Nominal -vs- Effective Wattage
What you are dealing with is the difference between Nominal and Effective wattage. The way to solve for this is as follows: wattage/12v = amps (OHMS law) in your case, 140w(nominal watts) /12v = 11.67amps. You then take the amps (11.67) x the voltage tap you intend to use (22v) to calculate Effective wattage; 11.67 x 22v = 256.67w.
As you can see the "Effective" wattage that you are asking the transformer to accommodate is actually 256.67w not 140w. 256.67w/12v = 21.39amps (OHMs Law). Since 8 gauge wire is rated to 40 amps you should be OK with your system.
Actual Amperage will be different than what is calculated above because the Ohm Law formula assumes that the system is operating at exactly 12v when more than likely it will be something different...but this will give you a good approximation. Also remember that 300w circuits are the equivalent of 25amps...NEC recommends not using more than 80% of the circuit which is 20amps. Based on the calculations above you would be only slightly more than the 20amp limit. So it will be critical to take a field amp reading so as to verify that you are within limits. This is what is termed "Dynamic Wattage" as it is the actual wattage requirement of the system as it takes all 7 causes of voltage drop into account not just distance, load, and wire gauge.
As you can see there is no "Rule of Thumb" for this situation...in our business it is always about the amperage of a system that drives what we do and how we do it.
The above calculation is part of the AOLP Test for Certified Low Voltage Lighting Technician (CLVLT). Good luck to all taking the test in Atlanta next week.
Gerry De La Vega
Terradek Lighting Inc.
AOLP Board Member