110 oven and safe connections

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Randy3000

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Jun 3, 2017
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I am planning a 9×9×4.5 oven to run on 110vac and want to heat treat mainly w2 and maybe some stainless steels. I saw a guy use 1250 watt heating element at 110v and reached 2000*F with these oven dimensions. I am thinking about getting a 1600 watt heating element from duralite.

I know you are supposed to make the connections from the wires to the element outside the oven so the heat doesnt wear out the connections and get the insulated wire too hot. I had the idea of using some nichrome wire to short out the coils sticking out of the oven so that they dont really draw much current and dont get too hot. But then i will have, i dont know, an inch or two of coils poking out of the oven. How should i protect these so i dont accidentally fry myself? I could make a little insulated box covering it and have the insulated wires come out from a small hole, but then that box may overheat.

I watched 2 videos where guys just leave it exposed, and i am just not that reckless. Any ideas?
 
You really should enclose or otherwise protect from contacting any energised & exposed component.
If overheating is concern, enclosure can be made of mesh. Primarily making it touchsafe what yourself or something tipped or whatever cant come to contact.
 
Whatever you do, you'll need to protect yourself against contact with live parts. I usually build a fingerproof cover that allows airflow for cooling.

The transition through the insulated wall needs to be prevented from getting too hot.

If you double the wire where it passes through the wall and twist it, there does not seem to be a problem. It seems to be a standard method for kilns.

Electrically, the 2 twisted wires have a combined resistance half that of a single wire. Only half the current flows in each wire, so only a quarter of the power is dissipated as heat. There are 2 wires, so each of them is producing 1/4 the heat of a single straight wire, making 1/2 the heat per inch combined. If you feel the urge, you can even go for 3 wires twisted together.

If you are not winding your own coils, or getting them wound to your specification, but are using coils off-the-reel, shorting the coils with nichrome wire might be fine. I'd probably try for something that would fit the inside of the coil quite tightly for the pass through the wall: maybe a piece of stainless allthread? I don't know whether the coils would stand straightening and then twisting together to make twisted tails.

If you are buying the coils off-the-reel on ebay or similar, check the specifications carefully. The ones I have seen are extremely thin and I'd not expect them to last. I built some HT ovens with 16 AWG coils. Most have been fine but 2 coils burned out. I now use 1.6mm Kanthal A1 wire (1/16", 14AWG).
 
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Duralite doesnt list the gauge of their elements, that i can find. Their 110-120v elements jump in coil diameter between the 1600watt and 1750watt, so i would assume their is a gauge difference there. I will try to talk to someone at the company and see.

EDIT
I feel the need to clarify this to others since it can be confusing, i think.

"Electrically, the 2 twisted wires have a combined resistance half that of a single wire. Only half the current flows in each wire, so only a quarter of the power is dissipated as heat. There are 2 wires, so each of them is producing 1/4 the heat of a single straight wire, making1/2 the heat per inch combined."

The 2 wires combined do indeed have half the resistance of a single wire, and this resistance is much lower than the heating element, and so the circuit is pulling the same current essentially that it was before. So each wire does get essentially half the current. Then using P=I^2*R shows the wattage quarters. This confused me for a second because i briefly thought halving the resistance would double the current and so the wattage would be the same. But the resistance of the wires is neglible compared to the heating element, so the current stays about the same.
 
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I measured and i am getting 122v at the outlet. I want to keep the amps low as possible, let us say 15amp. Then i basically need 8ohm of resistance. This will generate 1800 watts. Kanthal A1 wire at 1.6mm 14awg is 1.42 ohm/m, and so i need 5.63 meters of wire. If i coil the wire at 0.5" diameter tightly, then i will have about 141 coils. 141 coils is a length of about 8.9". I think my oven will have inside dimensions of 4.5"×4"×6" using 2.5"×4.5"×9" bricks. So if i stretch the coil to quadruple its length i should be able to cover the inside well and have a pretty toasty oven i think. I hope. This is just for folder parts.
 
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