The circuit I have all my hardware running off is only 15amp and I can't max out my 4xGPU system due to what I'm thinking is voltage drop.
What kind of cards are in that 4x GPU system? Is it really 8 GPUs on 4x 6990s?
If not, I don't think you should be pushing 15A unless you've got a bunch of other stuff on that circuit.
I ran two new dedicated 20A circuits for my rigs, right now I'm only using one for my miners - 4x 5830s and 4x 6950s, two AC high-velocity desk fans and a monitor, and I'm just pulling 13A at the socket. The fans pull ~3A each I believe.
Seems like you should be coming in well under that, but again a bunch of other stuff on that circuit will reduce your available power.
you're right, I'm not pulling that much off the miner with 2x5850s + 2x5830s -- about 625W/5.5amps. But everything else, a game machine, two routers, a switch, wireless access point, laser printer, media server and box fans all come of that circuit. Voltage is down to 108 :-/
Then again, maybe a couple of the PCIe extension cables need mole connectors to stablize the motherboard. They are on order.