Main Power Cable Size

Rule of thumb: square mm^2 size times ten is current capability.

So 2.5mm^2 speakercable can handle 2.5 x 10 = 25 ampere.
My Volvo's cable running from front to back (battery in the boot) is about 35mm^2, so 35 x 10 = 350 A.
Peak current is more.
When a cable is running next to other cables (that also cause heat), try to oversize them a factor of two. (Or halve the currentcapability found with the rule of thumb).

Biggest cables should be from battery to the startermotor AND DON'T FORGET THE GROUNDCABLE AS WELL.

Most alternators should cranck out about 100A, so 10mm^2 at the very, VERY least. Modern cars will have bigger alternators, my Volvo 140A, the campervan I drove last year even 180A.

My wishlist for wiring up a 30 year old BMW:
2Ga or 0Ga from battery to starterrelay and from starterrelay to startingmotor.
2Ga or 0Ga from battery to chassis ground.

4Ga from alternator to battery.
4Ga from battery to fusebox that feeds the car.

Don't forget that some cars even have ground cabling from alternator to engine, and even more often from engine to chassis. This is to not solely rely on the bolts holding the car together to also be excellent electricity conductors. Think rust causing higher resistance.

Have a look here to convert Ga's to mm^2 and to cable diameter. Cable diameter can be measured. I am assuming (but never measured) the Prius cables to be 10mm^2 or possibly 16mm^2.
They could do for most cabling, but I would double them up for the startermotor wiring (and grounding).
Also, I wouldn't be a fan of using orange wiring for 12V and for ground. That is asking for trouble. Normally red is 12V (or black with red tape and/or red connectors) and black is ground.

 

The orange color means nothing afaik. So just wrapping them in red should do.

My father-in-law used earth cable (green-yellow) for supplying a 230V~ lightbulb with power... The next person owning his house is going to be in for a surprise...

 

Any independent auto electric shop near you can sell you proper battery cable that's already black and red, and in a selection of gauges that are appropriate to the use. (The one near me, in fact, exclusively uses welding cable, which is even better, more strands that are finer, durable under flexing.) It's inexpensive enough that there shouldn't be much need to substitute salvaged wire that's of dubious suitability.

The Prius HV cable is shielded, so the central conductor is thinner than you'd expect in a simple wire with the same outward thickness. And there's no particular reason you'd use shielded wire for your 12 volt starting battery application. It's just excess complexity you don't need. (And, somebody else might be able to use that cable for its intended, HV purpose; that stuff is tough to find except in full expensive spools.)

-Chap

 

Your calculations are spot on imho. The wire from the hybrid battery to the Inverter in a Prius is not that thick maybe 4 or 2 gauge. Not big enough to service a classic starter motor. I would like a single O at least. Bigger the better. Stranded copper.