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I am creating a sub panel which will contain 20, 20 AMP circuits to provide power for 20, 1,000 watt, 120v stage lights. My calculations suggest that the light loads, which are constant, will require`20KW. 20KW at 120V means that I will be drawing 160 AMPS. My question is this: At the main panel, is it proper to feed the sub panel off of a double pole breaker that is rated for 80 AMPS? I came up with that by considering that 80 amps at 240v will be providing the equivalent wattage as 160amps 120v will need. Is this the correct way of looking at it?

  • What is the duty cycle of the lighting? Or will some lights be on at full power all the time and some at reduced levels? I did a high school stage a few years back a few more lights but at no time were they all on at the same time and when a majority of the lights were on they were not at full power (dimmed). Based on some additional information like above you may be able to use a smaller size, but since stage lighting changes based on the productions more lights might be needed... The last piece of information we will need how far is the sub from the main panel for voltage drop. – Ed Beal May 10 '18 at 21:06
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    20kw is 166.67A at 120V or 83.333 (repeating of course ) A at 240V if balanced perfectly. Derating by 125% for full continuous use (if applicable) gives 208.33 or 104.16 A respectively. – Harper May 11 '18 at 0:03
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    20,000 watts of continuous light will heat the place up like a greenhouse! – Tyson May 11 '18 at 1:53
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    assuming it's incandescent. If it's HID (that's a thing in theater) or LED, it'll light up the stage brighter than the sun! OP are we talking 1000W actual or 1000W equivalent? – Harper May 11 '18 at 6:11
  • @Harper My comment was a bit tongue-in-cheek.. something about 20 constant 1000watt stage lighting fixtures makes ya stop and wonder... – Tyson May 11 '18 at 15:06
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If your loads are evenly spread across two legs, i don't see any problem on using 80A double-pole breaker, just size wires properly. You'll have two 80A legs. But using 100A breaker on main panel will be better (it'll allow you to load the legs not perfectly even)

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