How to size the circuit breaker, fuses, cord and/or power supply of your kiln.

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How to size the circuit breaker, fuses, cord and/or power supply of your kiln.

This consists of the main power cord, receptacle, powerblock, wire, and breakers. 

Safety Warnings

NOTE: If you are not sure about some part or procedure in creating or testing a power supply line for a kiln, DO NOT GUESS! If you wire something improperly, you might just blow the circuit breaker, or burn the place down. Get an electrician or someone who knows.
NOTE: There are many different ratings on the wire and components that make up the supply line that need to be considered (along with proper and safe installation) when creating or testing a kiln power supply. It is beyond the scope of this manual to properly educate someone to fully understand the potential variations and variables involved in this. Often a building like a school or a recreation facility (or even a home) will be inspected for electrical code and fire code enforcement. If you do not own the building, or if the kiln is in anything other than a free standing private workshop for yourself, get permission to do what you want to do from the owner and get an electrician to properly wire the circuit so you are not liable. 

The Data Nameplate

A typical data nameplate:
Every kiln has a data nameplate, usually a sticker on the side of the control box, that specifies model number, serial number, voltage required, phase required, amperage required, watts produced and the recommended maximum temperature. This is the information to get from L&L for your particular kiln if you do have not received it yet or if it has no data nameplate. The information from the data nameplate is what you show to your electrician or use to procure the proper wire and components to create the power supply line. Here are some things to keep in mind if you are the “electrician”:

If you are the electrician:

As of January 2001, National Electric Code Handbook says that a resistive heater this size, on for more than three hours at a time, should be provided a circuit that is rated for 125% of the total amperage drawn by the unit when it is on high. So a kiln drawing 43.98 amps would require a service of no less than 54.98 amps. As wire and components for power supply lines are usually rated in increments of 10, the next higher rating is 60 amps. Therefore everything in the line must be rated at least for 60 amps. Note about the 50 amp power cord used on kilns with up to 48 amps: the cord is built to take at least 60 amps, (6AWG wire for the hot leads - good per NEC table 310- 16, 8 AWg for the ground, plus oversize copper blades on the plug, and high temp. rating.) In addition this configuration has been examined by UL and approved for use not only with L&L kilns but most other UL listed kilns on the market. Do size the rest of the circuit for 60 amps (i.e. the wires, circuit breaker, etc.).

Most L&L kilns require a specific voltage to operate properly. The Data Nameplate will say in the voltage column either 120, 208, 220, 220/110, 240, 380/220, or 480. Most of the USA is residential- 240 vac, with schools and industry- sometimes having 208 vac. Often, however, residential can be 208 (downtown NYC, south-central Idaho etc.). There are usually no issues with 120 volts. 220 and 380/220 are usually found overseas. 480 is sometimes available for schools or industry. The biggest problem we see is the issue of 208 vs. 240: The kiln that says 208 volts will overheat the elements and burn them out quickly if it is run on 240 volts. Older 208 volt kilns have 208 volt switches also. They will burn out quickly if run on 240 volts. If the kiln is made for 240 volts then it will have about 25% less power and a maximum temperature of about cone 5 (maybe) if run on 208 volts. You should only run a kiln on the voltage that the Data Nameplate specifies if you are to expect the best results. Plus or minus six or seven volts is OK, but keep in mind that the higher the actual voltage is over the recommended voltage, the higher the surface temperature of the element is at any given time, and therefore it is more likely to burn out early. Likewise, the lower the actual voltage is below the specified voltage, the lower the maximum temperature will be. Voltage is specific to the building and to time of day. It will probably be a bit lower during the day, and higher at night. You can carefully test it to see what it is with your multi-meter. It is costly to change the voltage, but this can be done with a transformer. You'll definitely need a good electrician, though. The cheapest way to fix the problem is to replace the elements (and possibly the switches) with the proper voltage ones. The switches are marked INF 240, INF 208, INF 120 or something similar. You will probably need to remove one to see the markings. The computerized controls will work on either 208 or 240 unless 120, 380/220 or 480 is specified. (Note that we use 240 volt INF switches now on 240 and 208 volt kilns. 208 volt INF switches have slightly different timing because of the way the voltage affects the internal heater in the switch).

Use copper wire

Always use copper wire with the highest temperature rated insulation you can find. 150°C is what the internal wiring up next to the kiln is rated for, so hook-up wire with that rating would be excellent; however, 105°C is adequate. Note: our power cords, which are approved by UL, are rated for 105°C. Keep in mind that for different amperage services, different thickness of copper hook-up wire is required. The following chart should give an idea of what is necessary:
Voltage is not really an issue here. Usually wire like this is rated for at least 300vac if not 600vac. Look for the wire specifications on the insulation or ask the supplier to be sure. The voltage rating is based on the electrical resistance of the wire insulation (to prevent voltage leak).

Voltage Drop over distance

Running power for your kiln over a long distance will result in a drop in voltage. The amount is about:
7 volts per 100 feet with 10 awg wire
21 volts per 300 feet with 10 awg wire
6 volts per 100 feet with 6 awg wire
18 volts per 300 feet with 6 awg wire
3 volts per 100 feet with 1 awg wire
9 volts per 300 feet with 1 awg wire

These estimates are dependent on the kiln operating at 50% to 100% of its capacity, with the temperature of the wire no more than 167°F. Be sure to test the voltage before the run has been installed so you know what you are working with.

See this link for voltage drop calculators and more information.

Power Cords

The industry name for the supplied power cord on many L&L kilns is: Nema 6-50P for all single phase 208, 220, and 240 models drawing less than 50 amps. A few exceptions are the later G models (GD, GT, GQ, and GS) with the 30 amp, four-prong plug. This is a Nema 14-30P plug. The Doll kilns and other 120 vac kilns use Nema 5-15P plugs for up to 15 amps, and Nema 5-20P plugs for 15 to 20 amps. The three phase Easy-fire kilns use a 15-50P cord. Various types of 30 amp plugs are used on the Liberty-Belle kilns. To get the appropriate receptacle just substitute the “P” at the end of the Nema code for an “R”. P for plug, R for receptacle.