More on amperage ratings for kilns

I’ve received some feedback from folks wanting more information about my previous post “The rule of 80% and kilns“.

First, here is a direct link to the NEC (National Electrical Code): http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=70 (scroll to near the bottom of the page and click on the link that says “View the 2008 edition of this document”)

The important sections are 210.21 and 210.23. In part, they say:

Table 210.21(B)(2) Maximum Cord-and-Plug-Connected Load to Receptacle

Circuit Rating …………………… Receptacle Rating ………………. Max Load (Amperage)
15 or 20 Amps ………………………. 15 Amps ……………………………… 12 Amps
20 Amps ………………………………. 20 Amps ……………………………… 16 Amps

210.23 Permissible Loads

(1) Cord-and-Plug-Connected Equipment Not Fastened in Place

The rating of any one cord-and-plug-connected utilization equipment not fastened in place shall not exceed 80 percent of the branch-circuit amperage rating.

As an example of what kiln manufacturers are selling, here is an excerpt from the Arrow Springs on-line catalog, spelling out the amperage requirements and plug connectors on their kilns: http://www.arrowsprings.com/html/kilns_controllers.html#AF66

Voltage, amperage, wattage and plug type of Arrow Springs ovens.
The following ovens operate on 120 volts and draw 14 amps, 1680 watts.
The plug and required wall receptacle configuration is NEMA 5-15, which uses a typical house receptacle (wall outlet) that is rated for 120 volts and capable of delivering up to 15 amps.
AF66, F66, AF99, F99, AF99-T4, AF138, AF138-T4, A1313 and A1813

The following ovens operate on 240 volts and draw 14 amps, 3360 watts.
The plug and required wall receptacle configuration is NEMA 6-20, rated for 240 volts and capable of delivering up to 20 amps.
AF1313, AF1313-C, AF1813 and AF1813-C

The following oven operates on 240 volts and draw 26 amps and 6240 watts.
The plug and required wall receptacle configuration is NEMA 10-30, 240 volts and capable of delivering up to 30 amps.
AF1818-C

The first paragraph tells us that the kilns draw 14 amps, yet are connected via a 5-15 plug — clearly a violation of the National Electrical Code. The proper connector to use is a 5-20.

The second paragraph shows the proper connector for the amperage, a 6-20, as 14 amps is only 70% of 20 amps.

The third paragraph also is incorrect, as the kiln draws 26 amps, and by the code should be on a 50 amp circuit. The correct connector is a NEMA 10-50 (there is no 40 amp connector available for the “10” series connectors.

I’m fairly sure that these violations of the NEC are because of ignorance of the code, however, they do create a clear and present danger to the user, by putting a large drawing device on a circuit that is not designed to carry the load.

And anyone who thinks that a violation of the NEC is nothing to worry about, remember my comments about keeping propane inside your house: the insurance companies will look for a reason to deny coverage in case of an “incident”. If a fire were ever to occur because of an incorrectly wired kiln plug, you can conceivably have your insurance cancelled and the loss not covered.

When shopping for kilns, be sure that the kiln is wired in accordance with the National Electrical Code. The Code exists for a reason: to keep you safe. If the kiln is not wired correctly, demand that it be corrected, or look for another supplier.

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2 thoughts on “More on amperage ratings for kilns

  1. I have a 36 amp kiln that the manufacturer suggested a 50 amp dedicated circuit. I hired an electrician, and installed the necessary plug and circuit.

    I tried to fire a load of glass last week, and the breaker tripped. I reprogrammed the kiln, started again, and it tripped.

    This morning, the electrician came back and checked the system. He used No. 6 wiring, which is the correct wiring. However, he put a meter on the wire that indicated it was drawing 49.2-49.8 amps!

    His solution was to install a 60 amp breaker, since the wiring can handle up to 65 amps. I’m a little concerned that the manufacturer says that it’s a 36 amp kiln, but it’s drawing much more power than that.

    Any thoughts?

  2. I am not surprised. I would certainly contact the manufacturer and let them know. They might have changed the design of the elements which could have caused the larger amperage draw, but then they should have updated their literature and information tags for the kiln.

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