inspector49

First of all, what's the overall size of your service to begin with? Secondly, while 4/0 aluminum might suffice for most 200A 120/240V services with a main service panel being behind or in close proximity to the overhead/underground service lateral, it's more than likely not going to be sufficient for the 250 foot run you indicated. The reason for this has to do with voltage drop. Interestingly enough, years ago I inspected a property with a 400A service that originated inside a pole barn. The home, being a considerable distance from the barn, was fed via 4/0 aluminum feeder conductors in an underground raceway that terminated inside the dwelling at a 200A main disconnect. Fortunately for the home owner, I uncovered a serious deficiency whereby the insulation on the 4/0 feeder conductors inside the panel had roasted to a crisp. Why? Because the 4/0 aluminum conductors were too small to compensate for voltage drop due to the distance from the pole barn to the dwelling. You also talk about bonding the disconnect to the panel? The term 'Bonding' ordinarily implies some type of mechanically connection, i.e., bonding a neutral bus, or an equipment busbar to/inside a service panel, etc.. In all due respect, do yourself a favor and look to hire a qualified licensed electrician!