%Voltage Drop and NEC

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pdemapan

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Currently I have the following electrical power layout for an airport project.
The utility company is providing a 75 kva transformer 13.8kV-120/240V single phase to power the airport. Since the transformer is too far away from this remote building we need to power, I am using a 75 kVA 240V - 2400V step up transformer to accomodate the voltage drop. Then i plan to have a 75 kVA 2400V-120/240V pad mounted step down transformer located outside the building. This will feed the building's main distribution panel. Now, there is a 0.58% voltage drop from the secondary side of the utility transformer to the step up transformer, there is a 1.97% voltage drop from the secondary side of the step up transformer to the primary side of the step down transformer, and there is a 0.58% voltage drop from the secondary side of the step down transformer to the main distribution panel inside the building. This is a total voltage drop of 3.13%. Does this mean that the branch circuits of the main distribution panel should not exceed (5% - 3.13% = 1.87%) in order to meet the NEC code. Does anyone have any suggestions or alternatives. Since i'm using all these transformers is there anyway around that 5% total voltage drop for main feeders and branch circuits?
 
As ryan mentions, voltage drop %'s are not a requirement.

You can significantly reduce voltage drop, by considering that the final step down trans at the building, would allow you to choose the primary tap, resulting in the desired secondary voltage.
 
When using transformers cascaded in this fashion, wouldn't you also need to consider the impedance of the transformers in the 'voltage drop' calculations?

-Jon
 
can you please explain this quote ron.

"the final step down trans at the building, would allow you to choose the primary tap, resulting in the desired secondary voltage."
 
Most transformers have multitap primary's, so that prior to loading, you set the primary tap so you get the secondary voltage you want. Generally they are is 1-2% increments.
 
By using transformers with as low an impedance as possible, you will get the lowest voltage variance from no-load to full-load through both transformers. Sizing the transformers larger than the building full-load would also help in that regard.

By using the primary taps on both transformers, you should be able to get the no-load (or the steady base-load) voltage into the building very close to what you want to start with. That would then be your starting point for voltage drop within the building wiring system.

However, I agree with HWS that it would probably be more cost effective to get the utility to run their primary to where you want the pad mounted transformer to be located outside the building.

If the utility won't do that, then you may want to consider running your own underground 13.8KV primary feed to a point where the utility will tie onto it, instead of doing this step-up, step-down transformer arrangement.
 
Patrick,
There is no NEC regulation regarding primary taps. It is a function of the equipment. Most transformers have these multiple taps onto the primary winding so that you can tweak the resulting secondary voltage.
 
Voltage drop is dependent on current flow. The higher the current flow the more voltage drop. No current flow = no voltage drop. Unless your load is a fixed current value, I do not understand how a transformer may be used to correct for voltage drop. As the load varies, the transformer will be providing either too much or too little correction. Installing larger wires or installing the transformer closer to the equipment would be a better solution.
 
The transformer tap setting can take care of a voltage drop condition at a given load. If you believe that the circuit will be 50% load on a regular basis, then you set the primary tap on the transformer to account for that. If the load varies widely, then you will have varying voltage drop problems that cannot be consistently taken care of with taps.
You will have the same problem with a utility transformer, with the exception that you will have to call the utility to adjust for long term load changes.
 
pdemapan
I am curious how you determined the voltage drop percentages you quoted. Are these measured or calculated. I am sure you are aware the voltage drop is proportional to load current.

As stated NEC does not specify voltage drop, you have to go to a higher standard to get that info like IEEE. As stated there are numerous ways to control voltage drop. Most economical are primary taps, transformer and load placement.

From IEEE you get two recommendations for voltage drop. 5% on non-sensitive loads, and 3% on sensitive loads as measured at the load device. Feeder design is 2 and 1% respectively, and the remaining on branch circuits.
 
FYI, the state of Florida has voltage drop requirements as specified in Chapter 13 of the Energy Efficiency Code.

Section 13-413.1.ABC.1.1 Feeders states "Feeder conductors shall be sized for a maximum voltage drop of 2 percent of design load." Section 13-413.1.ABC.1.2 Branch Circuits states, "Branch circuit conductors shall be sized for a maximum voltage drop of 3 percent at design load."
 
Does this airport only have the one building? What are the distances involved? Any runway lights? Any control tower? Why only single phase?
 
Sorry for the late response,

The distance from the utility transformer to the step up transformer is 400 ft. The distance from the step up transformer to the step down transformer is 3964 ft. And the distance from the step down transformer to the main distribution panel is 50 ft.

As for the transformers:
The utility transformer is 75 kVA, 13.8kV-120/240V single phase.
The step up tranformer is 75 kVA 240V-2400V single phase.
The step down transformer is 75 kVA 2400V-120/240V single phase.
And the total connected load of the main distribution panel is 57.8 kVA.

The voltage drop was calculated using IEEE std. 141-1993 (Red Book), Chapter 3, Section 3.11. Calcs were done at 75 degrees C with PVC underground conduit.

The utility company can't run their primary directly to the pad mounted transformer because the FAA won't allow it for whatever reason.

The main distributio panel loads include obstruction lights, room receptacles, room lights, precision approach pathway indicating lights, and a MALSR light system, including runway threshold lights. Hope i got everything.
 
Couple of comments:

1) Your wire will be cooler than 75C, so its resistance will be lower. This will pull a noticeable (but small) bit off of the voltage drop. 75C is commonly used for these calculations because of conductor self heating. But for very long runs where you are increasing the size of the conductors to reduce voltage drop, the self heating will be less. I am guessing that you are using #3 or #2 conductors for the 2400V link; these will be essentially at the surrounding soil temperature at 24A.

2) I did a bit of reading after my previous question. The 'impedance' rating of a transformer is the % voltage drop from no load to full load. 75kVA transformers will have anything from a 1% impedance to a 5% impedance or higher. The impedance of the three transformers in a row could easily exceed the impedance of the copper between them.

3) If for whatever regulatory reason, the maximum voltage that you can use is 2400V, I'd sure as heck try to get the utility to provide the 2400V directly, rather than using three transformers when two would do the job.

-Jon
 
If the FAA won't let the local POCO run their UG primary all the way to the building transformer, then consider running your own 13.8 KV UG from where the POCO is now going to set their pad mt. xfmr (or as close as they can/will run it to the building) to the building transformer location.

You're already running either UG 240V or UG 2.4KV that entire distance. You'll eliminate two transformers (the POCO and step-up) and will have far less voltage drop problems over the entire zero-to-full load range. Also you'll have less power consumption by eliminating the 2 transformers.
 
The voltage drop calcs are done at 75 degrees C because this is what the FAA has specified.

Can you please explain the transformer impedance. I'm not sure i understand what you mean.

I'm going to suggest to my boss increasing the cable size for the 2400V run from the step up to step down transformer. Actually, that is a #6 5kV cable which will need to be made larger.

I've been using IEEE Table 13 to calculate the voltage drop, however I noticed this table is for 600 V single-conductor whereas i'm going to be using 5 kV cable for the 2400V run. Will this make a large difference in the voltage drop?
 
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