voltage drop discrepencies

[danbones]

When checking different web sites and on line apps calculators I get very different results. some ask for the installation method and others do not. why is this calculation required if we are looking for v/d over a distance for cable, not amperage?
My installation is in PVC , 3 current carrying conductors, and is 480 volt 3 phase at 750 feet with a load of 240 amps. the S/W calculator comes up with 500MCM for less than 3% drop where many other apps come up with 350 MCM for under 3% Which is correct

 

[Adamjamma]

Well, I recall being told in eighties by my boss then that voltage is affected by the installation method. Wire heats up more in insulation or inside a conduit around another circuit, which is one reason why we do not put as many cables in. A conduit, the other reason in conduit fill is to make it easy to pull the cables through the conduit.

if you run your cables in the air, not on a wall but hung from the joists, like the old knob and tube was, it can run higher voltages over a distance than it does buried in conduit ins insulation. Thus, even though the resistance starts out the same, the heat does not build up as fast, causing resistance to go up and voltage to drop.

of course, all this is what I was taught while learning to nail studs and such remodeling an iron works in Virginia, so am sure someone else can explain it better, someone who is a real electrician and not studying the code finally.

 

[infinity]

Here's a similar thread from a few months ago. Be prepared there's quite a bit to digest.

http://forums.mikeholt.com/showthread.php?t=190485&highlight=voltage+drop+calculator

 

[Julius Right]

If we use NEC Table 310.15(B)(16) then for 75oC copper conductor temperature and 30oC ambient 250 MCM ampacity 255 A.
If we use NEC Table 9 only 500 MCM [Z=0.043 ohm/kft] DV=√3*0.043/1000*750*240 V] is suitable for less than 3% voltage drop.
Both resistance and reactance depends on conduit material[PVC, Aluminum, and Steel].
However the resistance depends on temperature ,on frequency and distance [skin effect,
proximity effect].The reactance depends also on conductor position in the conduit. NEC considers cradle position and in trefoil the reactance could be less.
For instance if the actual temperature could be 50oC 400 MCM will be suitable also.
The voltage drop depends on power factor too. In Table 9 pf=0.85.If the actual p.f.=0.72 then using the same Table 9 350 MCM will be o.k. The approximate formula it is:
√3xI[A]*length*[R*cos(φ)+X*sin(φ)]

 

[Ingenieur]

0.03 x 480 = sqrt3 x 750/1000 x 240 x R
R = 0.0462 Ohm/1k ft

500 mcm, Cu, pvc = 0.043, 2.8%
350 = 0.053, 3.5%
400 = 0.049, 3.2%

 

[Julius Right]

I said:
"If the actual p.f.=0.72 then using the same Table 9 350 MCM will be o.k."

I am sorry p.f.[cos(f)]=0.95 [it has to be -at least]:ashamed:

 

[Ingenieur]

a couple of things to consider
-this is not a requirement but recommended practice (might be bldg/energy code in some cases)
-is load continuous or peak, ie, 200 90% of the time, 240 only 10%
-is 240 the ampacity and sized for 125%, ie, load is 200
-if based on motor fla is the motor actually loaded 100%?
-as noted above, pf = ?
-what is the rated v of the load? ie a motor is 460 or 4%+ drop
-your service v will usually be >nominal 480, perhaps 490

comes down to engineering judgement