AC VD Formula vs DC VD formula

[lenardchristian]

I need your opinion on this matter guys.

What formula should we used to calculate the voltage drop, is it the AC VD Formula or the DC VD Formula?

 

[dereckbc]

Re: AC VD Formula vs DC VD formula

Makes no difference, the same formula is used for AC/DC. You just ignore the reactance.

 

[charlie b]

Re: AC VD Formula vs DC VD formula

Originally posted by lenardchristian:. . . is it the AC VD Formula or the DC VD Formula?

There are two AC formulas. What formula(s) are you looking at? What are you trying to do with them?

 

[lenardchristian]

Re: AC VD Formula vs DC VD formula

Originally posted by dereckbc:
Makes no difference, the same formula is used for AC/DC. You just ignore the reactance.

should we consider the reactance for calculating the voltage drop for ac circuits? my supervisor says no, but our client says that we have to consider the reactance of the cable. Whom do you think is correct?

 

[charlie b]

Re: AC VD Formula vs DC VD formula

Any circuit will be influenced by its total resistance, its inductive reactance, and its capacitive reactance. But how severely influenced? Is any one of these three so small as to be negligible? That depends on many factors. No one answer will fit all situations.

Originally posted by lenardchristian: . . . but our client says that we have to consider the reactance of the cable. Whom do you think is correct?

The client is always right. But sometimes you need to talk the client into changing his mind, after which, the client will still be right.

 

[dereckbc]

Re: AC VD Formula vs DC VD formula

The reactance portion is so small it can be ignored. If you run the equation both ways, you will likely come to the same size cable.

I agree the customer is right, but it probable comes out the same either way, the customer just feels warm and fuzzy.

 

[jtester]

Re: AC VD Formula vs DC VD formula

dereckbc
I believe you are partly right, but depending on wire size, both reactance and power factor of the load will make significant differences in voltage drop.
For example, from Tables 8 and 9, the dc resistance of 500 kcmil cu is.0258 ohm/m', the ac resistance in steel conduit is .029, and the reactance is .048 ohms/m'. You can see reactance is over 50% larger than resistance.
The Handbook suggests that voltage drop using dc resistance is not accurate for ac circuits, especially #2 and larger, and clearly ac resistance will miss the mark also.
Jim T

 

[rattus]

Re: AC VD Formula vs DC VD formula

Ask the client what he thinks the reactance might be and where he gets that number.

Then:

Z = sqrt(R**2 + X**2)

Z can then be used to compute voltage drop instead of R, but I think it is much ado about not very much.

It seems that romex cables or even wires in a conduit form a transmission line, but the wavelength at 60 Hz is so long that you can ignore this little tidbit.

 

[physis]

Re: AC VD Formula vs DC VD formula

There's no way to guess if the impedence is higher than the DC resistance because the load is not mentioned. Being as how the impedence can only be equal to or higher than the DC resistance, I would allow the customer to buy the bigger wire just to avoid insulting him. And bigger is good anyway.