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Old 06-22-2007, 12:32 PM   #1
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Long underground run


I am going to run electric to a pier from the main panel. I will be connecting to a 60 AMP sub panel at the pier. I believe I need to use THWN in a PVC conduit, bu am unsure about GA. The run is 350 ft. in an 18" trench. I have been told AWG 4 with a 6 ground upto AWG 8 with a 10 ground. I know power drops with distance, but I can't seem to get consistent answers. All advise is appreciated.

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Old 06-22-2007, 12:54 PM   #2
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Long underground run


It really depends on what you plan to power at the pier. Do you have any idea what the power requirements will be? Are you doing just lights and convenience receptacles or are you planning to run a motor such as an irrigation pump or boat hoist?
As a practical matter, at that distance, I wouldn't run anything smaller than #6 (you will need three) with a #10 ground.

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Old 06-22-2007, 03:48 PM   #3
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Yikes! That's a loooong run. You will have to up-size several gauges, I think.
Do you know what the load will be?

Remember, If you increase the size of the conductors, you will have to increase the size of the ground wire proportionally.

Let me crunch a few numbers real quick.
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Old 06-22-2007, 03:58 PM   #4
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I admit i'm a little rusty on voltage drop calculations, but, with a 40 amp load, I come up with #3 AWG, with a #6 ground.

If you want to be prepared for more load, you will have to increase the wire size more.
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Old 06-22-2007, 04:28 PM   #5
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Long underground run


Thanks for the quick reply.

I am planning on one 220 for a boat lift and then some dock lights.
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Old 07-01-2007, 02:51 PM   #6
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Thanks for the quick reply.

I am planning on one 220 for a boat lift and then some dock lights.
This project is more complicated than just finding the right wire size. I would first contact the AHJ as to his requirements regarding piers and floating docks.

NEC rules concerning docks and piers must be followed. This is a potentially dangerous project for someone who is not qualified.
Please get a qualified electrician for this job. And have the blessing of the local AHJ before doing any work.

This application could have lethal affect not just for your pier, but anyone who uses the waterway.
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Old 11-20-2007, 02:17 PM   #7
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J.V., Thanks for your concern, but you sound like a permit clerk. The work is being done by a licensed electrician and my neighbor is a county bulding inspector, so it will be code.

I was asking for educational purposes and thought this might be a good forum for enlightenment.
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Old 11-20-2007, 03:50 PM   #8
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J.V., Thanks for your concern, but you sound like a permit clerk.
Now that right there is funny. Showed you JV!
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Old 11-20-2007, 03:57 PM   #9
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Long underground run


The voltage drop will be relevant to the voltage at the wire runs source. By this I mean if the voltage at the panel where the feeder originates is 215 volts then that is about the lowest for a motor to operate at peak efficiency. This would mean I would have to put down wire that would'nt allow any voltage drop for 350 feet. This is of course considering you want the hoist motor to receive it's rated voltage.

When I calculate this for 40 amps demand (60 amp sub-panel) I get #3 awg copper conductors and a #4 awg copper ground wire considering 240 volts at the source.

8 awg copper needed for 40 amp circuit...no voltage drop
#10 ground needed

#3 awg copper for 40 amps at 350 feet from the main panel not over 3% VD at 240 volts.
? ground wire needed


#8 awg copper is 16510 cir-mil
#10 awg copper is 10380 cir-mil
#3 awg copper is 52620 cir-mil

52620/16510 = 3.187......so # 3 awg is a little over 3 times larger in cir-mil than #8 awg

This would mean we would have to increase the ground by a factor of 3.187

3.187 x 10380 = 33,081 cir-mil

Chapter 9 table 8 shows a #4 awg will be required for the equipment ground.

I can list a general guide line for the electrical installation for a private dock if you like . Just ask and I'll put it up.

Note the NEC doesn't address private docks this is an area that is local code sensitive.
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Old 11-20-2007, 04:39 PM   #10
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Long underground run


This wire is gonna cost you a few thousand dollars lol. Did you size this in copper because of the damp location and the corrosion factor in aluminum? Just curious.
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Old 11-20-2007, 04:51 PM   #11
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This wire is gonna cost you a few thousand dollars lol. Did you size this in copper because of the damp location and the corrosion factor in aluminum? Just curious.
If I got the bucks for a pier and boat lift....I'm running copper!
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Old 11-20-2007, 04:59 PM   #12
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If I got the bucks for a pier and boat lift....I'm running copper!
Yeah i guess if they have thier own private dock money is not an issue.
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Old 11-20-2007, 05:09 PM   #13
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Long underground run


For 350 feet run of wire you are talking about the size #3 which it will meet correct VD @240 volt [ 6.9v drop ,2.9% ] typically i keep at least under 3% drop the max.

but for privte docks there are some code it will cropped up and the materals you will have to use.

mantory GFCI is a must. [ note that in 08 code cycle the 240 v boat lift will required a GFCI so just a head up with it .]

it will have 4 wire circuit set up 2 hot 1 netual 1 ground the size will be #3 for hot and netrual but for ground can squeak by with #8 but rather upsized to #6 due the distance

[ the reason why i say upsized because i allready dealt with alot of parking lot luminaires they have pretty good distance to cover so that why i recomoned to upsize grounded wire a bit.]

Richyl .,, any underground conducts are automatic wet location it is not a damp location at all.

well with alum wires if done propely it should not be too bad but for docking area it will be much better stay with copper wire anyway.

Merci, Marc
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Old 11-20-2007, 07:12 PM   #14
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I can pretty well assure you that copper wire will be required. There will also need to be a grounding electrode at the sub-panel. The sub-panel likely will not be allowed on the dock.

Typical private dock wiring description (for example only) you must check with your local code requirements for specifics.
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Old 11-20-2007, 07:56 PM   #15
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Stubbie did you make that drawing yourself ???

if so you did a good job on it

Merci, Marc

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