What solar panel specs are used for calculating voltage loss

Well that needs to be clarified. The panel's IV curves have battery voltage as the V.

If the 12v panel is at 17v that is way past the 15v knee on the IV curve and you would not get any amps

There is no "working voltage" with PWM---that is MPPT language meaning the chosen Vmp.

smk is saying the controller is what reduces the voltage to battery voltage but the PWM controller does not have a buck converter in it, so it can't do that.

OTOH, an MPPT controller in its PWM mode does still have that buck converter in it and it still works that way, but now the chosen voltage for input is higher than Vmp was and nearer to Voc.

A PWM controller is not the same as an MPPT controller in PWM mode.

That's not true
I can measure 17v from panels at the controller any time there is good Sun
And 14+ volts out of the controller
You are confusing shunt controller with PWM
shunt controller connects directly to battery
aka switch close..switch open
Maybe some of these cheap Chinese PWM are really shunt controllers
Those that don't allow voltage settings ??

With PWM, panel voltage = battery voltage plus line loss, so howinheck can the controller input be at 16v?

I like smkettner's calculation. It reduces the chance of error. Be sure to really go overboard with corrosion proofing the battery end of the controller to battery leg.

My calculator says 30 amps 22' #8 wire will lose less than 1 volt.

So you will have 16+ volts hitting the controller at full amp rating. Controller clips voltage to ~14.8 and you are good to go in all conditions.

http://www.genuinedealz.com/voltage-drop-calculator

Even #10 wire at 44' loses 2.7 volts putting 15 to the controller (17.9v - 2.7v =15.2v) and still good to go.
(I recommend #8)

With the 12-12 PWM, you will use battery voltage for both the panel to controller path and for the controller to battery path.

Your "expected amps" at STC with two panels is twice the Isc of one panel (with panels aimed at a high sun) when the battery voltage is between 12 and 13

When battery voltage is between 13 and 14, the amps are a tad less (see the panel's IV curve)

For wiring set up, pick say 13.5v and 20 amps. Anytime amps are lower, line loss will be less. Most of the daytime, panel voltage will be between 13 and 14, so that is why I would pick 13.5.

CWJ8, and Mr Wizard you understood without me being wordy.

For the more challenged, especially me.......I've got 2 160w panels (a 3rd is unlikely) I'll be wiring 22' 1 way to a PWM controller, then another 2' to the batteries. Their specs are VOC-22.4v, VOP-17.9v, ISC-9.87a, IOP-8.89a. These specs are likely inflated and come from a perfect world without temperature variables. (Depending on wire size) I've no idea what voltage or amps can REALLY make it to the controller or batteries. Bigger wire size helps and with a voltage drop calculator I might not have to throw darts at a reasonable size to use. I'm wanting to know the likely real world volts and amps to use with a voltage drop calculator.

I used expected total output current of the panels vimp..The working current
Voltage drop is line resistance times amps
At 30 amps 14v or 30 amps 120v the drop is the same for the same cable...
But 1.0v means almost nothing to120v but means a great deal to 14v

Line losses from panel to controller mean very little to PWM, because anything over the output set point is not used
But line losses between controller and batteries mean a great deal. , lower voltage charges the batteries slower
If the set point is 14.2v
You want to get 14.2 at the batteries not 13.8 Or 13.2 etc..
Even with MPPT the controller output side is more important than the input side, and the higher the voltage on the input side the less effect the drop will have

Use Vmp. Probably will not matter much if using the typical PWM controller because you will just get Isc into the battery.

Its not worth even thinking about if only a few hundred watts on the higher voltage. Just run a 10 ga cable for every 10-12 amps. I ran the first 2 panels on one 10 ga cable. When I got the 3rd panel I ran a 2nd cable, which will be good for a 4th if ever.
Concentrate more on the total amps from the controller on their way to the batteries, that's where the loss will make a difference.