Avoiding voltage drop-Boat Dock

Hi,

First of all, I have a basic understanding of electrical systems and can do some things, but I also know my limitations. Running the service lines to the dock will be done ultimately by the electrician as part of our mini bedroom remodel job, but I would like to know more about what is involved.

I want to install two 30amp marine service recepticles on our dock. One for our boat, and one for friends' boats when they visit us. This is a private dock at our home, and not at a marina. The service run from the home service panel to the recepticles will be about 180 feet. Marine applications for 30 amp service requires 10 awg wires with 110 volts (tinned copper if stranded wire is used). I am concerned about voltage drop given the distance to the dock. The websites that calculate the voltage drop indicate I would need 6 awg wire for this project, which is really large and expensive.

Would it be better to pull 240 volts, utilizing 10/3 plus ground wire to the dock, and then step the voltage down to 110 for the recepticles. Can this be done?? Or is there a better way?????

Thanks.

 

Actually no transformer required. You would use a subpanel supplied with two hots (AKA 240v) one neutral (used with either of the hots to give you 120v) and a ground wire (AKA EGC, Equipment Grounding Conductor). To simplify the electrician would run a 4-wire 120/240v feed to a subpanel that would have breakers for each of your loads be they 120v or 240v.

 

The 10-3 feed would give you only 30 amps at the dock. You would also have voltage drop issues from a run that long.

 

Not too sure if the VD would be insurmountable at 240 volts. 3.8% IMO would be acceptable. Drops it to 230 volts. Thoughts???

 

The boat receptacle is only 120 volts. Using the 30 amps at 180' one way and #10 I get 9.23% loss. With #6 the VD is just under 4% and #4 2.38%. All based on copper.

This does not figure in when the second boat would be docked, although it could be on the other leg of the panel.

 

I have a further question based on Ray 2047's reply:

I understand the basic 240 volt concept with the 4-wire / 2 hots & a neutral & a ground wire system. However, when it is time to get it down to 120 volts for the 30 amp receptacle, I assume you would do this with single pole 30 amp breakers in the sub-panel. When this happens, will I really have two 120 volt applications for the entire 180 foot service run, with related voltage drops? Or does the 120 volt system start from the sub-panel to the receptacle?

 

To clarify, all residential electrical services in the US are single-phase 240V. It is delivered as split-phase power, with 120V available by connecting either of the split-phase legs to neutral, through a device.

Your 3-conductor, 4-wire supply to the subpanel at the dock is an extension of that system. As Ray said,The voltage you supply to any load there is determined in the same way it is in your existing panel: 240V by connecting the load to both legs of the supply, or 120V by connecting the load to one of the two legs and to neutral.

Note: In the subpanel, the neutrals must be isolated (i.e., not bonded; the grounds must be kept separate; you will need to install a GEC (a Grounding Electrode Conductor), in addition to the EGC extended in the supply from your main panel; and the subpanel must be bonded to ground.

 

Either way you're still looking at 120 volts and 30 amps to each boat. That means #6 at that distance.

(And yes -- boats with AC, water heaters, ovens, stoves, microwaves and coffee makers will use every bit of that ampacity and still want more!)

 

Now I am a little confused by Rick's reply. I thought the 240 volt run to the sub-panel eliminates the need for #6 wire. As far as power needs, I should clarify:


We are talking primarily sailboats here. Their power needs at any given time would be about 15 amps if someone is using a hair dryer, a small portable cabin heater, the infamous blender or turns on the electric water heater element. Most of the time, it is to maintain a couple of 4D or 8D deep cycle DC house banks and small starting batteries via the battery charger. All interior lighting and refrigeration is DC from the batteries. We visit other yacht clubs and marinas with 20 amp service and it is usually fine four our needs.

We are also talking about San Francisco Bay, so air conditioning is N/A (please consult the historical attribution to Mark Twain). A couple of bigger power boat buddies that visit do not plug in but use their generators.

Thoughts?

 

I think you're confused by the nature of electrical systems. 240 is the voltage across the two ungrounded conductors (the "hot" wires). Each ungrounded conductor is carrying one-half of the single-phase 240V service. To derive 120V, one leg of the split-phase conductors is connected to a device, along with a grounded conductor (a "neutral" wire).

That said, you're planning That's 30A of load for each 120V leg, or 60A for the 240. The voltage drop calculation can be done either way - 30A @ 120V or 60A @ 240V - and the answer will be the same.

Using 75[SUP]o[/SUP]C and copper conductors yields a voltage drop of 10.8% with 10AWG, 7% with 8AWG, and 4.4% with 6AWG. So, yes, as Rick said,

 

If each boat uses 15 to 20 amps, you can set it up with one 10-3 cable and 120/240 volts. One receptacle connected to black and white and the other connected to red and white for the two boats, and you will be drawing 15 to 20 amps at 240 volts altogether. Given the lesser number of amperes the voltage drop issues will be very much reduced compared with 30 to 50 amps at 120 volts.

No transformers used in this example.

Note that to draw 30 amps at 120 volts requires a subpanel at the outer end with individual 15 and/or 20 amp breakers (unless the boats have 30 amp cords and plugs going into 30 amp receptacles with no dock lights and no standard receptacles powered by the circuit). Provided that the breaker back at the main building is 20 amps or less you don't need a subpanel for the 120/240 volt line giving separate allotments of 15 to 20 amps at 120 volts to standard household receptacles and lights.

If you draw 20 amps down the 10 gauge wires you lose about 7 volts at either 120 or 240 volts (or any voltage). If you draw 30 amps you lose about 11 volts. Another volt might be lost in the boat's internal wiring. Actually the voltage drop issues with the 120/240 volt line are a little less when both boats are docked there and drawing power from opposite sides of the line (red/white and black/white). Voltage drop issues are also a little less if only one boat is there with its regular cord plugged into one receptacle and a temporary extension cord for some usage plugged into the other receptacle.

 

What threw us in the wrong direction was in your initial post:
Not a good idea to install 30-amp twist-locks unless they can provide the full 30 amps.

On the other hand, installing standard (20 amp) Edison receptacles and forcing the boaters to use the adapters will clearly indicate to them a max 20 amp service.

 

The long 10 gauge run, rated for 30 amps, may be breakered for 30 amps.

It would not be in that bad taste given that, with significant usage on both sides of the 120/240 volt line, the worst case 11 volt voltage drop will be split between both sides with a probable voltage drop to each 120 volt side in the 4 to 8 volt range which can be lived with. (The more equal the usage on each side, the more evenly the voltage drop will be split and, the lower the total amperes drawn by the greater of the two, the smaller the total voltage drop will be.)

The presence of voltage drop does not eliminate the need for a subpanel if the incoming line allows more than 20 amps and light fixtures or 15-20 amp receptacles are added.

 

I do not have a problem going to 20 amp receptacles out on the dock. All of us have Edison plug adapters, as well as 20 amp twist lock adapters due to other marinas we visit.

The path the circuit will need to take is from the main panel through the house and to the existing junction box in back on the house siding. It will then exit the j-box into the existing conduit that runs under the patio pavers to the dock (via another j-box and flex conduit at the gangway). Outside, I believe the wiring will need to be exterior rated. The portion outside the house is about the final 60 feet of the total 180 feet circuit.

My questions are these:

Can the portion through the house be heavier (say #6 or #8) interior-rated cable to the junction box? And if so,

Can smaller gauge exterior wire be used from the j-box to the receptacles (say #10)?

I like the idea of not using a sub-panel. But if one is still needed, it may be better to change out the j-box for the sub-panel on the house siding instead of at the sea wall. There is also a decommissioned galvanized sprinkler pipe system that can be used for unbonded grounding of the sub-panel.

Thanks for your time.

Thanks.

 

Yes you can use heavier wire for part of the run, and (unlike plumbing) it doesn't matter whether the heavier line is first or last along the route. The greater the amount of heavier wire on the route, the lesser the voltage drop issues.

If part of the run remains 10 gauge then the maximum breaker size is 30 amps. For a heavier wire inside going to the subpanel and 10 gauge wire outside to the dock the maximum 30 amp breaker can be in the subpanel.

If you put the subpanel on the side of the house you still may not install lights down at the dock without a 15 or 20 amp circuit coming from the subpanel.