Calling All Electricians

[Swee Pea]

I need to run some underground electric wire for a 50 amp circut; about 150 feet. I can buy 8-3 UF copper with a ground for over a dollar a foot. I can buy 10-3 UF copper with a ground for 34 cents a foot. The electric supplier said to buy double the length of the 10-3 and run it in parallel feed - running completely two 10-3 UF lines together in the same trench - at the box, you connect the two blacks to one hot, the two reds to the other hot, the two whites to the neutral, and the two copper grounds to the ground. This he said would be even a heavier connection and allow for a 60 amp circurt and is legal.

What say you?

John
Swee Pea

[Captains Cat]

John, I had a 400' run from my new house to the pier and wanted 80 amps down there, 40 amps on each side of a 240V circuit. I ran 4 wire (incuding a separate ground) from the house out there and used #2 copper. Your run isn't nearly that long nor is the amperage as high. There are all kinds of online calculators out there, that's what I used. It was expensive ($3.75 a foot) at Lowe's. I have about 75' of that left but it's not enough for you. Use a calculator (I'll look for one), don't scrimp on size, you will only do it once.

Run a three wire line plus a ground. Don't cut a corner and try to run two and play games with it, it's too important.

On edit: I think this is the calculator I used. Make sure you get the type designed for direct burial, I think it's UF-Bl

http://www.csgnetwork.com/voltagedropcalc.html

Charlie

[Swee Pea]

Thanks for the quick reply, Charlie.

All the needs at the end of the dock are 110, 30 amps max. That 10-3 with a ground copper is already run 250' UF direct burial. It will be connected to a 50 amp cut off. Then from the cut off to the temp meter 50 amp circut breaker (on a 100 amp service), 167' is where I want to keep the drop down. #8 will work, but two #10 runs connected parallel as I described in the original post would be better than a single #8. The question is, "is it legal?" The supplier says yes, that it is done all the time in commercial applications where really heavy wire is needed. According to the calculator link you supplied, if I'm using and reading it correctly, running the first 167" using #10 wire - 50 amp circut breaker, there would be a 12v drop -5% - I would have 228 v. This does not take into account doubling the #10 wire. #8 would yield a 3% drop to 232v. The dock run would drop it 18v to 214v. Pretty close (I think).

I think I'm good, but two #10 runs connected parallel as I described in the original post would be better than a single #8. The question is, "is it legal?" The supplier says yes, that it is done all the time in commercial applications where really heavy wire is needed.

John
Swee Pea

[Captains Cat]

Do you have a permit? Will it get inspected? Those are key questions to "is it legal"? If not, do what is safe and use lots of ground fault stuff.

Charlie

[thataway]

Not directly to answer our question--but have always run one size larger. I have found leakage in the direct burial cables--better in sealed conduit--large enough to allow for any heat.

The other issue, is that I always put in a second ground rod right at the dock. (even with a ground wire). I found voltage differentials in grounds--which can lead to problems with the boat.

[jkidd]

In a commercial install it would be in conduit. The problem is that down the road when one of the conductors rots through, now you twice the amperage going through the last conductor. I just put mine in conduit and didn't use the direct burial.

[Tim & Dave Kinghorn]

Hi John,

I'm confused. You have 250' of #10 already in to the dock, and you want to run another 167' from a main panel to a switched junction box. If so, you have a total run of 417' from the main to the dock panel with a 30A load.
This is a series circuit and the total voltage drop is in the single 120v leg. At 30A, the voltage drop with the single #10 would be 13v (120-13=107 at dock). If you ran #8 for the full 417', you would cut the the voltage drop to 10v. This assumes you started with a full 120v on the one leg at the main.
IN ADDITION, you are planning to tap 50A at the switched junction box. You could have a total load on the 167' of (30A +50A) 80A.

I share Bob's concern about proper grounding at the dock. Normally, the code would want two 10' copper rods spaced about 10' apart as a ground. Wiring the panel at the dock is very important. Normally, I believe, the neutral will float above the panel grounds. And, of course, you would protect with GFI's on your circuits.

I would strongly recommend getting a licensed electrician to review your plan. You could still do the labor, and I have always found that having the job inspected is a wise protection for you and others. Your boat will rely on the ground at the dock for its safety in case of a short as well as the safety of any swimmers where even a small current leak can prove fatal.

I am not an electrician! But I figured you'd want to hear different views.

Tim & Dave Kinghorn

[marvin4239]

Paralleling #10 wire in the application you describe is not legal. Also on a sub panel you must have and insulated ground wire. I think as others have said running pvc pipe and pulling single conductor THWN insulated wire would be a better option. Conductors in PVC rather than direct burial disipate heat much better. The wire you typically see at Home Depot or Lowes is THNN insulation. The THWN insulation is for wet locations which is what you will have in buried PVC (condensation). Your ground must only be connected per NEC to the main service ground which is typically two ground rods 6 feet apart. Adding a ground rod at your dock in my opinion is a good idea but it isn't legal either. Local codes may vary but what I describe is the minimum requirements.

[Swee Pea]

Thanks for the input so far.

I am taking your concerns to the electric shop and discuss the possibilities. It was funny, but I was talking about this to a friend today at school and the school's interperter overheard my conversation. She said she had to interpret a College class for electrican's and had to interpret the NEC code book to the students. She informed me that paralleling wire as I described was common in commercial applications and that topic was covered in the class. The only concen is if the local building inspectors are knowledgable in its application. So, I need to see if it is legal in my application.

Any more input that you may have will be greatly appreciated. This is what this site is all about!!! A place where we can go to discuss needs and "suck all the knowledge out of your brains" - for FREE!

John
Swee Pea

[marvin4239]

John it is true tha paralleling Large conductors is common but there is a limit to the size of the conductors that can be paralled. It's been many years since I've looked at NEC but I think you'll find that it applies to conductors 4/0 and larger. When you do parallel them they must be exactly the same length. Grounding can be sort of confusing also. If you speak to a local building inspector ask him the correct way to ground your installation. Typically all grounds must be connected in series. In other words it isn't legal to have one ground wire grounded at your service entrance and then another earthened ground at another subpanel. Theory is their could be current flow between the two grounds which is undesirebale.

[Minnow]

If I asked for advice from the counter guys at the supply house, I would have been sued out of business a long time ago.

Only conductors 1/0 or larger may be connected in parallel. See NEC 310.4(a).

As for heat dissipation, if you look at the tables in article 310, you will find that the ampacities for directly buried conductors are nearly twice that of conductors in buried conduit.

The wire must be protected by an overcurrent device at or below its' ampacity. In your case, the existing 10-3 should be fused at 30 Amp. and will give you a 30amp 120/240 volt branch circuit.

After reading your original post again, I'm also confused about the load. What kind of 50 amp circuit are we talking about? 50amp 240v? 50amp 120/240? Are we talking about dock wiring, or a piece of equipment?

There is always confusion about "slash rated" loads. These are things that have a nameplate stating 120/(slash)240, like dryers, cooktops and ovens. This means they need a full sized neutral. If it just says 240volt. then it doesn't need a neutral, like motors, welders, etc.

Tell us what the actual expected load is, and we can tell you what size wire you need.
As an aside, I almost always use aluminum conductors in conduit for large outdoor branch circuits.

[Will-C]

Hi,
In our business for longer runs we use 230 volt single phase and have the electrician pull three conductors two hots, and a ground. Make sure the wire size and breakers match the amperage draw. Breakers to be a sized 25% higher than expected amperage draw. Terminate at a subpanel and run your 120 volt off the single pole breakers in that panel. 230 draws half the amperage reducing wire size on the long run. I don't know your situation but we don't like direct burial wire. Use conduits and pull wires, pull a couple spares. At the devices at the end of the runs use #4 bare copper bugged off on the device to eight foot driven ground rods, electricity seeks ground. Use licenced electricians and get permits. Think about what you own and insurance ramifications if someone would ever get whacked. If your running this near salt water I would think about getting get copper wire that is tin coated to prevent the ineviable corrosion. my 2 cents.
D.D.

[Minnow]

Here's the deal with grounding, more is better.
The ground you're standing on needs to be at the same electrical potential (voltage) as the equipment grounding conductor of the circuit near you, for life safety.
This is done by re-establishing the circuit ground to earth remote structures. Ground rods are one step above worthless. Two is twice as good. Concrete encased electrodes (20 feet or more of re-bar in the footing) is about as good as it gets.

The confusion comes when we talk about bonding the ground to the neutral. It is normally done once only at the service entrance. From there to all downstream installations, the two are kept isolated so that the grounding conductor is not in parallel with neutral and is not carrying any current.
With some installations, a wire can leave a building as a "branch circuit", and miraculously enter another building as a "feeder". In that case. you would re-bond the neutral (groundED conductor) and the groundING electrode system.

The equipment grounding conductor normally does not carry current. It is only there to carry enough fault current (short) back to the overcurrent device to make it trip. The only time it's normal to see current on the ground is at the main service. The utility company grounds one leg of the transformer to create a neutral. Think ground rod at the pole. Then you run a 3-wire service to the house and ground the neutral again. What you now have is dirt electrically in parallel with the neutral wire. So, if you clamp an ammeter over your grounding electrode conductor, you will usually see an amp or two.

[Swee Pea]

Wow. I've got a lot to learn about electricity.

I am establishing electric service to my property. I built the temporary service to building specs, including a meter base and a service panel - just got it inspected and approved. It is a 100 amp temporary service. The electric company will bring in the meter and wire to connect (direct burial) to the temp, but their wire will support a 200 amp service when connected to the house, whenever that may happen.

From the temporary pole, I installed a 220 50 amp breaker to supply the electric on the dock (a fuse panel) - consisting of a Tide Tamer 10,000 lb. lift with two 120 motors. It will also have a 30 amp plug for shore power to the boat and a 15 amp plug for incidentals. All of the dock "devices" are 110.

The dock builder and I ran 10-3 copper with a ground uf for this supply about 270 feet to the entrance of the "boardwalk" over the protected area, where I will install a disconnect. At his suggestion and experience, the wire was NOT put in conduit but attached under the dock with plastic clamps - not staples which corrode. His position is that in his 25 years experience, he has never seen a piece of conduit not leak. In our location, the few if any times water will reach the wire would be in a hurricane or like incident. I chose to defer to his experience.

From the disconnect to the 50 amp breaker in the temporary service is the run I am concerned with. I was told that #8 wire is needed to carry the 50 amps to the cut off. No problem, I can do that. But there is this great deal on #10@ 34 cents a foot, that if allowed to parallel, would not only save me some money but also increase the wire rating - 2 #10 in parallel would be better than one number 8 - if it is doable and legal.

You all have given me a platform to think about and ask the local supplier.
I did not know that only conductors 1/0 or larger may be connected in parallel. See NEC 310.4(a). I will check it out. If that is the case, I will not do what is not by code. I will also compare with running wire in conduit and burying in the ground.

My thought is to carry a true 50 amps to the cutoff at the dock. Then I would have enough power to run down the dock. I just need the right run of wire from the breaker to the cut off that will deliver the power.

I need to get this settled because in a few weeks, I am going down to the property (Bath NC) and establish a water service to the property and to the dock. I already have the dock piped with 3/4" Schedule 40 pvc. It is time to connect to the water meter, run the water line to the "house" - temporary panel, and then on to the dock. I was told to use 1" pipe to the house then drop to 3/4 for the about 400 foot run to the dock. They said to use 3/4" on that run to increase pressure. This may be the subject for another topic, "Calling All Plumbers".

When I rent the "ditch witch" for the water line, I want to dig the ditch for the electric at the same time. Maybe I will just bury the two lines together --psych --- I know its a no no.

Thank you all for such great insight and knowledge. I will keep in touch.
And "keep your cards and letters coming"

John
Swee Pea

[Minnow]

Sorry, but if you really want 50 amps at your dock, the 8-3 UF isn't going to cut it. For UF, 340.80 points you back to table 310.16 which lists #8 UF at 40 amps. #6 gets you 55 amps.

As I mentioned before, you might want to price (2) #4's and (2) #6 aluminum type USE conductors in a 1 1/4 pvc conduit. (although it can be direct buried) Use anti-oxidation compound and a torque wrench to install.
Don't be afraid of aluminum-- the lugs in the meter socket, panel, disconnect, etc. are almost certainly made of aluminum. So are the utility service drop conductors and connectors. Why mix in dissimilar metals?

If you really want to drive yourself nuts, read through NEC article 555 on boatyards and marinas, especially if you have any wiring on floating docks.