A couple of points:
1. There is nothing wrong with having a fuse/breaker that is rated for LESS than the capacity of the wire - it is there to protect the devices, and the wire will hold up to any load that doesn't blow the fuse or trip the breaker.
2. NEVER protect a WIRE with a fuse/breaker that will supply more current than the wire ('downstream' of the protection) can handle.
3. Especially in a mobile installation, with vibration and grounded metal all around, always put the fuse/breaker as close to the source (batteries) as possible, to protect against fires if the insulation is compromised and the wire shorted. This is especially true if the wire is near hot or moving parts, or must pass through a metal panel. Notice how the breaker for some of the loads on your starting batteries is right next to the batteries: http://i175.photobucket.com/albums/w...s/DSC06827.jpg
A typical plan is to have a 60, 100, or 200 amp breaker at the battery, depending on the wire size, to protect the main feed. Then you can have smaller fuses/breakers at the distribution points based on protecting each of the loads.
4. Not all circuit breakers are DC RATED. Some or all Square D brand 'QO' model breakers are. Your helpful store man may not have steered you wrong on the model. Always check the labeling on any breaker before using it on DC. An AC only rated breaker may overheat on DC.
5. Since watts are volts times amps, a rule of thumb repeated over and over here is that it will take ten times the current at 12 volts to provide equivalent wattage to a 120-volt load through an inverter. With inverter loss figured in, eleven might be a better multiplier. As The Experience calculated, 40 amps at 12 volts will give you about 432 watts, or about 3.6 amps at 120 volts. (The 4 amps I posted earlier was a rough figure without any inverter losses built in.) If that is all you will ever need, no problem. But I suspect the day will come when you will want to plug in something substantial, even for a few minutes. If that happens, you will curse yourself for not up-sizing the DC breaker.
6. It's probably not a good idea to double up the dual 40-amp breakers on the same load to try and get 80 amps. It can be done, and the breakers are already paid for, but without load balancing there is no guarantee that each side will handle 50% of the load. It is true that splitting the load will give you somewhere between 40 and 80 amps before tripping, but there is no way to find out what it will be until your particular installation is complete, and with a steady load you measure current with a clamp-on meter on each breaker feed.
Here's why: Lets say the sum of the resistances of the input and output connections, jumper wires, plus the internal resistance of each breaker adds up to 0.04 ohms on the right and 0.02 ohms on the left. These are minuscule resistances, but the difference will cause two-thirds of the current to flow through the slightly lower resistance on the left, and one-third will go through the higher resistance on the right. The left breaker will trip at 40 amps, so you will get only 20 amps on the right side before the cut-off. This gives you only 60 amps total. High-power amplifiers and other devices with transistors operating in parallel generally have low-resistance, high wattage resistors feeding each one, so the resistance of each balancing resistor is much greater than the variation between devices and causes the current to even out.
Since you already have a (DC Rated?) enclosure for Square D breakers, why not swap out the double 40 for two individual breakers. One could be the supply for the inverter, and the other could be a master disconnect for 12-volt loads off of the house batteries, now or later. DC lighting??
I did add a ground strap to the inverter . . . . . Are you saying I should remove this ground to avoid potential problems?
I have read & talked to others that have told me they keep their off-grid setup completely seperate from the bus power, including no grounds whatsoever.
The AC safety ground MUST connect to the inverter when the inverter is the operating AC source, either through the third prong of an output outlet, or by a ground strap at the inverter. The same is true of a generator. The DC plus and minus can be isolated from ground if desired. But here's something to think about. If the third prong ground is tied to the DC minus inside the inverter, and the DC hot accidentally contacts the bus chassis in any way, all the energy in the batteries will go through your inverter to return to battery minus.
Ever see the insides of a radio after someone tries to start a diesel locomotive with a breaker open that disconnects the battery bank minus from the chassis? I have, more than once. When you use the case of the radio and the radio ground wire to (try to) turn over the big engine, the results ain't pretty.
Here's a plan: with the battery bank disconnected, measure the resistance from the inverter DC minus input to its AC safety ground. If it shows any connection, I would definitely put a high-current strap from battery minus to the chassis, to provide a high-current path to protect the inverter wiring from "oopses." If there is no connection, and you are confident your solar system is also ground-isolated, then you should have no problem leaving the house DC system isolated from ground.
But you may want to add DC devices in the bus someday, right?