I have a meter established at my garage that has 250 amp service. There is nothing except a single light bulb there. 100 and 190 feet away I have two piers. I need to establish. The pier 100 ft away is 100 feet long. The pier 190 ft away is 260 feet long. I need 6 30 amp plugs on short pier and 8 30 amp plugs on the 260 ft pier. I have 4 inch conduit to the start of each pier. I am thinking I should establish disconnects at the start of the pier and put the power panel out on the end of the pier. So first off does it sound like a good idea to use the disconnet. Secondly what size wire do I need to run that far? Of course boats are nothing like houses, most simply run a battery charger or refrigerator so actual usage is very low. The 30 amp twist lock is simply a marine standard connector. I currently have six boats sharing a single 20 amp circuit and it has never tripped. And lastly what is the difference electrically between aluminum and copper? I know that is a broad question so more specifically IE would I use #4 in copper and #2 in aluminum for a given power requirement?

The disconnect at the head end of each pier sounds like a good idea, thou I am not sure how practical it is. These disconnects need to be in gasketed enclosures that still allow manual operation of the switch. I believe these switches are expensive.

And if near salt water, you may need, per local code, NEMA 4X enclosures, which means all stainless steel. Now there is some bucks! A 30 amp disconnect can cost upwards of $300-400... each.

Before you get too involved, just in general, perhaps you should review Article 555, Marinas and Boatyards, of the National Electric Code 2005 edition, to see what applies to your situation.

You have some serious distances to plan for eliminating voltage drop. As you mention, the loads may not be that great, which is good, as voltage drop increases with increased amp draw.

Thou , to keep costs down, aluminum wire can be used for the larger feeders, Article 555 requires all equipment grounding conductors to be copper. Local codes may require all conductors to be copper. You know better than I that moisture, especially salt water, attacks aluminum.

Generally speaking the next size larger aluminum wire is used in place of copper wire, for ampacity. Best to consult with Table 310.16 for the proper size wire of either copper or aluminum.

Voltage drop is then calculated after the minimum size wire is determine to handle the demand load of amps.

I don't have enough information of the actual load in watts or amps of each outlet, and the voltage supplied to calculate what size feeders will be needed.

Plus all the branch circuit wiring and conduit sizes need to be calculated based on the distances from the distribution point and between each outlet.

This is a large complicated project that really needs to be all laid out, designed and calculated, and then seek the approval of local electrical inspector to be sure the design meets local codes.

I happen to live near a coast line with many harbors and marinas, lobster and fishing operations, and the local codes are fairly strict, and they do vary.

They say water and electricity don't mix. I say they both mix very well, all too well.

I wish I could be of more help,I hope I got you thinking in the right direction.

Thanks for the input it is very useful. Where might one find "Article 555, Marinas and Boatyards, of the National Electric Code 2005 "
Rob

Actually I have just learned recently, even thou I have looked for it before with no luck, the NEC 2005 can be reviewed in it's entirety at:

NFPA 70: National Electrical Code®

Click on Preview this document
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Click on table of Contents
Find Article 555 in the table of Contents and click on it.
And read away.

You should come to or get close to Art 555, just page through till you find it.

It works a bit cumbersome, but if you can get to it, sure beats buying it or going to the library.