mivey
Senior Member
If my calculations came out to the exact transformer kVA then I would be fine with it as I already have enough margins built into my loading calculations. A constant power light would not justify the 80% multiplier as it is not a 20% impact.
Anticipated the constant power light? Yeah, right. You said nothing about a constant load until Winnie mentioned the 200-277 volt Intellivolt ballast. Not really applicable for our 400 volt scenario anyway. We use 480 volt on highway lighting with long runs.
Given better data, I would prefer to put more thought into the pieces rather than just stamping a percentage that may not be appropriate on the whole package. I sure wouldn't knock off an extra 20% at the end just because I heard somewhere that some folks like to use 80% for other purposes.
The whole point is what Dennis pointed out in post #2:
Make the appropriate load adjustments and apply that to the transformer rating. Don't apply an 80% growth factor number just because it sounds like it fits. Even if the percentages come out the same, the reasons for using the 80% growth factor is wrong.
The 85% adverse condition factor is not a substitute for looking at the load either. That is for something completely different.
The OP will have to include that as part of his load calculation. I used fixtures that I typically see so his mileage may vary. As I stated, if he can provide better detail I can provide a better answer.
Voltage hike: Buy a transformer rated for the system voltage.
Losses in switch/fuse/breaker: Fractional. Included as part of line loss.
Changes in air density due to height: Buy a transformer rated for the elevation. Qatar's highest point is around 300 ft so most any off-the-shelf transformer will do.
Ambient temperature: Since most can handle 30C averages and 40C peaks, and since lights will be on at night when it is cooler, and since we can buy an 80C rise transformer with 220C insulation if required, an off-the shelf unit will probably do. Extra cooling options are available if needed. Shading rather than de-rating to keep out of direct sunlight if it is that hot. We can always buy a specialty transformer appropriate for ambients beyond what we can normally handle if needed.
I do, and I did use appropriate percentages. It was not a detailed calculation. The question sounded more like someone was wanting some preliminary information. As they provide more info we provide more detailed answers.
If I thought there were adverse conditions that were relevant, I would make the necessary adjustments. Seeing none, I choose to fully utilize the transformer for the loads we have.
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