Voltage drop calculation in 3 phase unbalance circuit

1. Jan 1, 2010

ben1000

Dear All,

I’m facing some problem when I try to determine the total voltage drop (%) from the source to the load.

Assume the condition as follow:

1. DB-A is serving 3 number of individual load (#1,#2 and #3) , each 100m away from DB-A, single phase, 5kw, pf=0.85, V=240V,by using individual cable 2Core, 16mm2 PVC/SWA/PVC copper cable (2.7mV/m/A).

2. The incoming power supply (415V, 3 phase) to serve DB-A is also 100m away, using 4Core, 16mm2 PVC/SWA/PVC copper cable (2.3mV/m/A).

For calculate total voltage drop from the source to the load #1. I’m using formula as follow:

Assume all three loads are on.
Total connected load under DB-A = 15kw = 15/0.85/0.7188= 26.085A

Load #1 = 5kw = 5/0.85/0.24 = 26.04A

VD (source to DB-A) = [(100m x 2.3mV/m/A x 26.085)/1000/415V] x 100%
= 1.45%

VD (DB-A to load #1) = [(100m x 2.7mV/m/A x 26.04)/1000/240V] x 100%
= 2.93%

VD total = 1.45% + 2.93 % = 4.38%

My question is how to determine the total voltage drop (%) at load #1 if the load #2 & #3 are off. Or let said load #2 and #3 is disconnected from DB-A
I’m confused which VD factor of cable and applied voltage should I applied when calculate VD (source to DB-A). 2.3mV/m/A or 2.7mV/m/A? 415V or 240V? The serving cable is 4 Core, but it just like using 4 Core as 2 Core by only serving load #1.

My second question is how about when there is unbalance load. Assume load # 1 now is 10kw, #2 and #3 are 5kw each. All single phase. How am I going to determine the V.D (%) from the source to DB-A? (Three load are on) should I assume it as balanced? If I do so, does it actually reflect the total V.D (%) suffer at load #1?