Hi Dave
This is for the primary battery wire which has a 300A terminal fuse. I used 60 minute duration, single wire, not in engine room. When terminated on a fuse I get 4/0. With no fuse I get 2/0. I agree that 300A for 60 minutes is unrealistic. if I drop it down to 15 minutes I still get 4/0, but at 10 minutes it drops to 2/0.
I was just surprised to find this discrepancy as I had always looked on ABYC as the safety standard but this got me to questioning it.
Blue Sea is giving a more specific rating, while ABYC and others are more generic. By specific, I mean the formula is including a time element and assuming the current draw is for that time.
As current is drawn through a wire, it heats up. The more current, the more heat. As wire temperature increases, resistance increases which causes the wire to heat up faster. To get around this, the wire has to be fatter.
The BlueSea calculator is telling you that for loads of 300 amps or less for a duration of 10 minutes or less there will not be a significant increase in resistance and the 2/0 cable is fine. However, if the load is longer than 10 minutes, resistance will increase and a larger cable is needed if you are to maintain the 3% voltage drop.
Try your calculations again, only this time use a 10% drop. At 60 minutes duration you'll need 3/0 cable, but at 15 minutes a 2/0 cable will be fine.
When I'm wiring devices on my boat, I almost always use larger wires than ABYC or BlueSea call for. The voltage drop is simply wasted energy that I worked hard to get. The more voltage I get to the device the less is wasted on heating wire.