How to calculate load of a 3 phase system

[Matt8445]

Hi all,

I just joined this because I am trying to figure out a formula to calculate my load. I am an entertainment electrician who wants to be more accurate in my calculations.

Most of our lights run off a 3 phase system where the units get 208v from combining 2 legs.

Most of our lights have specs on them saying for example @208v, 60hz they are rated at 937w, 4.6a, PF 0.98

Now our power distribution for these is wired as such. We take our service with our 3 hots, neutral and ground. Those are branched out to what we call soco cable connectors (not sure if this is common anywhere else) they are 6 circuit cables with 19 total wires one extra ground, no idea why that is another story.

So on the light end circuit one is wired to be pulling off Black and red, circuit 3 black blue, circuit 5 red blue and repeat for 2,4, and 6.

What I am trying to do is develop some type of worksheet to allow me to easily balance my loads.

What has really confused me is that some of the people I work with say to calculate load use the following formula.
W = V × I × PF × 1.73

So say using this on the light above I would get
W=V(208v)x I(4.6a) x PF (0.98) x 1.73.
This equals double my wattage.

I am just very lost and want to be more accurate in my ordering of power for events.

How can I get a formula to tell me if I enter the voltage, wattage, and maybe even the power factor to tell me how many amps I am drawing on each leg I am connected to.

That is what I am trying to do.

 

[desertfox]

Hi Matt

This seems a rather basic question for an electrician to be asking but the formula you are quoting is the total power in a three phase system with a balanced load.
Your power per phase is 937 watts which is the power rating of the light fitting.
I suggest you look a some books like Electrical Technology by Hughes.

regards

desertfox

 

[jghrist]

The 1.73 would need to be used if you used the line-to-neutral voltage of 120 volts. The system is 120/208 volt three phase. line-to-line (phase-to-phase) voltage is line-to-neutral voltage times sqrt(3) (approximately 1.73).

120·sqrt(3) = 208 (approximately)

It's best to add things up in VA instead of watts; that way it's easier to get total amps. Each light is 4.6·208 = 956.8 VA. If you have 30 of them, total load on the panel is 956.8·30 = 28704 VA or 9568 VA per phase. If they are evenly distributed by phase, the current in each phase will be VA per phase divided by volts line-to-neutral or 9568/120 = 79.7 A per phase. Note that this is the same as 4.6·30/sqrt(3). If you loads aren't evenly distributed by phase, use the largest phase. For instance for 31 lights, use 956.8·11 = 10525 VA per phase.

http://www.dcengineers.com/downloads.html

has Excel panelboard schedules. I can't vouch for them - just found them by Googling.

 

[stevenal]

208 is a standard rms voltage, not a peak voltage.
jghrist has the right approach.

 

[Matt8445]

Thanks that does make a bit more sense.

I know I am not a trained electrician. However I am having to learn certain things on the job and unfortunately some of our long time guys could not help me with this so just a friendly question and appreciate the help.

Thanks,
Matt

 

[desertfox]

Hi Matt

Now you have said that your not fully trained yet, puts a different angle on your situation.
Do they not send you to college to learn the theory?
Have a look at the link below it may help:-

http://www.radioelectronicschool.net/files/downloads/3phase.pdf

desertfox