208v question

Status
Not open for further replies.
coulter said:
Why wouldn't the open delta loads suffer from high source impedance similar to the Vbn loads? Rattus would likely say the impedance is 2Z. I can't tell, I haven't eeen a model or the math yet.

carl
Click to expand...

Yes, I would say that, but I haven't thought it out yet.
 
jim dungar said:
Circulating currents in a closed delta are one reason that a standard (single core) dry type 240/120 3PH 4W transformer is limited to 5% line to ground loading.
Click to expand...

5% ?!!
Line to neutral loads [on a centertapped delta] shouldn't be more than 5% of the total va ? . Are you sure of that ?
I would have never guessed that 10% line to neutral loads would be a problem
 
dnem said:
5% ?!!
Line to neutral loads [on a centertapped delta] shouldn't be more than 5% of the total va ? . Are you sure of that ?
I would have never guessed that 10% line to neutral loads would be a problem
Click to expand...

Please notice that I said this was for a dry type closed delta on a common core. Check the literature from Square D, Acme, Hevi-Duty etc for their catalog listed 480-240/120 3-phase units. The traditional limit hasbeen 5% total single phase loading.

An open delta does not have this restriction which is one reason most 240/120 3Ph 4w systems use individual transformers. For an open delta the common formula is Total transformer KVA = 2.5S+T
 
jim dungar said:
An open delta does not have this restriction which is one reason most 240/120 3Ph 4w systems use individual transformers. For an open delta the common formula is Total transformer KVA = 2.5S+T
Click to expand...

So a centertapped open delta has 57% less va than a centertapped closed delta but will tolerate a higher percentage of line to neutral load ?
Is this right ?
 
dnem said:
So a centertapped open delta has 57% less va than a centertapped closed delta but will tolerate a higher percentage of line to neutral load ?
Is this right ?
Click to expand...

Yes, an open delta has less kVA then an equivalent closed delta. The single phase restriction is dependent on the transformer bank construction.

The single phase restriction of a closed delta is primarily an issue with a single core (effectively 3 identical transformers). If the closed delta is constructed of 3 individual transformers it is possible to oversize any, or all, transformer in order to get the desired output capacity.

The times this has usually caused me problems was when a service was being upgraded and the customer wanted his own 240/120 3PH 4W transformer to feed his "old" distribution system and he thought he could get it from a "stock" unit. Other times it has been an issue is when a customer did not want to have an ungrounded or corner grounded 240V delta, so they bought a center-tapped unit figuring they could also use the 120V connections.
 
Wild Leg Phasor Diagram

Wild Leg Phasor Diagram

The attached diagram shows the qualitative effects of series resistance and leakage inductance on Vbn in a wild leg service with no other loading. It looks strange, but I do believe it is correct.

The load current is assumed to carry a phase angle of 90 degrees.

Comments welcomed.
 
bpk said:
I have never tried this but have always wondered if you have a piece of equipment (like an AC unit) rated at 208 volts typically it would be fed with 2 hots, if you had a high leg delta would it work the same to feed it with a neutral and a the 208v high leg? I have always thought it wouldnt make a difference but never knew for sure, as long as there wasnt any integral parts that needed 120v.
Click to expand...
I believe the high leg on a delta sytem is 190 volts to the neutral, too low for a piece of equipment rated at 208/240v. BillW
 
rattus said:
The attached diagram shows the qualitative effects of series resistance and leakage inductance on Vbn in a wild leg service with no other loading. It looks strange, but I do believe it is correct.

The load current is assumed to carry a phase angle of 90 degrees.

Comments welcomed.
Click to expand...
Well, I crunched some numbers for fun.

I'm not too happy to see one of the 120 volt legs increase and one decrease as it is not what I would have guessed, but I do not feel like re-drawing the diagram right now.

Just wondering if the numbers even make sense. If no one feels like tackling a similar calc, I'll re-calc my numbers later, maybe on the computer instead of paper to avoid calculator key-punch errors.

I assumed some reasonable %Z and %R for a 3-pot wye-delta bank and transfered everything to the load-side. I also assumed a neutral reference (don't have a cow carl), with the reference voltages of 120<180, 120<0, 207.8<90

With a resistance such that the high leg load was close to rated per-phase current: 116.1<180.79, 123.9<-0.74, 203.4<86.72 and IR = 94% FLC

With an impedance such that the high leg load was close to rated per-phase current: 121.3<-181.82, 118.8<-1.86, 196.3<91.09 and IZ = 91% FLC
 
say what ??

say what ??

Being a simple "dumb-schmuk" inspector, I take some consolation in the fact that there appears to be no 208 volt SP breaker, so "I'm covered".

To preserve what little sanity I have, I have convienced myslef that the rest of this thread was organized by a small group of brilliant soles who
are practicing a "new" langusge, known only to themselves.

Gawd, you guys can humble so many like myself that do well just to recognize the words.:smile:
 
augie47 said:
Being a simple "dumb-schmuk" inspector, I take some consolation in the fact that there appears to be no 208 volt SP breaker, so "I'm covered".
Click to expand...

While there may be no "load center" breaker rated for 208V 1Pole, there are many panelboard breakers that are (think of those used on 480Y/277V).:smile:
 
jim dungar said:
While there may be no "load center" breaker rated for 208V 1Pole, there are many panelboard breakers that are (think of those used on 480Y/277V).:smile:
Click to expand...
actually, I did think about that, but figured that the possibility of the 208 system being used with a 480/277 paneboard was remote. Then again, using the 'high-leg" and neutral is remote too, huh ? :smile:
 
augie47 said:
actually, I did think about that, but figured that the possibility of the 208 system being used with a 480/277 paneboard was remote. Then again, using the 'high-leg" and neutral is remote too, huh ? :smile:
Click to expand...

I still see projects with 240/120 3PH 4W several times a year in Wisconsin.
 
For the High Leg to Neutral.
This is a rather unique scenario, never seen it before, and
I would like to clear it up in my mind.

Interesting, approach, George.
Since you figured it 'mathematically',
could you provide the equation approach.
Perhaps a lead, and I'll look it up in my EL theory book.
 
I found a lead from GAR.

07-07-2008, 06:49 AM

If this is a delta system with one center tapped secondary to provide 120-0-120 single phase, then there is a wild leg. The wild leg voltage to the center tap (neutral) is 240*0.866 = 207.8 V. This you derive from a vector diagram. The 0.866 is 1/2 the sq-root of 3 and it is also the sin 60 deg. The sin of 30 deg is 1/2 and thus the 120 V.

This quote makes good sense. I can follow up.
 
I Hate to open a can of worms...but...

I Hate to open a can of worms...but...

Based on the last post does all of this mean the "Oregon Fudge Factor" only applies to a Delta not a Wye 208/120 configuration?

Also someone stated Sin 30 deg, I believe it is COS 30deg to calculate the phase angle..
 
Status
Not open for further replies.
Top