One 55 watt bulb would take approx 4 1/2 amps at 12 volts.  A pair of H2 bulbs would take about 9 amps.

The bigger the cable, the lower the wire losses due to heat.    However even 16 ga would be ok for 2 regular wattage lamps.  14 ga would be better.    Yes the ground wires need to be the at least the same size as the largest positive wires you use.

Steve

On May 2, 2013, at 4:01 PM, Jim Felder wrote:

> I have a number of good 40A relays that I'd like to use on a lighting
> project. How many H2 bulbs would it be safe to power a pair of H2 bulbs?
> 
> What wire size should provide power to the relays from the batteries using
> a single relay? What about two?
> 
> Would the wiring from the device to the relay be any different than the
> size from the relay to the battery?
> 
> Do the ground wires need to be the same sizes?
> 
> Where does one get the kind of female spade connectors with the little tang
> on the side that keeps them in the plastic relay mounts that VW uses?
> 
> Thanks,
> 
> Jim