Estrata Posted March 30, 2023 Report Share Posted March 30, 2023 (edited) Hello! I have a 50 year old kiln made by Knight in excellent condition - it was barely used and just stored in a basement for years. It has a NEMA 10-50 plug on it (the straight blade one). I am having an electrician come in to install the outlet (I have a double 50amp circuit from an old hot tub he will use, probably). My question is, do I have him install a an outlet for the 10-50? I've read they are not grounded and were meant to be phased out. Or should I replace the plug, and if so, what should use? Thank you! Edited March 30, 2023 by Estrata Add photos Quote Link to comment Share on other sites More sharing options...
Hulk Posted March 31, 2023 Report Share Posted March 31, 2023 Hi Estrata, Welcome to the forum! Likely the kiln tech experts will want to know specifics; does the kiln have a plate attached indicating the amps, phase, voltage, model number? Quote Link to comment Share on other sites More sharing options...
Bill Kielb Posted March 31, 2023 Report Share Posted March 31, 2023 @Hulk is spot on, post the design electrical requirements for best answer. Absent that, since you have an electrician coming, sizing for kilns comes under the continuous load definition for resistive loads meaning breakers sized for 125% of the load, not to exceed 150%. The potential problems would be the kiln was designed for 230v nominal system voltage so at 24p v it now will draw more amperage. Best way to know this is he should measure the resistance of the over all kiln load and adjust his breaker and wire size accordingly (still a continuous load under NEC) at the new system voltage of 240 volts. Second issue would be to confirm it is 240v 3 wire, meaning L1, L2 and a ground. If so, then the GFCI 50 amp breaker shown likely needs to be replaced because it will not have a load carrying neutral and the wattage at the higher system voltage will likely require a breaker rated higher anyway based on continuous load of its actual wattage. Best to post the design rating of this old kiln though as it is hard to speculate what the nameplate says. Quote Link to comment Share on other sites More sharing options...
PeterH Posted March 31, 2023 Report Share Posted March 31, 2023 Examples of Knight kiln-plates Quote Link to comment Share on other sites More sharing options...
Bill Kielb Posted March 31, 2023 Report Share Posted March 31, 2023 (edited) Looking at the name plates and following NEC for continuous load: The 115 volt kiln should be a design resistance of 4.25 ohms so correcting for the present 120v supply 120/4.25 = (Potentially) 28.2 amps. Min Breaker size: 28.2 X 1.25 = 35.29 amps Max breaker size: 28.2X 1.50 = 42.3 amps The single phase kiln = 40 amp single pole breaker, associated rated: wiring, cord and receptacle. Wires= (1) line, (1) Neutral and one ground wire. The 230 volt kiln should be a design resistance of 11.5 ohms so correcting for the present 240v supply: 240/11.5 = (Potentially) 20.87 amps Min Breaker size : 20.87 X 1.25 = 26.09 amps Max breaker size: : 20.87 X 1.50 = 31.31 amps The 230 v kiln = 30 amp two pole breaker, associated rated: wiring, cord and receptacle. Wires= Line 1, Line 2 and one (1) ground wire, No Neutral. Have your electrician double check all of this and confirm. Common would be a three prong cord end and matching receptacle with the appropriate amperage rating. Edited March 31, 2023 by Bill Kielb Quote Link to comment Share on other sites More sharing options...
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