[Russ]

I'm building playfair's DIY battery back-up today. I've got the same relay (120VAC 4pdt). I'm planning on running 5 things off of it: Heater (250watt), DIY wavetimer (Reeftech/MJ powerhead), 5volt DIY powersupply (to run 555-timer circuit in the DIY wavetimer), Mag7 return pump, airpump.

I've got a 500watt Cobra HG inverter (w/rebate it only cost me $40). I've opened it up, so that I can put the third leg of the relay in series with the inverter's power switch.

For the fourth leg of the relay, I was thinking I would put a small airpump that I had lying around in series. Then, when the power goes out, the small airpump will turn on automatically for oxygenation -- since my skimmer won't be running during power outages.

QUESTION: I guess I'm going to put a 5amp slow-blow fuse in series on the output. Do I put the fuse on the neutral or the hot side? Or doesn't it matter? Maybe I should go with a 7amp fuse?