Our local HD has some 240v high-output lights on sale and I'd like to install in our barn but I'm unsure what's involved. How do I tell what the draw will be for breaker and wire size? Is the a light switch for a 240v circuit?
Thanks for your advice.

The HO light has a ballast inside, and on the label it will. Or should, tell you the total amps that fixture draws. Add the quantity of fixture times the amp draw for each to arrive at the total circuit load.

For a 20 amp circuit, do not exceed 16 amp of load, so if the fixtures draw 1 amp each then only 16 fixtures per circuit, for example.

If you are using a 20 amp circuit, you need #12-2 cable, by the way, cannot use standard Rome NM-B , but can use NM-C in a barn, or UF cable, or PVC conduit. There are others methods but they are too expensive and not practical.

You can use a double pole 20 amp toggle switch to control the lights. This will have 4 terminals, and say "On" and "Off" on the handle. Read the instructions to know which set of terminals are line and load, usually top and bottom.

If this barn is for livestock, are you aware of how to create a Equipotential Ground Plane? Basically all this means that all metal the animals can come in contact with,that can likely become energized, including feed troughs, that are on concrete floor or in confinement areas, be bonded together and grounded to the electrical system. This will insure there are no stray currents or ground faults the animals can be exposed to, as animals are very sensitive to these currents.

No livestock - thank goodness - I can't even say "Equipontential"! As far as the wire type goes is BX (or AC as the Google tells me) OK? What am I trying to protect it from in the Barn? Is it damage or dampness? If BX won't work can I run PVC conduit inside the building? I thought there was some fire/gases issue or is that just inside a residential building?

Thanks for you great advice!

When someone says barn I think of livestock, if it is only for a garage or workshop, the BX or Romex cable is OK, so is PVC or EMT conduit.

PVC is not allowed in certain commercial buildings because of the gases it emits when it burns.

It is a barn and who knows if one day we'll have animals again so it's good to know what's required.
I remember reading in one of your - tkrussell's previous post an explanation of 1 phase, 2 phase and 3 phase - as it relates to voltage. The lights I want to install are Cooper's XF Falcon floodlights 250w MH - they say they are 277v and are "tritap 120/277/347"
I remember something about 277v not being 240v? Is the "tritap 120" a single leg of a 3 phase system not to be confused with one leg of a two phase system? Can the ballast be rewired? Can I make it work on a 2 phase system or is than impossible or stupid or both?

Thanks again for your thoughts