# THREE PHASE QUESTION...

• posted

We are having a problem at our church with incoming 3-phase electrical power. Does anyone know what the allowable voltage variation between the legs on incoming power is "industry acceptable"? We have what seems to be a great disparity--- 200v, 210v and 220v between the legs. As I recall it should be 208v on all legs, +/- what? That is the question.

PJPowell New Jersey

• posted

On 20 Sep 2004 17:22:57 GMT, None put forth the notion that...

If you have a delta system, you should have 120 to ground on two of the legs, 208 to ground on one of them, and 240 phase to phase. If you have a wye, you should have 120 to ground on all three phases, and 208 phase to phase.

• posted

Many electric utilities reference ANSI Standard C84.1-1995

208Y/120 Nominal System Voltage 200 Nominal Utilization Voltage

218Y/126 Maximum Utilization and Service Voltage

197Y/114 Minimum Service Voltage

191Y/110 Minimum Utilization Voltage

**With a voltage unbalance of 4.8%, 3ø motors should be loaded to ~77% of nameplate rating

?s falke

"N> We are having a problem at our church with incoming 3-phase electrical power.

• posted

Thank you both for the information. We now have something to back up our side of the discussions with the public utility.

Thanks again,

PJ New Jersey

• posted

| We are having a problem at our church with incoming 3-phase electrical power. | Does anyone know what the allowable voltage variation between the legs on | incoming power is "industry acceptable"? | | We have what seems to be a great disparity--- 200v, 210v and 220v between the | legs. As I recall it should be 208v on all legs, +/- what? That is the | question.

Those voltages are within the range usually allowed. But the fact that they are different at different phases is bad. Can you measure phase to ground voltages on each?

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Excuse me? In a std delta system you have no reference to ground at all. The system you describe sounds like a delta system with midpoint ground between two of the phases.

• posted

On 22 Sep 2004 13:03:20 -0700, bob peterson put forth the notion that...

I'm talking about a standard commercial 3-phase 4-wire 120/240 delta system. Since neutral is grounded, you'll also read the same to ground. As you probably know, standard 120/240 delta does have a center tap between two phases. The third phase (the high leg) will read 208 volts to neutral or ground.

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The most common delta system at 240 volts in the US is a 4-wire + ground configuration where one of the windings has a center tap which is grounded and thus resembles single phase 240/120 volt service. That allows using

240 volt delta loads as well as single phase loads. The voltage from that grounded center tap to the additional phase that is not in line with the center tapped winding is 208 volts. This configuration can be set up one of 3 ways, both involving a standard 240/120 volt center tap transformer. One uses 2 more 240 volt transformers to complete a delta. Another uses a single 208 volt transformer wired to the primary in a way to get a 90 degree phase shift (this arrangement is called a "Scott T"). The third is just an open delta with 1 additional 240 volt transformer instead of two.

A plus for this configuration is that the 240/120 transformer can be made to be a very large one, with smaller additional transformers for cases where the three phase load is small and the single phase load is large.

A negative for this configuration when closed delta is used is that upon one phase loss in the primary, the secondary can backfeed to the dead primary, usually blowing a transformer primary fuse.

Three 139 volt transformers could be used in a WYE configuration to make

240Y/139 which can run most 240 delta loads. Transformers to do this are made, but are usually custom and hard to find. It's half the voltage of the more common 480Y/277 volt system.

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