Skutt 822 wire gauge size vs. breaker size.

[cadenrank]

So, I don't have this kiln, but I have a kiln pretty much identical to it in size, and make up. I've been doing some calculations relative to putting new elements in this kiln, to get a more even, but still a nicely powered kiln, maintaining a safe amperage draw on the 40 amp, 8 gauge circuit that my kiln currently uses (at 30 amps currently). I think I have this part of a 30 amp, balanced (top and bottom elements hotter) configuration, covered, but have noticed something when looking at Skutt's 822 specs.

Skutt says recommended wire size is 8awg, and amperage pulled is 33.4 amps. and recommended breaker size is 50amps for their 822 model kiln. 

From my understanding, isn't 8 gauge wire not ideal to be on a 50 amp circuit? Even though the kiln isn't pulling 40 amps, I always assumed the 125% of rated load rule also applied to the conductors feeding the kiln, as well as the breaker. 

So my question here is, if this specification is not outside of best practices, could I theoretically use Skutt's element design in a kiln matching that size, on the existing circuit that I have, just by increasing the breaker size and not increasing the conductor size? The kiln itself's wires are all able to handle that added amperage, and the receptacle is already a 50amp receptacle, but I'm just curious what the groups thoughts on skutt's specs for this kiln are. 

After googling the 8 gauge wire information, I suppose it comes down to the type? I'm seeing the NM Romex type of 8/3 (which is what is run to my kiln) seems like it's max amperage is 40amps, but thhn seems to be rated for 55. Still curious on thoughts here. 

Edited December 13, 2022 by cadenrank

[Mark C.]

THHN is better wire (needs to be in conduit( this is more about the wire insulation  type and strands . The better insulation takes more heat and costs more.I use a lot of THHN in life when pulling wires to pump sheds and conduit runs.

NM Romex would be fine for a 30 amp kiln with the 125% rule

50 amps use #6 wire as that covers the 125% rule-you can still use the 40 amp breaker but it may trip and you will then need a 50amp

when it comes to wire size you are far better off to go big. (yes it costs more but kiln use demands it)

 

[neilestrick]

@cadenrank When looking at a wire ampacity chart, it's safest to use the largest wire size number for that amperage, regardless of which type wire you use, which means that for 50 amps you need 6 gauge wire. Always better to over-build.

(Edited for better wording)

[cadenrank]

53 minutes ago, neilestrick said:

regardless of which type wire you use, which means that for 50 amps you need 6 gauge wire.

You're referring to the circuit rating that the breaker would cover, when you say 50 amps? As in, even if the kiln wouldn't pull that? I understand the >125% <150% rule, but am just verifying that it does or does not apply to the conductor as well. I wish I would have run 6 gauge when I originally did the project like I did with my other kiln, but originally this kiln only pulled 24 amps, so I never thought I'd need the extra amperage there. 

Anyways, I plan to keep this kiln near 30 amps, I mostly just thought it was odd that skutt recommended 8awg and a 50 amp breaker for that specific model of kiln

Edited December 13, 2022 by cadenrank

[neilestrick]

14 minutes ago, cadenrank said:

You're referring to the circuit rating that the breaker would cover, when you say 50 amps? As in, even if the kiln wouldn't pull that? I understand the >125% <150% rule, but am just verifying that it does or does not apply to the conductor as well. I wish I would have run 6 gauge when I originally did the project like I did with my other kiln, but originally this kiln only pulled 24 amps, so I never thought I'd need the extra amperage there. 

Anyways, I plan to keep this kiln near 30 amps, I mostly just thought it was odd that skutt recommended 8awg and a 50 amp breaker for that specific model of kiln

The wiring should be rated the same as the breaker.

[Dick White]

I don't know why Skutt lists it that way. Skutt also does not list the breaker size, only the wire size and the plug configuration. Many kilns that do not actually pull ~50 amps still have a 6-50 plug on the power cord because that's their standard power cord inventory for all their medium and large kilns. The comparable L&L kiln (e18M) lists a 50 amp breaker and 6 ga. wire.

[cadenrank]

28 minutes ago, Dick White said:

I don't know why Skutt lists it that way. Skutt also does not list the breaker size, only the wire size and the plug configuration. 

Two of their distributors list 50 amp breaker in their specifications for the product. I thought that google information was from Skutt, but still. Giving the amperage, and the wire gauge still leaves questions following the 125% rule. 

41 minutes ago, Rockhopper said:

The reasoning is simple:  If the wire is rated lower than the amperage that the breaker will allow to pass through it, the wire itself becomes the 'fuse', and could get dangerously hot without tripping the breaker.  (You can use a heavier wire than required by the breaker, but not lighter - so 12ga on a 15amp breaker, or 6ga on a 40amp breaker is OK - but 14ga on a 20amp, or 8ga wire on a 50amp breaker would not be allowed). 

 

This makes sense to me. I just never really thought about the conductor needing the 125% increase too until I was reading Skutt's recommendations for references for this kiln. That being said, I also didn't think on it very long. Logic eventually would have told me that lower rated wires can't be used on higher rated circuits. But,  then I started to question if maybe my memory on 8 gauge NM cable's specs were wrong 

[Bill Kielb]

17 minutes ago, cadenrank said:

Two of their distributors list 50 amp breaker in their specifications for the product. I thought that google information was from Skutt, but still. Giving the amperage, and the wire gauge still leaves questions following the 125% rule. 

1 hour ago, Rockhopper said:

I think it is a tweener. The Skutt manual I saw listed # 6 wire, no breaker. Sizing this not less than 125% and not more than 150%. 33.4 amps  X 150% = 50.1 amp. So not more than 50.1 amps and not less than 41.75 amps (125%). One is left with choosing a 45 or 50 amp breaker each fit the rule.

The Skutt kiln sizing in the manual seems fine.

Edited December 14, 2022 by Bill Kielb

[cadenrank]

7 minutes ago, Bill Kielb said:

The Skutt kiln sizing in the manual seems fine.

This isn't what skutts distributors, or their main website/product page for this kiln shows.

In fact, I can't find a single product, technical, or kiln specific pdf from Skutt other than the one in your photo that shows 6awg for 822

Edited December 14, 2022 by cadenrank

[Bill Kielb]

39 minutes ago, cadenrank said:

This isn't what skutts distributors, or their main website/product page for this kiln shows

I just happened to look in the latest manual. The math, code requirements, and current Skutt manual seem to all agree so not sure what any distributor is saying but could be an error or a result of previously using lower system volts. Older manuals seem to show it as #8. I guess the lesson is always double check the math, pick an appropriate breaker and pick the wire. The more heating in the wire, the less heating in the kiln. Bigger is usually better for the kiln owner.

If you are using roamex then the wire size will likely be larger. The adjacent conductors in a confined space tend to heat more and are derated from free space. 

The rule about breakers is all about reducing the heating on the breaker so it remains dependable. Heating in wires and devices is not really good.

Edited December 14, 2022 by Bill Kielb

[Mark C.]

I'm sticking to the #6 wire and a 50 amp breaker. If you want a 40 amp breaker fine smaller breaker is the safety in this case

I have had 4 skutt kilns in 5 decades and they underrated the old ones badly. I blew breakers and melted a J box ,overheated wires in the 70'sand 80's Upsided the wires all to #6 -hardwired the kilns and zero issues since.

I said it before- bigger wire is always better.

This was my experience . I also learned the top cone the kiln plate  spec was overrated as well and was really not accurate, say cone 8 for example in an old 181 or 231 or a 1227 with 2.5 inch wall.

The plugs between sections wore out fast (from heat) as did the elements.

Of course Skutt discontinued this old plugs between sections to newer better materials

Skutt always overrated those old kilns on the spec plates

I have learned that spec plates are not always right. 

In todays kilns, get a cone 10 kiln for cone 6 fires not a cone 8 kiln for cone 6 fires.

Once you melt a few wires and boxes and toast a few breakers one usually gets it.

I think the newer Skutts are more acurate in terms of specs. Better 3 inch walls and wiring now.

Edited December 15, 2022 by Mark C.