Questions about kiln wiring components

[Jeff Thompson]

Thanks to some wonderful folks here, I'm well on my way to getting my old Paragon kiln retrofitted for digital control! But I had a few lingering kiln-wiring questions and figured it would be better to start a new thread. For context, my kiln will run single phase, 220V, around 40amps. I'll be firing typically up to cone 6 but would like to make sure I could fire hotter if needed.

I've done lots of low-voltage and house-type electrical, so I'm quite comfortable with working safely, but I am new to wiring kilns and just wanted to be sure I'm making safe choices! My big questions have to do with what kinds of temps I should expect inside the control box and selecting parts that will work under those conditions...

Wire/connectors for connecting elements
I've found wire from Skutt and other vendors that's intended for connecting the elements to power. I could for sure order that wire, but I use McMaster Carr a lot and found this 10-gauge wire that's good up to 55A at 600ºF and 840ºF max temp. Would that be appropriate?

Similarly, do I need to use special crimp-on connectors or will the normal, non-insulated ones work ok?

Terminal block
Like with the wire, I'd like to place a terminal block inside the control boxes to connect the power cable to the elements. McMaster has a few options...

• This standard-type one says it's good up to 250ºF
• This one is a little higher at 300ºF
• And this ceramic one is good to 840ºF

If this is all a bad idea and I should just order from a kiln supplier, feel free to yell at me

[Bill Kielb]

1 hour ago, Jeff Thompson said:

My big questions have to do with what kinds of temps I should expect inside the control box and selecting parts that will work under those conditions...

 

Well ventilated and thermally managed with wiring isolated from kiln infrared, hundreds of degrees above ambient. So most kiln manufactures create a channel adjacent to the kiln that creates an air gap (prox inch or two) where cooling air can rise from bottom to top freely for cooling. The remaining components and majority of the wiring are located on the other side of this partition and are also ventilated bottom to top.  Done well, a few hundred degrees over ambient.

1 hour ago, Jeff Thompson said:

e/connectors for connecting elements
I've found wire from Skutt and other vendors that's intended for connecting the elements to power. I could for sure order that wire, but I use McMaster Carr a lot and found this 10-gauge wire that's good up to 55A at 600ºF and 840ºF max temp. Would that be appropriate?

 

It’s never been worth my while, so I just always used MG wire which is Mica Glass usually rated continuous at 450C which is upwards of 800 f. I order from wire supply though, McMaster is probably pricey. Definitely overkill by many of todays standards and lesser rated wires will work. Make NO mistake, they are lesser rated so not necessarily better for temperature resistance but adequate. Popular now is silicone.  Wiring gauge is chosen by amount of current the wire will carry.

So for me, really good ventilation design and my wire is overkill, but my remaining parts can be standard stuff. And all will  last a long time. Your control box should be coolest furthest from the kiln wall and lowest where air comes in to ventilate. Ventilation (stack effect) and isolation are key. If your components can “see” the side of the kiln, they are not isolated from the infrared that will be emitted from it..

 

[neilestrick]

SRML wire will work just fine for element connections.  It's affordable and works very well. For element feeds you will typically use 12ga or 10ga, depending on amperage draw.

If you're using crimp connectors for the element connections, use hi-temp barrel connectors. Or you can also use copper set screw lugs, you just have to remove the offset tongue and use the connector part. I like non-crimping connectors because it allows you to take things apart if you need to.

Terminal blocks are usually located behind an insulating baffle, or at the bottom of the box where it stays cool, so any block should work fine. Just make sure it can handle the amperage.

[Jeff Thompson]

Thanks Bill and Neil!

Baffle:
I'll be putting everything in the original steel boxes that the switches were in, which are nicely vented all around. But they don't have a baffle, just an open box bolted to the side of the kiln. They do have a thin stainless U-shape piece with holes for the element wires – does that serve as a baffle? Would it be worth fabricating thicker baffles from steel? (I noticed the kiln sitter has a pretty beefy steel plate.)

Wire:
The wire from McMaster I linked to seems to be mica glass? Def not cheap but this silicone wire isn't much less. KilnParts.com has wire that's actually a good bit cheaper so I might go that way.

[Bill Kielb]

1 minute ago, Jeff Thompson said:

They do have a thin stainless U-shape piece with holes for the element wires – does that serve as a baffle? Would it be worth fabricating thicker baffles from steel? (I noticed the kiln sitter has a pretty beefy steel plate.)

Yes that U shaped piece divides the space it does not need to be heavy, just reflective. Making it heavier will not add to its effectiveness as a reflector. Making sure ventilation air can travel freely from  bottom to top will though.

[neilestrick]

You cannot just add electrical/electronic components to a manual kiln box or they will run too hot. The baffles where the elements connect don't count. If there are individual boxes for each section of the kiln, they won't get enough air flow to keep the parts cool. You need a full length box to create a chimney effect, plus an insulated baffle.  A better and easier option is to do a wall mount control box, or add a box to the front of one of the existing boxes, and put all the new components in there. All the wiring can run through a small hole between the boxes, and the parts will stay cooler. To fit a controller, 3 relays, and transformer you'll need at least an 8x8 box, but lay it all out on paper first and check what sort of depth you'll need for the relays and wiring to clear the back of the controller.

https://www.wireandcableyourway.com/srml-wire?gad=1&gclid=Cj0KCQjwrfymBhCTARIsADXTabl61VpL7i4-VK60nm3aljg6auX_piB5YWRIW83rqTWm53jLlhLIyz4aAs5PEALw_wcB

[Bill Kielb]

Separate box great idea!. Cooling from bottom to top still applies. Just a note, if installing wires through holes use appropriate bushing or electrical connector. Never wires through a hole in the steel. Chase nipples are cheap and an electricians safe way of routing machine wiring safely through an opening.

[Jeff Thompson]

Thanks again! Sorry for any confusion, lots of variables and parts here! I'm using a wall-mounted controller, so the boxes on the kiln itself are just for connecting the elements. I'm definitely planning to use conduit fittings etc to keep everything neat and safe.

TLDR, is there a rule of thumb for what temperature rating wire, connectors, etc should have?

[Bill Kielb]

Complicated question actually. Optimal operation is often considered 40c - 50c (105f x 122f). (For reference - 120f hot water is considered reasonably safe but feels VERY hot to the touch) most circuits are designed around the lowest rated connection in the circuit. Starting with 60c and has grown to 75c and 90c over time as materials improve. 150c rated wire nuts are often categorized as plenum rated. Ceramic wire nuts generally have higher operating temperature ratings. So exceed or meet 60c would likely be a material minimum requirement.

Best answer I can give, run everything as cool as practical. As things heat, resistance generally goes up which means a local voltage drop which means some watts locally which means more heat locally  …… Proper sizing and proper cooling are the best ways to minimize this. Wires rated higher in temperature will last longer, which is why MG would be better than lesser rated insulation. Passive cooling is dependable so that updraft thing is very useful dependable and free of operating cost but very dependent on ambient temperatures.  Kilns are weird animals, the element connections approach 2000f so better convective cooling helps the longevity of this connection and can enable lesser rated wires to operate satisfactorily. Element end connections can often be preserved longer with a bigger heat sink type splice just because of the additional cooling area and of course any improvement in convection helps lower the average temperature of this splice.

Old designs 60c, UL now I believe requires 75c for lugs / breakers. So all components can be designed to meet or exceed the lowest design temp in the circuit ….. which is often the lugs and breakers in the US.

A nice brief explanation of NEC temperature https://iaeimagazine.org/2009/september2009/where-a-successful-installation-begins-and-ends-understanding-nec-rules-about-wire-temperature-ratings-terminations/

A bit lengthy but the point is, lowest practical temp is generally best for longevity

Edited August 19, 2023 by Bill Kielb

[neilestrick]

Connections and wiring inside the wall mount box can all be standard stuff. All feeder wires going to the element should be insulated. Thermocouples need thermocouple wire, and should just be mounted to the side of the kiln outside the boxes. Kiln power cords should be rated for 105C, and if they use ring terminals they should be high temp. SEOW cords from McMaster work well. The ends in the control box should have insulating sleeves on them.

[Jeff Thompson]

I really appreciate the detailed help here! The costs are really adding up and, though I don't want to cut corners on safety at all, I wondered if you guys thought these would be ok:

• Mica wire, 450ºC, $26 for 10'
• High-temp wire, 250ºC, $44 for 25'
• SRML wire, "high temp", $56 for 25'
• Silicone wire, 392ºF, $41 for 50'
• And these high-temp ring terminals, $14 for 100

Obviously I'm more skeptical buying on Amazon than say McMaster, but the prices are so much lower.

[Bill Kielb]

31 minutes ago, Jeff Thompson said:

I wondered if you guys thought these would be ok:

For me,I go with mg @ 450c but shop the web. Usually 20-30ft of #12is a lot of replacement though. I just shop the wire suppliers for best price. $28.00 handles a lot of repairs for me.

 

[Jeff Thompson]

Popping back in to say a big thank you for all your help!