Hello, Thanks for your Electrical Help. This is in the USA.

I plan on installing (4) 1500 watt halogen lights in my pole barn. The distance from the 100 amp panel box to the pole barn is 135 feet. I am interested in using direct burial wire, but do not know what size wire or circuit breaker I should use. Also, should I install a sub panel in the pole barn then wire to the lights with 12/2? Thank you in advance.

Four 1500 watt lamps will draw 50 amps at 120 volts, or 25 amps at 240 volts.

P=EI or I =P/E
I= 6000 / 120 = 50 Amps
I= 6000 / 240 = 25 Amps

The circuit breaker and wire ampacity must be rated at least 25% higher than the load amps, and round up to the nearest standard size rating.

So, a 120 volt circuit will need a 70 amp breaker and #4 copper wire, and a 240 volt circuit will need a 40 amp breaker with #8 copper wire, as minimums.

Next to deal with is the voltage drop. These fixtures/lamps can be found rated 240 volts. I would not recommend delivering 120 volts that distance, as #3 copper will be needed, and #6 copper for a 240 volt circuit.

A 70 Amp 120 volt feeder is not practical. I would use 240 volt lamps/fixtures.

I would use #6- 3 plus ground to feed a four circuit panel, that would hold two 2 pole 20 amp breakers, each feeding two lights with #12 wire. Use a 2 pole 40 amp breaker at the main panel to feed this #6 cable and panel.

The two 2 pole breakers in the barn panel will act as the required disconnecting means.

This does not take any other load into consideration, unless other loads are used when the lights are not needed.

At least one ground rod will be needed to ground the equipment ground bar in the barn panel. The neutral white wire must be kept separated from the equipment ground. This is usually done by not installing the green Main Bond Jumper screw that comes with the panel, the equipment ground bar is furnished separately as an accessory.

Don't forget, direct burial cable needs to be buried 24 inches deep. That's quite a trench.

To clarify this:

Per NEC 2008 Edition Table 300.5 Column 1 for Direct Burial Cables, DB cables for a residential use one and two family can be at a minumim of 18 inch deep.

OP mentions barn, this may be a commercial barn, in which case, 24 inch deep is required. If this is residential, then 18" deep is allowed.