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Resistance in electrical equations
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You are currently reading a thread in /diy/ - Do It yourself

Thread replies: 12
Thread images: 3
Hello,

I have what seems like a really simple question but I have a hard time finding an answer just searching though the internet.

I am looking at a resistive circuit, the point of which is to generate heat.

I know that voltage = Current x Resistance

So, since I am using household voltage (120 volt nominal) and my circuit breaker says that it is rated for 15 amps per circuit with a maximal continuous load of 12 amps, does this mean that to reach the maximum allowable wattage I need a minimum of 10 ohms of resistance to run continuously?

Then, if I just need 10 ohms of resistance, and I have a material that is rated at 10 ohms of resistance per foot, do I just need a foot of material to complete the circuit?

I am building a heating element circuit as reference and don't want to damage anything or burn out my household wiring.
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The point of this question is to try and build an electrical furnace to melt metals up to copper so the furnace will need to get up to around 1100-1200 degrees centigrade.
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>>936617
ITT : How to burn your house down and collect insurance money.
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>>936623
Precisely what I'm not trying to do.

So am I right in my understanding?
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>>936628
i ^2 * R * t = Heat in Joules
Joules law
Use this
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>>936617
in theory yes that is exactly right

in reality your power cables add small resistances so the power might be slightly less.
then again your mains wont be exactly 120v, someone in charge should have a max range your voltage will be within , maybe 10% either way, design for this.

resistance will increase as heat increases, you should check the resistance per length at room temp and once heated to make sure you don't pull too much current at startup.

a thermal cutout is the usual kind of safety thing used in this type of thing, maybe consider it.

the breaker is there to protect the wiring in your house, nothing else. cable of a certain size will take a certain current but it depends how you install it. if you want to do your due diligence installing a heavy fixed appliance you should check your wiring in situ will take the load.
i.e. wire run on top of insulation should be derated. check your local electric code for more details and usually ready made derating tables.

the real test of if it works or not is just to get on with it. you have the materials i presume?

make sure you are within reach of an isolator that disconnects all live conductors, that means line and neutral. also have a fire extinguisher and an escape route ready when fucking about with mains especially high power/heaters.

keep your fucking hands and anything metal well clear when its live. ideally you should be able to lock off when working but a plug/socket should satisfy isolation requirements.

it sounds a bit tedious writing it all out but most of it is common sense.

also breakers/fuses have an operating curve, a 15A breaker wont trip immediately at 15A, it might trip after a while at 16A but after less time as current increases. check your curve if your interested.
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>>936617
>I know that voltage = Current x Resistance
V=IR

>So, since I am using household voltage (120 volt nominal) and my circuit breaker says that it is rated for 15 amps per circuit with a maximal continuous load of 12 amps, does this mean that to reach the maximum allowable wattage I need a minimum of 10 ohms of resistance to run continuously?
Correct

>Then, if I just need 10 ohms of resistance, and I have a material that is rated at 10 ohms of resistance per foot, do I just need a foot of material to complete the circuit?
Depends. Resistance increases as heat increases, which produces a "ballasting" action. This is why lightbulbs get hot and glow, until they stop getting hot. If your Element of Unknown Origin isn't designed to get that hot, or doesn't have enough surface area to sink the heat, it'll just melt.

You couldn't, for example, hook a micron-length of it up to a power station and use it to heat a city. While the power equations say you could, the heat wouldn't get out of the element at anywhere near the rate it was being generated inside it, and the element would turn to plasma.

>I am building a heating element circuit as reference and don't want to damage anything or burn out my household wiring.
If you overload the spur, it will trip. So long as all the stuff you built is outside, the only risk is that you'll set outside on fire.

1.44kw sounds like nonsense to me: my kettle is twice as powerful. But apparently it is possible to make a 1.5kw device that can get to 1200 degrees (just), so there you go.

http://chinajiuchen.en.alibaba.com/product/60089209584-212204840/220V_1_5Kw_2kg_Small_Zinc_Copper_Melting_Furnace.html
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Your math is good OP.
I'd only use 80% of the maximum rating though.

>material that is rated at 10 ohms of resistance per foot
Sounds to small to dissipate that much heat without melting
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>>936643

My basic test that I am going to run is an electric outlet with the wires attached to an industrial switch and then then thick wire and bolts which are connected to thin wire which is the heating element.

It will just be to test the theory out first, then I will build a proper kiln, get a variac and the proper intermediary fuses and what not.

>>936645
The fact that the resistance increases is a good thing in my case because that means it will draw less amperage which makes it safer for the house.

The NiChrome wire will be able withstand up to 1400 centigrade and I am aiming for 1200 centigrade to effectively melt up to copper, it will be mostly melting aluminum scrap I have.

I am building exactly the thing you posted from alibaba, except with things I already have and on the cheap.

Yea, it isn't the amount of power I draw, because the insulation and such that I will be using in the end will hold the heat so even if the build up is slow (my estimate it will take a little more than an hour to heat up and melt a full 10 pounds of aluminum)

Thank you! You have been most helpful.

>>936648
Yea I keep seeing very different charts that all follow that same guide line, I kept seeing that 24 guage would be good, because it needs about 10 amps to get up to 1200 centigrade, but your chart says it needs 7. So I am going with the worse estimate and getting 24 gauge because it is more resistive and therefore more safe in my understanding. In the worse case, i will get larger diameter wire and trial and error it until it works.

I want to put a video together when I am done to help people safely do this as well. I just think that for most people charcoal foundries are not easy to run and that this electrical one will be much simpler when it is set up and built.

Also the design I have in mind will be able to both open from the top and from the side to potentially be a forge for some light smithing work possibly.

Wish me luck =]
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>>936678
The temperature is dictated by the amount of insulation between the furnace and the outside air.

The rate at which heat is conducted by an insulator is proportional to the temperature difference. The temperature will rise until it reaches equilibrium, i.e. when the rate at which heat is conducted away is equal to the rate at which it is generated. The more insulation, the hotter the element will get.

Also, if you're planning on the current being close to what the breaker can handle when the wire is hot, it's going to draw significantly more than that when the wire is cold, so you'll need some form of current-limiting mechanism, e.g. a ballast inductor.
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>>936678
> less amperage
JUST FUCKING SAY CURRENT
You wouldn't say less ohmage or less wattage unless you are a pleb
Inb4 voltage.
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>>936678
>get a variac
If you want to vary the power output, you'd be better-off using different amounts of element. Run several elements in parallel, and then to reduce the power output, use a switch to disconnect some.

This eliminates power loss in your variac, something I think you can probably do without, seeing as you're using pretty-much all the power the socket can deliver.

Actually, I can think of no reason why you'd ever want anything other than "full power" or "no power". A variable thermostat is what you need.

Consider adding an Arc-Fault Circuit Interrupter for safety.
Thread replies: 12
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Thread DB ID: 477825



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