Use of 1.154 in single phase calc on three phase

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fsimmons

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Is there a good reason for using the 1.154 multiplier with 208 volt single phase loads when other three phase loads are connected to a three phase wye 208/120 transformer. Example: 8 amp motor load

(8 X 1.154 X 208) = 1920 vA instead of 8 X 208 = 1664 vA.

If this single phase line to line load was applied to a three phase transformer and there was no other load, it is conceivable that the transformer will see the load more appropriately as 1920 vA which is an increase of 256 vA.

Having said that, why would you use this multiplier ( 2/3 of square root of 3) in calculating a list of about 10 items where only one or two of the loads was single phase line to line load.

Can someone please set me straight on this one.

Also where is Rattus? Unable to make contact!!
 
hardworkingstiff said:
I think it is the 1.154. Remember the "Oregon Fudge Factor" thread?
Click to expand...

Not doubting you but I don't remember that one.

I don't follow the calculations threads as much as I should. (and it shows
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fsimmons said:
Having said that, why would you use this multiplier ( 2/3 of square root of 3) in calculating a list of about 10 items where only one or two of the loads was single phase line to line load.
Click to expand...

I wouldn't. I would try to spread the single-phase loads across all three phases as best I could.
 
fsimmons said:
If this single phase line to line load was applied to a three phase transformer and there was no other load, it is conceivable that the transformer will see the load more appropriately as 1920 vA which is an increase of 256 vA.
Click to expand...

Since 8-amps would be flowing through 2 out or 3 phases of the 3-phase transformer, I would conclude that the transformer would need to be rated for a 2,882 KVA load. 8*208*1.732=2,882 (each coil would have to be able to handle the 8-amps). This is different than the load the transformer would see.

I am curious, why do you think the transformer would see this as a 1920 VA load?
 
fsimmons said:
Is there a good reason for using the 1.154 multiplier with 208 volt single phase loads when other three phase loads are connected to a three phase wye 208/120 transformer. Example: 8 amp motor load

(8 X 1.154 X 208) = 1920 vA instead of 8 X 208 = 1664 vA.

If this single phase line to line load was applied to a three phase transformer and there was no other load, it is conceivable that the transformer will see the load more appropriately as 1920 vA which is an increase of 256 vA.

Having said that, why would you use this multiplier ( 2/3 of square root of 3) in calculating a list of about 10 items where only one or two of the loads was single phase line to line load.

Can someone please set me straight on this one.

Also where is Rattus? Unable to make contact!!
Click to expand...

You would apply this fudge factor to a single, unbalanced, 208V load. If other 208V loads tend to balance the system, you would not apply it all.

This factor arises because a single 208V load appears to draw 208VxI VA. But, the transformers think they are delivering 2x120VxI VA, and that is the crux of the matter. We try to not overload the transformers.

2x120/208 = 1.154
 
A transformer can only be overloaded if more than rated current flows through its windings. Fudge factors are used to simplify calculations. Phrases like "the load draws xVA, but the transformer sees yVA" often result in more confusion because the underlying cause is not understood.

The actual math reasoning is:
A 1664VA 208V single phase load will cause 8A to flow through each winding of a 208Y/120 wye connected transformer. Because the voltage across each winding is 120V each single winding size needs to be 960VA. Because there are three windings in wye, the resultant 3-phase transformer bank needs to be 2880VA in size. For watts loss calculations the effect is 2 times that of a single winding, or effectively equal to a single 1920VA transformer.

There, have I covered all of the answers so given so far? Pick which method you like best.
 
jim dungar said:
For watts loss calculations the effect is 2 times that of a single winding, or effectively equal to a single 1920VA transformer.
Click to expand...

Jim,
Could you expand on this a little? Where is the "watts loss" coming from? How did you calcluate it?

Thanks,
Lou
 
Jim, don't you mean to say that the single-phase load causes 8A to flow in two of the three transformers?

And, however you explain it, this is the technical basis for the Oregon Fudge Factor which some claim is non existent.
 
Rattus,
Yes, 8A does flow through two of the three windings.

Lou,
I am not talking about the actual heat losses of the transformer. My point is that because two windings are involved the effective losses are twice that of a single 120V transformer. In my opinion this number has nothing to do with the real world, except in the rare case of two transformers connected in an open-delta primary with an open wye secondary.

And I am one that does not find any reason to play with the numbers and create a fudge factor. All this factor does is tell you that the 3-phase transformer needs to be larger than the 1-phase load, it does not tell you the actual 3-phase size required.
 
e57,


Don't go stirring a horne's nest! If you must, here it is... The horse is waaaayyy past dead!

Use the local search in this forum on top of the page...
 
So a dead horse, not operable links to search pages on a dead topic of this very post here????? Still dont get the point, and guess I wont.....
 
e57 said:
So a dead horse, not operable links to search pages on a dead topic of this very post here????? Still dont get the point, and guess I wont.....
Click to expand...

It boils down to this. The State of Oregon, in one of their tests asks a question similar to the one below.

In a 120/208V wye, with a single 208V, 10A load, what is the apparent power delivered by the two transformers supplying that load?

Most would say 2.08KVA.

However, if one considers the transformers individually, one sees that each transformer delivers 120Vx10A = 1.2KVA for a total loading of 2.4KVA.

In other words,

Papp = 208V x 10A x 1.154 = 2.4KVA

We have dubbed this elusive factor, the Oregon Fudge Factor.

It appears to be a question to test one's understanding of phasors and apparent power. To that end, it works.
 
e57,

The link is to an eleven page thread here in the forum. Thought before it was rehashed, a viewing might be considered....

Not neccesary, Kingpb explains it quite well.
 
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