Help needed on electrical question

My home kitchen range plugs into a 3-prong 240v outlet (measured with a digital voltmeter). I am interested in purchasing a 240v 47amp electric kiln for "baking" ceramics (clay pots, etc.). Can I simply use my 240v range outlet for the kiln? Luckily the wall my kitchen range plugs into (the 240v outlet) has on the other side of it an enclosed outside porch, so I am thinking I would drill a hole through the wall, run the cord from the kiln through the wall, insulate the hole with polyurethane foam etc, so my kiln could use the indoor 240v outlet when needed (perhaps only once a month). My home wiring is 100amp, so am I correct in thinking that the 240v electric range outlet should function ok for the 240v 47amp electric kiln?
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What is the current rating of the breaker or fuse supplying that outlet, and what size wire from the breaker/fuse to the outlet?
MB
On 02/02/04 10:19 pm Beowulf put fingers to keyboard and launched the following message into cyberspace:

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Beowulf wrote:

Your kitchen range is probably on a 40A circuit, and you need 50A. And it should have the wrong plug. I think you'd be better off adding a new 50A circuit to your porch or garage. I've seen inexpensive NEMA 6-50R (welder) and NEMA 10-50R (range) outlets that fit in a recessed or weatherproof box, and they are available surface mount.
Best regards, Bob
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Generally, I'd agree. Welder outlets are certainly inexpensive.
However, an inspector may permit some finessing of the rules[+] here (w.r.t. utilizing stove outlet) assuming that (a) it won't be used much and (b) the circuit is already okay for 50A (or more) and (c) the inspector is comfortable with the idea in your specific circumstances.
Especially if the circuit isn't being used for a stove now.
Inspectors will allow considerable leeway sometimes.
_Ask_ before before making final choices. Ie: circuit rating and wire size.
I wouldn't normally consider doing this _unless_ the cost of running a new circuit is prohibitive.
As the stove is _probably_ 40A/8ga rather than 50A/6ga, it's unlikely to be useable. If the wire is 6ga, you may be in luck - ask your questions after finding this out.
[+] Notes:
1) "occasional use" may bypass the 80% ampacity rule (don't load heater circuits beyond 80% of rating, which implies it _should_ have 60A).
2) Our codes (and perhaps the NEC as well) _do_ permit _some_ fudging of receptacles, provided there's no possibility for dangerous confusion. Eg: assuming the ampacity and connection minimums are met, you can even use a 50A 240V receptacle for 120V/15A, as long as there are _no_ other 50A 240V receptacles in the building with something other than 120V/15A on them.[*] Thus, ordinarily, I wouldn't see a problem with a 240V/50A 4 wire plug & receptacle used used for a 240V/50A three wire device - you just leave the neutral unconnected to the device.
If it's the other way around (4 wire device on 3 wire circuit), forget it - no longer legal, and potentially dangerous.
[*] This sort of thing is sometimes recommended in circumstances where specific devices MUST be connected to specific outlets. But not usually 50A/240V 4-wire P&R for 15A/120V 3 wire - that's just plain dumb ;-)
--
Chris Lewis, Una confibula non set est
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The inspector won't really have much say about it if the kiln has a cord and OP uses an existing outlet; no permit would be required. But I doubt the range circuit is large enough. I'm also assuming the kiln does not require a neutral connection.
If the range outlet *is* wired with #6 copper or #4 aluminum but has a 40A breaker, that's when you ask the inspector about increasing the breaker to 50A and running the kiln with an extension cord. (Also ask the inspector about tapping the range circuit for a second 50A outlet -- make a branch circuit out of it.)
If the range outlet is wired with #8 wire or #6 aluminum, then there's no choice but run a new circuit (or buy a smaller kiln.)
Bob
Chris Lewis wrote:

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True enough, but...

Ditto. So it works out the same way.

Didn't the OP say the outlet was four wire?

I suspect an inspector would start getting antsy about putting an _additional_ 50A or 60A circuit on a 100A main...
Did someone say "load calculations"? ;-)
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Chris Lewis wrote:

I actually thought about that, but would anyone do a load analysis for an occasional heavy load like this?
Worst case is it trips the main breaker if someone uses the oven and the cooktop and the kiln all at the same time. (although that would be an argument for turning the range circuit into a 50A branch circuit with 2 outlets, so it only trips the one breaker...)
Best regards, Bob
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An inspector could go either way. If the homeowner _wasn't_ going to upgrade the main service (it's already below our minimum standards without the kiln!) an inspector probably wouldn't want to see a "new" circuit for it, even as an occasional load. The inspector (or permit office) might start thinking "home business", and that could open up a _real_ can of worms.

Which could become a major PITA is the mains are fuses...

I strongly suspect an inspector would be happier seeing a single circuit, either with just _one_ plug (especially if the OP doesn't have an electric range to begin with), or a big switch to switch between one and the other.
"One man shop"[+] rules best apply when it's a single room _shop_. Especially if it's a range in one room and a kiln somewhere else, the inspector is going to start worrying what the chances are that both will be run at the same time. In older systems, it's not a good idea to have systems that can overload circuits if you're not paying attention.
[+] In a "one man shop", all the major power tools can be on the same circuit, because only one of them is going to be turned on at a time.
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This is Turtle.
I think XYZBOB has the correct solution and it only being used once a month. i'm a tightwad but i found out it pays to do it right the first time and you don't have to deal with it again or other problem caused by the short cut.
TURTLE
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Your range outlet is probably 40A... It won't work.
Also, the code requires that high-current devices such as that kiln have thier own circuit. It also states that ciruits for resistive heating applications (such as the kiln) be sized so that they are only loaded to 80% of the circuit's capacity. Therefore you need a 60A circuit.
Something else: That kiln probably has a clearance-to-combustables requirement, be sure to keep that in mind when picking out it's location.

Be carefull with what you use around high-current wiring, some of those foams are actually quite combustable, with flashpoints at surprisingly moderate temperatures.
-- Steve
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My home wiring is 100amp, so am I correct in thinking that the

no
this depends on the breaker for the 240V circuit. In canada, for example, ovens are ona 240V circuit with a 40A breaker
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