3 phase Delta High Leg Single Phase Load

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Smart $

Smart $

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Location
Ohio
LarryFine said:
Yes, but is my thinking? Or does it place no undue thermal strain on the transformer?
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If by undue you mean compared to typical 1? and 3? loads...

Say you connect a 20A hi-leg to neutral load (4160VA@208V). There will be 10A flowing on all three windings (3@240V), which is equivalent to 7200VA of typical loads: 7200 ? 4160 = 173%
 
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mivey

mivey

Senior Member
rattus said:
Larry, draw the currents (I/2) as two loops, one CW and the other CCW. They combine in the load to form the load current (I) and split again at the CT.
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When Larry said "CT'd secondary" I immediately thought "current transformer" instead of "center-tapped". Either one you talk about, the current spilts and combines.
rattus said:
If we assume that Vbn = 208V@90 with a resistive load, the phase angle of the load current will also be 90 degrees.

This means that (I) will be 90 degrees out of phase with Van and Vcn. (I) will be 30 degrees out of phase with Vba and Vbc. Good reasons not to use the high leg voltage.
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Are you saying it is bad for a current to be out of phase with the source voltage or some of the source voltage internal components?
 
N

nakulak

Senior Member
phasor drawings, current calculations, thermodynamic effects of variant magnetic flux differentials, and quantum mechanics of all of the above are nice to know, but in deference to oversimplifying this quagmire:

is there really any difference between
1) the unbalance of the single phase loads (120)
and
2) the unbalance of some 208 loads.

Or to put it another even more simple way (and this is my real question)

Let's say this tranny feeds one panel. The jman hooking up the loads is probably going to check the a,b,c phases on the line side with his amprobe to see if the thing is mostly balanced. So, in doing so, let's say he gets withing 10%, is the transformer going to be similarly balanced regardless of whether or not there are 208 loads ? (I say yes, but I really am interested in hearing if I am wrong)
 
mivey

mivey

Senior Member
Smart $ said:
If by undue you mean compared to typical 1? and 3? loads...

Say you connect a 20A hi-leg to neutral load (4160VA@208V). There will be 10A flowing on all three windings (3@240V), which is equivalent to 7200VA of typical loads: 7200 ? 4160 = 173%
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That is a good way to think of it. Remember to square Smart$'s current results to get the heat loss ratio of 300%.

Heat loss = R*I^2

For single-phase high-leg:
4160 / 207.8 = 20.0148 amps => heating current = 10.0074 amps for each winding with a heat loss comparison of: 3*R*10.0074^2 = 300.444*R

For 3-phase:
4160 / 3 / 240 = 5.7778 amps => heating current = 5.7778 amps for each winding with a heat loss comparison of: 3*R*5.7778^2 = 100.148*R

For single-phase 240:
4160 / 240 = 17.3333 amps => heating current = 17.3333 amps for one winding with a heat loss comparison of: 1*R*17.3333^2 = 300.444*R

So it would appear for a given kVA, the high-leg has about same losses as the single phase. I'm sure the other internal reactances have some input and it might be a little different for non-resistive loads but I don't feel like crunching the numbers right now.
 
R

rattus

Senior Member
mivey said:
When Larry said "CT'd secondary" I immediately thought "current transformer" instead of "center-tapped". Either one you talk about, the current spilts and combines.Are you saying it is bad for a current to be out of phase with the source voltage or some of the source voltage internal components?
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mivey, I am saying it is inefficient. If we assume a single 208V load drawing the maximum current from the delta. Two of the xfmrs are delivering 87% of their possible real power--not so bad, but the other xfmr is delivering 0%, but we suffer the same iron and copper losses in all three.
 
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mivey

mivey

Senior Member
rattus said:
mivey, I am saying it is inefficient. If we assume a single 208V load drawing the maximum current from the delta. Two of the xfmrs are delivering 87% of their possible real power--not so bad, but the other xfmr is delivering 0%, but we suffer the same iron and copper losses in all three.
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I agree that using 3-phase is going to be more efficient, but we don't always have that choice and are stuck with a single phase device.

As for a single phase load and feeder, I would think the 240 volt option would be better than the 208 volt option but there may be extenuating circumstances there as well. There does not appear to be any real "danger" involved in choosing the 208 volt option.

As for the metering, I don't think with a form 8S it would matter as it has 3 CT's and the associative voltages. I would have to go back and look at the other forms to see if it would matter. Some meters do not follow Blondel's Theorem and that could lead to metering errors if the underlying assumptions are no longer true.
 
V

victor e

Member
Location
Southern Cali
Delta High Leg Single Phase load

Delta High Leg Single Phase load

This is funny because, I called the power company to service their transformer today. They told me told 210v to ground was normal? Ive never came across this before. Does the high leg have any negetive effects on 3phase motors (ac units, hood vents,etc)?
 
Smart $

Smart $

Esteemed Member
Location
Ohio
victor e said:
Does the high leg have any negetive effects on 3phase motors (ac units, hood vents,etc)?
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None at all. 3? motors utilize line-to-line connections for voltage and current. The hi-leg has the "odd" voltage only when referenced to the center-tap of the split-phase winding. 3? motors are not directly connected to that terminal or its conductors.
 
ohmhead

ohmhead

Senior Member
Location
ORLANDO FLA
Well question 3 phase delta closed we attach breaker to hi leg b .


All 3 phase loads are balanced all single phase loads are balanced next we have by example a dead short between B hi leg and c center .


A major fault at this point which breaker would trip first the single phase B on hi leg or the primary breaker or the secondary breaker ?

If we have a series rated system or a combine rated cordinated system which would or how would this effect the trip issue ?

Does that smaller breaker having a higher impedance meaning breaker contacts resistance during that fault on B phase now installed at higher voltage rating but now installed at a lower voltage on B phase what issues if any would be needed do you see a problem ?


Would that effect time to trip or would that B phase breaker blow out of panel explode during that point in time before secondary main breaker tripped ?

Just a electrician wondering again.
 
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Smart $

Smart $

Esteemed Member
Location
Ohio
mivey said:
...

For single-phase 240:
4160 / 240 = 17.3333 amps => heating current = 17.3333 amps for one winding with a heat loss comparison of: 1*R*17.3333^2 = 300.444*R

...
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This is incorrect.

Even though it is a single phase load, the current is carried through all three windings of a "closed" delta secondary: 1(1/R + 1/2R) = 2R/3.

So for single phase 240, the heat loss comparison is: 1*2R/3*17.3333^2 = 200.296*R

Your assessment would be correct for a single phase load connected across one winding of a an open delta secondary, but it would again be incorrect if connected across the missing winding terminals.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
ohmhead said:
...Just a electrician wondering again.
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While I can't answer your question, I was pondering something myself...

It seems to only be mentioned that one cannot connect a "slash" rated breaker on the hi-leg for these bastard 208V single phase loads. However, you cannot use a "slash" rated breaker for any load which is supplied in part by the hi-leg. This includes all 3? loads, all A-B and B-C line-to-line loads, and any would-be B-N load, where B is the hi-leg. The stipulation regarding these breakers is that no voltage to ground exceeds the lower of the slash rating... so that would exclude all the branch circuits mentioned. That leaves only A-C, A-N, and C-N circuits as suitable for slash rated breakers.
 
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mivey

mivey

Senior Member
Smart $ said:
This is incorrect.

Even though it is a single phase load, the current is carried through all three windings of a "closed" delta secondary: 1(1/R + 1/2R) = 2R/3.

So for single phase 240, the heat loss comparison is: 1*2R/3*17.3333^2 = 200.296*R

Your assessment would be correct for a single phase load connected across one winding of a an open delta secondary, but it would again be incorrect if connected across the missing winding terminals.
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That's what I get for typing without thinking about the parallel path. I just had a single coil on the brain and neglected the others.
 
mivey

mivey

Senior Member
ohmhead said:
...next we have by example a dead short between B hi leg and c center ....or would that B phase breaker blow out of panel explode during that point in time before secondary main breaker tripped ?

Just a electrician wondering again.
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You would have to compare the trip curves at the indicated fault current. Just thinking about utility breakers, it is possible that the curves cross at high fault levels or you have an instantaneous trip such that the up-line breaker trips.

I don't have any stats handy, but bolted faults are rarely seen (like with an installation wiring cross-up or something). Most faults have some impedance and would not be the same as a "dead short". The stats may be a little different in LV work but I don't know.

I doubt anything is going to blow out of the panel if you have the correct breakers installed.
 
ohmhead

ohmhead

Senior Member
Location
ORLANDO FLA
mivey said:
You would have to compare the trip curves at the indicated fault current. Just thinking about utility breakers, it is possible that the curves cross at high fault levels or you have an instantaneous trip such that the up-line breaker trips.

I don't have any stats handy, but bolted faults are rarely seen (like with an installation wiring cross-up or something). Most faults have some impedance and would not be the same as a "dead short". The stats may be a little different in LV work but I don't know.

I doubt anything is going to blow out of the panel if you have the correct breakers installed.
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Well i asked a GE rep /engineer today about this post and the hi leg issue . Said he get some info for me but said If it was used on the Hi leg single pole 240 volt brk would need to also have on its label a 1 ph 3 ph rating on that single pole breaker listed for both . And yes a slash rated would not work but a 1ph 3 ph sticker on that breaker would . Heres that breaker
Dispatcher.jpg


He also said thats not common to use a hi leg in that fashion but he also likes selling Ge breakers and panels . And added this one will work and GE has lots more in stock !
 
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