calculations for stock speaker wire

Sorry to bring up speaker wire again, but I have some technical questions about stock wiring safety when using aftermarket amps, and I can't seem to find a definitive answer.  I think my stock wiring is 18 gauge, which I thought about using rather than re-wiring through the doors; but going through some calculations, I have a few concerns.  I have searched this forum enough to know that 16 gauge is probably best, but I haven't found much on the safety of the lower gauge wiring if I choose not to re-wire.  What are its limitations, etc.  I'm probably just confusing myself, but here is how I looked at it...

Assuming I have a 200W RMS per channel amp going to 4ohm speakers.  Simply using ohms law,:

I=V/R

For the 200W RMS amp seeing a 4ohm load per channel then,

200W=V^2/4ohms

so, V=28volts output per channel

If I then calculate the current for that voltage with the 4ohm speaker load, I get,

C=28v/4ohms = 7amps

Therefore, it looks to me that the speakers will draw 7 amps per channel at 200RMS.  My next assumption is that I need a speaker wire that will handle that current.

Looking at some wiring charts/calculators, it appears that speaker wire of 18 gauge would be insufficient to handle 7amps of current, potentially creating a safety issue.  For example, if I plug in these numbers (assume no more than 12ft of stock wire will be used per channel, 18 ga wire) into some of the wire calculators at the www.bcae1.com  site, I get circular mils/amp lower than 300, and a warning that this is too low (should be greater than 300). 

On the other hand, I've heard all over the place (this forum, other forums, etc) that 18 gauge wire would be ok for a 4ohm load (see the chart at the bottom of this page https://www.the12volt.com/info/recwirsz.asp  which doesn't even mention current).  And this chart ( http://www.cablesandconnectors.com/wiregauge.html) indicating that 18ga is ok for 18 amps chasis wiring or 2.3 amps for power transmission.  I assume speaker wire is a short-run chasis type application, which leads me to believe 18gauge would be fine for most any speaker wiring.

Needless to say, I'm confused about it.  Can someone with more practical knowledge of this, give me a more definitive answer, or show me where my logic is off or on target?  I know I can avoid all of this by just re-wiring with 16 gauge wire, but my goal is to try to verify whether or not the stock wiring is safe for the current it will potentially handle.  I'm not really worried about any small drop in output that comes with using the low gauge wire.  Thanks for any help.

I did not look at your calculations closely.  All seems in order.  A 4 ohm speaker is not 4 ohms at most frequencies.  At certain frequencies it could be as high as 20 phms.  You really need to use a test tone at different frequencies and use a digital volt meter to determine voltage drop.  Set the volume and read at the amp and without adjusting the volume, read it with the same meter at the speaker.  You will be shocked to see how little voltage drop you will see.  If you do not have access to a test tone, you can use music.  The process is a bit different but just as effective.  Meter set to AC volts.  One lead on the positive terminal of the amp.  Other lead on the positive terminal of the speaker.  This will tell you the voltage drop on the positive wire.  You will need to multiply x 2 to get total voltage drop.  Woltage will be lost on the negative wire as well.

i am an idiot wrote: I did not look at your calculations closely.  All seems in order.  A 4 ohm speaker is not 4 ohms at most frequencies.  At certain frequencies it could be as high as 20 phms.  You really need to use a test tone at different frequencies and use a digital volt meter to determine voltage drop.  Set the volume and read at the amp and without adjusting the volume, read it with the same meter at the speaker.  You will be shocked to see how little voltage drop you will see.  If you do not have access to a test tone, you can use music.  The process is a bit different but just as effective.  Meter set to AC volts.  One lead on the positive terminal of the amp.  Other lead on the positive terminal of the speaker.  This will tell you the voltage drop on the positive wire.  You will need to multiply x 2 to get total voltage drop.  Woltage will be lost on the negative wire as well.

Ok.  Thanks for he feedback.   I think I see the flaw in my assumption.  The 4ohms is only a"nominal" impedance, and varies greatly with frequencies. a point I simply overlooked.   I would assume then that the 18 gauge wire would indeed be ok for the stated application, because the likelihood of the speaker see a lower impedance and thus higher current draw for extended periods of time does not present a big problem.   Agreed?

Agreed.