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Old 10-04-2009, 03:16 PM   #1
 
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Dock electricity


I need to run a 120 volt 20 amp line to my boat lift motor; the distance is 360 feet. What gauge wire do I need to do this? Many thanks.
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Old 10-04-2009, 04:35 PM   #2
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(5% of 120)/20A(720) = 0.4 ohms/1000' = #6 or larger copper just for voltage drop.
Ampacity is another issue.
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Old 10-04-2009, 04:54 PM   #3
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(5% of 120)/20A(720) = 0.4 ohms/1000' = #6 or larger copper just for voltage drop.
Ampacity is another issue.
I am not sure what you mean by this. Voltage drop is directly proportionate to ampacity.


Fish, do you have the lift already? If not then a 240v motor will have much less problem with voltage drop as opposed to a 120v motor.
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Old 10-04-2009, 04:59 PM   #4
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This is kinda obvious, but make sure it's GFCI protected.
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Old 10-04-2009, 05:02 PM   #5
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Originally Posted by Speedy Petey View Post
I am not sure what you mean by this. Voltage drop is directly proportionate to ampacity.
Voltage drop @ a given current is dictated by physics; ampacity is dictated by the NEC.

Regarding safety, people swimming in electrified salt water are safer than in electrified fresh water. The salt water shunts the current around the people.
You can't be too careful with the electrical safety of your dock setup.

Last edited by Yoyizit; 10-04-2009 at 05:06 PM.
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Old 10-04-2009, 05:04 PM   #6
 
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Have not bought motors yet. I am told that I can get a 240 volt motor and need 15 amp supply. This makes sense?
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Old 10-04-2009, 06:26 PM   #7
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Have not bought motors yet. I am told that I can get a 240 volt motor and need 15 amp supply. This makes sense?
Depends on the motor. 1/2hp, 5hp, 15hp? See what I am getting at.

Either way, like I said, voltage drop at 240v is MUCH less severe for a given load.
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Old 10-04-2009, 06:52 PM   #8
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Do yourself a favor and install a subpanel at the dock. The initial cost of the wire may be a little more(four conductors) but you'll have a lot more options in the future. You can add a light and receptacle, etc. If you're going through the trouble of trenching 300+ feet, you'll only want to do it once.
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Old 10-04-2009, 08:08 PM   #9
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A one horse motor can lift 5500 pounds one foot in 10 seconds.
The lift might have a way to accommodate smaller motors, by altering its geometry.
If you're not in a hurry you can go with half the horsepower, ~half the amperage, smaller wire and twice the time to lift the boat.

Last edited by Yoyizit; 10-04-2009 at 08:17 PM.
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Old 10-04-2009, 09:47 PM   #10
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Quote:
Originally Posted by Yoyizit View Post
Voltage drop @ a given current is dictated by physics; ampacity is dictated by the NEC.

Regarding safety, people swimming in electrified salt water are safer than in electrified fresh water. The salt water shunts the current around the people.
You can't be too careful with the electrical safety of your dock setup.
Actually, it's a little bit of both. (Physics and NEC.) Just to disprove Yoyizit's contention --that Ampacity is (entirely NEC). Try to run a 100 Amp. load on a #18 wire. Then again. What Speedy Petey was saying is that the calculation for Voltage Drop comes after figuring the Ampacity for the load. (No matter what) Don't Drink and Drive, Ever!!!
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Old 10-05-2009, 11:32 AM   #11
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Wire size and distance is the least of your concerns at the moment. Are you familiar with this type of application? You need to get together with you're local inspector and see what his requirements are. Installing electrical apparatus near or around water is dangerous and could cause shock or death if not installed correctly.
What type of dock is this? Floating or permanent? If it's floating, you really have alot to consider. Do you know what a datum plane is? A permit and inspection would alleviate any fears I have regarding giving you advice.
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Old 10-05-2009, 11:50 AM   #12
 
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good advice thanks to all
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Old 10-05-2009, 03:53 PM   #13
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http://books.google.com/books?id=Z4D...20dock&f=false
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Old 10-05-2009, 06:53 PM   #14
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Quote:
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Although it is certainly good information, Article 555 does not apply to private docks or boat lifts at a single family dwelling.
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Old 10-06-2009, 11:25 AM   #15
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Quote:
Originally Posted by Speedy Petey View Post
Although it is certainly good information, Article 555 does not apply to private docks or boat lifts at a single family dwelling.
That's why I only do voltage drops

The NEC is almost more like a legal document than an "E= IR" document.
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