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I have a saltwater dock that extends 625 feet from shoreline with an elevated junction box at the dock entrance, which is located an additional 175 feet from the dock entrance to the main panel of my house.

I plan to completely renovate the house including the house electrical system in a couple of years. I want to wire the dock so that eventually I have 240 volts for a 12.8 amp load for dual 1 HP motors for a future mechanical electric 10,000 pound capacity boat lift. I also want to have 120 volts to receptacles for various small electrical appliances, a ceiling fan, and lights on the dock.

I would like to wait on installing the boat lift until the house has been renovated, but would like to have available the 120 volts for outlets, fan, and lights now. I want to run the final wiring down the 625 foot dock to the elevated junction box at this time rather than later to reduce the cost. I am anticipating needing a sub panel at the far end of the dock.

Using the Southwire calculator, I calculate a 4 AWG cable for the 800 foot run (625' + 175') with a 15 amp breaker at the sub panel with less than a 3% voltage loss. If I use the 80% rule for load on a single circuit, this bumps the amperage to 16 amps and the Southwire Calculator recommends a 3 AWG cable to keep the loss at less than 3%. I understand that having a much greater voltage drop significantly shortens the life of the motors, which are about $500 each and difficult to replace, but I understand that the cost of much larger wire is significant as well. I plan to place a separate grounding rod installed in the soil at the entrance to the dock and plan to use copper wiring for the project to avoid corrosion problems.

Specific Questions:

  1. Would 6 AWG 3 wire with ground cable be adequate or should I use 4 AWG 3 wire with ground cable or even a 3 AWG 3 wire with ground cable for the 625 feet dock run at this time anticipating the future boat lift?

  2. Can I temporarily run 10/2 AWG with ground cable for the 175 feet from the house to the junction box at the dock entrance in order to have 120 volt outlets and lights on the dock now if I use the larger 6 AWG cable, or 4 AWG cable, or even 3 AWG cable for the 625 foot dock run? Or would a 12/2 AWG cable from the house be adequate for the temporary run if I use LED lighting, a small ceiling fan, and limit the wattage of the small appliances?

  3. I plan to have an electrician install the sub box and make hook-up to the house main panel, but is there anything in particular that you would recommend about the wiring or breakers in the sub panel which would provide the 240 volt circuits for the 2 motors and provide 120 volt circuit for the outlets, fan, and lights in a manner to avoid any imbalance in the electrical circuit?

  4. Lastly, will code allow me to run a water line in the same trench as the temporary electrical line from the house as long as I keep the water and the electrical line 12 inches apart? If I use an electrical PVC conduit for the temporary electrical line, can the water and power be placed only 6 inches apart?

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    80% is for continuous loads. You haven't described any of those. Southwire calculator is designed to sell you more and bigger wire, by pretending 3% is a rule (which it's not, unless you are in Canada.) If they can lead to to copper they will, too. Aluminum and sensible voltage drop and not applying a derate that doesn't apply should save you a lot of money. Best way? 480V-600V feed and a marine-exposure rated transformer... ;^)
    – Ecnerwal
    May 22, 2023 at 14:18
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    Voltage drop is a physics effect based on actual amps and importantly, it does not know how to read. Thus it does not care what Code says or the breaker handle says :) So no need to apply 125% to it except in abundance of caution. May 22, 2023 at 22:19
  • I wonder how a voltage drop would shorten the lifetime of the motor. May 23, 2023 at 5:43
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    @SimonRichter motors are not resistive loads. Aa a result, as supply voltage goes down, current goes up -- not down, as ohm's law might lead you to assume -- leading to deterioration of windings and other components from overheating. Given low enough voltage, the motor may never achieve enough speed to disconnect the startup windings on larger motors where those are often present, leading to potentially an even faster failure. May 23, 2023 at 10:41

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@Ecnerwal 's answer is excellent - 2 AWG aluminum is the way to go and meets the basic 240V 15A < 3% guideline and allows for plenty more current with a somewhat higher voltage drop. More than adequate as long as you don't plan on charging an electric boat at 40A.

Two additional things to keep in mind:

  • Everything at a dock should be GFCI protected. Required for a long time for 120V 15A and 20A circuits. Likely required under newer code for the hardwired boat lift, but even if not it is highly recommended. An extra $100 for the breaker, but worth it in terms of life safety.
  • Conduit with individual wires instead of cable. Conduit has a big advantage of allowing you to replace wires later if you need to upsize. So you can start with 2 AWG aluminum and later upgrade if you add more large loads like electric boat charging (seriously, it is a thing). Just make sure your conduit is large enough. Conduit also helps protect against critters of all types and allows you to use a shallower trench (6" to 18" depending on type vs. 24").

As far as water lines in the same trench as cable or conduit? I don't know the answer to that. But I do know that all outdoor cable and conduit/wires is required to be wet-rated. Conduit is assumed to be filled with water (yes, that does sound crazy, but it is true) due to condensation, groundwater, etc.

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Don't use copper wire for this. Aluminum is absolutely the right stuff for a fat/long feeder.

One of the most dangerous modern uses of electricity is at boat docks. And stuff gets damaged, so when wires are near water, I like to see them GFCI-protected too.

But I'm a little concerned about GFCI protection for an 800' run. That may not be able to hold, due to capacitive coupling along such a long wire.

So I think I might go a different way, with city approval of course. I would obtain two 5 KVA transformers (5000 watts) - they're a readily available and well-stocked size. Some readers are thinking I'm about to suggest wiring the transformers back to back to step up to 480V for the long haul for cheap wire -- close, but without the 480V. This time I'm here for the isolation. But yeah, it saves a wire both in the 175' and 625' run.

Foremost, the transformer out at the end of the dock will be a separately derived service. That means it's isolated from its supplying wires, and it must establish its own grounding and neutral, using its own grounding rods. That means the "electrical system" at dockside is isolated from shoreside and bonded to local water, which means a variety of faults cannot energize the water. Voltage gradients (difference in ground potential between house and dock) won't matter.

For the run down the dock, I want to use the "back to back" transformers to isolate that from absolutely everything. The wire run between transformers is referenced to nothing - it is an isolated system. Is it a hot and neutral? Is it two hots? It's undefined and that's the point. It is not related to "ground" (the dock metal or conduit shell) in any way whatsoever.

See, in an isolated system, "the first ground fault is free". Imagine one of the hot wires touches the dock - so what? Now instead of being ??? voltage to the dock, the faulting wire is 0 volts to the dock, and the non-faulting wire is 240V to the dock. The system would not become menacing unless both hot wires faulted. And you can check for that "first ground fault" simply by checking voltage periodically with a low impedance voltmeter between the dock and each inter-transformer hot wire, or by doing a "megger test" between either hot wire and ground/dock, as that would test the whole loop, including the transformer primaries, for isolation.

As far as the back-to-back voltage, I'd go as low as I could - aluminum is cheap. Unfortunately the readily available transformers do not allow a 120V primary and 120/240V secondary, so I think the minimum configurable will be 240V. I suppose someone could use a 2.5 kVA transformer in "buck" configuration to lower it to 120V.

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    I entirely see the point of two transformers primarily for isolation, but given you have two transformers, why not run the wire from home to dock-end at 480V? What have I missed. May 23, 2023 at 11:34
  • Also, second para, bold text: should start "separately" (too small a change for me to make) May 23, 2023 at 11:34
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    I don't grok why you want isolation and grounding? Usually you use only one of them. If you want a ground referenced system, why not use the grid and provide local bonding to water and pier surface?
    – vidarlo
    May 23, 2023 at 11:53
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    @Martin Belt and suspenders, and also if you want to add active sensing later to that line, it's a lot easier to acquire 240V gear than 480V gear. May 23, 2023 at 18:08
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    @Vidario I want isolation on the transmission, and then local grounding on the pier-end distribution. As Jon Custer says, you can have voltage gradients across the ground, and ground rods are not infinite capacity. Attempts to equalize earth voltages 800' apart by running wires between ground rods a) is useless, and b) will not be perfect, causing local voltage gradients near the rods. May 23, 2023 at 18:27
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4AWG copper is very expensive these days.

2-2-2-4 "Mobile home feeder" with aluminum conductors happens to be a good price point, and will also get you under 3% drop for an 800 foot run, not that you need that unless you are in Canada.

If you're not in Canada, you can very reasonably feed that with a 30A dual breaker (it's 90A wire) and have around 6% voltage drop, which is perfectly fine for "real life and not Canada" conditions. Even 40A would be fine for a feed breaker, firmly less than 10% down and likely considerably better since your planned loads are much lower - but that breaker is merely to protect the wire, and the breakers protecting the loads will be in the sub-panel on the dock.

Standard advice on sub-panels is to buy a larger one than you think you need. Feeding it with a 30A (or 40 or 50A - just breaking 10% drop for a 50A load, which you shouldn't hit) does NOT mean you need a 30 or 40 or 50A sub-panel, any amperage as large as, or larger than the feed breaker is fine, and 100A is often inexpensive due to volume. Assuming each motor should have its own dual breaker, you need a 6-space panel minimum to allow for the 120V loads, so getting a 12-space would allow for "things you haven't thought of now" better than a bare minimum 6 or 8 space. Make the feed a dual-pole GFCI breaker at the house, given 625 feet of salt-water dock and everything out there should be GFCI protected.

Put in at least two 120V circuits since two is literally the cost of a breaker (since the feed wire sees the same load from a 120V on one leg or two 120V on two legs) and your loads should be pretty much balanced, not that it matters much, since your heavy loads are 240V and are therefore inherently "balanced."

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Starting two single phase 1HP motors will be difficult at best due to the starting current of the motors. You might want to check into a single phase VFD with 3PH 200 Volt motors. The VFDs will give you a soft start and the 200 Volt motors will give you some protection from the excessive voltage drop of the long runs. Don’t forget to get a permit and have it inspected if applicable.

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