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Discussion Starter #1
Hey gang, I have a quick question for you.

I should state that I am an accomplished handyman and have done quite alot of construction and electrical work that stood up to inspection. This time i have a unique situation and need some advice.

I currently have a 240 volt circuit that has a continuos load of 18 amps for about 14 hours a day (kiln). I ran 10 gauge wire for this circuit. I am planning on installing a split air conditioner as my studio is quite hot. It pulls a maximum of 8 amps (240) I dont have space in my panel for another double pole breaker (no I am not exceeding the overall load rating) so I wanted to run a 12/2 line to the new air conditioner (20') and run it to the same breaker. Is this going to cause a problem? The breaker is rated for 40 amps.

I seem to recall reading about 5 years ago that I read something about having to use the same gauge wires when they both connect to one breaker.

Thanks!
 

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Semi-Pro Electro-Geek
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There are several problems here. First of all, #10 wire must be protected by a 30A or smaller breaker. You can't use #10 on a 40A circuit. For that kiln, you need a circuit ampacity of at least 23A, so you could just replace that 40A breaker with a 30A and you'd be fine. The air conditioner almost certainly states on the label (or in the manual) that it must be on a dedicated circuit of a certain ampacity. The allowable circuit breaker size will be specified by the manufacturer. It will probably also specify the required wire size. You'll have to find a way to get another 2-pole breaker in there. Perhaps you could use tandem breakers elsewhere in the panel?
 

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Allowable breaker size means that the appliance must not be plugged into a circuit with a higher breaker rating. Also plugs andreceptacles come in different prong arrangements for different breaker (and circuit) ratings and you must not install receptacles on circuits higher than the rating. (You may install 15 amp receptacles on 20 amp circuits provided you install at least one duplex or two singles per circuit.)

So an air conditioner rated for a 20 or fewer amp circuit can't be plugged into a 30 amp kiln circuit even if the two items together draw well under 30 amps.
 

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My idea was the same as mpoulton's... consolidate two breakers into a tandem breaker and dedicate a circuit to that air conditioner.
 

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Discussion Starter #5
Got it. So if I understand correctly, the problem here is two fold. 1. If there was a short in the kiln the 10 gauge wire would melt before the breaker popped and 2. Same deal with the AC although it might fry the AC because the breaker refused to pop?

I really don't want to install two more tandems but indeed this seems the safe and proper route.
 

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What brand of panel do you have? Some breakers are listed to allow two wires. If this is what you have then maybe you could consolidate some exiting circuits to free up some space.
FYI - Just so you learn something new...
consider for a moment that your kiln is off and you had tapped that #12 into the 40 amp breaker. Now your #12 circuit is breakered at 40 amps but your wire is only rated for 20 amps. See the problem?
 

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Discussion Starter #7
It's a square d panel. Yep, I do see the bigger problem. I short sonething, burn down the house and the insurance company says tough luck. Reminds me of my favorite saying, everything is fine, until it isn't

Thanks again !
 

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What brand of panel do you have? Some breakers are listed to allow two wires. If this is what you have then maybe you could consolidate some exiting circuits to free up some space.
FYI - Just so you learn something new...
consider for a moment that your kiln is off and you had tapped that #12 into the 40 amp breaker. Now your #12 circuit is breakered at 40 amps but your wire is only rated for 20 amps. See the problem?
12 wire in just not rated for 20 amps, when using it with motors the rating is higher. If an ac unit says min amps is 25 max 30, than 12 wire could be used.
 

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Discussion Starter #10
Let me ask you this. And I realize I'm still under rated on my 10 gauge to 40 amp breaker but given the conservative nature of NEC I should probably be ok if I left it


What if I installed a fusable (30 amp I believe is the one they had on the shelf) pullout box before the AC? Seeing as I have to install one anyways....

My thought is that if there was some kind of short in the condenser it would pop the 30 amp fuse, there by preventing a large load on the 12 gauge wire. The pig tail from the cutoff box to the condenser has what appears to be 10 gauge wire.
 

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Just seems like it would be easier to spend a couple bucks pulling one breaker for a tandem and moving the wiring a few inches.
 

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Discussion Starter #14
Well. I need to pull two breakers and move four wires in order to fit a new 240 but your probably right

Thanks to everyone that helped
 

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You could also pull a new 6/3 cable from your panel to the studio and install a subpanel there. Kiln, AC, and any other needed circuits could then be run from it. The existing 40A breaker for the kiln would be replaced with a 50 or 60A. No tandems required (there's a good chance your panel won't accept them anyway.)
 

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Isn't the Min amps and Max amps referring to the breaker size?
Not quite.

The minimum Amps determines the wire size.

The maximum Amps determines the breaker size.

For example: Suppose you had a unit with a nameplate that stated minimum circuit amps = 30, max breaker = 50.

You could wire said unit with a #10 conductor, protected by a 50 Amp breaker under such a scenario.
 
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