Using a 250w you are very unlikely to have a fire. It likely only pulls about 3 amps(if that). So it's just like running two and a half standard 100W incandescent bulbs on that breaker/circuit.

8 hours of HPS to veg will be a huge issue for you. First off the plant goes into flower at 12/12 because it takes 12 hours for the flowering hormones to build up enough to trigger flowering. Eight hours of light means 16 hours of dark. So without more daylight hours your plant is going to stretch like crazy.

Add to that. A HPS light is not meant to veg a plant. That is a flowering light. Even with 18/6 the plant will stretch under that color bulb! You want a metal halide bulb to veg the plant.

Budsndrums is correct on the yield. No one can tell you what you'll get. Plus under your conditions(less then flowering hours for light), no one here could even tell you if you'll succeed at all!