load calculation on a 3 phase 480/277 200amp panel.

[green2012]

I have a three phase 480/277 600amp panel. There is a 200 amp 3 phase breaker feeding a lighting panel. When first installed they were using highbays and such, but they recently did an upgrade to thier lighting and the owner insists that he is using half the power he was to light his shop. We are considering changing the 200 amp breaker for a 100 amp for several reasons and need to perform a load calculation to see if he is under 100 amps. I am used to working residential and havent worked in a 3 phase system for a while. My guess would be to do it the exact same way. Put a amp probe over each phase and see what the draw is. If each of the 3 legs read under 100 amps we are good to downgrade the breaker? Am I missing anything? Is there a better or more efficient way? What would you guys do?

 

[charlie b]

I don't understand. Why do you think anything has to be done at all? Suppose you were to discover that the present lighting load is under 100 amps. What would be wrong with leaving the 200 amp breaker in place?

 

[jumper]

I don't understand. Why do you think anything has to be done at all? Suppose you were to discover that the present lighting load is under 100 amps. What would be wrong with leaving the 200 amp breaker in place?

Ditto to what Charlie said, the size of the downstream breaker does not affect the calculated load on the main service/600A panel.

 

[charlie b]

To answer your question, your approach would give a good indication, but it would not suffice as a design input for any changes you might have planned. You would be measuring at one point in time. Unless you could be certain that all lights that are powered from this panel are on, and are at their maximum brightness, when you take the measurement, then your measurement would be insufficient. A better approach would be to measure maximum demand (in units of KVA) at 15 minute intervals for a 30 day period, as is described in 220.87. Even if you do not intend to add any load to this panel, you can use the highest value measured during the 30 days, multiplied by 1.25, as the "existing load." That, at least, would be a defensible result, one that could serve as the basis for the next design changes.

 

[green2012]

Its not a matter of need. The owner needs to add a compressor and needs a 200 amp three phase breaker. The breaker costs 1150.00. He wants to use the 200 amp breaker that he already has and put his lighting panel on a 100 amp breaker ( 300.00 breaker) and save himself 850.00 in installation cost. If the lights are all on and they are at full brightness, would my above mentioned method be a good indication of power usage. Amp probe each leg and make sure that each leg measures under 100 amps? Like I said I dont normally work with 3 phase panels and just want to be sure that Im taking an acurate measure and arent forgetting any math that should be applied. On that note: a 3 phase 480/277 200 amp panel can handle how many watts. 480 x 200 x 1.73?