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(In the US, Ohio): We had our electric to our home professionally redone a couple years ago, so that we could hook up a kiln in the garage. This all works great. We recently got another smaller kiln, but the electric cord had a plug that had the angled pieces instead of all straight.

Can I replace the plug with a straight one so it works in the existing 240 V outlet? I know an electrician did this with our dryer a few years back.

Here's the details:

Existing (larger) kiln is:

240 V AC, 2 W (???), one phase, 48 A, 11,520 W (doesn't mention Hz)

New kiln (smaller, whose plug I want to replace) is:

240 V AC, one phase, 60 Hz, 30 A, 7200 W

The Outlet

The outlet is on a 60 A breaker which is exclusively for the kiln.

Both kilns will not be running at the same time. We'd just like to use the same outlet and venting and manually switch them as needed.

Existing kiln, plug, and outlet:

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The new kiln with plug we'd like to replace to work with existing outlet:

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  • How will you prevent both kilns from being powered at the same time? Simply having one socket that can take either plug but not both ?
    – Criggie
    Jan 1, 2022 at 5:28

3 Answers 3

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You MUST change the plug

The plug, as provided on the new-to-you equipment, is a "NEMA 10-30" type. It is obsolete and dangerous, and does not contain a ground wire.

(That wire is neutral).

The kiln manufactures supply NEMA 6 receptacles appropriate to the unit, so clearly, "monkey business" has gone on here.

enter image description here

What's the story? Grounding. Safety grounding became mandatory in the 1960s (NEC 1966). However there was still a large inventory of ungrounded 30-50A cabling in the manufacturing pipeline, so an exception was made to allow ungrounded wiring for dryers and ranges only until wire stocks ran out. New wire was required to be grounded, but the old 3-prong NEMA 10 connector could be used for awhile to accommodate dryers and ranges with the old cords. It was expected these would be wired with "/3 w/ground" cables, so the socket could be changed to NEMA 14 in the future. Unfortunately, this permission was wildly abused. Appliance manufacturers and builders dragged their feet on switching to NEMA 14. "/2 w/ground" cable was illegally used. Finally around 30 years ago, NEC "put its foot down" and banned NEMA 10 absolutely.

The plugs and sockets are still sold, strictly for direct replacement of a broken plug or socket. This too is wildly, laughably abused.

Several communities, such as welders and kiln people, have "adopted" the defective NEMA 10 as "their private socket" even thought it is wrong. Usually this involves cutting off the correct socket and then putting "Amateur's choice" of socket, typically based on the existing sockets they have in their home for dryers and whatnot. They really should be using the NEMA 6 type receptacle.

The manufacturers insist not to do that, and Paragon voids the warranty when you do.

The difference is that the neutral pin on a NEMA 10 socket is wired to the neutral of the house, not the ground. In sub panels and outbuildings, there's a big difference. Neutral can be energized and float at 120V under a number of common wire failures - that's why we put insulation on it. Ground should be safe at all times.

Breakers must match plug size

We see a lot of blatant "ignoring of Code" among kiln and welder folks, and your current sockets are no exception.

I would agree that ordinarily, a 125% derate would need to be applied by default. However member J... makes a really good point in comments:

Kilns aren't flat, though - usually kanthal elements and those change resistance about 1.6x from cold to operating temperature. That's a short startup (<3h) so it behaves like inrush.

That would mean kilns would be at 100% nameplate ampacity for less than 3 hours, and then would fall below 80% nameplate, which means the 125% derate isn't really applicable. Annoyingly, nothing about this is mentioned on the equipment nameplates, and it would help a lot if it said "Circuit breaker: 50A".

As things are, the kiln manufacturer is resting entirely on their admonishment not to change the plug, and the assumption that NEMA 6-50 receptacles are wired with 50A breakers (which is not a valid assumption at all, see below.)

However, socket sizes must match the circuit breaker, unless an exception is specified in NEC 210.21. The only exception is that there is no NEMA specification for a 40A socket, so 50A sockets are used on 40A circuits. Since the units ship with NEMA 6-50 receptacles, they need 50A circuits.

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Your 30A Paragon kiln would work better on a 40A circuit, but we presume that TUV (an alternative to UL) certified it also for a 50A circuit, as that is plausible for them to do.

Option 1: Change the breaker to 50A ($12)

The 60A breaker is illegal on a NEMA 6-50 receptacle. If you want to directly feed the receptacle you must change the supply breaker to 50A. This is more "monkey work", some monkey oversized the breaker in an effort to avoid "nuisance tripping", but as established above, that shouldn't happen.

Option 2: replace the 50A receptacle with a subpanel and 2 breakers and receptacles (under $100)

This spares you having to change the 60A breaker, e.g. if you don't have access to it. Install two 50A breakers in the subpanel. You can run 1 kiln at a time. Really. (it doesn't sound right, but neither I nor anyone else has found any prohibition on oversubscribing panels with loads you do not intend to use simultaneously. If you do, then yes, the subpanel must support the load to be served).

I presume this subpanel will not have a neutral. Leave the neutral bar empty and disconnected, and make all ground splices on a separate ground bar. That will deter anyone who thinks they can put a 120V circuit there (they will anyway, but at least it will be their fault instead of yours).

From this subpanel, wire two NEMA 6-50 sockets. One to each breaker. (they can't share).

The reason not to just swap plugs everyday is that those large sockets and plugs are not made for daily swapping (to the lament of EV drivers who also want to use their dryer; but their salvation is that multiple sockets are allowed on a 30A circuit. Not a 40/50A, sorry.)

Option 3: 90A feeder to subpanel for simultaneous use

I say 90A feeder not 100, because #2 aluminum wire is a cheap commodity. #1 aluminum is 100A but a little harder to find. 90A should suffice given the nameplates on the devices.

Other than that, sub panel as above.

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    "...avoiding the cost of NEMA 6-60 plug" Which is pretty much infinity. I don't think such a thing actually exists. The only 60A rated NEMA plugs out there are 3-phase twist lock or something with a neutral (15-60, etc).
    – J...
    Jan 1, 2022 at 22:48
  • I like the idea of a sub panel, I would suggest getting a transfer switch but wiring it up "backwards" so that instead of one output to two inputs it's one input to two outputs. The 60 amp breaker in the main panel to a sub panel with a 60 amp and a 40 amp breaker might raise eyebrows if there is an inspection. A transfer switch is just a sub panel with a mechanical device preventing both breakers being on at the same time, so the cost difference over a sub panel should be minimal.
    – MacGuffin
    Jan 3, 2022 at 4:53
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    @J... nono... what you're observing applies to motor loads that is, NEC article 430. It's a gory mess because motors need startup current, so breakers can be oversized and UL gets involved in saying how much. However kilns are flat resistive loads, so yes, you derive from nameplate. UL approving a 48A kiln for 50A breaker #skeptical forums.mikeholt.com/threads/kiln-as-a-continuous-load.97049 Jan 3, 2022 at 19:48
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    @J... well that might be cause for UL to rate them like that. Still, the 2 manuals I could find don't have the pages of boilerplate I expect in a UL approved manual, but neither one has breaker specs. They do say "data plate overrides manual". but both data plates are silent on breaker size, so you can only go by nameplate and standard derates. They imply "factory shipped plug indicates circuit size" (ambiguous for 6-50) and says "changing plug voids warranty", but the Paragon kiln does not ship with a NEMA 10 plug. What a mess. Jan 3, 2022 at 20:26
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    @J... Here the UL listing and approval to ship with a 6-50 is the best argument I think. I can accept UL overriding 425.4(B) (is continuous) based on facts and circumstances of a piece of equipment, as that is in their purview. However overriding 210.21 (50A breaker==socket) isn't something UL can do. I perceive breaker enclosures as "2-space subpanels"... Jan 3, 2022 at 21:37
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The two factors for circuit size are the Installation and Operating Instructions which are part of the UL/TUV Listing, and the Electrical Code, which defers and specifies that the Listing must be complied with.

I don't find the actual Instructions for either, but I do find spec sheets online for both the Skutt and the Paragon kilns. These may be inaccurate or out of date so if the actual instructions say otherwise follow the instructions.

Both kilns specify a NEMA 6-50, receptacle fed by #6awg wire, and are suppose to be fed by 50 amp breakers. It appears the Paragon has had the plug field changed out of compliance with the instructions, and the wrong size breaker is feeding the existing kiln. The bottom of the page for the Paragon says it "assumes" the kiln will be plugged into a dedicated circuit. I think if that is consistent with the actual instructions then you could comply changing the breaker to a 50 amp and using a single receptacle and just plugging in the kiln you are using.

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    Probably best (unless actual printed instructions came with it, that were the ones it came with originally) to check with Paragon directly, since the instructions online may reflect "same model number, updated hardware" (why they don't update the model number with the hardware, dang if I know, but I've seen it too often to get complacent about website instructions being correctly applicable.) i.e. tell them you are asking about a 2014 TNF82, serial number 445381, not just a TNF82.
    – Ecnerwal
    Jan 1, 2022 at 1:51
  • @Ecnerwal Yes, I was less than clear when I referred to "actual instructions" in the second paragraph. If they don't have them they should get originals from manufacturer for both. Jan 1, 2022 at 2:01
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    I agree that it appears the plug was changed after the fact. A device drawing 30 amps continuous should have a 40 amp breaker and a 6-50 or 14-50 plug. A 10-30 amp plug on this was a hack job, someone trying to get by on an existing 10-30 dryer outlet. The 10-30 outlet is not supposed to be on anything built after 1996, after that 30 amp circuits should be a 6-30 or 14-30. But with heating devices there is an 80% derating, meaning a 40 amp breaker and 6-50 outlet/plug. If the manual says a 50 amp breaker is good then I'd say a 50 amp breaker is good.
    – MacGuffin
    Jan 1, 2022 at 3:46
  • "It appears both of the kilns have had the plugs field changed out of compliance with the instructions" - no, just the blue Paragon kiln. The red kiln has the correct 6-50 plug and OP has the correct 6-50 receptacle. There are only two problems here - first the blue kiln needs a 6-50 plug (OP's question - yes it can be replaced) and the outlet should be breakered at 50A instead of 60A. OP's electrician probably picked 60A based on a derate of the 48A nameplate of the Skutt, and that's not crazy.
    – J...
    Jan 1, 2022 at 13:31
  • @J... Thank You, my mistake about the wrong plug on the Skutt, confused the 6-50 and 5-50. I edited to reflect. If the Instructions didn't (or maybe don't) specify the 50A receptacle and I had to just install to Code then I would have a difficult time using a 60A breaker in conflict with 210.21(B)(3) and if 125% circuit rating was required for the 48A kiln then I would think it needs to be hard wired, but I don't see where it is a 125% circuit. Kilns are thermostat controlled and cycle like an oven so not 3 hour constant rated, not a motor, not fixed electric space heating, not a transformer. Jan 1, 2022 at 15:43
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You can do it but I would recommend that you simply add an additional circuit. The Kiln power cable on the smaller unit more than likely will not be able to safely trip the breaker if something faults. It does not look to be that difficult of a job as everything appears to be on the surface. If this was done recently then you should have open breaker slots. The end result is you could optionally run both if you have enough power available. I am not up to date on the local code in your area so this is my best suggestion. There are many other ways but I am the most comfortable with this, it is safe.

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    (Not sure who downvoted you, wasn't me!) This is in Ohio in the US. I'll update that on the question.
    – MikeJansen
    Dec 31, 2021 at 21:36
  • The outlet is on a 60 amp breaker which is exclusively for the kiln.
    – MikeJansen
    Dec 31, 2021 at 21:44
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    I agree with @Gil. The new kiln takes half the power of the circuit. If something happen inside the kiln, then it could take all of the 60 amps before the breaker would even think of tripping. If the two kilns were closer together in power amounts, would be closer to being okay.
    – crip659
    Dec 31, 2021 at 22:45
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    The reason for the suggestion to bring in a smaller breaker would be to trip faster on an overload scenario. 15 amps is the lowest common breaker size in the US (don't know if that's by rule or because by rule the smallest wire gauge is 14). If you want lower overload protection, you need a fuse on the plug or in the thing you are protecting. This is common in the US on Christmas lights (3 amps). , My understanding is that in other counties there are fuses in many other cords.
    – Edwin
    Dec 31, 2021 at 23:29
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    The 60W lightbulb is part of a lamp that has (unless it came "direct import" from Amazon/Alibaba) been tested by UL to destruction on a 15 or 20A circuit and passed. Not the same thing at ALL as a 30A kiln on a 60A breaker. Fairly sure the new kiln instructions will specify a maximum breaker size (or simply a breaker size) and it won't be 60A. Having seen the arc in my 50A range's oven when it blew the oven element which was quite happily burning away without tripping the breaker, I will pile on the recommendation to install a correctly protected outlet for the 30A kiln.
    – Ecnerwal
    Dec 31, 2021 at 23:53

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