Wiring requirements for an electric kiln
When we upgraded our electrical wiring last year, we included 240V wiring to the garage for future installation of an electric kiln. Now we are looking at different kilns, and trying to select a kiln that will be large enough but still safe with the wiring that we got.
We have a 4ga feeder from the main panel to a subpanel in the garage, on a 60A breaker. The garage subpanel has a 20A breaker for the lights and general use receptacles, and a 50A breaker for a dedicated 240V circuit for the kiln. The kiln receptacle is only about 12-18″ from the subpanel. I assume the wire for that is 6ga, although I don’t know for sure. (I know about the feeder because the electrician told me the wire was good for 70A even though the breaker is 60A). The kiln receptacle is 6-50, which is what most of the kilns we were looking at a year ago specify.
So here’s the question. Many of the size kilns we are looking at say they draw 11.5kW and 48A. One brand of kiln specifies 4ga wire and a 50A breaker, and another specifies 6ga wire and a 60A breaker. What could account for the difference, if they both draw 48A?
This is not a motor, so I wouldn’t think there would be a huge induction draw on startup. There could be some variability in the draw depending on where you are in the firing cycle, but why a larger breaker with a smaller wire? The ampacity table I’m looking at gives 55A as the 60 degC (140 degF) rating for 6ga wire.
The ambient temperature will be increased some due to proximity to the kiln, but I don’t know by how much. This effect can be limited by placing the kiln as far away from the receptacle as the cord will permit and leaving the garage door open when firing (probably a good idea anyway for ventilation). Does the short length of the wire make any difference?
BTW, I assume they’re talking about the branch circuit when they specify the wiring size, but could they talking about a field-wired cord for the kiln itself???
Please enlighten me.
Thanks,
Rebeccah
Replies
I think you are allright with what you have.
For the most part, your arrangement sounds fine. The difference in the wiring specs isn't very much- some of the kilns just might have a little more element area (for a faster warm-up), or the manufacturer based his recommendation on different wire types.
IF you wired things right, then it should be easy to make any changes.
By "Right", I mean that the wires are run in pipe, and preferably behind a layer of drywall. I cannot approve of using Romex, or any other cable type, for this application due to the increased temperature in the area.
The pipe would also make it easy to pull larger wire, if you need to.
Where the pipe comes out of the wall, there should be either a metal junction box projecting from the wall, or a disconnect switch. (A disconnect switch is not absolutely required, as your panel is so near.) From this box, you would use metal flex to "hard wire" the kiln in place. Again, due to the heat in the area, I cannot recommend using a cord & plug.
Wires like THHN are rated for up to 90 degrees Centigrade, or over 190 degrees Fahrenheit. If your instructions specify a higher rated insulation, 'appliance repair' shops have the fiberglass-wrapped stuff, which you would then use for the short run from the J-box to the kiln.
Likewise, most wire nuts are rated at 105 C, or 220 F.
Why don't I feel ordinary kitchen range cords are good enough? While they probably would work OK, a kiln is typically operated for much greater periods than a kitchen range. Kitchen ranges also often have vent hoods.
But, as I said at the beginning, what you have will probably work fine. I just wanted to "fine tune" the design for others who aren't already presented with a completed job.
Thanks, guys.I just looked again, and it's the box of 4 120V receptacles that's 12-18" from the subpanel. The 6-50R receptacle is connected directly to the subpanel with a metal connector about 1 1/2" long. There's no drywall, and the feeder comes into the subpanel from the back via a metal pipe on the outside of the garage.Rebeccah
My wife just bought a (used) Skutt kiln that uses 48A@240V, single phase. I put in 4ga wire because the branch run is long (~40'), but 6ga is fine for short runs.
Now since this is a 48A continous load, according to the NEC it must be used on a 60A branch (80% of 60 is 48). You'll notice the kiln makers' recommend a 60A breaker. But here's the gotcha: Skutt (and every other kiln manufacturer I've seen) ships this with a 50A plug. 'problem is, according to the NEC, you can't put a 50A receptacle on a 60A branch circuit.
Since I wanted to do a code-compliant install, I put in a 60A receptacle, and installed a 60A plug on the kiln cord. I think I found out why the kiln makers' stick with 50A plugs: since the 50A are used on kitchen stoves, they are commodity parts and cheap; on the other hand, 60A components are not widely used, so they tend to be quite expensive. There also is no single phase, 240V-only NEMA receptacle. There is a single phase, 120V/240V 60A one (14-60), so that's the one that can be used, with the neutral connection in the plug left unused.
Thanks, Barry. Your explanation makes sense.Rebeccah
OK, here's a different question. I'm looking at this used kiln that a guy in Newark (CA) is selling for his brother. It's a 7- or 8-year-old Skutt KS1027, which is a pretty standard size, draws 48A, comes with a 6-50P plug and all. The guy doesn't know anything about kilns, and neither do I (my fiance is the artist, I'm in computers), although I've at least been doing some web research. He mentions that when his brother first got the kiln, they tried plugging it into the dryer receptacle and there was smoke, but that there were 4 wires and "to convert it to 240, we had to disconnect the ground and then it didn't smoke any more" (or words to that effect). This is a guy who thought it was a 250A kiln when I asked him over the phone, thought maybe Sitter Kiln was the brand when I asked him (it has a Kiln Sitter controller on it). In context, I thought he was saying something about rewiring the plug, and it just didn't make any sense at all. Maybe he was talking about the wiring of the receptacle?What do you think he probably meant?And do you think it likely the kiln was damaged when they first plugged it in? It appears to be in reasonably good shape, but it's been in storage for 7 or 8 years and I have no idea if it actually works or not.Thanks,Rebeccah
This is probably old news to you, but if you're looking at buying a used kiln, you should try to find a plug in, so you can check out all the elements. They should all glow an even orange if all the switches are turned up to high, and they should come on fairly quickly.
Elements aren't all that expensive to replace, but it should be factored in to the price. Most of the expense is in the bricks/structure and the kiln sitter. You should make sure that works, too.zak
"When we build, let us think that we build forever. Let it not be for present delight nor for present use alone." --John Ruskin
"so it goes"
Well, we picked the kiln up Saturday and spent the weekend doing the following:
- making a 30"x30" wheeled platform (metal frame, metal wheels, drywall platform surface) for it to replace the 18"x18" metal frame stand it came with;
- inspecting and cleaning the sections;
- replacing a few inexpensive pieces that were missing (one cone rest and the peep hole plugs, and the gauge for checking the kiln sitter adjustment);
- checking the adjustment on the kiln sitter (it was OK);
- turning the receptacle right side up (the electrician had asked which orientation I needed, and I hadn't known at the time, so he left enough slack in the box that I could just turn the receptacle + cover around as a unit);
- wrapping a damaged area of the cord with electrical tape (previous owner had let the cord rest on something hot - maybe the kiln itself?, damaging the outer insulation. Inner insulation and paper appear intact, though);
- and plugging the thing in and turning it on.The first element to glow hot on high was the bottom element, followed soon after by the top one - this is normal, the top and bottom elements on this kiln are hotter than the middle four. The four middle elements took about 20-25 min to glow dull red uniformly, which may be a little slow, but we'll know better when we do a test cone firing and see if it gets to cone 10 or not.No breakers tripped, and neither the cord nor the controller apparatus got hot in the brief period of time we had the kiln turned on. We will of course watch all of these things closely when we do the test firing, too.DH and I are so excited! The kiln was only $300 including shelves and posts (which we need), as well as a bunch of plate holders, glazes, and molds (which we don't) and a bunch of low-fire cones (which we don't need *and* are old and probably no good any more anyway). I spent about another $100 building the wheeled stand and about $50 in miscellaneous parts. It's so cool that we're actually going to get to use the special wiring I had done with the service upgrade 2 years ago.:-PRebeccah
drywall platform surface
Is this in contact with the bottom of the kiln? I don't think this is a good idea. Remember drywall is surfaced with paper. It works well in a fire-resistant wall because the gypsum core contains water, that boils off in the presense of intense heat. But eventually, it does boil off, the gypsum core becomes weak, and the paper backing can ignite.
With the original Skutt base, most of the bottom surface was exposed to the air, mostly I guess for thermal isolation and to allow connection of the venting system.
I was wondering if someone would notice that and comment.Yeah, I came to the same conclusion. I'm going to come up with something else. I think I've got it figured out.Rebeccah