Can I unplug my kiln?

[TLDinNC]

Hello all- I recently purchased a Skutt KM1027 that is NOT hardwired. As a hobby potter still fairly new to pottery, I will be firing infrequently. My question is this- is it OK for me to only have the kiln plugged in when I’m firing? I am asking because the “kiln expert” from the retailer where I purchased the kiln told me that I would shorten the life of the kiln’s plug and cord by plugging and unplugging it and he strongly recommended against doing so. He also wasn’t thrilled that I purchased the stand with casters so that when not in use, I could roll the kiln back toward the wall.   Can anyone offer opinions on this?  Thanks so much. 

[Bill Kielb]

All of the 1027 models require a 50 amp plug. He is correct in that the plug is an ok means of occasional disconnect but if it is exercised often will wear out. The part that is most susceptible to wear and tear is really the receptacle more so than the plug.. There are several styles available usually discernible by the weight of the material they are made of and of course the purchase price. In my opinion best to ensure the receptacle and plug  are truly heavy duty construction. And of course when your electrician installs that all connections are appropriately tight. Checking the tightness of the connections over time is a thing as they loosen the connection will generate heat  destroying the connection further.

Having said all that, it sounds like you need the portability so a plug is the way to go. A good quality plug and good quality receptacle carefully unplugged each time with an eye toward loose connections or signs of overheating will last a long long time.

[Hulk]

...as for moving the kiln around on rolling stand, perhaps the concern is subjecting the kiln to bumps, vibration, etc.? My 1027 is over thirty years young - it survived the pickup truck trip from Lompoc last April, so there's that - however, not looking to bump it around!

btw, methinks a single phase 1027 requires a sixty amp breaker (draws 48A)? ...the three phase would be forty (draws ~30A)?

 

[Bill Kielb]

4 minutes ago, Hulk said:

btw, methinks a single phase 1027 requires a sixty amp plug (draws 48A)? ...the three phase would be forty (draws ~30A)?

Might be, if so good catch. I was going with some  googled Skutt stuff

[PotterPutter]

My kiln is also on casters so I can move it out of the way when it's not firing. It has been moved 100 times, only a few feet, and it's perfectly fine. Just move it slowly and carefully and you should not have a problem. 

[Hulk]

Fifty amp plug, aye that; I was thinking breaker, my bad!

[TLDinNC]

Hulk and Bill, it is a 60 amp breaker- already installed with the extra weight copper wire (I forget that terminology).  I’m confident the electrician installed it correctly as we have lots of potters here in NC and it was far from his first rodeo. My concern was keeping the kiln plugged in when not in use- for example, the strong t-storms that are going to blow through here today. I don’t want the circuitry to get fried if we have a power surge.  

PotterPutter, that’s exactly how I am/will be using the rolling feature- just to back it up 3 feet to the wall when not in use. The “kiln expert” felt I might wear out the cord by flexing it too much.

Thanks for the input!

[Mark C.]

Do  not worry about the plug -if it wears out they are cheap and easy to replace. Keep the contacts shiny for best connections-you could also use an electrical contact grease to keep oxidation down. Over time I have cooked those 50 amp receptacles.Usually from oxidized terminals.Cords can flex and if they wear out are all an easy fix

[Denice]

My Skutt 1027 has a 50 amp breaker,  I  have had it on a 50 amp breaker for 30 years.   When I built my new studio a told the electrician I wanted a 60 amp,  he explained why I should stick with the 50 amp.   He did use a extra heavy wire,   I also unplug it a lot,  I have two kilns that use that same outlet.   I have to replace the receptacles now and then but like Mark said they are easy to fix.   Denice

[Bill Kielb]

• I am guessing the Skutt 50 amp receptacle in their specification is fine based on load diversity. If an electrician installs a 60 amp breaker then everything downstream from the breaker must be rated 60 amps or more, else the breaker will not protect what it is supposed to.  Which means a 18-60R receptacle or greater rating. Heavier wire is always permitted and often required depending upon distance from the load to source and the calculated voltage drop. Heavier wire - lower voltage drop over long distances.

 

[Denice]

I believe that is the same information he told me.  He is a master electrician and a journeyman plumber  and grew up in a heating and air conditioning business.   He was one semester from graduating with a electrical engineering degree when he had to go home for a emergency in the family.   He would argue with the inspectors when they  thought they found something wrong,  he would quote chapter and verse out of the code book.   My husband use to work with him,  he wired the big amusement rides my husband wrote the service manuals.    Denice

[Lbegley]

I used to plug and unplug my kiln all the time, and the plug/outlet did, in fact, fail.  It began smoking, and the plastic melted and if I hadn't been right there, I think it might have caught fire.  Scared the crap out of me!

I was firing every week and unplugging and moving it each time, and it was a few years before the problem, so it did take a lot of wear and tear before it wore out. 

 

[Dick White]

Denice and Bill, the notion of needing consistent rated ampacity of everything in the circuit (i.e., 60A breaker requires a 60A receptacle, or the 6-50 plug on the kiln should not be fused higher than a 50A breaker) is incorrect. The code is not about protecting the end device (that's UL's job), but about protecting the wiring from overheating and catching fire inside the wall. For example, bathroom and kitchen circuits are required to be 20A, but the receptacles are all ordinary 5-15s. The issue with kilns and other constant draw devices that leads to the 125% rule has to do with heat dissipation of the wire. There are no perfect conductors in general commerce, so all conventional wires generate some heat as current passes. Larger wire generates less heat for the same amount of current, and if a current is intermittent as in most situations, the wire can cool between uses. The engineers who wrote the code determined the maximum ampacity that can be carried by any particular wire gauge (and in a particular installation type, e.g., open air, enclosed raceway, conduit) in normal usage without overheating. However, if a particular circuit is intended for an end device that will draw full ampacity constantly (as when the kiln is running at full heat for the last several hours of the firing), the circuit must be upsized to 125% of the device rating so that the circuit won't overheat as there won't be any intermittent cooling time. Circuit breakers protect not only from the overcurrent surge of a short circuit, but also from overheating due to a long draw of a lesser current. The largest plug-in kilns are typically are 48A or so with 6-50 (or 15-50 3-phase) plugs, but the servicing circuit must be rated for 60A for the heat dissipation. (And if the circuit is longer than 50' from the panel, the wire gauge should be upsized to account for voltage drop, but the breaker need not be increased.) Thus, the 6-50 receptacle on the 60A circuit is ok, and the 50A breaker (even with heavier wire) on the kiln circuit is not ok.

[Bill Kielb]

@Dick White

Hmm, 

Works  for me;  the kiln will never draw 48 amps continuously ever for extended periods , since the duty cycle of the relays will limit this to no more than 10 second intervals  for mechanical relays so I was speculating that the Skutt chart was correct and the plug would be 50 amps.

as far as the nuances of the electrical code, it literally has been 40 years since I was one but  our practice was to always make sure everything downstream was rated  equal or higher than the breaker protecting it. Your 15  amp device installed on a 20 circuit was a prime  no no back in the day  for us and we would always ensure the circuit wired with #12 wire had a device rated as such and our inspectors would double check us.

As a matter of fact it was always an important point of inspection that the 20 amp circuits be wired with #12 wire (20 amp) and not #14  (15 amp) for the same reason. Too many fires from over heating due to contractors  trying to save money on copper.

When multiple devices occur on one circuit often we find diversity taken as in three receptacles on a single 20 amp breaker. In a commercial environment each  still are rated for the maximum load though (20 amps each) and the breaker protects everything to 20 amps.

I am not really interested in if I can find an exception that allows a 60 amp breaker to use a 50 amp receptacle, in my experience it is less safe. As to an entire machine and all the internal wiring needing to be rated to the sum total of the draw, I agree the interior wiring of the machine will vary. In this case though, the machine has to contain a catastrophic failure to its maximum possible load.

As far as sizing breakers and all downstream wiring and devices, I would  never ever advocate for a 15 amp  wiring or receptacle used on a 20 amp circuit. Seen too many failures from this very thing.

Just my experience though

I figure I should add for your exception ( Home duplex receptacles) just looked this up and it is possible because home duplex devices need to be rated 20 amps clear path in their internal wiring size as part of a manufacturing requirement  to allow this exemption. As to commercial outlets probably not the case. As to undersized wire, never the case. I think unlikely this exception applies to 50 amp single receptacles.

[Dick White]

interesting discussion, Bill, always learning something new from you. Regarding the continuous draw, a manual kiln on high will be a continuous 48A draw, so the circuit needs to have 60A capacity. Even with digital controls, the relays will cycle, but I doubt an exactly-sized wire (8ga copper for 50A) inside the wall will cool enough between the 10-second cycles to mitigate the overheating. I've heard of this 125% rule not only for kilns and similar high demand heating devices, but for commercial lighting, e.g., a shopping center store with the entire ceiling of light fixtures on one circuit. Add up all the fixtures and wire/fuse it at 125% because that circuit is full-on all day and half the night. The kilns are manufactured with 50A cords and 6/50 plugs (consistent with the 48A rating), but the whole idea behind NEMA plug design is you can't plug together incompatible amps or voltage. So the manufacturer's 6-50 plug won't go into  the higher rated 14-60 receptacle even though the circuit before it must be wired/fused at 60A. And of course, when we get into kilns bigger than 50A (single phase), the 125% rule pushes us to something more than 60A service, and there are no plugs/receptacles above 60A, thus direct wire is the only option.  Regarding the Skutt chart, it only specs the receptacle and minimum wire gauge, no reference is made to fusing. Both the L&L and Paragon sites show specs of seemingly mismatched 6-50 receptacle and 60A breaker/6ga wire.

And that design factor you looked up for home duplex outlets needing full 20A internal capacity despite the 5-15 face explains why all the ("modern") kitchens and bathrooms I've lived in had 12ga wire and standard 5-15 outlets. Never saw one with a T-slot that could take a 5-20 plug. And on the other hand, our brand new school studio has nothing but 5-20 duplex outlets - there are a lot of them and I'm pretty sure each of the 15 wheels (for which there is a separate outlet for each) does not have a separate breaker in the utility closet (which we, appropriately, can't get into to check). Could very well be solely because of the difference between commercial vs. residential codes/practice.

cheers on this rainy day here.

[Hulk]

"Regarding the Skutt chart..."

What Bill posted includes wire and plug - it's from the Skutt website; what I posted is an excerpt from a Skutt .pdf file that does specify the breaker - it can be found here:

https://skutt.com/pdf/catalog/cat10_notes.pdf

There's notes there on long runs -> bigger wire, local code, Canada, etc.

My 1027 has three rotary switches; when they all turned t'high, I believe that's full gas, 48A if'n it's in good trim, aye?

The ol' knob an' tube had the advantage of nice cooling, zapping roaches and mice ...nah, let's go all modern!

[Bill Kielb]

4 hours ago, Dick White said:

interesting discussion, Bill, always learning something new from you. Regarding the continuous draw, a manual kiln on high will be a continuous 48A draw, so the circuit needs to have 60A capacity. Even with digital controls, the relays will cycle, but I doubt an exactly-sized wire (8ga copper for 50A) inside the wall will cool enough between the 10-second cycles to mitigate the overheating. I've heard of this 125% rule not only for kilns and similar high demand heating devices, but for commercial lighting, e.g., a shopping center store with the entire ceiling of light fixtures on one circuit. Add up all the fixtures and wire/fuse it at 125% because that circuit is full-on all day and half the night. The kilns are manufactured with 50A cords and 6/50 plugs (consistent with the 48A rating), but the whole idea behind NEMA plug design is you can't plug together incompatible amps or voltage. So the manufacturer's 6-50 plug won't go into  the higher rated 14-60 receptacle even though the circuit before it must be wired/fused at 60A. And of course, when we get into kilns bigger than 50A (single phase), the 125% rule pushes us to something more than 60A service, and there are no plugs/receptacles above 60A, thus direct wire is the only option.  Regarding the Skutt chart, it only specs the receptacle and minimum wire gauge, no reference is made to fusing. Both the L&L and Paragon sites show specs of seemingly mismatched 6-50 receptacle and 60A breaker/6ga wire.

And that design factor you looked up for home duplex outlets needing full 20A internal capacity despite the 5-15 face explains why all the ("modern") kitchens and bathrooms I've lived in had 12ga wire and standard 5-15 outlets. Never saw one with a T-slot that could take a 5-20 plug. And on the other hand, our brand new school studio has nothing but 5-20 duplex outlets - there are a lot of them and I'm pretty sure each of the 15 wheels (for which there is a separate outlet for each) does not have a separate breaker in the utility closet (which we, appropriately, can't get into to check). Could very well be solely because of the difference between commercial vs. residential codes/practice.

cheers on this rainy day here.

Likely, always exceptions usually have their upsides and downsides.  I just never undersized the devices but it is interesting that they made the manufactures rate them at 20 amps regardless. Back in the day I was a city of Chicago union electrician and we had a chip on our shoulders as always doing things code or greater, I have an old perspective. I returned to college majoring in electrical engineering and minoring in civil and taught as an adjunct in  a JR college for twenty years as ........ you guessed it in HVAC.  Go figure that mess! 

Good point on manual kiln and I would have to look at breaker manufacture to see if a 50 amp breaker needed to be able to sustain 50 amps and trip at  a maximum of 125%.  Etc.... Don’t know and always try and advise on the conservative side and wire on the conservative side.

I think most productive point  is plug and receptacle are fine but take obvious maintenance and monitoring.

your school (commercial)  outlets likely are three duplex on a circuit all on a three phase network sharing one neutral so likely 9 receptacles sharing one #12 neutral.

fun stuff - first nice sunny day here in a long while!

[Denice]

My kiln receptacle is about 30 feet from the breaker panel.   Since I have been firing my kiln for 30 years with a 50 amp breaker I will try not to worry about it.  It is a manual kiln so I don't sleep or leave when firing.  It is need of repair right now,  my last firing had the bottom and top elements worked and the middle ones were unresponsive.  I have about 10 firings on this new set of elements.   My husband is going to look at it when he gets the chance,  good thing I have a smaller new kiln to use.   Denice

[Bill Kielb]

I think 30 years with your 50 amp breaker is a positive sign that you were protected! Good luck with the troubleshooting.

[neilestrick]

Electrical code, which is part of the National Fire Code (for good reason) ,states that continuous duty resistance heating appliances should be on a breaker that is 25% greater than the draw of the appliance. On manual kilns, that draw is definitely continuous. I have seen a couple of instances where  a kiln manufacturer argues that since digital kilns cycle on and off, they do not qualify as continuous duty, and therefore don't need to follow the 125% rule. This allows them to advertise 15 amp kilns as needing a 15 amp circuit meaning the customer doesn't need to upgrade their wiring to use it. Other kiln manufacturers think this is foolish, and I agree, because the kiln could be fired at full on, without the relays cycling, and things should be done in such a way that it is safe in every possible firing, not just the 'typical' firing schedule. Plus, at the high end of a firing, the relays may not cycle at all for long periods.

Many 48 amp kilns will run just fine on a 50 amp breaker, but it's not up to code or installed according to manufacturers recommendations, and therefore could be trouble if there's ever an insurance claim because of damage caused by the kiln, whether or not that was the cause of the problem.  48 amp kilns come with 50 amp power cords, and some electricians have issues with hooking up the 50 amp cord on a 60 amp breaker. Those 50 amp cords have #6, 105C wires, so they can handle the 60 amps. That is how they are able to get UL certification for kilns with those cords. Your kiln should always be installed according to the manufacturers recommendations. They have done the work to make sure your kiln works safely. Your electrician does not know better. The only time it should done differently is if there are local codes that are different than the national codes. But in that case, the local codes will be more strict, not less.  If you electrician won't do it right, find another electrician. This forum is a source of information for many first time kiln owners. When it comes to hooking up kilns, we should never recommend doing less than what the manufacturer recommends.

As for unplugging the power cord, the best option is to install a disconnect on the line, so you don't have to unplug it to kill the power. But if you do have a disconnect, make sure you're holding it by the plug, not the cord, when you unplug it.

I'm not a big fan of rolling kiln stands, but they work as long as you move them slowly so they don't bounce around. As for it wearing out the cord, if it's a molded plug, where the cord and plug are all once piece molded plastic, yes, the joint between the mold and plug will wear out faster if you're unplugging it a lot. Usually when kiln power cords come apart, it's where the plug and cord meet up. When that happens, it must be replaced. If it's the type where the plug is attached to a cord, two separate pieces hooked together, it's less of an issue. Just be sure that when you roll the kiln away from the wall, it's not so far that the plug is pulling out from the outlet at all. Kiln cords are usually pretty stiff, so there's a short limit to how far way you can get before it's a problem.

[Bill Kielb]

2 hours ago, neilestrick said:

Electrical code, which is part of the National Fire Code (for good reason) ,states that continuous duty resistance heating appliances should be on a breaker that is 25% greater than the draw of the appliance. On manual kilns, that draw is definitely continuous. I have seen a couple of instances where  a kiln manufacturer argues that since digital kilns cycle on and off, they do not qualify as continuous duty, and therefore don't need to follow the 125% rule. This allows them to advertise 15 amp kilns as needing a 15 amp circuit meaning the customer doesn't need to upgrade their wiring to use it. Other kiln manufacturers think this is foolish, and I agree, because the kiln could be fired at full on, without the relays cycling, and things should be done in such a way that it is safe in every possible firing, not just the 'typical' firing schedule. Plus, at the high end of a firing, the relays may not cycle at all for long periods.

Many 48 amp kilns will run just fine on a 50 amp breaker, but it's not up to code or installed according to manufacturers recommendations, and therefore could be trouble if there's ever an insurance claim because of damage caused by the kiln, whether or not that was the cause of the problem.  48 amp kilns come with 50 amp power cords, and some electricians have issues with hooking up the 50 amp cord on a 60 amp breaker. Those 50 amp cords have #6, 105C wires, so they can handle the 60 amps. That is how they are able to get UL certification for kilns with those cords. Your kiln should always be installed according to the manufacturers recommendations. They have done the work to make sure your kiln works safely. Your electrician does not know better. The only time it should done differently is if there are local codes that are different than the national codes. But in that case, the local codes will be more strict, not less.  If you electrician won't do it right, find another electrician. This forum is a source of information for many first time kiln owners. When it comes to hooking up kilns, we should never recommend doing less than what the manufacturer recommends.

As for unplugging the power cord, the best option is to install a disconnect on the line, so you don't have to unplug it to kill the power. But if you do have a disconnect, make sure you're holding it by the plug, not the cord, when you unplug it.

I'm not a big fan of rolling kiln stands, but they work as long as you move them slowly so they don't bounce around. As for it wearing out the cord, if it's a molded plug, where the cord and plug are all once piece molded plastic, yes, the joint between the mold and plug will wear out faster if you're unplugging it a lot. Usually when kiln power cords come apart, it's where the plug and cord meet up. When that happens, it must be replaced. If it's the type where the plug is attached to a cord, two separate pieces hooked together, it's less of an issue. Just be sure that when you roll the kiln away from the wall, it's not so far that the plug is pulling out from the outlet at all. Kiln cords are usually pretty stiff, so there's a short limit to how far way you can get before it's a problem.

Not really disagreeing here

Not bad but Article 100 centers around the 125% rule based continuous load which is defined under NEC as 3 hours or more. Kiln manufactures may be able to qualify for the 50 amp breaker with a 14 second duty cycle on an automatic kiln that maxes out at 48 amps. Mechanical relays will not tolerate less than 10 seconds so 14 seconds or more is where we tend to see them. The real issue was that of using a 50 amp rated receptacle on a 60 amp circuit which is less desirable than using a 60 amp rated receptacle on a 60 amp circuit. A 60 amp breaker will not provide more safety than a 50 amp especially with a  receptacle rated at 50 amps even if the wiring is rated at 60 amps. (Power cord and feeds from the breaker included) There is an exception to the code with respect to 15 amp devices on a 20 amp circuit in that manufactures are mandated to ensure the 15 amp receptacle they  provide must be capable of carrying 20 amps. Not sure this exemption applies to a 50 amp receptacle, but why would anyone risk it.

I believe you are absolutely correct in that upsizing to a 60 amp breaker is permissible under the code, that is correct but it does not enhance safety, only avoids nuisance trips and still should require all downstream devices be capable of the additional heating that  they will experience waiting for the breaker to trip. That would be a reason why a kiln manufacture would attempt to certify at the lesser breaker amperage. Their design would only need to withstand the heating of 50 amps, not 60 so to speak.

As in all thing lots of twists and turns and it turns out that there are two types of breakers, 100% and the old 80% (which is the inverse of 125%  interestingly).  A 50 amp 100% breaker withstands 50 amps continuous load, so no upsizing is necessary. Here is the catch which breaker do you have?

So everything has an element of correctness to it including upsizing to 60 amps when necessary and allowable. My only point to stress is a 50 amp rated device (receptacle and plug) is not safer on a 60 amp circuit breaker, period no two ways about it and suggesting it is better is probably erroneous.  If the user has the need to jump a full size breaker higher because of nuisance tripping or manufacture rating then best to make sure these devices will handle the additional heating they will see with a 60 amp breaker.

The conservative advice in my view is if ya have to use a 60 amp breaker, make sure everything connected to it will be rated for 60 amps. Your kiln manufacture likely had to certify it would withstand the additional heating why make your plug and receptacle be the weakest link? Buy a 60 amp Plug and receptacle.

The 125% rule for sizing circuit breakers was developed to fill the gap in sizing based on expected heating of the breaker.  The actual  rule for sizing - Circuit breaker sizing = 100% X intermittent load + 125% continuous load. In the event there is no "three hour minimum" continuous load,  guess what 100% of rating works. The rule enhances safety but really enhances diversity with little or no cost to reducing safety. An argument can be made that the breaker is less susceptible to overheating and failure which can enhance safety but no mistake a 60 amp circuit has significantly more potential than a 50 amp circuit, always.

Now if the kiln comes with a 50 amp plug but 60 amp rated cord, 50 amp plugs do not fit 60 amp receptacles for good reason. I guess trust in the fact that the 50 amp plug will handle the load. If you are adding your own plug and receptacle 60 amp rated plug and receptacle is definitely safer, more expensive, but safer for sure.

Safer is better, so I agree advise conservatively which would be a 60 amp rated plug and receptacle, it is a fairly complicated web of exceptions and sizing and codes, so following the manufactures specification is probably best since they certified on some basis. As to a 50 amp plug that is preinstalled on a 60 certified device, I am not sure how I feel about that. I guess not impressed now that we are thinking about it.

[Dick White]

Not trying to defend the kiln manufacturers, but if they felt obliged to put 60A plugs on their power cords (which in turn would oblige us to put a 14-60 receptacle on the wall), now we are all at 4 wire configurations. Given the basic price of a kiln (and for the user, the price of the installation), another wire shouldn't be a deal killer. But then I don't own a kiln company. I just fix them.

[Bill Kielb]

I agree, cost is a marketing concern for them. I do not believe the plug would be fully populated but the higher expense is considerable. I could understand them trying to certify as low as practical, It makes me wonder why not insulate better and reduce the wattage. Must be plain old more costly.