Marina shore power for boats

[hardworkingstiff]

I wire a lot of marinas and I would like to get the opinions of what an acceptable voltage drop is. I'll have cable runs of 300', 400', + long. It is often feeding a power center that has two 50 amp 120/240 volt receptacles on it. Boats come in all flavors. Speaking with boat manufacturers, they tend to operate without problems down to 200 volts. They (boat manufacturers) load the boats differently. Some load all 50 amps, others only 30-40 amps.

The dock builder is always telling me that boats don't really use much power. I am always in a quandary about the best size wire to run. If you feed one power center with a 100 amp circuit (not increasing wire size) and only one boat is plugged in (50-amp receptacle) then you are fine. Add the 2nd boat, and you are fine if they do not draw 100% loads at the same time. You could always loop feed the power centers (usually no more than two so you get a 200 amp circuit and 4 50-amp receptacles), but the material costs go up because the larger cable is more than twice the smaller, and the labor for the larger cable is a lot more than one of the smaller cables (plus other factors I won't get into here).

What I have been recommending as a minimum is to size the wire so at 100% load on the circuit, the voltage drop is not more than 10% with the power company maintaining 240 volts. The power company around here delivers about 246-252 during "normal" voltage, so we have a little fudge factor here. I then try to explain how voltage drop can be an issue, but that the demands of the boats (unknown at construction) will be the determining factor. Then I give a price to upgrade wiring (still can't usually get it to 3%) to help compensate for voltage drop.

Charlie B does not agree with this approach and went through how to calculate feeder sizes in another post, and this is not about that. This is about trying to be practical, yet safe. If it costs the customer 33% more (going from $140,000 to $186,000) to get a 5% calculated drop on the feeders or a 61% increase (going to $225,000)(in lieu of a 10% drop, all three scenarios are calculated at 100% of the rated amperage of the receptacle) when we don't know the boats will actually pull 100% of the 50 amps, am I wrong to offer to wire the dock at the 10% voltage drop calcuation?

Anyway, am I being practical, or am I doing my customer a disservice.

 

[bphgravity]

Re: Marina shore power for boats

Until very recently, I was a real voltage drop nut. I even submitted proposals to the NEC for voltage drop requirments. This was before I had several conversations with several different engineers and manufacturer's of all types of different equipment.

The fact of the matter is that even though equipment is becomming more and more electronic and containing sensitive components, they are also becomming more and more resilient to voltage spike and drops. At least thats what they claim.

When its all said and done, a happy medium is usually the best bet. 3% is not very practical and the cost assoicated with the initial material installations outweighs the savings. At the same time, providing some efficentcy and adequacy is good for the user. I like your method of providing several optional bids describing the voltage drop characteristics of each and how the costs are assoicated. Let the builder or owner make the call.

 

[charlie b]

Re: Marina shore power for boats

Originally posted by hardworkingstiff:Charlie B does not agree with this approach . . . .

He is talking about this thread.

 

[nicknorth]

Re: Marina shore power for boats

I did a RV park a few years ago. Pretty much the same deal as you have. We oversized all the wire. All the RV sites had 240 volt 50, 30amp, 120volt 30, and 20 amp outlets. I would say about only 10% of the customers use the 50amp, and of those 10% only 2% of those maybe ever draw more than 40 amps. I think it would be a wasted to oversize everything from out I have seen. I would think that boats would be about the same. The only real load is going to be heating/cooling.

 

[charlie b]

Re: Marina shore power for boats

Of course it would be oversized, if you followed the code to the letter. It is the same with dwelling units. We calculate the load for an apartment building per Article 220, and the power company gives us a transformer rated at 40% of the calculated service load. Experience has shown that the amount of load that is actually running at any one time is far less that the NEC calculation process would predict.

But that does not give us (with ?us? being defined as designers and installers) the privilege of downsizing the installation. If there is any conservatism in the calculation process, it is not given to us to expend as we will. We have to follow the NEC calculation process, and we have to install what it tells us to install. Overkill? Yes. Too expensive? Yes. Owner unhappy? Yes. So be it! The NEC is interested in safety, not in cost, and not in happy owners.

The power company is not constrained to follow the NEC. They are free to take advantage of the built-in conservatism of the calculation process. They are free to install smaller transformers. They take the risk of having to spend extra money to repair or replace a transformer that fails due to overload. We don?t have that freedom.