Watts = Volts x Amps

You know your wattage so just slide your voltage into the equation. Size the fuse about 1.5 times the number and install it in your power line. This should do the trick. Probably end up being about 5 amp.

Have a good grow!:jointsmile:
Psycho4Bud Reviewed by Psycho4Bud on . Home Made Ballast I built a MH 400w ballast (really just put parts together) with a GE ballast and cap. I would like to fuse it to protect against shorting or fire.....where in the line should I put the fuse and what value should I use? Thanks in advance. Rating: 5