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Another silly question for the deep dive experts. We often toss about the 125% rule for wiring kilns, e.g., a 48 amp kiln needs a 60 amp circuit. Reading around in the code book to find the exact passage that requires that, I see in 210.9(A)(1)(a) that the wiring of a branch circuit containing a continuous draw must be wired at 125% of the continuous load in accordance with a wire table later in the book. Does this require that only the wire be 60A-capable (6ga plus whatever for voltage drop due to length), or does this 125% include the breaker too,  or would a 50A breaker and 6ga wire be acceptable? Thanks.

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Ok I’ll give you my understanding

I Think reading the code,  125% of continuous load plus 100% of the non continuous load with a continuous load being defined as 3 consecutive hours. The rule is in place to protect the breaker from overheating with the continuous load on it. There  are 100% rated breakers that can be loaded as such. We usually find these breakers 250 amps and above though. So for most breakers this rule is actually the same as the do not load a normal breaker greater than 80% of its capacity. To me the wire size has always matched the breaker size or is even heavier. No sense in finding out the wire is a better fuse than the breaker.

So bigger breaker and wire to match the breaker.

I have not read the code in several years though, so some of this could have changed or be nuanced.

Edited by Bill Kielb
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Looks to me that NEC guidelines for continuous load applies to the OCPD as well.

...am seeing discussion regarding 100% rated continuous load CB vs. "standard" 80% rating, meh.

...there have been changes to the language over the years, hence I'm not listing code section numbers...

OCPD vendors publish selection tables; the ones I'm seeing reference 80% continuous load reqs.

e.g. UL489_US-Breaker_Wire_Size_Chart.pdf (usbreaker.com)

Heading up for air now.

 

ocpd over current protection device

cb circuit breaker

80 and 125 are inverse

Edited by Hulk
link to vendor selection table, replete with code references and upside down 125 is 80 examples...
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The  wire and breaker must be rated at 125%.

I do run into a number of 48 amp kilns that are on a 50 amp breaker, and I always tell them to swap it out for a 60 amp breaker. The problem is that the breaker will wear out running so close to its max, and if you're running slightly high voltage, like 245 amps, which is very common, then it ups the amperage draw even closer to 50. Also, the amperage is higher when you first turn the kiln on and the elements are cold, so it may trip the breaker at that point if the elements are rolled according to 'hot resistance' numbers.

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I agree with Neil. 125% overrated. 

If I recall correctly, kilns fall under Article 424 'Fixed Electric Space-Heating Equipment' which includes electric boilers.

I would consider it a continuous load as defined in Art 100 "A load where maximum current is expected to continue for 3 hours or more."  But many disagree, as it can cycle on/off within that 3 hours, but I'd find it unlikely at startup.  I'm ignorant of kiln internal workings.

Typically you can't exceed a manufacturers rated name plate, and that seems to make most inspectors comfortable.  Art.422.11(A) 'shall not exceed manuf. nameplate rating' (paraphrased)

If no nameplate or manufactures info is available, I'd add up total wattage, divide it by available voltage and with that amperage I'd add 125% and size my branch circuit and breaker (overcurrent protection device) based on that.

 

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Thanks folks for all your input. The thing that is driving this question is a bit of a tif going on over in another pottery group. Somebody bought a new kiln and their regular household electrician refused to put a 60 amp breaker and wire on a 6-50 receptacle, insisted it had to be a 60 amp outlet or a 50 amp breaker. Another self-described "master electrician and hobby potter" opined that 50 amps is all that is needed. His defense against the continuous load requirement is that he has heard the relays in his kiln clicking so that makes it not a continuous load. As if the short pause, if any when coming into mid-fire or high-fire maturity, is enough for the cable to cool off... And what about manual kilns that are simply full on high for the last 6 or more hours of the firing? And the rest of it is just good common sense - listen to the kiln manufacturer and don't use the breaker as a convenient on-off switch. Anyway, on to the next battle.

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My manual kiln is always full on for at least-5 6 hours.48 amp draw  on a skutt 1227. I toasted a few 50 amp breakers in the 70's and 80s before swapping up to 60 breakers

My fire right controller switched it on and off (switches are on full) for say2-3 hours until its on full.

Kilns are outside most electricians experience I have found and they are not well understood. You can use that 50 amp breaker but it will fail in time.

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1 hour ago, Dick White said:

Somebody bought a new kiln and their regular household electrician refused to put a 60 amp breaker and wire on a 6-50 receptacle, insisted it had to be a 60 amp outlet or a 50 amp breaker. Another self-described "master electrician and hobby potter"

The codes have been in effect for many years originating in resistance heating that would be on for long periods of time. You could also remind him of the 80% rule which most electricians know by heart.. His 50 amp breaker can only carry 9600 watts or 40 amp load ….. so now what is an electrician to do but go read the codes and learn them. He can always fix his plug issue by hardwiring the kiln into a disconnect satisfying all concerns.

Edited by Bill Kielb
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1 hour ago, Dick White said:

Somebody bought a new kiln and their regular household electrician refused to put a 60 amp breaker and wire on a 6-50 receptacle, insisted it had to be a 60 amp outlet or a 50 amp breaker.

I've run into a couple of electricians like that. But here's the thing- if the electrician pulled a permit, the inspector would defer to the manufacturer's recommendations, and that's exactly what the electrician should do. Plus, the 50 amp cord has 6 gauge wire rated to 105C, which can handle the 60 amps just fine.

1 hour ago, Dick White said:

His defense against the continuous load requirement is that he has heard the relays in his kiln clicking so that makes it not a continuous load.

This argument is so weak. Just because his firing schedule doesn't keep it full on for 3 hours doesn't mean that you couldn't run it for 3 hours full on. And like you said, the cycling of the relays doesn't allow enough time for anything to cool off enough to matter, especially if you have solid state relays with a 1/2 second minimum cycle time.

4 minutes ago, Bill Kielb said:

He can always fix his plug issue by hardwiring the kiln into a disconnect satisfying all concerns.

Exactly. The customer should be putting in a disconnect anyway, so the cost difference should be minimal. However I would still fight it and continue to bring up the manufacturer's recommendations. Or ask him to get a permit and ask the inspector why you're not allowed to install a UL listed appliance according to the manufacturer's recommendations.

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7 hours ago, neilestrick said:

I've run into a couple of electricians like that. But here's the thing- if the electrician pulled a permit, the inspector would defer to the manufacturer's recommendations, and that's exactly what the electrician should do. Plus, the 50 amp cord has 6 gauge wire rated to 105C, which can handle the 60 amps just fine.

Manufactures recommendations - This is generally good advice however the inspector should follow the adopted codes, after all these are recommendations for a reason. “Mfg requirements can be lax or more restrictive than the NEC, however they cannot contraindicate any other codes.” Many manufactures defer to being in accordance with local codes somewhere in their documentation, as its nearly impossible for them to know the application of all.

A recent paragon listing (I believe) I looked up just deferred that there might be a 125%  requirement for continuous load.

Bottom line manufactures do a pretty good job but a solid knowledge of the current code  generally provides the safest path. For kilns this would be a non issue had it not been for the use of a plug as an acceptable means of disconnect. Most electricians use this rule to avoid a more costly disconnect, yet few potters I would say regularly unplug their kiln before loading it. Yet still manufactures likely have some reference in their material  to remove power from the kiln prior to loading and unloading.

In other countries, lid switches and positive means to denergize have been in effect to combat this loophole so to speak and protect the end user but then again they can have 230v single phase on lightly insulated lamp cord to which everything eventually gets put on a GFI breaker to sort of make it safer again.

Complicated stuff!

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20 hours ago, Dick White said:

Thanks folks for all your input. The thing that is driving this question is a bit of a tif going on over in another pottery group. Somebody bought a new kiln and their regular household electrician refused to put a 60 amp breaker and wire on a 6-50 receptacle, insisted it had to be a 60 amp outlet or a 50 amp breaker. Another self-described "master electrician and hobby potter" opined that 50 amps is all that is needed. His defense against the continuous load requirement is that he has heard the relays in his kiln clicking so that makes it not a continuous load. As if the short pause, if any when coming into mid-fire or high-fire maturity, is enough for the cable to cool off... And what about manual kilns that are simply full on high for the last 6 or more hours of the firing? And the rest of it is just good common sense - listen to the kiln manufacturer and don't use the breaker as a convenient on-off switch. Anyway, on to the next battle.

His clicking relays is moot. Art 424.3 (B) "Fixed electric space-heating equipment and motors shall be considered continuous load." (125%)

 

I would not put a 50 amp receptacle on a 60 amp circuit.  Art 210.21(B)(3)  . . . "or, where rated higher than 50 amperes , the receptacle rating shall not be less than the branch-circuit rating."

As far as disconnecting, if it's in a dwelling (residential bldg) it can be disconnected at the main service disconnect 424.19(C)(3)  HOWEVER   I personally would install a disconnect within sight of the kiln (you can use a breaker rated as a disconnnect) and I would keep it out of the heat affected zone (so I don't have to derate the equipment.)  This is just for both convenience and safety IMO and what I would do on my own.

I wouldn't be quick to disparage the electrician.  As Bill mentioned it's complicated.  Also, the code is written in legal'esce language and I wish they would name kilns specifically, most sparkys don't associate kilns with 'fixed electric space heating equipment' and rightfully so.  It's confuzzling.  Additionally, most industrial electricians are rarely sent out to a house to install equipment, and even many contractors do either residentilal/commercial/industrial/hospitals depending on locale and demand.

BTW, my NEC is a 2014 so all this could have changed and not be current, so check the updated versions to confirm.  My guess it hasn't changed much.

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Yes, most sparkies are responsible within their area of expertise and don't immediately recognize how different a kiln is from just a big hair dryer in the bathroom. And not being familiar with the Skutt et al websites where explicit specification are listed, they would look at the rating plate and do what seems normal for the numbers on the plate. It's the few who show up in a pottery group and insist that because they have a license they know what they are talking about and and it absolutely must be fused at the rating plate. As noted, most sparkies are responsible and will adjust their opinion (and work) when the strange nuance of 125% is shown to them, but every once in a while we get into arguments about it. I don't have a license, but I can read the code and from that understand why Skutt et al say what they say. Thanks for everybody's help and input,

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On 6/22/2021 at 11:10 AM, Mark_H said:

I would not put a 50 amp receptacle on a 60 amp circuit.  Art 210.21(B)(3)  . . . "or, where rated higher than 50 amperes , the receptacle rating shall not be less than the branch-circuit rating."

The only crazy exception I ran into pertained to 15 amp residential receptacles that were actually mandated to be made capable to pass through 20 amp  at the manufacturing level. Very surprised when I read the reason that was ok. Guess it’s ok though  a true 20 amp plug won’t fit a 15 amp receptacle.

Edited by Bill Kielb
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Bill, exactly.  15amp receptacles can be on 20 amp circits, just like in your kitchen counter outlets or bathroom circuits.

3 hours ago, neilestrick said:

Makes me wonder how much stuff out there isn't actually up to code simply because the code is so large and confusing.

A metric poop-ton Neil, but code here has been taken to a new draconian level here in CA IMO.  Liability only drives the bldg depts and if anything deviates from it they want a prof. engineer stamp.  They used to make a 'residential' code book for homeowners, so folks could do their own home improvements and not have to wade through the larger code books.  I don't know if that still exists, and even that was the UBC (building code) not  mech/elec/plumbing.

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22 hours ago, Dick White said:

Yes, most sparkies are responsible within their area of expertise and don't immediately recognize how different a kiln is from just a big hair dryer in the bathroom. And not being familiar with the Skutt et al websites where explicit specification are listed, they would look at the rating plate and do what seems normal for the numbers on the plate. It's the few who show up in a pottery group and insist that because they have a license they know what they are talking about and and it absolutely must be fused at the rating plate. As noted, most sparkies are responsible and will adjust their opinion (and work) when the strange nuance of 125% is shown to them, but every once in a while we get into arguments about it. I don't have a license, but I can read the code and from that understand why Skutt et al say what they say. Thanks for everybody's help and input,

Typically, the nameplate rating is what we use or manufacture's recommended circuit size for the appliance.  If Skutts' tech data recommended to size the circuit bigger than nameplate and it complied with code I would size it bigger. 

The problem is most companies throw in the 'must comply with local codes' to cover their rears, throwing the liability to the building dept who then tosses it on down to the contractor who tosses it to his liability insurance company, who he pays dearly for, which passes it on to the customer.

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