Yet another Amperage question

[MountainMudworks]

So, I've been learning a lot about electricity while I get everything in order for setting up my (yet to arrive) new kiln. It's a skutt 1227, which draws 48a. I fully understand that this is to be wired with 125% of that, and it will be. But I am curious about the amperage draw for different firings, will my kiln always reach that 60a max? I know it would for sure if I was firing to ^6 or so, but I'll probably never fire above ^04. 

 

Again, my kiln will be wired appropriately for it's maximum draw. I'm just wondering if there is any difference in the draw between different types of firings.

[neilestrick]

With digital controllers, the elements are always on or off. There is no 'low' or 'medium' where the amperage draw would be different. So even if you just turned it on and it's at 150F degrees, it's pulling the full amperage. The kiln cycles the elements on and off to control how quickly the kiln heats up. If you're firing quickly, they stay on longer than if you're firing slowly, and at the high end of the firing they have to stay on longer to maintain the heat. So whether you're firing to cone 019 or cone 12, the amperage draw is the same every time the elements kick on.In your Skutt kiln, all of the elements turn on at the same time. The only exception to that would be in a kiln that has multiple zones, like an L&L or ConeArt, where each section of the kiln cycles independently of the others. In those cases it's possible that only one section of the kiln may be on at a time (briefly), but that section will be at 100% of its amperage draw when it's on.

Your kiln will always pull around 48 amps, never 60 unless something is wrong. The 60 amp breaker is required because the typical breaker is only rated to run continuously at 80% of its rating.

[Bill Kielb]

2 hours ago, MountainMudworks said:

I fully understand that this is to be wired with 125% of that, and it will be.

An interesting point here is the 125% sizing is based on preserving the integrity of the breaker under high stress situations. So your kiln should never draw 125% it’s a way to protect the breaker which could heat once placed in a typical enclosure. In fact, it is not really different than the rule not to load a breaker more than 80% of its capacity. It’s simply the reciprocal.
 

Oops, should have read through Neil’s response to the end!

[MountainMudworks]

3 hours ago, neilestrick said:

 So whether you're firing to cone 019 or cone 12, the amperage draw is the same every time the elements kick on.

Your kiln will always pull around 48 amps

Thanks Neil, this is the info I was looking for. 

Am I understanding correctly if I were to say: it always pulls about 48 amps, but because of heat resistance building in the circuit, it becomes more 'difficult' to draw it? Namely at, say, hour 10 of a firing it's pulling the same 48a but a 50a breaker would overheat and trip even though the amperage is technically below it's capacity? 

Just trying to wrap my head around these concepts, it's pretty interesting stuff that I've always taken for granted before looking to put a miniature sun inside my house!

[Bill Kielb]

16 minutes ago, MountainMudworks said:

50a breaker would overheat and trip even though the amperage is technically below it's capacity? 

 

The 50 amp breaker performance could degrade because it will heat up in operation, meaning trip early or worse fail to trip. The rule provides additional insurance that the breaker will perform over its normal lifespan for this type of load. Also known as a continuous load which has a specific definition of whether something could continuously operate for a period of time.

Just to add, the breaker is a thermal device so it will heat up in normal use.

[neilestrick]

There is heat buildup, but also it's never exactly 48 amps. If your voltage is a little high it'll pull more and trip the 50. I've seen many 48 amp kilns run on 50 amp breakers and they work, although it's not up to code. I've also seen 48 amp kilns trip a 50 amp breaker.

[MountainMudworks]

59 minutes ago, Bill Kielb said:

The 50 amp breaker performance could degrade because it will heat up in operation, meaning trip early or worse fail to trip. 

Im assuming tripping early would be a much more common issue than failing to trip? The trip is based off of heat, or is it a different fail state that causes it?

56 minutes ago, neilestrick said:

 I've seen many 48 amp kilns run on 50 amp breakers and they work, although it's not up to code. I've also seen 48 amp kilns trip a 50 amp breaker.

Yep, I just inspected the setup I've been using in my uncle's house for the last two years. A skutt 1027 and a ceramicrome something (1031). Both are on 50a breakers and I have personally fired them both a hundred or more times without any issues. Didn't even know to look until recently! 

He's a mechanical engineer, and a very smart man who's been doing this for several decades. Said he's always run them this way without problems. I was surprised to see how under code it is, but, I guess if it ain't broke ....

Either way, I have mine all wired up to code in my garage, so these questions are all theoretical to sate my curiosity

[neilestrick]

24 minutes ago, MountainMudworks said:

Said he's always run them this way without problems. I was surprised to see how under code it is, but, I guess if it ain't broke ....

If they are wired up to the 50 with 6 gauge wire as they should be, then you can just change the breaker to a 60, as it also uses 6 gauge wire.

I've also seen 48 amp kilns that are wired to a 50 amp breaker that work fine for years, but then don't work when the elements are changed.

[Bill Kielb]

29 minutes ago, MountainMudworks said:

Im assuming tripping early would be a much more common issue than failing to trip? The trip is based off of heat, or is it a different fail state that causes it?

Likely true, but I would be hard pressed to assume anything as I have seen many old breakers fail to trip as well.

[MountainMudworks]

3 minutes ago, neilestrick said:

I've also seen 48 amp kilns that are wired to a 50 amp breaker that work fine for years, but then don't work when the elements are changed.

Yep, I'm sure that we could, and maybe should upgrade the breakers, but I'm not actually in charge of them and he's got them where he wants/is unconcerned as he is unlikely to run the kilns as much as I have!

Luckily, when we did change elements we had no issues. We never fire above ^04 which is part of why I asked this question, I thought perhaps our luck was due to lower/shorter firings affecting the draw. These kilns have never tripped a breaker in my experience.

[cadenrank]

3 hours ago, neilestrick said:

but then don't work when the elements are changed.

This would be because the new elements would have lower resistance? And thus higher amperage draw? 

By this logic, the reason elements 'age' is because their resistance increases, and as that increases, the amount of amperage being drawn lessens, which lowers their output? 

That makes sense to me, just never really thought hard on the electrical reason why elements age. 

[Bill Kielb]

4 hours ago, cadenrank said:

That makes sense to me, just never really thought hard on the electrical reason why elements age. 

Good thinking! And as they age they corrode and the diameter of the element wire gets thinner which leaves less cross section (area):for the electrons to flow through (basically less metal) therefore their resistance goes Upppppppp!