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I am installing lights on 30' light poles for beach volleyball courts. What size breaker and wire for runs up to 315'? 100 amp service provided to the panel.

Can 20 amp breaker be used without noticeable voltage loss for four 500W LEDs?

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Note that any wire you install will need to be wet-rated. That means either individual wires in conduit or direct bury cable. Distance to bury varies depending on type of conduit, cable, etc. Normal indoor non-metallic cable ("Romex") is not an option. GFCI protection is required, and for this use, even if you are installing a receptacle, I highly recommend either a GFCI/breaker or an indoor GFCI/receptacle to provide the GFCI protection as 315' is a lot of wire to worry about and it doesn't take much to zap someone.

A few scenarios:

  • Lighting only - 2A

Assumptions: LED lighting, 2A or less total current requirement, 15A breaker, 14 AWG wire

This is a deliberate choice, specifically to keep the voltage drop low. This will work on 14 AWG (the standard for 15A circuits in general) with a 2.81% (below the mostly mythical 3% voltage drop "ideal").

120V x 2A = 240W of lighting. With LEDs, that is a lot of light. For example, you could get 3 of these 80W, 10,000 Lumen flood lights:

big light

or you could get a whole bunch of much smaller lights to put at various spots along the 315' distance.

  • Lighting only - 5A

Assumptions: LED lighting, 5A or less total current requirement, 15A breaker, 12 AWG wire

This is another deliberate choice: keeps to under 5% voltage drop and uses 12 AWG wire which is standard for 20A circuits and often used on 15A circuits as well.

This is 2.5 x as much power - 600W total. That's 7 of those big lights. Or up to 50 of these 1000 Lumen, 12W motion sensor lights:

small light

With something like that, you could put a light every 10 feet and light up an the entire 315' path easily, with power to spare.

  • Receptacle at the end

This is where voltage drop becomes an actual problem. A 120V receptacle at the end of the 315' distance should be ready to handle anything. A cordless tool battery charger or other small stuff is no challenge. But what if you want to plug in some power tools? Let's limit this to 12A - the continuous rating for a 15A circuit, and 5% voltage drop. Now you need 8 AWG wire to get the job done, which will cost quite a bit extra for 315'. If you actually have a real need for 12A and a maximum 3% voltage drop then you have to go all the way to 6 AWG wire, which is crazy.

So if this is a "light post" for "light" then 14 AWG on a 15A circuit will probably be enough and 12 AWG on a 15A circuit will given you plenty of capacity for more lights added later. But if "light post" means "light post with a general purpose receptacle" then you definitely need much larger wire.

Based on latest edits, I think this may be 4 x 500W of lighting. If so, that may be a problem. There is an 80% derate required for continuous loads (3 hours or more). If that applies here then the maximum on a 20A breaker would be 20A x 0.8 = 16.0A. 2,000W / 120V = 16.66A, which is more than 16.0A.

Due to voltage drop and derate concerns, I suggest a Multi-Wire Branch Circuit or MWBC. This uses one extra wire (2 hots, 1 neutral, 1 ground instead of 1 hot, 1 neutral, 1 ground) but can provide both 120V and 240V in the same circuit. That has a big advantage here because by providing 1 extra wire you cut the current requirement in half, which really solves the problem. In addition, if your lights are wired 240V (2 hots) instead of 120V (1 hot and neutral) then you get better voltage drop. And you can still provide a convenience receptacle (recharge your volley ball players' iPhones). Some math:

  • 2,000W (the 4 x presumed 500W LED lights) / 240V = 8.33A
  • 8.33A, 12 AWG, 240V - voltage drop = 3.80% - that's pretty close to the "ideal" 3%.

If you use a 20A circuit then you can have a 120V convenience receptacle. Can't do that with a 15A circuit as you can't have general purpose receptacles on a circuit with > 50% of capacity of hardwired loads.

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  • Downvoter care to explain? I did answer the question: wire size and breaker size, and also included lots of explanation. Not sure what's wrong with that. Commented Jan 12 at 2:52
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    Beyond a smack on the wrist for trying to answer such a vague question, I have no idea why the DV...
    – FreeMan
    Commented Jan 12 at 13:32

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