Voltage Drop Calculations

[Karl H]

"Most" people in the field use Ohm's Law when sizing conductors
for voltage drop.Such as,

Evd=IxR

VD=(2xKxIxL)/CMIL

VD=(1.732xKxIxL)/CMIL

CMIL=(1.732xKxIxL)/VD

Distance=(CMILxVD)/(2xKxI)

Distance=(CMILxVD)/(1.732xKxI)

I=(CMILxVD)/(2xKxL)

I=(CMILxVD)/(1.732xKxD)

These calcs are all assuming a unity power factor.

What about AC Reactance?

Would anyone be willing to share with me VD calcs

that include AC Reactance in the formula.

Or is AC Reactance not an important factor?

I want to learn more than I already know.

Even if what I learn teaches me, I'm just another

poor sap trying to invent a better mouse trap.

 

[Pierre C Belarge]

AC reactance is not necessary in performing standard voltage drop calculations for properly installed conductors.

 

[Smart $]

Would anyone be willing to share with me VD calcs

that include AC Reactance in the formula.

See http://www.dolphins-software.com/voltageDrop.htm

Or is AC Reactance not an important factor?

Importance is relative. If one or two wire sizes is going to make or break the job, I'd say it is relatively important.

Using the DC voltage drop formulae you posted above will generally yield a larger than necessary conductor size, when dealing with smaller conductors. However, that is not true for the larger sizes of conductors.

 

[Karl H]

AC reactance is not necessary in performing standard voltage drop calculations for properly installed conductors.

Thank you for posting Pierre.

So reactance does not resist current significantly enough
for XL XC to be a varible in a VD calc?

I'm just curious and that's all.

 

[Karl H]

See http://www.dolphins-software.com/voltageDrop.htm



Importance is relative. If one or two wire sizes is going to make or break the job, I'd say it is relatively important.

Using the DC voltage drop formulae you posted above will generally yield a larger than necessary conductor size, when dealing with smaller conductors. However, that is not true for the larger sizes of conductors.

Yes that is one of my points it is a DC Formula .How different would
the AC formula be? Since Unity Power factor is'nt reality in most
AC circuits.Is the reactance impedance not significant to when it comes to
the over-all impedance of the circuit not to be included in the formula?

 

[S'mise]

I remember seeing some vd calc examples in the back of the NEC that involved reactance, If that helps you.

 

[Smart $]

Yes that is one of my points it is a DC Formula .How different would
the AC formula be? Since Unity Power factor is'nt reality in most
AC circuits.Is the reactance impedance not significant to when it comes to
the over-all impedance of the circuit not to be included in the formula?

The AC formula is on the web page of the link I provided.

The significance of the reactive impedance varies with the size of conductors, raceway type (i.e. material), and PF of the load. So in some cases it is significant, others not.

Because voltage drop is a design issue rather than an NEC requirement, accuracy is not a major concern for most.

 

[Karl H]

The AC formula is on the web page of the link I provided.

The significance of the reactive impedance varies with the size of conductors, raceway type (i.e. material), and PF of the load. So in some cases it is significant, others not.

Because voltage drop is a design issue rather than an NEC requirement, accuracy is not a major concern for most.

Yes I will make the install safe per the NEC even if I have to
exceed the NEC's minimum safety requirments.Now we are into
design.Is the reactance not a factor in function?

 

[gar]

090110-1759 EST

Karl H:

Earlier I posted a comment, but it is not here.

Without going back to the source information here is the essence:
185 ft length of Romex has an inductance of about 185*1.45 micro-henrys.
The resistance of 185 ft of 14-2 Romex is about 1 ohm.
The inductive reactance of 0.265 milli-henrys is about 0.1 ohm.
Thus, the wiring inductance is generally ignored for your type of voltage drop measurement due to the cable. Not so at 10 kHz and this length.

.

 

[ohmhead]

Voltage drop 3 phase formula

Voltage drop 3 phase formula

Well here is a formula CM=1.732x KXQXLXI /VD to find (Q) Q= DC/AC i would include ac resistance into formula today we need to but i maybe incorrect comments? best to ya

 

[Karl H]

Thanks to all. You all have been very helpful.

 

[Snorks]

Since Reactance is sensitive to voltage it is not a major factor in low voltage systems. In medium and high voltage systems and lighting strikes reactance becomes very important with impedance (resistance) becoming a lessor factor.

Also, 2KIL / Cmils is only part of the DC voltage drop formula. The other part is application of ambient temperature adjustment formula for the resistance value.

Additional information along with indept details can be found in the IEEE Red Book, standard 141.

I hope this clears this thread up a little bit.

 

[jspak]

I just had a similar question come up with the difference between dc resistance and effective impedance.

I have a 230' long feeder, 480A, 208V/3phase needing 2% voltage drop. Using the dc formula, (2) sets of 350 kcmil will work, but using the effective impedance (NEC Table 9), it won't work. I've found that it does make a difference.