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Old 01-29-2017, 09:58 PM   #1
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Default Safety Switch vs. Breaker disconnect (electric kiln)

I'm quoting a branch circuit for a 240v 13kw electric kiln in a residential basement that requires a 70-amp breaker. I believe a 70-amp enclosure with 70-amp circuit breaker, which would cost about $70 (coincidence?), would suffice for the local disconnect. However, I see non-fused safety switches out there at considerably more money: $200 and up. Opinions?
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Old 01-29-2017, 10:08 PM   #2
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I'm quoting a branch circuit for a 240v 13kw electric kiln in a residential basement that requires a 70-amp breaker. I believe a 70-amp enclosure with 70-amp circuit breaker, which would cost about $70 (coincidence?), would suffice for the local disconnect. However, I see non-fused safety switches out there at considerably more money: $200 and up. Opinions?
I would have no problem putting in a CB enclosure to act as the local disconnect. The only time I use a one arm bandit in residential, is if I need fuses. I might bump up to a 100 amp circuit because its close to the 70 amp rating. Someone with a code book handy might look this up.

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Old 01-30-2017, 05:45 AM   #3
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If it's 13kw wouldn't it be 55amps?



A non fused disconnect runs @ $10 at the Depot.

Never put more than a disconnect on a kiln yet and have done several in schools.
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Old 01-30-2017, 07:24 AM   #4
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Its continuous, so 125% gets you up to 67-68 for the conductors. If the thing ever got replaced, the circuit would be too small. That's where my head was at.
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Old 01-30-2017, 11:05 AM   #5
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Its continuous, so 125% gets you up to 67-68 for the conductors. If the thing ever got replaced, the circuit would be too small. That's where my head was at.
I gave an attempt at humor there with the ac disco, but my point was if you have a breaker at the source you only need a disco at the kiln itself.

While the breaker has to be rated to cover the 67-68 amps would a basic disco also have to be?
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Old 01-30-2017, 11:59 AM   #6
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The breaker has to protect the conductors. So it can be smaller than the max the wires can handle. But of course, if it's too small it will not carry the load. The wires have to be larger than the load, 125 %. And can be as big as you want. The disco has to carry and interrupt the load. So it should be sized to it. I believe because of the nature of the load, it also has to be at 125%. I could be wrong. I'd make sure the 13kW is a standard size. If it ever changes to say a 15kW, the wire and disco would be under sized.
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Old 01-30-2017, 12:11 PM   #7
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The breaker has to protect the conductors. So it can be smaller than the max the wires can handle. But of course, if it's too small it will not carry the load. The wires have to be larger than the load, 125 %. And can be as big as you want. The disco has to carry and interrupt the load. So it should be sized to it. I believe because of the nature of the load, it also has to be at 125%. I could be wrong. I'd make sure the 13kW is a standard size. If it ever changes to say a 15kW, the wire and disco would be under sized.
I wasn't sure on this either. Code is my weak point and at this stage will remain that way.

We used 100 amp discos on the kilns we put in schools that were 208v-240v.
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Old 01-30-2017, 01:35 PM   #8
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Its continuous, so 125% gets you up to 67-68 for the conductors. If the thing ever got replaced, the circuit would be too small. That's where my head was at.
If replaced, more likely with a smaller kiln after the HO sees the electric bills.
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Old 01-30-2017, 03:22 PM   #9
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NEC rule on disconnects is minimum 115% of the load. Conductors are 125% of the load, and if using a fused disconnect as the feeder, you would need to size it for the protection for the conductors, but a NF disconnect at the load only needs to be 115% (or if it's a motor, it must have the HP rating of the motor). Bottom line though, because disconnects come in 30, 60 and 100A ratings, 60A is less than 115% of 55A, so you would need to use a 100A NF switch.

Chances are about 99% that the customer's load center will need a feeder breaker in it (you didn't say), 70A would be the best choice. No need for more fusing or CB in that case, so a cheap general duty non-fused disconnect would be fine.

But since there is no "70A" NF switch, a 100A switch will indeed cost you more than $70. So although you don't need the breaker, if it's cheaper, I'd do it.

Just make sure it has a hasp for a Lock Out padlock, that's the point of a local disconnect.
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Old 01-30-2017, 10:13 PM   #10
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Another item to look at when selecting a disconnect is it's listed to make or break the load.
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