208v load balancing

[Alex QP]

Hey Everyone,

I've scoured the threads on this forum and many others but I still seem to end up confused when calculating amperage draw on 208v moving lights.

to keep the numbers simple in this example I will have 6 x 2000W fixtures on a standard splay. Phasing is as follows:


C1:XY
C2:XY
C3:XZ
C4:XZ
C5:YZ
C6:YZ

Now I can find the current draw of a light by dividing the wattage by the voltage: 2000w/ 208v = 10a (I'll round to 10 to make this simple)

Now I place the current draw of each phase out in a chart that the lights will be plugged into on a splay:

X	Y	Z
10	10
10	10
10		10
10		10
	10	10
	10	10
40	40	40	Sum total
24a	24a	24a	TOTAL

once I do this I add the total of the current draw of each phase and then divide it by the square root of 3 and that gives me 24a current draw on each phase. I've been told this is correct from a colleague. Though everything I've researched on this forum is leading me to multiply the total current draw on each leg instead of divide. That will leave me with 69a per leg (X, Y, and Z) it can't be that each leg is already drawing 69 amps with only 6 lights. ( 207a total) That's rightly absurd in my estimation. Or do I have it right by dividing that total of 69a across all 3 phases and each are sharing the load by 24amps?

If anyone can tell me where I'm going wrong and where I'm going right with this that would be greatly appreciated!

Thanks!

 

[derekleffew]

Stop calling it a splay. You know the proper term. https://www.controlbooth.com/threads/local-custom-or-standard-practice.48572/

Did you see this thread? https://www.controlbooth.com/threads/208v-power-load-balancing.35498/

 

[Alex QP]

Stop calling it a splay. You know the proper term. https://www.controlbooth.com/threads/local-custom-or-standard-practice.48572/

Did you see this thread? https://www.controlbooth.com/threads/208v-power-load-balancing.35498/

Will you meet me half way if I call it a "break out splay?"

I did read that thread yes,

"WRONG! The total(s) on each leg must be multiplied by the constant: 1.732 [=SQRT(3)], to determine the actual load, in order to account for the "across both legs" portion"

This is the part that confuses me if I'm honest. So in my example if I multiply my totals of 40a x 1.732 I get 69 amp load per leg. Now if X Y and Z are seeing 69 amps each that's a total draw of 207 amps no? from 6 lights?

I know somewhere I'm thinking about this terribly wrong.

 

[danTt]

As I understand it, You're double counting the lights. The phase cancellation/1.73 cancellation occurs rather than "seeing" 10a from both sides of each light. In this case it would be 20*1.732/leg, not 40.

See also https://www.controlbooth.com/threads/208v-and-load-calculation.30192/

 

[Alex QP]

As I understand it, You're double counting the lights. The phase cancellation/1.73 cancellation occurs rather than "seeing" 10a from both sides of each light. In this case it would be 20*1.732/leg, not 40.

See also https://www.controlbooth.com/threads/208v-and-load-calculation.30192/

Thanks for the reply

I think I get it now, so the idea is that 34.64a could technically be drawn from any leg at any time. So the only discrepancy in my mind is, if that's the total draw per leg, and I multiply that total by 3 (for each phase) I now arrive at 103.92a. All that from a set of fixtures that should be drawing 60a total. As a means to double check my work do I divide 103.92/1.732 to get back to what should be a total 60 amps?

-AQP

 

[Malabaristo]

It sounds like you just don't know when to stop

I'm assuming the reason you're doing the calculation is to figure out how big the 3-pole breaker feeding this needs to be. If that's the case, then it's important to note that a 3-phase breaker is listed by the maximum current per phase. That means your answer is that you need a breaker greater than 35A for these six fixtures (or more likely >44A for continuous use since most breakers will be 80% rated). Trying to get back to a 60A total just confuses things because that would only apply if they were all on the same phase.