# Adding Currents from different Voltage Sources (3-Phase)

Discussion in 'The Projects Forum' started by engrBecks, Apr 4, 2016.

1. ### engrBecks Thread Starter New Member

Mar 31, 2016
3
0
Hello awesome people!!

I am trying to get an estimate of the total current drawing for a lab.
I have the following plugs in the room:

480V, 20A , 3Φ
208V, 33A, 3Φ
120, 10A, 1Φ
Assume all the outlets are in use. How can I estimate total current drawing??
My guess: Do single phase analysis since it is assumed to be all balanced loads, making total max current 63A ?? I know net phasor current is ZERO, but I am trying to get an estimate of max current drawing of the room.
Another guess: Add induvidual power from each connection, then walk back and get the current from source? HELP

2. ### BobaMosfet Active Member

Jul 1, 2009
110
11
You cannot estimate current draw-- current isn't something pushed through equipment. The equipment uses only what it needs. Given your example, you have 20+33+10 Amps available (max) for use-- but whether or not you use all of that, or need more than that, is determined by what equipment is in the lab.

If you have this situation in real life, this is called a power-study. Since it may not be possible to find power stickers on every piece of equipment, and you need to know for billing purposes how much current you're using, you measure this at the load center that is feeding just that room.

Voltage and phase is irrelevant at this point because this is purely an amperage question.

If you cannot measure current draw specific to this room from the load center, another way is to turn everything on in the lab, and look at your meter reading. Then turn everything off and look at the meter reading again. The difference is your answer. Answer will likely be in Watts, so you'll need to convert.

3. ### Dyslexicbloke Active Member

Sep 4, 2010
420
19
Just to add to what was said above....

Why do you want to measure the current? for billing or for capacity calculations

You could easily make up some Leeds, plug and inline socket, that allow a clip on ameater to be safely used to measure the current of anything you have connected but that will not help if its actually power you realy want to know about.

For AC circuits power W is seldom simply a function of Volts and Amps because loads are usually reactive uniess they are simple heaters.
VA is called aparant power and isnt metered.
VAr is called reactive power and is the portion of VA that is reactive current and isnt metered.
The power factor is an expression of this proportion as a factor.
W is called Power/Watts and is metered, it is V x A x Power factor.

For a 3 phase circuit. W = avg(V) x avg(A) x sqrt(3) x avg(PowerFactor) assuming that V is measured between phases.
If V is measured between Phase and Nutral then 3 single phase calculations are done and the results summed.

I would be happy to help further but you will need to explain your actual goals.
Al

4. ### tcmtech Well-Known Member

Nov 4, 2013
2,114
1,752

480 VAC 20 amps 3 phase = 480 VAC 60 amps single phase or 28,800 Watts.
208 VAC 33 amps 3 phase = 208 VAC 99 amps single phase or 20,592 Watts
120 VAC 10 amps 1 phase = 1200 Watts.

Total power 50,592 Watts.

5. ### Dyslexicbloke Active Member

Sep 4, 2010
420
19
Not witstanding the fact that you are confusing the rating with the actual current that calculation is wrong anyway...
You can add up the currents and then multiply by average voltage but if you do that you must then devide by route3

when you do this you get VA which must be multiplied by the powered factor to get W.
granted a resistive load will have a powerfactor of 1 so the term can be ignored but 33A at 208V phase to phase ovr 3 phases is actuall about 11.9 kW

note that. 208V L1 - L2 is 120V L1 - N (208 / sqrt3)

As I said above if you want to simply add up W in a three phase circuit you must use L - N voltage not L - L as this ensures that the sqrt term is applied before you start.

Sqrt3 is about 1.7 so its a big error if you get it wrong.

Al