or 3 phase you would need for motors or other equipment. Be careful and have a prosporus and Happy New Year.

steveormary