OK. For many years I ran a cheap halogen spottie mounted in the cavity behind the plate with two scrrws sits beside rear number plate on the back of a Brumby.

 

It was simply powered direct from reverse light power, no relay and with both factory reverse bulbs. No problems encountered

 

I am attempting to not fit same to new Brumby. I am nearly finished grafting halogen 55 watt from same spottie into the bulb holder of the factory reverse light to make factory reverse brighter in just one side, plan to add reflector material inside as well.

 

Question is am I loading this circuit near to its factory limits by adding this 55 watt bulb? Not that I drive in reverse for long.

 

Not having clear lenses don't expect same resulting light.

 

Any calculation results welcome

Use a relay.  Use the original wiring to trigger the relay and then use a fused dedicated wire that will power the light. 

Check out chart in center of this web page:

http://www.offroaders.com/technical/12-volt-wiring-tech-gauge-to-amps/

and

https://www.wirebarn.com/Wire-Calculator-_ep_41.html

 

It is apparent that load wattage determines wire size at low voltages.

P=I x E = E/R = I x I x R.

P = power in watts, E = volts, I current in amps, R = resistance in ohms.

Also, E = I x R to determine resistance, knowing other two values.

I wouldn't put a 55w halogen in a stock plastic housing with a plastic lens. A stock reverse bulb is what, maybe 10w? You're going to melt the light really, really quickly. Try to find some bright LED thing to stuff in there if you want it to look stock, don't put a halogen in, though.