Lighting Transformer and Voltage Drop Calculation

[Electriman]

Hi,

I have a 112.5 kVA transformer which is 600' away of my switchgear. The actual load that is fed by this transformer is 80 kVA. My question is how should I calculate the voltage drop to size the primary cable?

If I condsier the actual load and 2% voltage drop then I can size a 4/0 cable for primary, but if I consider 112.5 kVA as a load then I need to select the a 350Kmil cable for 2% voltage drop.

What do you think which one make more sense?

 

[victor.cherkashi]

2% vd of 120v or 2% of 277v. It is big difference

Sent from my Nexus 5 using Tapatalk

 

[Smart $]

Voltage drop results from actual load.

Any anticipated future load?

 

[big john]

It's entirely possible I'm doing my math wrong, so I'd like to see how you got your numbers, but I can't figure how you would need anything as large as 350s even for full transformer capacity.

Shoot, it looks to me like if you used your calculated load and allowed the 5% taps on the transformer to make up the voltage drop you could do this with 1/0/copper.

 

[Electriman]

It's entirely possible I'm doing my math wrong, so I'd like to see how you got your numbers, but I can't figure how you would need anything as large as 350s even for full transformer capacity.

Shoot, it looks to me like if you used your calculated load and allowed the 5% taps on the transformer to make up the voltage drop you could do this with 1/0/copper.

I am using a software that calculates the conductor size. And the 350 kcmil gives me 1.9% voltage drop. There is always extension in the plant but I am not sure if if they use this panel.

 

[templdl]

Hi,

I have a 112.5 kVA transformer which is 600' away of my switchgear. The actual load that is fed by this transformer is 80 kVA. My question is how should I calculate the voltage drop to size the primary cable?

If I condsier the actual load and 2% voltage drop then I can size a 4/0 cable for primary, but if I consider 112.5 kVA as a load then I need to select the a 350Kmil cable for 2% voltage drop.

What do you think which one make more sense?

You are concerned about voltage dop but did not consider the primary t as p as that would be included on the transformer to compensate for voltage drop. The transformer most lonely has either +-(2)2-1/2% full capacity taps or +(2)-(4)2-1/2%full capacity taps which would give you the opportunity to correct for 2-1/2%, 5% or 7-1/2% and up to a 10% voltage drop.
So when you consider upping the cable size you may not have to add the extra cost of the larger cable that may not be necessary.

 

[Electriman]

You are concerned about voltage dop but did not consider the primary t as p as that would be included on the transformer to compensate for voltage drop. The transformer most lonely has either +-(2)2-1/2% full capacity taps or +(2)-(4)2-1/2%full capacity taps which would give you the opportunity to correct for 2-1/2%, 5% or 7-1/2% and up to a 10% voltage drop.
So when you consider upping the cable size you may not have to add the extra cost of the larger cable that may not be necessary.

Thank you for bringing it up. I was not sure if such a small transformer, it, actually has a tap changer. So you are sure that this transformer has a tap changer, am I right?

 

[big john]

Not a tap changer, just permanent wiring taps. That's what those 7 points are on each of the high-side windings in this photo:

 

[Electriman]

Not a tap changer, just permanent wiring taps. That's what those 7 points are on each of the high-side windings in this photo:

Understood. Thanks.

 

[templdl]

Understood. Thanks.

The NP shown illustrates +(2)2-1/2% and (4)2-1/2% full capacity taps whith would allow for correction of up to 10% below normal voltage.
Please don't make it more difficult than needed.

 

[iwire]

You are concerned about voltage dop but did not consider the primary t as p as that would be included on the transformer to compensate for voltage drop. The transformer most lonely has either +-(2)2-1/2% full capacity taps or +(2)-(4)2-1/2%full capacity taps which would give you the opportunity to correct for 2-1/2%, 5% or 7-1/2% and up to a 10% voltage drop.
So when you consider upping the cable size you may not have to add the extra cost of the larger cable that may not be necessary.

Please don't use transformer taps to overcome voltage drop, that results in over voltage at times of low loading.

 

[GoldDigger]

Please don't use transformer taps to overcome voltage drop, that results in over voltage at times of low loading.

If there is a single load, such as a motor and no components that would be damaged by a small overvoltage, then transformer taps can be used to establish optimal operating conditions.
If, on the other hand, there are multiple loads which continue to operate while a larger load comes and goes, taps will not serve as well as decreasing VD.

 

[iwire]

If there is a single load, such as a motor and no components that would be damaged by a small overvoltage, then transformer taps can be used to establish optimal operating conditions.
If, on the other hand, there are multiple loads which continue to operate while a larger load comes and goes, taps will not serve as well as decreasing VD.

Well we have been told this is a 'lighting transformer' with 80 kva of load on it.

I am not picturing a single 80,000 watt lamp running 24/7.

 

[GoldDigger]

Well we have been told this is a 'lighting transformer' with 80 kva of load on it.

I am not picturing a single 80,000 watt lamp running 24/7.

It is likely that most, but not all, of the lights will be turned in and off at the same time. And in that case the question still arises as to whether some if those remaining lights, such as incandescents, will have their life reduced unacceptably by the overvoltage.

 

[iwire]

It is likely that most, but not all, of the lights will be turned in and off at the same time.

There is no way in the world we have enough info to say that. I would think with the energy codes becoming more prominent it is likely that they will not operate together.

And in that case the question still arises as to whether some if those remaining lights, such as incandescent, will have their life reduced unacceptably by the overvoltage.

It is not really the life of the equipment I would worry about as much as the customer perception of quality when they see lights dimming/brightening.


I fully understand there are times and circumstances to use the taps, they are there for a reason. I just do not believe for most building wiring systems jumping the taps up is the way to go. In most cases I rate that with turning up the dials on a breaker or removing GFCI because it is tripping.



The one time I recall doing so was for a customer whose small commercial UPS kept taking over on under-voltage. The low voltage was tracked back to the utility and they where contacted. Basically they said yes, we know, we are working on it, don't expect improvement until the next summer.

My choice would have been to widen the parameters on the UPS but the IT dept was not for that. So we did raise the taps on the transformer supplying the UPS. I never heard anymore about it.

The OP has a new installation here, why not run the right size wire now? Save the taps for the added load in the future.

 

[templdl]

Please don't use transformer taps to overcome voltage drop, that results in over voltage at times of low loading.

Then transformer taps are not necessary because of a voltage drop for fear of an over voltage. What is the allowable percentage of voltage fluctuation, +-10%? Remember that you don't have to opt for the taps that would correct for a voltage drop based upon full load but a compromise. At what expense is there to reduce a voltage drop to an acceptable level? What is acceptable?
Also, it is not unusual to change the taps down the road should the average transformer load change.
Transformer taps have been used for a long time for voltage correction and transformer loads and not consistent and secondary voltage variations are the result. It is a matter of compromise.

 

[kwired]

Use of these taps is no different then using buck-boost transformers. If you have a relatively steady load and steady voltage you maybe consider using them for voltage drop compensation or just general voltage correction. If you have fluctuating loads and as a result fluctuating voltages - you over/under compensate during peak/low peak periods and need to consider what problems those conditions may develop.

 

[iwire]

Use of these taps is no different then using buck-boost transformers.

Actually it is entirely different, at least how I use them.

We use them to buck or boost a steady non-changing voltage, say from 208 up to 240.

I have never used one, or recommended one to compensate for voltage drop.

 

[iwire]

Then transformer taps are not necessary because of a voltage drop for fear of an over voltage. What is the allowable percentage of voltage fluctuation, +-10%? Remember that you don't have to opt for the taps that would correct for a voltage drop based upon full load but a compromise. At what expense is there to reduce a voltage drop to an acceptable level? What is acceptable?

Again, my concern is that the result of this band aid approach is that the lighting will dim and brighten as the load changes.

Also, it is not unusual to change the taps down the road should the average transformer load change.

I understand we both have long experience in the trades, and perhaps in your world that is true. In my world of office buildings and retail that is not true at all.

Transformer taps have been used for a long time for voltage correction and transformer loads and not consistent and secondary voltage variations are the result. It is a matter of compromise.

Why are we suggesting such compromises in a new installation?

This conversation is funny to me as typically it is the electricians looking for a shortcut not the engineers.

 

[kwired]

Actually it is entirely different, at least how I use them.

We use them to buck or boost a steady non-changing voltage, say from 208 up to 240.

I have never used one, or recommended one to compensate for voltage drop.

It is same concept, just a tap to a different potential in the same coil. Bucking/boosting a steady non-changing voltage is what I was trying to indicate that they are for.

If you use them for voltage drop compensation on a non steady voltage you will over or under compensate at different load levels.

If voltage is going to remain at a particular level the majority of the time, you at least need to compensate from that level, but beware of what the high/low may be at other times and plan accordingly if it will be a problem.