Ponds need electricity to run pumps, filters, heaters and lights. Since water and electricity do not mix well, this entails the installation of a ground fault circuit interrupter, GFCI outlet. This type of circuit is extremely sensitive, and when it comes in contact with moisture or if cut it will automatically shut off. Outdoor wiring also requires conduit to enclose exposed cables or the use of direct burial cable.
Check your local codes to confirm how far outlets need to be from water and how deep you need to bury your cable, or if the cable even needs to be buried. Then cut the power to the circuit at the breaker.
Call the local utility companies to determine where water, power and gas lines are buried on the property.
Inspect all devices you are adding to the circuit, such as the pump, and determine whether that circuit can handle the new electrical load by adding up the watts each appliance uses. If the circuit has a 20 amp fuse, then multiply 20 amps x 120 volts. This is the amount of watts the circuit can handle. In this case it is 2400 watts. If the amount of watts is less then 2400 watts then you do not need a new circuit. . If the new device wattage is greater, then you will need to add a 30 amp fuse.
Find a good location somewhere 6 feet from your pond where your GFCI outlet will go and hammer in a stake to mark the spot. Check your local electrical code for permits and outlet requirements.
Install a 3 foot 4x4 cedar post at the marker so that 20 inches are above ground. Dig a post hole with post hole digger 16 inches down. Throw a bag of post hole cement into a bucket and mix with water following the directions on the bag. Pour the cement into the post hole. Use a level to check for plumb.
Measure the distance with a tape measure from the old outdoor outlet to the new outlet next to the pond. This will let you know how much cable to buy.
Dig a trench 18 inches deep for the pump with a trench shovel. This should satisfy local codes. If the trench distance is greater than 50 feet consider renting a power trencher ($65.00 to £48.70 per day as of December 2009).
Mount the outlet at least 16 inches off the ground on a wood post. Enclose all exposed above ground wiring in ¾ inch PVC grey electrical conduit. Use 12 gauge direct burial or underground feed for below ground to connect to the new outlet unless the local building codes call for all wire to be in conduit.
Pop out the bottom hole cut out of the old outlet box and connect the black and white wire to the load terminals with a Phillips screw driver on the GFCI outlet. This will extend GFCI protection to the new device. Load terminals are for outgoing wires and Line terminals are incoming terminals. If the old outdoor outlet is GFCI then the new outlet is protected and does not need a GFCI receptacle.
Enclose the new receptacle in a weatherproof housing so the pump plug and any other device are not exposed.
Pump wires and any other device wires can be hidden under a rock or behind bushes. They do not have to be buried.
Check your local codes and your city's building department to see if permits are needed.