Wiring a kiln


Old 08-20-07, 08:43 PM
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Join Date: Jul 2006
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Exclamation Wiring a kiln

We have a kiln that is rated at 48a/208v. Last week I wired it up using #6 stranded THHN wire in a 3/4"ID plastic conduit. (two #6 conductors plus a #10 neutral) The run is about 30', all in the open. The circuit is wired through a 2-pole, 240v, 50a, Crouse-Hinds (E13207) circuit breaker. (The CB was pre-existing and had been previously used for a welder.)

Our service panel voltage is 240-245.

I had previously checked the amps, at the kiln HIGH setting with a Fluke ammeter and observed 53 amps. I could also detect a slight temperature rise in the conduit run ... nothing serious, maybe ten deg ... barely detectable.

Today, while running the kiln on HIGH power, after about 40 minutes, I heard a funny "scratchy/frying" sound from the breaker panel, followed in about a minute with the breaker tripping. The breaker seems to be burned-out (looks just fine) but will not reset ... even a hour later.

Here's the question: I'm thinking now that the circuit is undersized and probably should be #4 wire and a 60a CB. I'm also thinking that the conduit size should be larger, too.

Any help and suggestions would be greatly appreciated.
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Old 08-20-07, 08:59 PM
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If it pulls 48 amps at 208 volts, it will pull about 55 amps at 240 volts or almost 57 amps at 245 volts. Furthermore, 48 amps at 208 volts is only 10KW. But 56.5 amps at 245 volts is almost 14KW--a lot more.

Does it indicate that it is okay to run it at 240 volts? If not, I wouldn't. The kiln itself may not safely tolerate this many amps and this much heat.

A 50-amp breaker will tolerate 53 amps quite a while before tripping. By the time it trips, it'll be pretty hot and take a while to cool down before allowing a reset (especially in hot weather).
Old 08-20-07, 10:38 PM
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Kilns are kind of special duck in that they are resistive loads and are generally not dual rated for voltages. This meaning that you cannot operate a 208 volt single phase kiln on 240 volt single phase. It draws to many amps as John has laid out. You must run it at 208 volts. This is not a common voltage for a single family residence. Most residences are 120/240 volts so your in a bit of a jam using that kiln. Your going to burn up the elements at 240 volts.

Assuming you get one that is rated 240 volts you need to size the conductors at 125% of the rated amperage of the kiln. Kilns are considered continuous loads.

So if you get one that is 48 amps at 240 volts you need conductors that will be rated for 60 amps (1.25 x 48). #6 thhn like you have is fine. But you will need a 60 amp breaker.

Your conduit size is fine. It isnt necessary to run #4 but it also won't hurt anything if you do.... stay with the 60 amp breaker though. I'd check your heating elements real close to make sure none are burned through.

How did you come about getting this 208 volt kiln? Did a vendor sell it to you?

Old 08-21-07, 10:06 AM
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Thanks for the info

John & Roger: Thanks for the helpful info. I'm glad I don't need to buy larger wire!

Here is some follow-up info:

We bought the kiln on eBay. The seller was Washington State Surplus Sales and was sold in a "as is ... no guarantee" basis. It is well-used and as far as I know, the elements may have been replaced several times. The prior use was probably in a school art studio. I'd guess it is about ten years old, but in good shape.

I removed the CB from the panel and this morning, it still won't reset. Trash! I'm off on a quest to find a 60a replacement.

The kiln has three segments, each with it's own LOW/MED/HIGH switch. I probably will not need all three on HIGH, except possibly when my wife want to fire her projects at "cone 10" ... around 2000 deg.F

I'll do a little experimenting with the heat settings and with the 208/240 difference, it may not be needed to use the HIGH setting.

The heating elements are easily replaced, so if one burns out, I can replace it with a more suitable voltage rating. Since the kiln is fairly old, there are no digital, or other complicated controls, the voltage difference isn't as important ... Just turn it on, select the heat setting and wait for a small ceramic cone, placed in the kiln, to melt and trigger the off switch.

I have also added an Omega digital thermometer and datalogger and a probe suitable for up to 2450 deg.F. I'll be able to monitor performance.
Old 08-21-07, 10:26 AM
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I would recommend that you not use the HIGH settings at all; it will probably wreck the heating elements to operate at a wattage that is so much greater than rating.
Old 08-21-07, 02:29 PM
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If used in a commercial setting such as a school as you suggest then the place may have had 3 phase which might explain the voltage. If the elements are easily replaced it might be best to replace them now with elements intended for 240V.

Just a quick clarification. In your post you mentioned a neutral. A device such as your kiln that does not use 120v does not have a neutral. It has a ground. If you are feeding from a subpanel this must be on the ground bar. Color should only be green if bare wire isn't used.

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