[jebradl]

I am having some trenching done to add a waterline close to my garden. Since I will already have the trench, I'd like to also run an electric line to a utility shed.
Questions:
1. Since the water line will be plastic of some sort, and not a conductive metal, does it matter how much spacing there is between the wire and the waterline? (If code requires it, I guess that it does matter.)
2. The measured distance from the service at the meter box to the shed is 410 feet. With the buried depth plus the box height added in, it will be about 430 feet of wire. I have a small kiln, that draws 23 amps, but could potentially have one that draws approximately 50 amps, so I'd like to put at least a 60 amp service in the shed. What gauge and type of wiring should I use?
3. Is underground wiring OK, or should it be run in conduit?

[tkrussell]

Looks like no one wants to give you the bad news.

You would need minimum of #2/0 copper or #4/0 aluminum, based on the information provided and some assumptions.

400 feet is a long distance to deliver 120 volts, and keep voltage drop (Vd) limited to 5%.

Voltage drop is a tricky issue to deal with. The load amps is the most variable, but most important to get accurate, as, no load amps equals no Vd, and as amps rise, volage drop increases.

So the max amps needs to be assumed, to arrive at the smallest wire possible, but large enough to limit voltage drop. Miscalculating Vd will result in wire too small, and Vd too high, and poor operating results at the end of the circuit. Low voltage will affect motor operation and life of the motor, for example.

Do a Google search for Online Voltage Drop Calculator, you will find several.

I used the following variables:

120 volts single phase
60 amps
5% Vd
400 feet
copper wire

#2/0 copper is the answer.

A simple formula recommended by IEEE and NEC is:

Vd= 2 x length x amps x ohms per 1000 foot of wire / 1000

This formula does not consider other important factors dealing with AC power, such as inductance and in rush starting current of motors, which would increase wire size.

Some may say to use 240 volts as a baseline. This is incorrect, as due to actually delivering 120 volts also, using a hot wire and a neutral. Using 240 volts will size the wire and limit Vd for only 240 volt loads. Any 120 volt loads will see a Vd too high.

Have fun with the calculators, get the variables accurate, and the result will be an accurate wire size.

As to your other two questions:

There is no NEC code to keep wire/conduit away from other utility lines in a trench. Common sense prevails here. Most contractors will keep electric on one side of the trench, and other lines at the opposite side. Telephone and data lines are kept away 2 inches from power lines. There may be local codes dictating this distance.

Either direct buried conductors or conduit is allowed, the risk is up to you. Direct buried conductors, even if surrounded by a bed of clean sand, can become damaged. Conduit helps protect wires, but include the additional labor and material costs, and must be large enough to allow the wires to be pulled that distance with out friction damage to the conductors. A long run such as this would have two or three handholes to assist with pulling.

A popular direct buried cable is 4 wire aluminum wire called URD, you need 4 wires to feed a panel in a remote building/location.

You have a tough job with some tough decisions ahead of you.

[stanfortyman]

Some may say to use 240 volts as a baseline. This is incorrect, as due to actually delivering 120 volts also, using a hot wire and a neutral. Using 240 volts will size the wire and limit Vd for only 240 volt loads. Any 120 volt loads will see a Vd too high.

I am one of those "some". Most every electrician I have ever know has used 240v in figuring VD for feeders.

I respectfully disagree that this using 240v "incorrect". It is NOT at all incorrect, it is a difference of interpretation. To boldly say it is incorrect would require validation. This "validation" does NOT exist.

HOW can you use 120v as the baseline for determining VD when the VAST majority of the load will be 240v. Besides that fact, this is a 120/240v feeder. Just by it's nature we assume most of the load IS balanced.
You would have to go out of your way to intentionally imbalance this feeder.

I know we have been down this road before, but this way the OP does not have to search old discussions to find it.




For an estimated 425' run, you can use #2 to stay within a reasonable voltage drop.

You say the 410' is from your meter to the shed. How are you intending to come from the meter? You CANNOT tap right off the meter. You must come from a panel and have a breaker for this sub-feed.

[jebradl]

...

You say the 410' is from your meter to the shed. How are you intending to come from the meter? You CANNOT tap right off the meter. You must come from a panel and have a breaker for this sub-feed.

My meter has a breaker panel.

[stanfortyman]

My meter has a breaker panel.

OK then. :)