Question is, I am installing sixty 400w fixtures for parking lot lighting. Add up the wattage and you have 24kw. Now this is a three phase 208/120v system. Since the load is so high I am going to wire for 208v. In order to size the contactor, when I do my division, do I multiply 1.73 for three phase, which gives me 66 and change or do I leave out the 1.73, because the fixtures are single phase? which would give me 115a.
Unless you are installing incandescent fixtures, the load will be more like 460W per fixture (HPS or MH). Be sure to load the circuit to 80% (or less) of the circuit rating if the load is continuous (more than 3 hrs at a time).
[This message has been edited by electure (edited 12-21-2001).]
When sizing conductors, disconnect switches, contactors etc. I believe the amp value on each ballast must be used, not the wattage of the lamp. For example a 400 watt metal halide fixture operating at 208 V. has a ballast amp value of 2.30 or 478.4 watts per fixture. Multiply 478.4 watts by 60 fixtures = 28,704W / 208V x 1.73 = 79.73 Amps.
A 100 Amp three phase contactor should work fine.
Use 3AWG,THHN Conductor Size.
Assuming the single phase fixtures are balanced properly on the three phase system.
[This message has been edited by Frank Cinker (edited 12-22-2001).]
Yes, if they are supplied by a 3Ø system, you will divide by 1.732 460 / 208 = 2.21A (load per fixture) 2.21A X 60 = 132.6 (total load) 132.6A / 1.732 = 76.6 (load @208v 3Ø) 76.6A X 1.25 = 95.6A (if a continuous load)
We must be careful! I've had my otherwise correct blueprints rejected from plan check 'causa my nasty habit of calling it THHN. They don't buy "You know what I mean". Here's a couple of things that might help (or maybe not). Rather than a timed panel, (I'm doing some guessing here), you might consider putting in a large box with listed multi pole lighting contactors rated per your individual branch circuits. Use a timeclock and photocell to operate this. A contactor can be used to operate certain security lights that stay on all night via the P/C. Others can be turned on by P/C, and off by the time clock (just series the P/C and T/C, set the T/C earlier than dark). You're not limited to 20 amp circuits here if these are HID fixtures, it can be up to a 50A, but you certainly CAN'T use a 100A circuit with #3 wire for anything other than a feeder to smaller C/Bs or fuses.
Rojosy, The timed panel approach is not that uncommon out here. It's generally used in larger shopping centers where the tenants have their own meters, and the site lighting/signage power is provided by the landlord. With this setup, all the lighting/signage is sure to come on at once. A smaller panel is usually non-switched, and provides the power for irrigation controls, timeclock power, etc. I like the setup I mentioned above much better. (I sure hope that nobody had in mind a single 100A contactor with that circuit run around the parking lot)
[This message has been edited by electure (edited 12-23-2001).]