Lighting load question

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kenzolee

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Location
US
My electricians engineer made a load calc. The math is counter intuitive to me.

He claims that 3 1000w lights on the same leg of a 480Y system is 3000/277
But that 3 1000w lights, one on each phase, is 3000/480v

Is this correct?

This is relavant as Intend to have 250+ 1000w 277v lights.

Am I looking at 250000w/277v or 250000/480v

My main is 800 amp 480v

Thanks.

Kenzo

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JFletcher

Senior Member
Location
Williamsburg, VA
If the lights are 277V, (phase-->N), they will use 1000W/277V = 3.6A each
If they are 480V (phase-phase), they will use 1000W/480V =2.1A each

3ph power is calculated like this:

P = V x A x PF x 1.73

250kw/(277 x 1 x 1.73) = 522A

Note that 277V x 1.73 = 480V. 1.73 is the sq rt of 3, which is used when 3phase power is available.
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Still the same

(1000/277) + (1000/277) + (1000/277)

Yup.

Only in the first case the current in the single line is 3000/277 while in the other case each of the three hot wires carries 1000/277. :happyyes:

If he were looking at wiring equivalent wattage 480W bulbs in a line to line configuration then the calculation would be different than for the bulbs you propose wired line to neutral. But the end result in current in the line wires would end up exactly the same for the same total wattage, as long as the loads are distributed as equally as possible over the three phases.
On the other hand, the simple, intuitive, way of converting VA numbers on the panel schedule (assign half the VA to each end of the bulb connection) might end up with different numbers because the simple way is wrong.
Watts is watts.
Not necessarily a bad engineer, but one who has not taken the time to really understand three phase systems.
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
My electricians engineer made a load calc. The math is counter intuitive to me.

He claims that 3 1000w lights on the same leg of a 480Y system is 3000/277
But that 3 1000w lights, one on each phase, is 3000/480v

Is this correct?

This is relavant as Intend to have 250+ 1000w 277v lights.

Am I looking at 250000w/277v or 250000/480v

My main is 800 amp 480v

Thanks.

Kenzo

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You won't be looking at 250,000/277 if you are looking for the amps per phase. 250/3 phases = 83 lights per phase. 83*1000= 83,000

83,000/ 277 = 300 amps per phase so an 800 amp service is fine if the lights are all you have. This seems to be a great case for LED lighting
 

JFletcher

Senior Member
Location
Williamsburg, VA
You won't be looking at 250,000/277 if you are looking for the amps per phase. 250/3 phases = 83 lights per phase. 83*1000= 83,000

83,000/ 277 = 300 amps per phase so an 800 amp service is fine if the lights are all you have. This seems to be a great case for LED lighting


hmm, where is my math off then? see post #4.

should I have used 480V instead of 277V for the calculation?
 

kenzolee

Member
Location
US
You won't be looking at 250,000/277 if you are looking for the amps per phase. 250/3 phases = 83 lights per phase. 83*1000= 83,000

83,000/ 277 = 300 amps per phase so an 800 amp service is fine if the lights are all you have. This seems to be a great case for LED lighting
Thanks.
So, 300 amp per phase x 3 is 900 amp, no? Greater than 800amp. What am I missing?

Oh and 1000w because the are for greenhouse. In the crop being grown, the added yeild by the hid vs led will pay for the power.




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GoldDigger

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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Thanks.
So, 300 amp per phase x 3 is 900 amp, no? Greater than 800amp. What am I missing?

Oh and 1000w because the are for greenhouse. In the crop being grown, the added yeild by the hid vs led will pay for the power.




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The phrase 300 amps per phase is one that is annoying to many of the members here and can lead to confusion since a phase is traditionally used to refer to the voltage and/or current from line to line.
Better would be 300A on each hot wire.
Given that a three pole breaker can carry up to its rated current on each of the three poles, an 800A breaker could carry 800A of non continuous or 640A of continuous current on each phase wire.

And whatever else you do, when you wire the 1000 watt bulbs from line to neutral (to get the 277 volts they need) you divide by 1000 by 277, not 480.
Any member that tells you otherwise is confused or has not followed the entire thread.

BTW, the standard description for your service is "480Y/277, three phase four wire".
 

GoldDigger

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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
If the lights are 277V, (phase-->N), they will use 1000W/277V = 3.6A each
If they are 480V (phase-phase), they will use 1000W/480V =2.1A each

3ph power is calculated like this:

P = V x A x PF x 1.73

250kw/(277 x 1 x 1.73) = 522A

Note that 277V x 1.73 = 480V. 1.73 is the sq rt of 3, which is used when 3phase power is available.
You have made a common error in your calculation.
P = V x A x PF x 1.73 requires the A to be amps on each hot wire while the V is the line to line (= phase) voltage. Using the right values in the equation gives you 250kW/(480 x 1.73)= 250kW/(277 x 3)
 

kenzolee

Member
Location
US
Ok thanks. I also find that people I speak with don't agree on per phase or total. That's why I confirmed here! Many thanks for setting me straight.

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