hid lighting calculation

[litungping]

I am having trouble figuring out the calculation on a warehouse space that is fed with a 225 main,120/208 3 ph 4 wire panel. They want 70kw of hid lighting.
I use the formula W/ V x pf =A
70kw/ 208 x 1.732 (360.256) = 194.3a p/phase
1st- why do we divide by 208 & not 120v? (the ballast are 120v/1,000w )
2nd - if I de-rate to 80% of the main (225) I come up with 60kw & 167a p/ph. using above formula.
3rd- art 220.18(b)-Maximum loads: states to go off rated amps of ballast, if so we are talking 9.5a at 120v. If I go by this article than I can only install 55kw
and also I am having trouble finding where in the code it says how many neutral conductors are required. Is it 1 p/ 3ph, I thought 2008 code required neutrals for every hot? there also are existing loads I will have to account for.
and one last thing...... I had another Jw tell me I can add a 100a sub panel if I need additional ckts and that this would not affect the loading of the 225a panel, that just sounds wrong to me, but at this point my head is spinning.
thanks for the help

 

[weressl]

I am having trouble figuring out the calculation on a warehouse space that is fed with a 225 main,120/208 3 ph 4 wire panel. They want 70kw of hid lighting.
I use the formula W/ V x pf =A
70kw/ 208 x 1.732 (360.256) = 194.3a p/phase
1st- why do we divide by 208 & not 120v? (the ballast are 120v/1,000w )
2nd - if I de-rate to 80% of the main (225) I come up with 60kw & 167a p/ph. using above formula.
3rd- art 220.18(b)-Maximum loads: states to go off rated amps of ballast, if so we are talking 9.5a at 120v. If I go by this article than I can only install 55kw
and also I am having trouble finding where in the code it says how many neutral conductors are required. Is it 1 p/ 3ph, I thought 2008 code required neutrals for every hot? there also are existing loads I will have to account for.
and one last thing...... I had another Jw tell me I can add a 100a sub panel if I need additional ckts and that this would not affect the loading of the 225a panel, that just sounds wrong to me, but at this point my head is spinning.
thanks for the help

Why wouldn't you use 208V ballast?
The 80% rule would apply to the individual circuit loading, 2 or 3 pole breakers in this case, Not to the panel or mains rating.
208V 1000W HPS would be 5.4A, just snugly fit 3 lamps per 20A breakers. (I personally would not worry about the 0.2A overage, the Inspector may not agree.
Metal Halide would be 5.3A and that would keep you to the letter of the law.

So that would give you roughly 260A and ~93kVA, for 72kW 24 evenly balanced circuits. (So if you drop 2 fixtures, the results would be somewhat less.)

If you stick with the existing 225A panel and you have NOTHING else on, then the maximum fixtures you can install would be 63 1000W HPS at 221A and ~79kVA. The maximum fixtures you can add would be reduced by the other loads you have on the panel. It is also 'good practice' not to load the panels to 100%.