saluki4 Posts: 1, Reputation: 1 New Member #1 Sep 5, 2007, 11:36 AM
Under ground wire
Hi, I'm new to this so thank you in advance for any and all advice. I live in Massachusetts and I want to run some wire under ground from my house to a shed and a gazebo in my back yard. I'm planning on building a water garden next to the gazebo and need electricity for the pump. It's a long way out - maybe 75 to 100 yards. I was planning on digging the trenches and then have an electrician do the actual hookups. My questions are : What kind of wire would I need? How deep would the trench have to be without conduit and how deep if I put it in conduit? Are there any other factors I have to consider? Thank you, Steve.
 peaden1711 Posts: 27, Reputation: 0 New Member #2 Sep 5, 2007, 12:23 PM
10-2 should be enough with out dropping very much amperage. Trench should be 24" deep without conduit , it should be 14"with conduit. You can use regular 10-2 if using conduit, were you would have to use direct burial 10-2 without conduit
 tkrussell Posts: 9,659, Reputation: 725 Uber Member #3 Sep 5, 2007, 02:33 PM
Correction to some incorrect information given.

300 feet is a very long run for a 120 volt circuit. Before anyone speculates on the voltage drop the amperage draw must be known.

The maximum voltage drop for a branch circuit is 3%. To keep at or below 3%, or 3.6 volts, the circuit cannot draw more than 5 amps. If the load increases above 5 amps, the voltage drop will increase, the voltage at the end of the circuit will begin to reduce down to a point that may damage equipment, esp motors.

Keep in mind that when motors start, they draw 6-10 times the running current. This causes the voltage drop to be extremely excessive at the most important time full voltage is needed.

Cable should not be pulled through conduit that distance. "Regular cable" probably means Romex, which cannot be installed in a buried conduit at all. A buried conduit is considered as a wet location, and Romex is only allowed in dry locations. Pull cable through a conduit will damage the cable due to friction, unless the conduit is grossly oversized.

If you use conduit, use copper wire with THWN insulation. Conduit needs to be buried a minimum of 18".

If you use direct buried UF cable,24" deep is correct.

If you protect the entire circuit with a maximum of 20 amp GFI breaker, and this is residential, you are allowed to bury cable 12" deep.

You have several options available. Have your electrician do the voltage drop calculations based on the load at the gazebo to determine the minimum size wire needed to keep the voltage drop at or below 3%.

If the run crosses any driveways, should use a conduit sleeve to protect the cable and allow for easy replacement of the cable should it become damaged.

For this installation, I personally would install 1" PVC conduit, buried 12" deep, with a GFI breaker at the panel, and pull 2-#8 THWN for hot and neutral, and 1-#10 THWN green for the equipment ground.
 tkrussell Posts: 9,659, Reputation: 725 Uber Member #4 Sep 6, 2007, 01:44 PM
Correction to my advice. If residential, direct burial cable may be 18 inch deep, as well as conduit.
 MOWERMAN2468 Posts: 3,214, Reputation: 243 Ultra Member #5 Sep 7, 2007, 04:44 PM
10-2 should be enough with out dropping very much amperage. Trench should be 24" deep without conduit , it should be 14"with conduit. You can use regular 10-2 if using conduit, were you would have to use direct burial 10-2 without conduit
national electrical code 2002, section 300.5 / #3. Service Conductors. Underground service conductors that are not encased in concrete and that are buried 450mm(18") or more below grade shall have their location identified by a warning ribbon that is placed in the trench at least 300mm(12") above the underground installation.

voltage drop = current in amperes x resistance in ohms.
for example, the voltage drop over a 200' long #14 copper, 2 wire cable supplying a
1000 watt floodlight is calculated as follows.

Current = 1000 watts / 120 volts = 8.33 amperes
Resistance of solid #14 copper wire = 2.58 ohms / 1000 feet @ 77 degrees F.
Resistance of power line = 2 x 200' x 0.00258 ohms / foot
=1.032 ohms

Voltage drop = 8.33 amperes x 1.032 ohms = 8.60 volts
Percent voltage drop = 8.60 volts / 120 volts = 7.2%

The 7.2% drop is over the maximum 2% so either the wattage of the bulb must be decreased or the diameter of the wire must be increased. [ a decrease in wire gauge # ]. If number 8 solid copper wire were used in the above example the voltage drop would have been 1.8%.
 MOWERMAN2468 Posts: 3,214, Reputation: 243 Ultra Member #6 Sep 7, 2007, 04:46 PM
It is also recommended that you check with your local electric company and see what the codes are in your area. I only provided the national code of 2002, these too probably have changed.
 tkrussell Posts: 9,659, Reputation: 725 Uber Member #7 Sep 7, 2007, 07:38 PM
Section 300.5(3) refers to service entrance conductors only those that come from a utility transformer to the first means of disconnect. These conductors have no short circuit protection.

300.5(3) is not intended to infer that all underground conductors must have the warning tape, as feeders to a subpanel will have short circuit protection.

Local codes may require warning tape over feeders with short circuit protection.

The maximum recommended voltage drop to the farthest outlet is 5 %, which can comprise of 3% for feeder and 2% for branch circuit, or vice versa.

I have no idea where the data of 2.58 ohms / 1000 feet comes from, or why #14 is used as an example.

For AC power, the values listed in Table 9 should be used, which include the impedance of copper wire. Since direct bury cable is suggested, the value of ohms listed for PVC conduit is applicable, and #14 copper wire would be 3.1/ 100 feet of Ohms. For an inductive load, best to use 2.7 ohms Z/1000 foot.

For a purely resistive load, Table 8 is applicable, which #14 solid is 3.07ohms/1000 foot. All close to the 2.58 ohms value given, but the actual resistance or impedance is higher and would result in higher volts dropped.

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