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I live in the United States. I've purchased a kiln for heat treating knives and want to determine the best size for the breaker. The kiln runs off of a 6-50 receptacle and rated usage is 240V 33A. I want to select the best breaker size.

Nominally 240V x 33A = 7920W. As measured my lines run at 123.4V. 7920W/123.4V would equal 32.09A. With 80% breaker utilization that'd require a breaker size of 40.11A. So, would a 40A breaker be okay to use to keep the expensive machine optimally protected? Or would it be proper to assume the nominal 240V requiring a 41.25 amps on a 33A load and use a 45A breaker? Or Should I just size it up to the 50A max that the plug and receptacle allow?

The project includes wiring a 100A subpanel from a 80A breaker with 4 gauge copper thhn2 through 18 feet of 1" emt conduit. Then wiring the kiln from the subpanel with 6 gauge copper thhn2 through 5 feet of 3/4" emt conduit. Other circuits on the panel will include a 20A 240V circuit with a single 6-20 receptacle for a belt sander and some standard 20A 120V receptacles. The wiring and conduit is all indoors in my garage. Does the panel need to feed the panel with a 100A breaker and 3 gauge copper thhn2?

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    Panels are fed by breakers up to the listed size. You need to start at the beginning with a load calculation to see how much power you have to spare. 80 amps is almost half of a common 200 amp house service, so it depends on how you use power first before adding that much extra.
    – crip659
    Commented Apr 28 at 19:56
  • "wiring a 100A subpanel from a 80A breaker" sounds like an 80A subpanel.
    – Hart CO
    Commented Apr 28 at 20:02
  • I will guarantee you that your supply voltages vary, depending on the use of other people on your distribution branch. Measure on a different day or at a different time, and they will not be 123.4V 99 times out of 100. They will typically be within 6 volts above or below 120V (or 12V above or below 240V measuring the 240V supply directly.)
    – Ecnerwal
    Commented Apr 28 at 20:03
  • @HartCO this is normal use of language to describe the nominal Amperage of the panel selected for the sub-panel (where historically the consensus is that you might as well use a 200A panel for a garage shop, given the tiny price differential, due to more breaker spaces) and so long as it is greater than or equal to the supply breaker, you're good. The supply breaker describes the supply amperage chosen, and sets the minimum size of the wiring to the panel.
    – Ecnerwal
    Commented Apr 28 at 20:08
  • @Ecnerwal Oh thanks, so that's basically the rating for the bus bars in the actual panel? Makes sense, I've only ever heard people refer to the subpanel's service amperage since that is the key variable for ensuring all components along the way are appropriately sized.
    – Hart CO
    Commented Apr 28 at 20:38

1 Answer 1

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If it's UL or ETL or other Nationally Recognized Testing Laboratory listed, you follow the manufacturer's instructions, which will normally tell you exactly what breaker to use. The instructions are part of the listing. Not having instructions is ...suspect.

As far as the NEC is concerned, lacking manufacturer's instructions otherwise, (did it come from across the ocean with only a CE mark, or no mark?) 40.000A computed is 40A and 40.001A computed is 45A for the breaker and wire sizing. NEC is concerned with you not burning down the building. Protecting the machine is a problem for the machine designers.

Being in your garage, under current code you'll need GFCI protection for all the receptacles. That may be optional on 240V if your Local Area Having Jurisdiction has not adopted current codes (some are quite far behind.) GFCI for 240V circuits comes at the breaker, for a considerable upcharge. That will enhance protection of the machine somewhat, though it's mostly reducing the odds of killing people that it got into code.

Unless you love spending money you don't need to, 80A (or 90A) feed means 2AWG aluminum wiring. Learn to find and follow the torque recommendations of the terminals you are attaching the wires to (which will be aluminum themselves.) Neither too tight nor too loose. That applies to copper wiring as well. According to http://conduitfillcalculator.com (I use it, but no other affiliation) three 2AWG will just pass in 1" EMT if the insulation type is XHHW/XHHW2 and just fail for THWN-2 - either would be using the conduit for ground. If you want a separate ground wire or can only find THWN-2 wire, you'd need to bump the conduit up one size, or go back to paying too much for copper wire.

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