I am switching my greater than 50 amps manual kiln from 208 volts to 240 volts. What do I need to do to the kiln?

Change Elements

  1. All the elements will need to be replaced.
  2. For example the ohms on the J2927 208 volt element circuits should be around 10.4. If you put 240 volts to each circuit it will pull around 23 amps per circuit- which is too much.
  3. Should not be more than 20 amps per circuit for the J2927 208v kiln.

You May Need To Change Relays

  1. Greater than 50 amp manual kilns were made with voltage specific relays and switches too. It is hard to say what will ultimately happen if you put 240 volts to a 208 volt switch or relay.  The switch might cycle on and off faster and probably burn out more quickly, and the magnetic field created by the relay coil would be stronger than it needed to be and probably burn out the relay sooner. The best thing is to swap these out for the right voltage if the relay coils are not multi-voltage.
  2. If you are switching from 240 volts to 208 volts you will not get to temp if you are trying to get to cone 5. Using the J2927 example- on 208 volts the 240 volt elements will make the kiln draw about 16-1/2 amps per section which is about 20% lower than it should be. Electrically this is ok, but max temp will be about Cone 1.
  3. For the relays and switches you will want to replace the relays. The switches will be ok running on 208 volts. The relays however will produce a smaller magnetic field and may not be able to completely close the relay contacts. When this happens the relay can chatter which quickly overheats the contacts and often fuses them shut allowing the kiln section to remain on High all the time, even when the kiln-sitter is off.

Check Electrical Specifications

  1. Changing voltage sometimes means pulling more amperage. Be sure to check the electrical specs for your kiln as it will be after the change- i.e. if you are changing to 240 volts look for the electrical specs your kiln as a 240 volt kiln. Be sure your breaker and wire size is large enough. Breaker size is determined by the new amp rating of the kiln, multiplied by 1.25 (125%) and then rounded up to the nearest breaker size. Wire size is tied to the breaker size:
20 amps
12 gauge copper
30 amps10 gauge copper
40 amps8 gauge copper
50 amps6 gauge copper
60 amps6 gauge copper
70 amps4 gauge copper
80 amps3 gauge copper
90 amps2 gauge copper
100 amps1 gauge copper
125 amps1/0 gauge copper (pronounced "one aught")
150 amps2/0 gauge copper (pronounced "two aught")
175 amps4/0 gauge copper (pronounced "three aught")
200 amps250 MCM copper wire (MCM = Thousand Circular Mils)
225 amps   
350 MCM copper wire