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ncc74656

do i understand how DC and AC current works?

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I would like to ask this question as a thought experiment to determine if I am properly understanding the following relationships with wiring and power.

 

                Our speakers get AC out of our amps and our amps get DC from our power system. The DC power does not have frequency as it is a single flowing direct current and as such the flow of electrons through any wire have ZERO skin effect as skin effect is only applicable with frequency of transmission.  What does play a part in power capacity however is the material of which the wiring is made of and how it is made. The total measured DC resistance of a cable of X size will be lower in copper material than it will be with aluminum. Also the resistance of said wire (given that it is the exact same diameter) will be lower with a single solid strand than with a multi stranded wire.  

                Wire used in AC systems such as our speakers will gain resistance with frequency and will also increase its skin effect with frequency.  Our subs normally operate from 20-120hz (in most installs) and thus the skin effect of signals even up to 150hz leaves a skin depth of .244" useable on the cable. this means that in a 1" diameter cable that only 1/4 of it would be useable for current transfer. Given that most of us (myself included) use only up to 8GA wire (for amp to speaker) and given that 8ga wire is .1285" in diameter that there is no conceivable way that skin effect will play any part in the transmission of power through this wire. one would need to be at 550hz or greater to the sub before the very center of this 8ga wire would no longer be used. In mid range and tweeter applications we use much smaller wire say perhaps 16ga for mids and 24ga for tweeters. this puts the skin effect at 118% the diameter of a 16ga wire for the mids and 102% for the tweeters. in this scenario there is no way that I can envision skin effect will have any bearing on the power requirements for signal transmission in car audio.

 

                Now in power wires we have CCA and OFC, CCA being aluminum wrapped with copper and OFC being supposedly 99.9% or greater free of oxygen and being pure copper in construction. If one were dealing with high frequency AC current the CCA or even CCS would not matter as the copper on the outside would be the only part of the wire to see electron flow however in our car audio application it is the thermal inductance that factors into the CCA wire. meaning that the CCA wire is 25% less conductive than the copper wire due to aluminum having ~25% less conductivity than copper and a greater propensity for capacitance. thus given 2 runs of identical gauge wire but one being CCA and one being OFC and both having 80% of rated AWG power handling capacity the CCA wire will reach a temperature considerably higher than the OFC and thus will have a greater resistance to the flow of electrons and deliver less power to the amp with a greater demand on the electrical system.

 

 

 

edit: it is recommended that the ground for the amps be less than 3' long. why does this matter as the frame or body it is grounding to is much greater than 3' in length. would not a run of wire to the battery terminal and a ground to the frame that leads to a ground to the battery be the same thing as it is roughly the same distance?

Edited by ncc74656

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From what I skimmed you got it for the most part.

 

#1 Skin effect has minimal effect in the audio range

 

#2 CCA doesn't do anything for skin effect at high frequency (if it were used there) because the wire isn't Litz construction...  CCA is a much poorer performer than copper of equivalent AWG under all circumstances.

 

#3 The ground is part of the overall circuit.  It has the same current flowing as the positive lead, it's length isn't limited as long as you still have the overall desired DC resistance.

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i am still unclear on a few things, the discussion has continued on another forum but i wanted to throw this up here to maybe get some more speciffic car audio people to chime in on what i might be wrong about.

 

from what i gather using litz wire will allow each individual strand to have its own surface area and thus greater capacity for current due to the skin effect being limited with in each strands jacket. so a litz wire would have a much greater capacity for current transfer than a stranded or solid core wire but at what frequency would this take effect? in the 10's of mhz or into the ghz range?
 
if i have a 10ga sold and a 10ga stranded is it true that hte solid will have greater capacity due to the lack of air pockets in the wire (taking 12V DC voltage or >1mhz frequency)? and secondly would it be true that the stranded wire would behave with skin effect as many individual wires or would the outside area of the entire stranded bundle be what gets the skin effect? so in other words would each one of the 150 strands have its own independent skin effect going on or would the 10ga wire as a whole have just 1 skin effect? (assuming the frequency is with in a range to allow for skin effect, say up into the khz or greater range).
 
i want to get some documentation of all this and print it out so i can be sure i fully understand all of this. then i want to sit down with my boss and get this all straight as what he is telling me is contrary to what im told on here and what i have found with my own research.
 
 
EDIT::: after much research tonight i have come to the following conclusions, please correct them if they are wrong:
 
solid wire is cheaper to make and smaller in diameter than stranded wire. for short runs stranded wire is better than solid when it comes to load capacity but at longer distances the solid wire is more durable, better protected, and due to skin effect vs increased resistance on the line it is preferred over stranded. (solid wire has less resistance than stranded). most homes use solid wire due to the termination needs of outlets and receptacles having ports designed for solid wire. i can not find any codes in my local or state area that stipulate in wall wiring must be solid so long as all receptacles are approved for solid or stranded termination.
 
the skin effect depth changes based on the resistance of the wire so it is not a uniform measurement that can be applied to any gauge of wire but must factor in the relative inductance along the line. with that in mind however it seems that a 21ga wire would have next to no negative effects caused by skin effect at 20khz due to the skin effect being at minimum the rough diameter of that wire. at 100hz for a subwoffer on AC the minimum skin effect would be .22" and an 8ga stranded wire is .128" in diameter so in that application skin effect would still have no bearing. is this correct?
 
rapid fire Q/A:
stranded wire must be large diameter than solid core to have the same load capacity?
 
only litz wire provides a skin effect benefit due to more usable skin effect surface area?
 
solid wire is cheaper and more durable than stranded which is why its used so much in homes?
 
skin effect should never be in the discussion with pure DC current as it plays no part?
 
if by the math the skin effect depth at 10khz is .029" and a cable of .028" diameter is used the skin effect will effectively use the entire wire just like a DC signal would?

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Skin effect plays no part in DC.

 

Wire cross sectional area doesn't change due to solid or stranded construction if it is the same Awg.  The physical size changes, but only due to strand construction not being 100% solid (there is air or something else between the strands)...

 

If the stranded wire isn't Litzed in some way, it doesn't really do much to combat skin effect vs. solid core.

 

Skin effect doesn't really effect us enough in the audible range to be a huge concern.  It is a huge concern in the Radio Frequency spectrum.

 

Solid wire is less durable due it to being more prone to fractures due to increased bending radius.  It is cheaper to make, that is why it is used in homes.  It is also used where it is never bent/moved.  The power lines outside are always bending and moving due to the weather, that is why they are stranded.

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A good example of where skin effect plays a role in car audio is the transformers inside the switching power supplies of our amplifiers.

 

They switch at frequencies above the audible spectrum.  The transformers (the donut shaped ferrite cores with all the red/brownish wire wrapped around them) have multiple strands of individually insulated (with red/brownish varnish) wires.  They are built this way to improve the transformer efficiency at the higher frequencies they operate at.  The litz construction reduces the losses due to skin effect.

 

I would post a pic, but this site has so many goddamn extension criteria limitations it isn't worth my time, lol.  Here is a link:

 

https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcRM_xAQvPDTtRuJwnXwrJ7nItzHIuM2OwMgBACWqsY4HonqnU1VLg

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A good example of where skin effect plays a role in car audio is the transformers inside the switching power supplies of our amplifiers.

 

They switch at frequencies above the audible spectrum.  The transformers (the donut shaped ferrite cores with all the red/brownish wire wrapped around them) have multiple strands of individually insulated (with red/brownish varnish) wires.  They are built this way to improve the transformer efficiency at the higher frequencies they operate at.  The litz construction reduces the losses due to skin effect.

 

I would post a pic, but this site has so many goddamn extension criteria limitations it isn't worth my time, lol.  Here is a link:

 

https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcRM_xAQvPDTtRuJwnXwrJ7nItzHIuM2OwMgBACWqsY4HonqnU1VLg

So just for future reference when does skin effect come in? I know outside the audible hearing range but is there an exact point where it could happen?

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It happens above DC.  It is proportional to frequency, it doesn't really "come in".

 

You have to calculate to figure out how much effect it has, there is help with this online.

 

The thing to remember is, it really isn't a big factor in the audible range...

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It happens above DC.  It is proportional to frequency, it doesn't really "come in".

 

You have to calculate to figure out how much effect it has, there is help with this online.

 

The thing to remember is, it really isn't a big factor in the audible range...

Yeah I was just curious in general. I know people who think skin effect happens but saying "within audible range" doesn't seem to convince them otherwise. So if there was a number like 50Khz or x number it'll be a better wow factor.

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Also, current still flows below the skin depth, this isn't a hard cut-off point.  

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