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#1 neilestrick

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Posted 12 August 2014 - 09:31 PM

There's been some talk on the forum lately about new kilns and appropriate breaker/fuse sizes, so I thought I'd share an odd experience I had today with a customer's kiln.

 

I was doing a checkup on an older Paragon (pre-1994) with the oldest digital controller I'd ever seen. It had an old phone pad for the buttons- only the second one of those I'd ever seen. There was no cone fire mode, only custom program, and that would only allow one step- X rate of climb to Y degrees and off. So no slow heating with a faster climb in the middle or slowing at the end or anything like that. So that was pretty neat to see.

 

But the main reason I'm writing is to discuss the voltage situation at the customer's house. I checked the service voltage to her kiln and it read 259 volts! It should be at 240, give or take 3 or 4 degrees. Typically when I see high voltage it's 245-248. The most I'd ever seen was 251. So this was crazy high.

 

This was a big deal because the electrician who wired up the outlet for the kiln put in a 50 amp breaker. If this had been a normal 23x27 kiln that pulled 48 amps at 240 volts, it would have pulled close to 52 amps at 259 volts- too much for the 50 amp breaker. The customer's kiln was rated for 45 amps at 240 volts, so at the higher amperage it would be just barely under 50 amps, but still very likely to flip the breaker.

 

Electrical code says that resistance heating appliances like kilns should be on a breaker that s 25% greater than the actual draw of the kiln, so a 45 amp kiln should be on a 56.25 amp breaker, which doesn't exist, so you go up to the next size which is 60 amps. 48 amp kilns also need 60 amp breakers, although I have several customers who run them on 50 amps without problem, but clearly their voltage isn't too high.

 

You really can't control how high the voltage is, short of calling your electrical company and demanding that they do something about it, which they most likely will not do. However, I did suggest to my customer today that she call them because the voltage is so very high, and could possibly be a sign of a problem in the system somewhere. And she needs to call her electrician and have a larger breaker put in, which could also mean larger wire if they did not use wiring large enough for 60 amp service.


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#2 Biglou13

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Posted 13 August 2014 - 08:57 AM

The studio I work hat has high voltage also, not 250V. Is there another fix other than from electric company. And is there problem wit adding next higher breaker. What's the harm on kiln for increased voltage? I'm not doubting your meter but is there any way to do QC on them.
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#3 neilestrick

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Posted 13 August 2014 - 09:26 AM

I had one kiln that ran at 26 volts and was tripping the 30 amp breaker because they had higher than normal voltage, so they went to a 40 amp breaker to solve the problem. Code says that for overload protection you should not go more than 50% more than the actual draw, so 26 amps + 50% is 39 amps, so that was okay. The only worry about the kiln itself is that the elements will run hotter than usual if the voltage is high, so element life could be decreased. But if it's only a few volts high then it's not a big deal. There are too many other variables to be able to pin down short element life on voltage issues. On a manual kiln it will fire faster with the higher voltage, but on a digital kiln it won't be any different since the kiln cycles on and off to control the rate of climb.

 

The customer I worked with yesterday was the first house on the supply transformer, in a fairly densely populated area in Chicago- lots of houses/row homes packed close together. So it could be the electric company had it ramped up to make sure people at the end of the line were adequately supplied. It was also the middle of the day, when usage was low. In the evening I would expect her voltage to drop considerably. It's not that there was anything dangerously wrong, just that we needed to make sure that kiln was on the appropriate breaker. There was no wiggle room due to the higher than normal voltage.


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#4 Bob Coyle

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Posted 13 August 2014 - 03:12 PM

Neil, I noticed that when the kiln elements on my kiln are on high range that the voltage drops almost 8 volts from no load. So, should the lower voltage be the one you use to calculate amps and watts?



#5 neilestrick

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Posted 13 August 2014 - 04:27 PM

Neil, I noticed that when the kiln elements on my kiln are on high range that the voltage drops almost 8 volts from no load. So, should the lower voltage be the one you use to calculate amps and watts?

 

Do you mean for rolling your own elements, or just calculating firing costs?


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#6 Marcia Selsor

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Posted 13 August 2014 - 07:32 PM

Neil, 

Thanks for the insight from your professional expertise. It is really valuable information to know when thinking about installing kilns.

I hope lots of people read this.

Thanks again.

Marcia



#7 schmism

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Posted 13 August 2014 - 09:52 PM

While I will always advocate using a properly sized breaker, keep this in mind.

 

With an electric controller, specifying a ramp rate of something like 150°  or 200°/hr  is hardly useing 100% of the output of the kiln.   If you never call for a "max" climb rate, its likely you will never ask for 100% output from the coils meaning you dont draw full amps meaning you would not trip the 50amp breaker (even with the 7% over voltage condition listed).

 

In short, if you never ask for max climb rate, you never ask for 100% usage then you never ask for max amp draw.



#8 Mark C.

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Posted 13 August 2014 - 11:31 PM

I will say I do not have a newer computer controller on any of my electrics. I do have a ramp up fire right electric controller but I thought they both are similar in the fact that near the end of fire the kiln is on all the way to achive max temps?

Or another way to say it is near the end of say a cone 6 fire wouldn't the kiln be on full most if not all the time to reach max temp?

Yes the climb rate is not maxed but sooner or later does not the kiln need to be full on (meaning max amps) near the end?

Mark


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#9 Bob Coyle

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Posted 14 August 2014 - 09:14 AM

 

Do you mean for rolling your own elements, or just calculating firing costs?

 Mostly rolling my own.  I want to max the allowable amperage so I get the fastest ramp possible at high end. Right now I can just barely pull 100F / hr.

 

Also, doesn't the resistance increase quite a bit at high temperatures?  I never saw this calculated in.



#10 Dick White

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Posted 14 August 2014 - 09:29 AM

I'm not a licensed electrician nor do I play one on TV, but I've installed and repaired kilns and fired them, and this is one area that is not well understood by most potters. We think of kilns as just another electric tool, like our wheels. We turn them on, we use them, and they do what they are supposed to do. Until they don't.

 

I think of electricity and wires like water and hoses. Voltage is the water pressure and amperage is the volume of water flowing through the hose. You can create water pressure by forcing it through a small nozzle and squirt it quite a distance. That's voltage. If you want just a modest amount of water for your flower pots, you can use a garden hose, but if you want to put out a fire, you need a lot of water and a big fire hose. That's amperage. If you connect a garden hose to a fire hydrant and turn it on full blast, the hose will break because it can't carry the volume or pressure. That's an electrical fire inside your wall.

 

Electricity flowing through a wire will (except for superconductors) generates some waste heat. For a high amperage device, like a kiln, you need a big wire, not only to pass the large volume of electricity, but also to dissipate the heat. If the wire is too small, you not only lose some voltage at the business end of the circuit, but you raise the amount of waste heat being generated along the wire. Too much, and you have a fire. The notion of the necessary size of the wire includes all connections along the way. If any connection is loose or frayed, that miniscule section of the total wire in the circuit is too small and that's where the fire starts. But back to the issue of waste heat, the electric code requires wires of a certain size for a particular amperage, but that calculation is based on intermittent use in a normal operating environment. You put a piece of bread in the toaster and the wires provide a lot of amperage to the toaster. The wires heat up a little bit, but the toast is done in a few minutes and then the wires cool off. For a device that has a lengthy constant draw, such as a kiln, a kitchen oven, or even the fluorescent lights in the ceiling of an office or store, there is no off time for the wires to cool. So the electric code requires those wires to be capable of carrying 125% of the rated amperage of the device(s) on the circuit. That's why, as Neil pointed out in the beginning of this thread that a 45/48 amp kiln needs a 60 amp circuit.

 

Now on to voltage. That's the 'pressure' in the wire. Some wires, such as the elements in the kiln, are intended to generate heat. But if you put more voltage through them than they are designed for, they will get hotter, and vice versa, less voltage than designed for, not as hot. For a manual kiln on full high, 208V elements in a kiln on a 240V circuit will get hotter and fire faster, or 240V elements in a kiln on a 208V circuit will not get hot enough and fire slower. The issue with kiln elements is that they wear out over time. A tiny bit of the metal burns off each time you use it, until there isn't enough left to generate the needed heat. A 208V element on a 240V circuit will fire faster, but wear out sooner, and a 240V element on a 208V circuit may never get hot enough to finish a cone 10 firing. Or a properly sized element may be so old and worn it takes forever or can't finish the firing.

 

So where does a controller and the ramp rate fit in? The controller will turn the elements on only long enough to cause the temperature to increase (or decrease) at the programmed rate and then it clicks off until it needs a bit more heat. The issue with heat and electricity, however, is that the amount of electricity required for a particular amount of heat is not a linear relationship. The amount of electricity required to raise the kiln one degree from 100F to 101F is quite small compared to the amount of electricity required to raise the kiln one degree from 2000F to 2001F. So as you get to the end of the firing, the amount of electricity needed to keep it going is massive. The elements are on full power full time. And if your elements are worn or you have 240V elements on a 208V circuit, no matter how much power you have, they are not going to get hot enough to raise the temperature at the programmed ramp rate. And now you have the infamous ERR1.

 

Hope this helps people understand why their kiln circuits are what they are.



#11 neilestrick

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Posted 14 August 2014 - 10:06 AM

While I will always advocate using a properly sized breaker, keep this in mind.

 

With an electric controller, specifying a ramp rate of something like 150°  or 200°/hr  is hardly useing 100% of the output of the kiln.   If you never call for a "max" climb rate, its likely you will never ask for 100% output from the coils meaning you dont draw full amps meaning you would not trip the 50amp breaker (even with the 7% over voltage condition listed).

 

In short, if you never ask for max climb rate, you never ask for 100% usage then you never ask for max amp draw.

 

You're confusing amps with kilowatts. Kilowatt hours are what you pay for, and during most of the firing you're not using 100% of what's listed on the kiln, which is the draw when the elements are all on high. On a digital kiln you can roughly calculate your kwh usage by breaking down your firing time into quarters and assuming 25%, 50%, 75% and 100% kwh draw during each quarter. As Mark said, during the last quarter of a firing, the kiln cycles off very little.

 

Amperage draw is 100% whether the kiln is on for a split second or 20 hours. Amperage is the flow of electricity, similar to the rate of flow of gallons of water per minute through a pipe. It doesn't matter how long you have the pipe open, the flow is the same. In fact, with kilns the amperage draw is slightly higher when the elements are cold, because the resistance changes as the elements heat up (about 4%). Within a few minutes of being on, the elements warm enough that the amperage drops to its functional level. So you actually get the highest amperage draw the first time the elements click on at the start of the firing when the elements are cold.


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#12 neilestrick

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Posted 14 August 2014 - 10:22 AM

Neil, I noticed that when the kiln elements on my kiln are on high range that the voltage drops almost 8 volts from no load. So, should the lower voltage be the one you use to calculate amps and watts?

 

I just reread this. The voltage should not drop due to anything going on with the kiln. As described in my previous post, there will be a slight change in the element resistance and therefore amperage draw in the first minutes of a firing, but the voltage will not change because of that. If you're getting a voltage drop, it's likely due to the usage in your neighborhood. It's not uncommon to see drops in gas pressure and electrical voltage in the evening, when everyone gets home from work and starts running all their appliances. I would use the lowest voltage number for calculations.


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