Pole Lights Calculation

[Sahib]

Silly would be buying 25% more transformer than you will ever need. Do you buy five new tires for your car when it is time for new tires?

You did not understand my reasoning here. So go through my post below and start your argument from there.

Take, for example, transformer capacity 180 KVA and pole light KVA=18. Then the no. of poles=180/18=10. But this assumes there is no power loss in the connecting cables, fuses etc., This assumption is not always workable and so there is concern for transformer overloading.

 

[mivey]

You did not understand my reasoning here. So go through my post below and start your argument from there.

Take, for example, transformer capacity 180 KVA and pole light KVA=18. Then the no. of poles=180/18=10. But this assumes there is no power loss in the connecting cables, fuses etc., This assumption is not always workable and so there is concern for transformer overloading.

I understood what you said about line losses. I just disagree with an assertion that line losses are that big. They are certainly not 25%. Refer back to my post:

... Besides, the line loss is probably 3-4% as a worst case and probably more in the 2-3% or less in practice.

 

[weressl]

You did not understand my reasoning here. So go through my post below and start your argument from there.

"Take, for example, transformer capacity 180 KVA and pole light KVA=18. Then the no. of poles=180/18=10. But this assumes there is no power loss in the connecting cables, fuses etc., This assumption is not always workable and so there is concern for transformer overloading."

As it was pointed out none of the above losses are significant to be concerned about transformer size increase. So why argue with something that is nonsensical. That would be stupid on its own.

 

[weressl]

Silly would be buying 25% more transformer than you will ever need. Do you buy five new tires for your car when it is time for new tires?

Nope, he only buys 4 for his three-wheeler.:lol:

 

[Sahib]

I understood what you said about line losses. I just disagree with an assertion that line losses are that big. They are certainly not 25%. Refer back to my post:

The line losses are not that big; let us take a conservative 5%. The losses in associated switchgear including fuses/breakers, control fuses=.......% and power loss due to temporary primary voltage hike during grid light load condition (an adverse environmental condition)=........% (Fill up yourself please).

As it was pointed out none of the above losses are significant to be concerned about transformer size increase. So why argue with something that is nonsensical. That would be stupid on its own.

Then the following would be an example of prime stupidity in this forum.

I would not load up a transformer to it's full capacity .

 

[mivey]

As you wish:

The losses in associated switchgear including fuses/breakers, control fuses=...INCLUDED IN THE GIVEN LOSS PERCENTAGE....% and power loss due to temporary primary voltage hike during grid light load condition (an adverse environmental condition)=...INCLUDED IN THE GIVEN LOSS PERCENTAGE.....% (Fill up yourself please).

 

[electrofelon]

Wouldn't one need to know the characteristics of the load to figure out if it adds or subtracts to the load seen by the transformer?

 

[winnie]

There are a huge number of factors which will influence the performance of a system, and _might_ influence it in a significant way.

A great number of the factors which influence a design are so subtle as to have a maximum range of effects which are smaller than unknowns inherent in the design. In this case, it is wasted effort to focus on the subtle effects, because effort spent there will give benefits totally swamped by random noise.

A good engineer will have judgement about which design aspects need to be concentrated upon, and which can simply be ignored as too small to matter. But as a cautionary tail, sometimes issues dismissed as insignificantly small can rear up and bite one's donkey.

A good engineer will also judge some things to be potentially significant, but decide that it is cheaper to overdesign the system rather than do an exact design.

To the OP, the huge unknown is the power factor of the lamp system in question, and weather or not the 1.5KW number is power _to the lamp_, or power to the lamp system (including ballast).

Once you know the VA of each lamp system (lamp, ballast, controls, etc) then it is a simple matter of adding up the VA numbers to get the total VA consumed by a set of 12 lamps (on a single post) or a set of lamp posts.

Now, these lamps systems are going to have a tolerance on their power consumption. It is very unlikely that a 1.5KW lamp is going to consume 1500.000W of power. Similarly the VA going into the ballast will not be an exact number, but somewhere in a range of values that depend upon things like applied voltage, ambient temperature, age of the lamp, burn time, etc. So even though you can simply add up the VA of each lamp system to get a good idea of total consumption, you _must_ expect that the consumption will be some % off from your calculated value.

Similarly, the transformer will almost certainly have a tolerance on its power rating, and the power rating itself will be based upon assumptions that leave lots of room for 'fudge'.

You could start doing a detailed tolerance stackup, to make sure that the maximum _actual_ KVA of your lamp system is less than the minimum _actual_ KVA of your transformer. Or you could do the easy thing and make your design load some fraction of the transformer rating. A bunch of the guys here have suggested using a 200KVA transformer at 180KVA...but note that power companies often use transformers at well _above_ their nominal ratings.

As to the relevance of power loss in the wiring to the lamps, I believe that is something that could potentially (and counterintuitively) be a problem. In particular, some modern lamp and ballast combinations are constant power devices, consuming more current as the supply voltage goes down. The lamp continues to perform adequately even with horrible voltage drop on the supply conductors. At the same time, outdoor lighting applications often involve large distances where the cost of conductors is a major factor, and some amount of voltage drop almost needs to be designed in to optimize the economics of the system. In this case (and the OP never specified what sort of lamps were being used), voltage drop in the supply conductors could significantly increase transformer loading.


-Jon

 

[weressl]

As to the relevance of power loss in the wiring to the lamps, I believe that is something that could potentially (and counterintuitively) be a problem. In particular, some modern lamp and ballast combinations are constant power devices, consuming more current as the supply voltage goes down. The lamp continues to perform adequately even with horrible voltage drop on the supply conductors. At the same time, outdoor lighting applications often involve large distances where the cost of conductors is a major factor, and some amount of voltage drop almost needs to be designed in to optimize the economics of the system. In this case (and the OP never specified what sort of lamps were being used), voltage drop in the supply conductors could significantly increase transformer loading.


-Jon

I was with you until "horrible" was introduced.

As engineers, or even 'just'designers or good tradesman, we like to deal with data, real values. Could you quantify "adequately", "horrible" and
"significantly increase"?

 

[electrofelon]

Wouldn't one need to know the characteristics of the load to figure out if it adds or subtracts to the load seen by the transformer?

Poorly worded. Meant to say "...... to know if the losses from VD will translate to load on the transformer " . Without running some numbers, I would think the more VD with a resistive load, the less power the transformer would supply.

 

[winnie]

I was with you until "horrible" was introduced.

As engineers, or even 'just'designers or good tradesman, we like to deal with data, real values. Could you quantify "adequately", "horrible" and
"significantly increase"?

Well, I will need to continue to hand-wave to some extent, but I'll try to provide a more concrete example.

It won't match the OP, but should suffice to give an idea.

Hubbel makes Metal Halide ballasts with the IntelliVolt feature. The ballasts are rated to operate over a nominal voltage range of 208-277V, with tolerance beyond the nominal range. In their operating range, the ballasts maintain a power factor of at least 90% with constant input power.

A lighting system built using such ballasts, with 277V at the supply end, could tolerate voltage drop down to 208V with _normal_ light output.

I would call full nominal light output 'adequate' performance.

I would call a 25% voltage drop horrible. I've not decided on the proper threshold that separates acceptable versus tolerable versus horrible voltage drop.

Given constant power, a 25% voltage drop to a constant power load corresponds to an increase of 33% in current drawn from the supply. The significance of this would depend upon the size of the supply and who pays the bills

-Jon

 

[Sahib]

Cyber warriors please excuse me for not continuing to argue with you in an attempt to find some common ground.

So winnie, for pole light with constant power characteristic, the 80% transformer loading holds or a lower/higher percentage for transformer may be adopted depending on the voltage drop in the cables.

 

[topgone]

Well, I will need to continue to hand-wave to some extent, but I'll try to provide a more concrete example.

It won't match the OP, but should suffice to give an idea.

Hubbel makes Metal Halide ballasts with the IntelliVolt feature. The ballasts are rated to operate over a nominal voltage range of 208-277V, with tolerance beyond the nominal range. In their operating range, the ballasts maintain a power factor of at least 90% with constant input power.

A lighting system built using such ballasts, with 277V at the supply end, could tolerate voltage drop down to 208V with _normal_ light output.

I would call full nominal light output 'adequate' performance.

I would call a 25% voltage drop horrible. I've not decided on the proper threshold that separates acceptable versus tolerable versus horrible voltage drop.

Given constant power, a 25% voltage drop to a constant power load corresponds to an increase of 33% in current drawn from the supply. The significance of this would depend upon the size of the supply and who pays the bills

-Jon

The voltage dropped and the current increases--> there's nothing in there that tells people there will be overloading of a transformer rated to supply a certain number of HID lamps in your example! HID lamps will always approximate its rating because the ballast limits the current across the arc, else those lamps will explode if ballast is damaged! Maybe you are explaining the metal halide lamp current draw during starting at lower than normal supply voltage, IMO.

 

[mivey]

Well, I will need to continue to hand-wave to some extent, but I'll try to provide a more concrete example.

Failed the slump test but let's do some supposing for fun.

I would call a 25% voltage drop horrible. I've not decided on the proper threshold that separates acceptable versus tolerable versus horrible voltage drop.

I would give you about half that for the sake of discussion and trying to create an extreme but unlikely example. The problem with trying to get to 25% is that you run into an ampacity wall for reasonable distances. If you allowed some goofy near-mile distances you could get there but how practical is that when you consider that running MV for long distances would be cheaper?

Even using this extreme scenario, we move from a normal, expected 3-4% size impact for source vs. load (probably 1-2% less in real practice) to a 6-8% size impact. A long way from the 25% (1/80%) size impact goofiness. Probably not cost effective to use a LV-only solution for these scenarios anyway.

The significance of this would depend upon the size of the supply and who pays the bills

-Jon

As you might expect, allowing for more voltage drop would save you money when you consider the bottom line including installation cost and cost of losses. But this is true only if you stay with a LV-only solution for the extreme cases.

We normally see most highway specs limiting voltage drop to around 3-5%. I'm sure if you could find a wide-range ballast you could talk them into a different spec. If I were reviewing the plan, I would ask at what point it makes sense to incorporate some MV gear. I can tell you it will be long before we have 20% losses.

 

[Sahib]

Failed the slump test but let's do some supposing for fun.

I would give you about half that for the sake of discussion and trying to create an extreme but unlikely example. The problem with trying to get to 25% is that you run into an ampacity wall for reasonable distances. If you allowed some goofy near-mile distances you could get there but how practical is that when you consider that running MV for long distances would be cheaper?
As you might expect, allowing for more voltage drop would save you money when you consider the bottom line including installation cost and cost of losses. But this is true only if you stay with a LV-only solution for the extreme cases.
I'm sure if you could find a wide-range ballast you could talk them into a different spec. If I were reviewing the plan, I would ask at what point it makes sense to incorporate some MV gear. I can tell you it will be long before we have 20% losses.

This does not make sense, for application of MV instead of LV becomes practical only when long distance as well as large current is involved. Long distance feeder for highway pole lights? Yes. Large current? No.
As a side note take the operation of a voltage regulator in a large buiding complex for example, it also can operate with an exteme input voltage drop of 25% or more and I can imagine you suggesting to the people using it to scrap
the regulator and go for MV instead.

Even using this extreme scenario, we move from a normal, expected 3-4% size impact for source vs. load (probably 1-2% less in real practice) to a 6-8% size impact. A long way from the 25% (1/80%) size impact goofiness.

Please explain a bit more.

 

[mivey]

This does not make sense, for application of MV instead of LV becomes practical only when long distance as well as large current is involved. Long distance feeder for highway pole lights? Yes. Large current? No.

200-300 amps can be large enough current. Big wire can get expensive real quick.

As a side note take the operation of a voltage regulator in a large buiding complex for example, it also can operate with an exteme input voltage drop of 25% or more and I can imagine you suggesting to the people using it to scrap
the regulator and go for MV instead.

Easy enough to do if the economics are right.

Even using this extreme scenario, we move from a normal, expected 3-4% size impact for source vs. load (probably 1-2% less in real practice) to a 6-8% size impact. A long way from the 25% (1/80%) size impact goofiness.

Please explain a bit more.

For a reasonable marginal installation scenario, I would expect the line loss to cause the transformer size to be increased by 3-4% at the most. For an installation with these loads and voltages that we might see in actual practice, I would expect the transformer size to be increased by 2-3% or less due to line losses.

For the not-so-realistic scenario of a 10-15% voltage drop on an un-realistically long LV line, I would expect the line loss to cause the transformer size to be increased 6-8%. A line loss scenario that would require the transformer size to be increased 25% would be some goofy scenario that would not or should not be installed due to it being impractical and way too expensive.

 

[winnie]

Mivey,

Granted the concrete was friable and the example is most likely pretty unlikely.

The point that I was trying to make is that the effect of voltage drop on transformer loading is _intuitively_ unimportant, but that in 'corner cases' our intuition might be wrong.

I agree that for long distances, higher voltage (or simply larger gauge wire) will likely make more economic sense once you consider the cost of electricity, and at some level a 25% voltage drop system emotionally offends me

But consider a situation where two different bean counters are in charge of installation and operation costs. You can deliver quite a bit of power using only 12 AWG wire at 480V, and if you are willing to tolerate losing 25% of that power to heating the wire, you can go for quite a distance. Compare the price of 4000 feet of 12AWG with 1000 feet of single phase MV and a transformer......

I disagree with your final calculation. Assuming a constant power load, transformer loading will increase by 1/(fraction voltage remaining). So if you have a voltage drop of 10% with a constant power load, I would expect transformer loading to increase by 11%.

-Jon

 

[weressl]

Mivey,

Granted the concrete was friable and the example is most likely pretty unlikely.

The point that I was trying to make is that the effect of voltage drop on transformer loading is _intuitively_ unimportant, but that in 'corner cases' our intuition might be wrong.

I agree that for long distances, higher voltage (or simply larger gauge wire) will likely make more economic sense once you consider the cost of electricity, and at some level a 25% voltage drop system emotionally offends me

But consider a situation where two different bean counters are in charge of installation and operation costs. You can deliver quite a bit of power using only 12 AWG wire at 480V, and if you are willing to tolerate losing 25% of that power to heating the wire, you can go for quite a distance. Compare the price of 4000 feet of 12AWG with 1000 feet of single phase MV and a transformer......

I disagree with your final calculation. Assuming a constant power load, transformer loading will increase by 1/(fraction voltage remaining). So if you have a voltage drop of 10% with a constant power load, I would expect transformer loading to increase by 11%.

-Jon

Another thing to remember is that street lighting on is roughly 1/3 of the day, so to recoup the energy saving to offset the copper cost is considerably longer than with a constant, around-the-clock loading.

 

[mivey]

But consider a situation where two different bean counters are in charge of installation and operation costs. You can deliver quite a bit of power using only 12 AWG wire at 480V, and if you are willing to tolerate losing 25% of that power to heating the wire, you can go for quite a distance. Compare the price of 4000 feet of 12AWG with 1000 feet of single phase MV and a transformer......

I'm thinking we are more in the 2000 feet of 500 MCM or 2000 feet of 6x2/0. #12 is not enough mass.

I disagree with your final calculation. Assuming a constant power load, transformer loading will increase by 1/(fraction voltage remaining). So if you have a voltage drop of 10% with a constant power load, I would expect transformer loading to increase by 11%.

I agree. The other was probably the more like the I^2*R loss I estimated.

 

[mivey]

Another thing to remember is that street lighting on is roughly 1/3 of the day, so to recoup the energy saving to offset the copper cost is considerably longer than with a constant, around-the-clock loading.

We use 10-12 hours per day.