Modernized Engine Wiring

Nope, naddah, no way a #12 will carry 200 amps at 12 volts.

First, circuits are not rated in watts. Watts is a rate of power usage. You can use Volt Amps (volts times amps) as this nomenclature does not have a time component.

Direct current behaves differently in a conductor than alternating current, and requires a larger conductor as length of circuit increases. The increase in wire size needed for DC is rapid within very short distances.

Conductor size increases rapidly as length of DC circuit increases. In other words let’s assume a 20 amp load @ 12 v: 3 feet of #18 conductor will be safe to carry a 20 amp load; a circuit of 25 feet needs a #10 gage conductor to carry a 20 amp load. This footage includes the path back to battery negative terminal.

Here is a chart the maybe helpful to sizing 12v DC conductors:
http://www.offroaders.com/tech/12-volt-wire-gauge-amps.htm

In addition to the distance parameter, one has to take into account the effect temperatures above 20 degrees centigrade (70* F) where a conductor is used. The higher the temperature the less amps the conductor can carry safely. Most automotive wire sold in auto parts stores is rated at 90* C., or about 200* F. Under hood temperatures easily reach 90* C. A conductor rated 20 amps at 20* will only safely carry about 60% of the load at 90* C ambiant, so now because of heat, the conductor is only capable of about 12 amp capacity; this called derating.

I hope this helps.