ECN Forum Forums General Topics of the Trade General Discussion Area Need 3 phase neutral calculation help

Thanks for all the replies. Yes the fixtures are already installed(20+ year old supermarket.) 185 total fixtures.4 bulb fixtures w/ 2 rapid-start magnetic ballast per fixture, using 4 T12 32W bulbs. 1.2 amp(per fixture) is what my fluke was reading. Did not see any THD% anywhere on ballast, but do remember seeing power factor of .90. Does not say that they are heavy-duty, so I assume they are not. Local supply house says it would be cheaper in long run to change all 185 fixtures out to electronic ballast w/ T8 32W bulbs. The bulbs now are 32W. Wouldn't wattage use be roughly same?

The old ballasts are more than likely to be magnetic; not electronic.

Personally....IMHO I would attempt to sell the owner/tenant a energy efficient lighting job.

The T-8 lamps and electronic ballasts are energy efficient alternatitives, and can save energy $$$ for the person/company paying the electric utility bill.

Your old fixtures, based on the info above use 144 watts each, As I said oin my first reply, the Elec Ballasts are under 120 watts, some are as low as 91 watts.

The difference is approx 30 watts ea. times 185 fixtures for 5500 watts +/- or 5.5 KW.

5.5 KW for a 12 hr 'on' equals 66 KWHR of 'saved energy' at say .14 cents equals $9.25 per day at 12 hrs on. Work it out to a 30 day period and you have $275.00 approx.

Punch it out further, $ 3300.00 for a year.

Also, your POCO may be offering a rebate to offset cost of energy efficient lighting.

Please note the figures I used above are approximations; your local POCO KWHr fee may be higher or lower. The actual wattage figures are dependent on equipment you select. IF you have access to a supply house, see if they have a lighting guy who does energy comparisons; most do it free to you, and can be a big help if you want it.

Not only can the neutral get hammered by those narrow pulses (at up to 300% of the current of one circuit), but both the neutral and phase lines get hit further with increased I2R losses due to the high current peaks. The narrower the pulse, the higher the current, and you square that for the losses before averaging it back out over time. So the end result is when a pulse is N times narrower, you have N times more I2R loss (N^2 loss during the pulse times 1/N time ratio). These losses will also affect the OCPD thermal element. So what should be 15 amps of load can trip a 20 amp breaker when it has peaks 3 times normal (9 times the dissipated watts for 1/3 time). It basically becomes a derating issue when it gets bad.

Fortunately switch-mode power supplies for computers are getting better.

This is one of the reasons why they never even brought a neutral into computer room panels back in the olden days.
You were just asking for trouble.

Many mainframes I used to work on were powered through motor-generators at higher frequency output, typically 400 Hz. That doesn't eliminate the issue, but at least it confines it.

If the source transformer was a wye/star secondary, there would still be issues at its neutral. If all your pulse current loads were connected L-L at 208V on a 208Y/120 system, you still have current peaks crossing the neutral junction in the transformer 360 times a second. Those junctions need to be beefed up.

Even then, harmonics could add 15% to the current on the delta connected line wires. And the I2R effect of narrow high current peaks is still there.

But at least you don't have to beef up an entire neutral run if you haven't connected to it. And power supplies are doing better these days, better approximating a sine wave in current drawn.