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I have a home pottery studio which has its own 100A sub panel, properly permitted and installed. There is one 240v 50A circuit that runs to the kiln room and powers a kiln. Plus a handful of other circuits which power lights, plugs, etc

We are adding a second smaller kiln to that same room, which requires 240v 30A. We will NEVER run both kilns simultaneously. As the subpanel is a couple rooms away, with no crawl space or attic, pulling a new circuit from the subpanel to the kiln room is difficult/expensive.

Can I /should I:

  1. Add a subpanel to the kiln room with a 50A circuit for the old kiln and 30A circuit and 14-30 receptacle for the new kiln.
  2. Simply unplug the old and plug the new small kiln into the existing 14-50 receptacle when needed (using a 14-50 to 14-30 adapter as necessary)
  3. Add a DPDT switch ( like a manual generator transfer switch) which would power the 50A receptacle or the 30A receptacle (but not both at the same time)
  4. Other suggestions?
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    How many watts are the kilns in question? – ThreePhaseEel Mar 18 at 0:25
  • What type and size wire are feeding the existing 50A kiln? – NoSparksPlease Mar 18 at 4:57
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    The big one is 11.5 kW, the new small one is 3.6 kW. I believe the run is in 6/3 copper Romex on a 60 A breaker. – Stevo Mar 19 at 3:19
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Your easiest bet will be a cross between 1 and 3

Your easiest and simplest bet is a cross between approach 1 and approach 3, using a subpanel with a breaker interlock between the two breakers in it to keep you from overloading the feeder by running both kilns at once. The good news is Siemens, Square-D QO, and Eaton all have fairly nice interlock kits available now, and for decent prices too, while a basic subpanel (you could get away with a 4, 6, or 8 space unit here since you aren't going to want to feed general load from this panel) isn't expensive either.

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  • The interlock isn't technically required, right? The knowledge that both kilns won't run at the same time is sufficient to allow provisioning them together, and there's no safety issue because the 50A breaker would blow immediately if you tried? – Glenn Willen Mar 20 at 0:32
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    @GlennWillen -- breakers don't trip that fast on overload, but you could technically omit the interlock here and still be within Code as 220.60 only says "where it is unlikely two or more noncoincident loads will be in use simultaneously". The interlock is cheap insurance, though. – ThreePhaseEel Mar 20 at 0:38
  • That's true, I shouldn't have said "immediately". Assuming the kilns use substantially all of their allocated breakers (80%, presumably?) then it ought to trip within a few minutes. But kilns seem like a very specialized kind of equipment which would only be used by an expert in a very deliberate way. E.g. there's no risk of like, a random visitor trying to turn on both kilns at once, or a future owner of the building trying to use the kilns. So the interlock seems probably unnecessary in this specific case. – Glenn Willen Mar 20 at 1:26
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    @GlennWillen both kilns going full blast draw just under 63A, putting you just over 78.5A if you count the 1.25 continuous load factor in. That puts us at 1.3x of nominal breaker rating, which for a 60A QO breaker, will trip it anywhere between oh, 1-1.5 minutes and 15-odd minutes from when the overload starts. (You haven't quite even hit the 135% "must trip" threshold from UL489, so there's no actual guarantee of a trip) – ThreePhaseEel Mar 20 at 2:00
  • @ThreePhaseEel Thank you for the information, just so I understand, you are suggesting using a subpanel which contains a 60A circuit for the large kiln, and a 30A circuit for the small kiln. A mechanical interlock kit prevents turning on both branches at once... correct? – Stevo Mar 21 at 0:47

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