I have an electric pottery kiln rated 240v, 11,520va, which comes out to be 48 amps. This kiln can run easily 7 or 8 hours at a time. I realize my branch circuit conductors and OCPD have to be sized 125% of the load. My question is, does a device only have to be rated for the rating of the load, allowing me to have a 50amp cord end plug into a 50amp rated receptacle? If I assume this is an appliance, then I believe 422.33(C) would apply and allow me to do this. Or would I have to "hard wire" this? Any thoughts would be appreciated.
CAN KILN BE CORD & PLUG CONNECTED?
- Thread starter SPierce
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K8MHZ
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I used to repair kilns and nearly all of them were cord and plug connected.
Your best bet would be to contact the manufacturer with the model of the kiln and find out how it came from the factory. They may even have replacement cords with plugs available.
All kilns eventually need to be re-bricked and have elements changed. That would be a real PITA if it were hardwired.
K8MHZ
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Speaking of warm, there may be an ambient temp correction factor that may need to be dealt with.
K8MHZ
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Janus 27, 11,520 watts, uses a NEMA 6-50P plug.
http://www.paragonweb.com/Ceramic_Kilns.cfm
If no other info about the OP's kiln could be found, I would go with that.
http://www.paragonweb.com/Ceramic_Kilns.cfm
If no other info about the OP's kiln could be found, I would go with that.
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K8MHZ
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Also, I don't suggest anything smaller than a #6 cable.
I'm not so sure a kiln is a continuous load. The heating elements will cycle on and off once it has reached operating temperature. I'm assuming that doesn't take 3 hours... Every kiln I've seen had a cord and plug. If it has a 6-50 plug, then a 50A circuit breaker would be expected on a 6-50 receptacle circuit.
K8MHZ
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If the temp controls are duty cycle and HI is 100% and the max temp is done by a 'kiln sitter' that disconnects the power when the desired temp is reached (common, uses cones with calibrated melting points) then the kiln may draw full power for up to 8 hours. That isn't how it's supposed to be done, but it's possible, so that is what I would size the unit for.
Under normal operation, the unit is ramped up over time then set to HI and left (sometimes more than 3 hours) at that temp until the kiln sitter shuts the unit off.
But we need to size for worst possible scenario and as such I would consider the kiln a continuous load.
The manufacturers generally consider it a continuous load. It may not be on for 3 hours without ever shutting off, but they cycle off and on constantly, even if only a few seconds in between cycles They have custom computer timers to ramp up temp in a certain way based on the clay type and whether it is a bisque or glaze fire.
I strongly suggest hardwiring a Kiln, avoid the 6-60R. That is the single biggest point of failure. Even if they never unplug it, the receptacle's metal tabs get brittle from the temperature changes being in proximity of the kiln and the load its under.
K8MHZ
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Depending on the size and temp, this is a typical procedure:
Dry out. Low temp for 12 hours.
1st ramp. Low/Med temp for 3-6 hours
2nd ramp. Med for 3-4 hours
Final fire. High until shut off, 3-6 hours.
Some ceramics need a temp of 2350 F. That's a bunch.
Dry out. Low temp for 12 hours.
1st ramp. Low/Med temp for 3-6 hours
2nd ramp. Med for 3-4 hours
Final fire. High until shut off, 3-6 hours.
Some ceramics need a temp of 2350 F. That's a bunch.
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My brother and his wife used to have a ceramics business, the elements in their kiln ran continuous for many hours until a disposable melt out element failed and shut them down. It was nothing like an oven.
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nhfire77, you mentioned the 6-60R, which brings up another issue. If I am to size my circuit conductors and OCPD at 125%, then I cannot use a 50 amp plug and receptacle since 210.21(B)(1) says my receptacle has to have a rating not less than the branch circuit. So I would have to either hardwire, with disconnect, or use a 60 amp receptacle, which I don't know if they make a 250v, 3-wire receptacle. Thoughts?
PetrosA
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Commercial grade kilns that I've installed were hard wired with a disconnect and the manufacturers specified copper only wiring to them, even ahead of the disconnect. Check the instructions carefully before wiring because they will use any variation from them to void the warranty.
Tony S
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I?ve installed kilns and furnaces from 30KW to 2.6MW. I go by what the manufacturer say is the correct cable size.
I may not agree with it but the balls in their court if/when it goes wrong.
I may not agree with it but the balls in their court if/when it goes wrong.
I believe you are right, hardwire it.
I love learning new technical jargon.
A bunch- also known as Cone 10. In electric kilns for porcelain glaze oxidation.
K8MHZ
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That's the Kiln Sitter. The 'melt out' is called a cone.
K8MHZ
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I fixed kilns for a few years. We re-bricked them, installed new elements (that we made) and fixed connections. I don't recall ever having to repair a NEMA plug.
I worked on Ameco, Crusader and Paragon kilns plus we built our own. Our biggest customer was the school system.
When it comes time to re-brick, the kilns have to be moved. Also, in schools, homes and pottery shops, they are moved just to move them. That's why they are cord and plug connected.
What brand kilns were you working on where there were problems with the plug and receptacle? (Just curious)
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K8MHZ
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