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Old 04-17-2008, 01:51 PM   #1
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240 circuit kiln


Ive got a 240v, 48A, 11540W Kiln that needs to be hooked up. Would the best be to run #6-4 Romex on a 50A Double Breaker? Or would a #8-4 Romex on a 40A Double Breaker work? Thanks

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Old 04-17-2008, 03:09 PM   #2
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240 circuit kiln


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Ive got a 240v, 48A, 11540W Kiln that needs to be hooked up. Would the best be to run #6-4 Romex on a 50A Double Breaker? Or would a #8-4 Romex on a 40A Double Breaker work? Thanks
I would use a #6 romex on a 60 amp 2 pole (double) breaker. Does the unit require 4 conductors? If so, the romex would be called 6-3 with ground.

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Old 04-17-2008, 04:32 PM   #3
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240 circuit kiln


You can use 6/2 NM. Most kilns are straight 240v. No neutral is required and you can use the black and white as the two hots.

With the cost of copper wire these days you will see a noticeable difference in the price.
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Old 04-17-2008, 06:12 PM   #4
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240 circuit kiln


Kilns are generally classified as continuous loads by the manufacturer and usually require connection to 90C conductors. So a 48 amp 240 volt kiln would require copper conductors rated at 60 amps. #6 romex is 55 amps. Use 6 awg THHN in conduit for 65 amps and ocpd of 60.

Most kilns will not allow connection to AL.
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Old 04-17-2008, 06:25 PM   #5
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240 circuit kiln


I should say most kiln installation instructions will prohibit aluminum wire.

The kiln could care less.....
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Old 04-17-2008, 09:06 PM   #6
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240 circuit kiln


Thank You...I will use 6-2 copper romex w/ground on a 60A double breaker. Could I just hard wire it also instead of looking for the matching outlet? It has a plug on it already that looks like a dryer plug, 3-prong.
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Old 04-17-2008, 09:09 PM   #7
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240 circuit kiln


What is the model an name of the kiln?
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Old 04-17-2008, 09:50 PM   #8
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240 circuit kiln


I will have to get back with you on that. Its at another location from my house. But I know its going to be used to heat pottery.
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Old 04-17-2008, 09:59 PM   #9
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240 circuit kiln


Without knowing the model and the manufacturer there is no definitive way to give you a 'for sure' answer. If the kiln is considered a continuous load you must use conductors that are rated 60 amps, 6/2 romex is not.

I have checked several manufacturers since you started the thread and all are showing their kilns continuous loads. the ones I've been able to verify that are 48 or so amps require #6 copper thhn on 60 amp ocpd.

Yes you can hard wire the kiln.
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Old 04-18-2008, 09:57 AM   #10
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240 circuit kiln


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Originally Posted by Stubbie View Post
Without knowing the model and the manufacturer there is no definitive way to give you a 'for sure' answer. If the kiln is considered a continuous load you must use conductors that are rated 60 amps, 6/2 romex is not.

I have checked several manufacturers since you started the thread and all are showing their kilns continuous loads. the ones I've been able to verify that are 48 or so amps require #6 copper thhn on 60 amp ocpd.

Yes you can hard wire the kiln.
Stubbie, according to 310.15 #6 romex is good for 65 amps. Romex is a 90 degree conductor, using the 75c column I get 65. So #6-2 with ground romex on a 60 amp breaker will be ok good.
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Old 04-18-2008, 10:17 AM   #11
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240 circuit kiln


Hi John

To my knowledge romex has never been allowed to use the 75C column. See
334.80. The section that is specific to NM cable will override that in 310.15.
Yes, the conductors are rated 90c however they are not handled like thhn or thwn as for ampacity.

The 90 c rating can only be used when NM conductors are derated for bundling or conduit and with NM cables the final derating cannot exceed the 60C column.

The overcurrent can be 60 amps for #6 nm-b but only because the next size up rule is applied since the ampacity is only 55 amps for #6 nm-b column 1 60C conductors.
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Old 04-18-2008, 10:50 AM   #12
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240 circuit kiln


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The overcurrent can be 60 amps for #6 nm-b but only because the next size up rule is applied since the ampacity is only 55 amps for #6 nm-b column 1 60C conductors.
Bingo!




There is pretty much NO situation where we can use the 90 deg C column to figure circuit amperage, since there is basically nothing that is rated for 90 deg C other than the conductors themselves. Most breakers and terminals are rated for 75 deg C.
As Stub stated, because of 334.80 we MUST use the 60 deg C column for figuring ampacity of NM circuits.
True, we do use the actual conductor temperature column for derating purposes.
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Old 04-18-2008, 11:00 AM   #13
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240 circuit kiln


The 'size up rule' is found in 240.4(B)

It states "the next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to bo used provided all the following conditions are met: (1) the conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads. (2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments). (3) The next higher rating selected does not exceed 800 amperes."

The standard ampere ratings are found in 240.6(A) It goes 50 then 60, 55 is not one of them, so a #6 NM could be legally protected by a 60 amp breaker.

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Old 04-18-2008, 11:05 AM   #14
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240 circuit kiln


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Originally Posted by jambud View Post
Thank You...I will use 6-2 copper romex w/ground on a 60A double breaker. Could I just hard wire it also instead of looking for the matching outlet? It has a plug on it already that looks like a dryer plug, 3-prong.
Hard wire it. Or do you feel the need to move it around and fire in the Livingroom or the Kitchen on occasion?
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Old 04-18-2008, 12:00 PM   #15
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240 circuit kiln


Jambud

In case I haven't been clear... my point is that a 48 amp continuous load requires conductors rated 125% of the continuous load (1.25 x 48 amps = 60) and the overcurrent protection for a continuous load must also be 125%. #6 romex is only rated 55 amps. You need conductors rated 60 amps. If you want to use a cable then get #6 awg copper SER . Your kiln will be required to have 90C or 75C conductors but if your running from an old breaker or fuse box be sure that it is rated for 75C connections for ampacity purposes usually it states this on the panel cover sheet. If you can not confirm 75C for the panel box then you must use 60C ampacities and a #4 conductor will be required.


Last edited by Stubbie; 04-18-2008 at 12:03 PM.
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